Architecture and programming of 8051 MCU's TOC Chapter 1 Chapter
2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7Chapter1:
Introduction to Microcontrollers 1.1 What are microcontrollers and
what are they used for? 1.2 What is what in
microcontroller?IntroductionIt was electricity in the
beginning....The people were happy because they did not know that
it was all around them and could be utilized. That was good. Then
Faraday came and a stone has started to roll slowly...The first
machines using a new sort of energy appeared soon. A long time has
passed since then and just when the people finally got used to them
and stopped paying attention to what a new generation of
specialists were doing, someone came to an idea that electrons
could be a very convenient toy being closed in a glass pipe. It was
just a good idea at first, but there was no return. Electonics was
born and the stone kept on rolling down the hill faster and
faster...A new science - new specialists. Blue coats were replaced
with white ones and people who knew something about electronics
appeared on the stage. While the rest of humanity were passively
watching in disbelief what was going on, the plotters split in two
groups - software-oriented and hardware-oriented. Somewhat younger
than their teachers, very enthusiastic and full of ideas, both of
them kept on working but separate ways. While the first group was
developing constantly and gradually, the hardware-oriented people,
driven by success, threw caution to the wind and invented
transistors.Up till that moment, the things could be more or less
kept under control, but a broad publicity was not aware of what was
going on, which soon led to a fatal mistake! Being naive in belief
that cheap tricks could slow down technology development and
development of the world and retrieve the good all days, mass
market opened its doors for the products of Electronics Industry,
thus closing a magic circle. A rapid drop in prices made these
components available for a great variety of people. The stone was
falling freely...The first integrated circuits and processors
appeared soon, which caused computers and other products of
electronics to drop down in price even more. They could be bought
everywhere. Another circle was closed! Ordinary people got hold of
computers and computer era has begun...While this drama was going
on, hobbyists and professionals, also split in two groups and
protected by anonymity, were working hard on their projects. Then,
someone suddenly put a question: Why should not we make a universal
component? A cheap, universal integrated circuit that could be
programmed and used in any field of electronics, device or wherever
needed? Technology has been developed enough as well as the market.
Why not? So it happened, body and spirit were united and the first
integrated circuit was designed and called the MICROCONTROLLER.1.1
What are microcontrollers and what are they used for?Like all good
things, this powerful component is basically very simple. It is
made by mixing tested and high- quality "ingredients" (components)
as per following receipt:1. The simplest computer processor is used
as the "brain" of the future system.2. Depending on the taste of
the manufacturer, a bit of memory, a few A/D converters, timers,
input/output lines etc. are added3. All that is placed in some of
the standard packages.4. A simple software able to control it all
and which everyone can easily learn about has been developed.On the
basis of these rules, numerous types of microcontrollers were
designed and they quickly became man's invisible companion. Their
incredible simplicity and flexibility conquered us a long time ago
and if you try to invent something about them, you should know that
you are probably late, someone before you has either done it or at
least has tried to do it.The following things have had a crucial
influence on development and success of the microcontrollers:
Powerful and carefully chosen electronics embedded in the
microcontrollers can independetly or via input/output devices
(switches, push buttons, sensors, LCD displays, relays etc.),
control various processes and devices such as industrial
automation, electric current, temperature, engine performance etc.
Very low prices enable them to be embedded in such devices in
which, until recent time it was not worthwhile to embed anything.
Thanks to that, the world is overwhelmed today with cheap automatic
devices and various smart appliences. Prior knowledge is hardly
needed for programming. It is sufficient to have a PC (software in
use is not demanding at all and is easy to learn) and a simple
device (called the programmer) used for loading raedy-to-use
programs into the microcontroller.So, if you are infected with a
virus called electronics, there is nothing left for you to do but
to learn how to use and control its power.How does the
microcontroller operate?Even though there is a large number of
different types of microcontrollers and even more programs created
for their use only, all of them have many things in common. Thus,
if you learn to handle one of them you will be able to handle them
all. A typical scenario on the basis of which it all functions is
as follows:1. Power supply is turned off and everything is stillthe
program is loaded into the microcontroller, nothing indicates what
is about to comePower supply is turned on and everything starts to
happen at high speed! The control logic unit keeps everything under
control. It disables all other circuits except quartz crystal to
operate. While the preparations are in progress, the first
milliseconds go by.2. Power supply voltage reaches its maximum and
oscillator frequency becomes stable. SFRs are being filled with
bits reflecting the state of all circuits within the
microcontroller. All pins are configured as inputs. The overall
electronis starts operation in rhythm with pulse sequence. From now
on the time is measured in micro and nanoseconds.3. Program Counter
is set to zero. Instruction from that address is sent to
instruction decoder which recognizes it, after which it is executed
with immediate effect.4. The value of the Program Counter is
incremented by 1 and the whole process is repeated...several
million times per second.
1.2 What is what in the microcontroller?As you can see, all the
operations within the microcontroller are performed at high speed
and quite simply, but the microcontroller itself would not be so
useful if there are not special circuits which make it complete. In
continuation, we are going to call your attention to them.Read Only
Memory (ROM)Read Only Memory (ROM) is a type of memory used to
permanently save the program being executed. The size of the
program that can be written depends on the size of this memory. ROM
can be built in the microcontroller or added as an external chip,
which depends on the type of the microcontroller. Both options have
some disadvantages. If ROM is added as an external chip, the
microcontroller is cheaper and the program can be considerably
longer. At the same time, a number of available pins is reduced as
the microcontroller uses its own input/output ports for connection
to the chip. The internal ROM is usually smaller and more
expensive, but leaves more pins available for connecting to
peripheral environment. The size of ROM ranges from 512B to
64KB.Random Access Memory (RAM)Random Access Memory (RAM) is a type
of memory used for temporary storing data and intermediate results
created and used during the operation of the microcontrollers. The
content of this memory is cleared once the power supply is off. For
example, if the program performes an addition, it is necessary to
have a register standing for what in everyday life is called the
sum . For that purpose, one of the registers in RAM is called the
"sum" and used for storing results of addition. The size of RAM
goes up to a few KBs.Electrically Erasable Programmable ROM
(EEPROM)The EEPROM is a special type of memory not contained in all
microcontrollers. Its contents may be changed during program
execution (similar to RAM ), but remains permanently saved even
after the loss of power (similar to ROM). It is often used to store
values, created and used during operation (such as calibration
values, codes, values to count up to etc.), which must be saved
after turning the power supply off. A disadvantage of this memory
is that the process of programming is relatively slow. It is
measured in miliseconds.
Special Function Registers (SFR)Special function registers are
part of RAM memory. Their purpose is predefined by the manufacturer
and cannot be changed therefore. Since their bits are physically
connected to particular circuits within the microcontroller, such
as A/D converter, serial communication module etc., any change of
their state directly affects the operation of the microcontroller
or some of the circuits. For example, writing zero or one to the
SFR controlling an input/output port causes the appropriate port
pin to be configured as input or output. In other words, each bit
of this register controls the function of one single pin.Program
CounterProgram Counter is an engine running the program and points
to the memory address containing the next instruction to execute.
After each instruction execution, the value of the counter is
incremented by 1. For this reason, the program executes only one
instruction at a time just as it is written. Howeverthe value of
the program counter can be changed at any moment, which causes a
jump to a new memory location. This is how subroutines and branch
instructions are executed. After jumping, the counter resumes even
and monotonous automatic counting +1, +1, +1Central Processor Unit
(CPU)As its name suggests, this is a unit which monitors and
controls all processes within the microcontroller and the user
cannot affect its work. It consists of several smaller subunits, of
which the most important are: Instruction decoderis a part of the
electronics which recognizes program instructions and runs other
circuits on the basis of that. The abilities of this circuit are
expressed in the "instruction set" which is different for each
microcontroller family. Arithmetical Logical Unit (ALU)performs all
mathematical and logical operations upon data. Accumulatoris an SFR
closely related to the operation of ALU. It is a kind of working
desk used for storing all data upon which some operations should be
executed (addition, shift etc.). It also stores the results ready
for use in further processing. One of the SFRs, called the Status
Register, is closely related to the accumulator, showing at any
given time the "status" of a number stored in the accumulator (the
number is greater or less than zero etc.).
A bitis just a word invented to confuse novices at electronics.
Joking aside, this word in practice indicates whether the voltage
is present on a conductor or not. If it is present, the approprite
pin is set to logic one (1), i.e. the bits value is 1. Otherwise,
if the voltage is 0 V, the appropriate pin is cleared (0), i.e. the
bits value is 0. It is more complicated in theory where a bit is
referred to as a binary digit, but even in this case, its value can
be either 0 or 1.Input/output ports (I/O Ports)In order to make the
microcontroller useful, it is necessary to connect it to peripheral
devices. Each microcontroller has one or more registers (called a
port) connected to the microcontroller pins.
Why do we call them input/output ports? Because it is possible
to change a pin function according to the user's needs. These
registers are the only registers in the microcontroller the state
of which can be checked by voltmeter!Oscillator
Even pulses generated by the oscillator enable harmonic and
synchronous operation of all circuits within the microcontroller.
It is usually configured as to use quartz-crystal or ceramics
resonator for frequency stabilization. It can also operate without
elements for frequency stabilization (like RC oscillator). It is
important to say that program instructions are not executed at the
rate imposed by the oscillator itself, but several times slower. It
happens because each instruction is executed in several steps. For
some microcontrollers, the same number of cycles is needed to
execute any instruction, while it's different for other
microcontrollers. Accordingly, if the system uses quartz crystal
with a frequency of 20MHz, the execution time of an instruction is
not expected 50nS, but 200, 400 or even 800 nS, depending on the
type of the microcontroller!Timers/CountersMost programs use these
miniature electronic "stopwatches" in their operation. These are
commonly 8- or 16-bit SFRs the contents of which is automatically
incremented by each coming pulse. Once the register is completely
loaded, an interrupt is generated!If these registers use an
internal quartz oscillator as a clock source, then it is possible
to measure the time between two events (if the register value is T1
at the moment measurement has started, and T2 at the moment it has
finished, then the elapsed time is equal to the result of
subtraction T2-T1 ). If the registers use pulses coming from
external source, then such a timer is turned into a counter.This is
only a simple explanation of the operation itself. Its somehow more
complicated in practice.
A registeror a memory cell is an electronic circuit which can
memorize the state of one byte. Besides 8 bits available to the
user, each register has also a number of addressing bits. It is
important to remember that: All registers of ROM as well as those
of RAM referred to as general-purpose registers are mutually equal
and nameless. During programming, each of them can be assigned a
name, which makes the whole operation much easier. All SFRs are
assigned names which are different for different types of the
microcontrollers and each of them has a special function as their
name suggests.Watchdog timerThe Watchdog Timer is a timer connected
to a completely separate RC oscillator within the
microcontroller.If the watchdog timer is enabled, every time it
counts up to the program end, the microcontroller reset occurs and
program execution starts from the first instruction. The point is
to prevent this from happening by using a special command. The
whole idea is based on the fact that every program is executed in
several longer or shorter loops.If instructions resetting the
watchdog timer are set at the appropriate program locations,
besides commands being regularly executed, then the operation of
the watchdog timer will not affect the program execution.If for any
reason (usually electrical noise in industry), the program counter
"gets stuck" at some memory location from which there is no return,
the watchdog will not be cleared, so the registers value being
constantly incremented will reach the maximum et voila! Reset
occurs!Power Supply CircuitThere are two things worth attention
concerning the microcontroller power supply circuit:
Brown outis a potentially dangerous state which occurs at the
moment the microcontroller is being turned off or when power supply
voltage drops to the lowest level due to electric noise. As the
microcontroller consists of several circuits which have different
operating voltage levels, this can cause its out of control
performance. In order to prevent it, the microcontroller usually
has a circuit for brown out reset built-in. This circuit
immediately resets the whole electronics when the voltage level
drops below the lower limit.Reset pinis usually referred to as
Master Clear Reset (MCLR) and serves for external reset of the
microcontroller by applying logic zero (0) or one (1) depending on
the type of the microcontroller. In case the brown out is not built
in the microcontroller, a simple external circuit for brown out
reset can be connected to this pin.Serial communication
Parallel connections between the microcontroller and peripherals
established over I/O ports are the ideal solution for shorter
distances up to several meters. However, in other cases, when it is
necessary to establish communication between two devices on longer
distances it is obviously not possible to use parallel connections.
Then, serial communication is the best solution.Today, most
microcontrollers have several different systems for serial
communication built in as a standard equipment. Which of them will
be used depends on many factors of which the most important are:
How many devices the microcontroller has to exchange data with? How
fast the data exchange has to be? What is the distance between
devices? Is it necessary to send and receive data
simultaneously?One of the most important things concerning serial
communication is the Protocol which should be strictly observed. It
is a set of rules which must be applied in order that devices can
correctly interpret data they mutually exchange. Fortunately, the
microcontrollers automatically take care of this, so the work of
the programmer/user is reduced to a simple write (data to be sent)
and read (received data).
A byteconsists of 8 bits grouped together. If a bit is a digit
then it is logical that bytes are numbers. All mathematical
operations can be performed upon them, just like upon common
decimal numbers, which is carried out in the ALU. It is important
to remember that byte digits are not of equal significance. The
largest value has the leftmost bit called the most significant bit
(MSB). The rightmost bit has the least value and is therefore
called the least significant bit (LSB). Since 8 digits (zeros and
ones) of one byte can be combined in 256 different ways, the
largest decimal number which can be represented by one byte is 255
(one combination represents zero).ProgramUnlike other integrated
circuits which only need to be connected to other components and
turn the power supply on, the microcontrollers need to be
programmed first. This is a so called "bitter pill" and the main
reason why hardware-oriented electronics engineers stay away from
microcontrollers. It is a trap causing huge losses because the
process of programming the microcontroller is basically very
simple.In order to write a program for the microcontroller, several
"low-level" programming languages can be used such as Assembly, C
and Basic (and their versions as well). Writing program procedure
consists of simple writing instructions in the order in which they
should be executed. There are also many programs running in Windows
environment used to facilitate the work providing additional visual
tools.This book describes the use of Assembly because it is the
simplest language with the fastest execution allowing entire
control on what is going on in the circuit.
Interrupt-electronics is usually more faster than physical
processes it should keep under control. This is why the
microcontroller spends most of its time waiting for something to
happen or execute. In other words, when some event takes place, the
microcontroller does something. In order to prevent the
microcontroller from spending most of its time endlessly checking
for logic state on input pins and registers, an interrupt is
generated. It is the signal which informs the central processor
that something attention worthy has happened. As its name suggests,
it interrupts regular program execution. It can be generated by
different sources so when it occurs, the microcontroller
immediately stops operation and checks for the cause. If it is
needed to perform some operations, a current state of the program
counter is pushed onto the Stack and the appropriate program is
executed. It's the so called interrupt routine.
Stackis a part of RAM used for storing the current state of the
program counter (address) when an interrupt occurs. In this way,
after a subroutine or an interrupt execution, the microcontroller
knows from where to continue regular program execution. This
address is cleared after returning to the program because there is
no need to save it any longer, and one location of the stack is
automatically availale for further use. In addition, the stack can
consist of several levels. This enables subroutines nesting, i.e.
calling one subroutine from another.
2.1 What is 8051 Standard?Microcontroller manufacturers have
been competing for a long time for attracting choosy customers and
every couple of days a new chip with a higher operating frequency,
more memory and upgraded A/D converters appeared on the
market.However, most of them had the same or at least very similar
architecture known in the world of microcontrollers as 8051
compatible. What is all this about?The whole story has its
beginnings in the far 80s when Intel launched the first series of
microcontrollers called the MCS 051. Even though these
microcontrollers had quite modest features in comparison to the new
ones, they conquered the world very soon and became a standard for
what nowadays is called the microcontroller.The main reason for
their great success and popularity is a skillfully chosen
configuration which satisfies different needs of a large number of
users allowing at the same time constant expansions (refers to the
new types of microcontrollers). Besides, the software has been
developed in great extend in the meantime, and it simply was not
profitable to change anything in the microcontrollers basic core.
This is the reason for having a great number of various
microcontrollers which basically are solely upgraded versions of
the 8051 family. What makes this microcontroller so special and
universal so that almost all manufacturers all over the world
manufacture it today under different name?
As seen in figure above, the 8051 microcontroller has nothing
impressive in appearance: 4 Kb of ROM is not much at all. 128b of
RAM (including SFRs) satisfies the user's basic needs. 4 ports
having in total of 32 input/output lines are in most cases
sufficient to make all necessary connections to peripheral
environment.The whole configuration is obviously thought of as to
satisfy the needs of most programmers working on development of
automation devices. One of its advantages is that nothing is
missing and nothing is too much. In other words, it is created
exactly in accordance to the average users taste and needs. Another
advantages are RAM organization, the operation of Central Processor
Unit (CPU) and ports which completely use all recourses and enable
further upgrade.2.2 Pinout DescriptionPins 1-8:Port 1Each of these
pins can be configured as an input or an output.Pin 9: RSA logic
one on this pin disables the microcontroller and clears the
contents of most registers. In other words, the positive voltage on
this pin resets the microcontroller. By applying logic zero to this
pin, the program starts execution from the beginning.Pins10-17:Port
3Similar to port 1, each of these pins can serve as general input
or output. Besides, all of them have alternative functions:Pin
10:RXDSerial asynchronous communication input or Serial synchronous
communication output.Pin 11:TXDSerial asynchronous communication
output or Serial synchronous communication clock output.Pin
12:INT0Interrupt 0 input.Pin 13:INT1Interrupt 1 input.Pin
14:T0Counter 0 clock input.Pin 15:T1Counter 1 clock input.Pin
16:WRWrite to external (additional) RAM.Pin 17:RDRead from external
RAM.Pin 18, 19:X2, X1Internal oscillator input and output. A quartz
crystal which specifies operating frequency is usually connected to
these pins. Instead of it, miniature ceramics resonators can also
be used for frequency stability. Later versions of microcontrollers
operate at a frequency of 0 Hz up to over 50 Hz.Pin
20:GNDGround.Pin 21-28:Port 2If there is no intention to use
external memory then these port pins are configured as general
inputs/outputs. In case external memory is used, the higher address
byte, i.e. addresses A8-A15 will appear on this port. Even though
memory with capacity of 64Kb is not used, which means that not all
eight port bits are used for its addressing, the rest of them are
not available as inputs/outputs.Pin 29:PSENIf external ROM is used
for storing program then a logic zero (0) appears on it every time
the microcontroller reads a byte from memory.Pin 30:ALEPrior to
reading from external memory, the microcontroller puts the lower
address byte (A0-A7) on P0 and activates the ALE output. After
receiving signal from the ALE pin, the external register (usually
74HCT373 or 74HCT375 add-on chip) memorizes the state of P0 and
uses it as a memory chip address. Immediately after that, the ALU
pin is returned its previous logic state and P0 is now used as a
Data Bus. As seen, port data multiplexing is performed by means of
only one additional (and cheap) integrated circuit. In other words,
this port is used for both data and address transmission.Pin
31:EABy applying logic zero to this pin, P2 and P3 are used for
data and address transmission with no regard to whether there is
internal memory or not. It means that even there is a program
written to the microcontroller, it will not be executed. Instead,
the program written to external ROM will be executed. By applying
logic one to the EA pin, the microcontroller will use both
memories, first internal then external (if exists).Pin 32-39:Port
0Similar to P2, if external memory is not used, these pins can be
used as general inputs/outputs. Otherwise, P0 is configured as
address output (A0-A7) when the ALE pin is driven high (1) or as
data output (Data Bus) when the ALE pin is driven low (0).Pin
40:VCC+5V power supply.2.3 Input/Output Ports (I/O Ports)All 8051
microcontrollers have 4 I/O ports each comprising 8 bits which can
be configured as inputs or outputs. Accordingly, in total of 32
input/output pins enabling the microcontroller to be connected to
peripheral devices are available for use.Pin configuration, i.e.
whether it is to be configured as an input (1) or an output (0),
depends on its logic state. In order to configure a microcontroller
pin as an output, it is necessary to apply a logic zero (0) to
appropriate I/O port bit. In this case, voltage level on
appropriate pin will be 0.Similarly, in order to configure a
microcontroller pin as an input, it is necessary to apply a logic
one (1) to appropriate port. In this case, voltage level on
appropriate pin will be 5V (as is the case with any TTL input).
This may seem confusing but don't loose your patience. It all
becomes clear after studying simple electronic circuits connected
to an I/O pin.
Input/Output (I/O) pinFigure above illustrates a simplified
schematic of all circuits within the microcontroler connected to
one of its pins. It refers to all the pins except those of the P0
port which do not have pull-up resistors built-in.
Output pinA logic zero (0) is applied to a bit of the P
register. The output FE transistor is turned on, thus connecting
the appropriate pin to ground.
Input pinA logic one (1) is applied to a bit of the P register.
The output FE transistor is turned off and the appropriate pin
remains connected to the power supply voltage over a pull-up
resistor of high resistance.
Logic state (voltage) of any pin can be changed or read at any
moment. A logic zero (0) and logic one (1) are not equal. A logic
one (0) represents a short circuit to ground. Such a pin acts as an
output.A logic one (1) is loosely connected to the power supply
voltage over a resistor of high resistance. Since this voltage can
be easily reduced by an external signal, such a pin acts as an
input.Port 0The P0 port is characterized by two functions. If
external memory is used then the lower address byte (addresses
A0-A7) is applied on it. Otherwise, all bits of this port are
configured as inputs/outputs.The other function is expressed when
it is configured as an output. Unlike other ports consisting of
pins with built-in pull-up resistor connected by its end to 5 V
power supply, pins of this port have this resistor left out. This
apparently small difference has its consequences:
If any pin of this port is configured as an input then it acts
as if it floats. Such an input has unlimited input resistance and
indetermined potential.
When the pin is configured as an output, it acts as an open
drain. By applying logic 0 to a port bit, the appropriate pin will
be connected to ground (0V). By applying logic 1, the external
output will keep on floating. In order to apply logic 1 (5V) on
this output pin, it is necessary to built in an external pull-up
resistor.
Only in case P0 is used for addressing external memory, the
microcontroller will provide internal power supply source in order
to supply its pins with logic one. There is no need to add external
pull-up resistors.Port 1P1 is a true I/O port, because it doesn't
have any alternative functions as is the case with P0, but can be
cofigured as general I/O only. It has a pull-up resistor built-in
and is completely compatible with TTL circuits.Port 2P2 acts
similarly to P0 when external memory is used. Pins of this port
occupy addresses intended for external memory chip. This time it is
about the higher address byte with addresses A8-A15. When no memory
is added, this port can be used as a general input/output port
showing features similar to P1.Port 3All port pins can be used as
general I/O, but they also have an alternative function. In order
to use these alternative functions, a logic one (1) must be applied
to appropriate bit of the P3 register. In tems of hardware, this
port is similar to P0, with the difference that its pins have a
pull-up resistor built-in.Pin's Current limitationsWhen configured
as outputs (logic zero (0)), single port pins can receive a current
of 10mA. If all 8 bits of a port are active, a total current must
be limited to 15mA (port P0: 26mA). If all ports (32 bits) are
active, total maximum current must be limited to 71mA. When these
pins are configured as inputs (logic 1), built-in pull-up resistors
provide very weak current, but strong enough to activate up to 4
TTL inputs of LS series.
As seen from description of some ports, even though all of them
have more or less similar architecture, it is necessary to pay
attention to which of them is to be used for what and how.For
example, if they shall be used as outputs with high voltage level
(5V), then P0 should be avoided because its pins do not have
pull-up resistors, thus giving low logic level only. When using
other ports, one should have in mind that pull-up resistors have a
relatively high resistance, so that their pins can give a current
of several hundreds microamperes only.2.4 Memory OrganizationThe
8051 has two types of memory and these are Program Memory and Data
Memory. Program Memory (ROM) is used to permanently save the
program being executed, while Data Memory (RAM) is used for
temporarily storing data and intermediate results created and used
during the operation of the microcontroller. Depending on the model
in use (we are still talking about the 8051 microcontroller family
in general) at most a few Kb of ROM and 128 or 256 bytes of RAM is
used. HoweverAll 8051 microcontrollers have a 16-bit addressing bus
and are capable of addressing 64 kb memory. It is neither a mistake
nor a big ambition of engineers who were working on basic core
development. It is a matter of smart memory organization which
makes these microcontrollers a real programmers goody.Program
MemoryThe first models of the 8051 microcontroller family did not
have internal program memory. It was added as an external separate
chip. These models are recognizable by their label beginning with
803 (for example 8031 or 8032). All later models have a few Kbyte
ROM embedded. Even though such an amount of memory is sufficient
for writing most of the programs, there are situations when it is
necessary to use additional memory as well. A typical example are
so called lookup tables. They are used in cases when equations
describing some processes are too complicated or when there is no
time for solving them. In such cases all necessary estimates and
approximates are executed in advance and the final results are put
in the tables (similar to logarithmic tables).
How does the microcontroller handle external memory depends on
the EA pin logic state:
EA=0In this case, the microcontroller completely ignores
internal program memory and executes only the program stored in
external memory.EA=1In this case, the microcontroller executes
first the program from built-in ROM, then the program stored in
external memory.In both cases, P0 and P2 are not available for use
since being used for data and address transmission. Besides, the
ALE and PSEN pins are also used.Data MemoryAs already mentioned,
Data Memory is used for temporarily storing data and intermediate
results created and used during the operation of the
microcontroller. Besides, RAM memory built in the 8051 family
includes many registers such as hardware counters and timers,
input/output ports, serial data buffers etc. The previous models
had 256 RAM locations, while for the later models this number was
incremented by additional 128 registers. However, the first 256
memory locations (addresses 0-FFh) are the heart of memory common
to all the models belonging to the 8051 family. Locations available
to the user occupy memory space with addresses 0-7Fh, i.e. first
128 registers. This part of RAM is divided in several blocks.The
first block consists of 4 banks each including 8 registers denoted
by R0-R7. Prior to accessing any of these registers, it is
necessary to select the bank containing it. The next memory block
(address 20h-2Fh) is bit- addressable, which means that each bit
has its own address (0-7Fh). Since there are 16 such registers,
this block contains in total of 128 bits with separate addresses
(address of bit 0 of the 20h byte is 0, while address of bit 7 of
the 2Fh byte is 7Fh). The third group of registers occupy addresses
2Fh-7Fh, i.e. 80 locations, and does not have any special functions
or features.Additional RAMIn order to satisfy the programmers
constant hunger for Data Memory, the manufacturers decided to embed
an additional memory block of 128 locations into the latest
versions of the 8051 microcontrollers. However, its not as simple
as it seems to be The problem is that electronics performing
addressing has 1 byte (8 bits) on disposal and is capable of
reaching only the first 256 locations, therefore. In order to keep
already existing 8-bit architecture and compatibility with other
existing models a small trick was done.What does it mean? It means
that additional memory block shares the same addresses with
locations intended for the SFRs (80h- FFh). In order to
differentiate between these two physically separated memory spaces,
different ways of addressing are used. The SFRs memory locations
are accessed by direct addressing, while additional RAM memory
locations are accessed by indirect addressing.
Memory expansionIn case memory (RAM or ROM) built in the
microcontroller is not sufficient, it is possible to add two
external memory chips with capacity of 64Kb each. P2 and P3 I/O
ports are used for their addressing and data transmission.
From the users point of view, everything works quite simply when
properly connected because most operations are performed by the
microcontroller itself. The 8051 microcontroller has two pins for
data read RD#(P3.7) and PSEN#. The first one is used for reading
data from external data memory (RAM), while the other is used for
reading data from external program memory (ROM). Both pins are
active low. A typical example of memory expansion by adding RAM and
ROM chips (Hardward architecture), is shown in figure above.Even
though additional memory is rarely used with the latest versions of
the microcontrollers, we will describe in short what happens when
memory chips are connected according to the previous schematic. The
whole process described below is performed automatically. When the
program during execution encounters an instruction which resides in
external memory (ROM), the microcontroller will activate its
control output ALE and set the first 8 bits of address (A0-A7) on
P0. IC circuit 74HCT573 passes the first 8 bits to memory address
pins. A signal on the ALE pin latches the IC circuit 74HCT573 and
immediately afterwards 8 higher bits of address (A8-A15) appear on
the port. In this way, a desired location of additional program
memory is addressed. It is left over to read its content. Port P0
pins are configured as inputs, the PSEN pin is activated and the
microcontroller reads from memory chip.Similar occurs when it is
necessary to read location from external RAM. Addressing is
performed in the same way, while read and write are performed via
signals appearing on the control outputs RD (is short for read) or
WR (is short for write).AddressingWhile operating, the processor
processes data as per program instructions. Each instruction
consists of two parts. One part describes WHAT should be done,
while the other explains HOW to do it. The latter part can be a
data (binary number) or the address at which the data is stored.
Two ways of addressing are used for all 8051 microcontrollers
depending on which part of memory should be accessed:Direct
AddressingOn direct addressing, the address of memory location
containing data to be read is specified in instruction. The address
may contain a number being changed during operation (variable). For
example:Since the address is only one byte in size (the largest
number is 255), only the first 255 locations of RAM can be accessed
this way. The first half of RAM is available for use, while another
half is reserved for SFRs.MOV A,33h; Means: move a number from
address 33 hex. to accumulatorIndirect AddressingOn indirect
addressing, a register containing the address of another register
is specified in instruction. Data to be used in the program is
stored in the letter register. For example:Indirect addressing is
only used for accessing RAM locations available for use (never for
accessing SFRs). This is the only way of accessing all the latest
versions of the microcontrollers with additional memory block (128
locations of RAM). Simply put, when the program encounters
instruction including @ sign and if the specified address is higher
than 128 ( 7F hex.), the processor knows that indirect addressing
is used and skips memory space reserved for SFRs.MOV A,@R0; Means:
Store the value from the register whose address is in the R0
register into accumulatorOn indirect addressing, registers R0, R1
or Stack Pointer are used for specifying 8-bit addresses. Since
only 8 bits are avilable, it is possible to access only registers
of internal RAM this way (128 locations when speaking of previous
models or 256 locations when speaking of latest models of
microcontrollers). If an extra memory chip is added then the 16-bit
DPTR Register (consisting of the registers DPTRL and DPTRH) is used
for specifying address. In this way it is possible to access any
location in the range of 64K.2.5 Special Function Registers
(SFRs)Special Function Registers (SFRs) are a sort of control table
used for running and monitoring the operation of the
microcontroller. Each of these registers as well as each bit they
include, has its name, address in the scope of RAM and precisely
defined purpose such as timer control, interrupt control, serial
communication control etc. Even though there are 128 memory
locations intended to be occupied by them, the basic core, shared
by all types of 8051 microcontrollers, has only 21 such registers.
Rest of locations are intensionally left unoccupied in order to
enable the manufacturers to further develop microcontrollers
keeping them compatible with the previous versions. It also enables
programs written a long time ago for microcontrollers which are out
of production now to be used today.
A Register (Accumulator)
A register is a general-purpose register used for storing
intermediate results obtained during operation. Prior to executing
an instruction upon any number or operand it is necessary to store
it in the accumulator first. All results obtained from arithmetical
operations performed by the ALU are stored in the accumulator. Data
to be moved from one register to another must go through the
accumulator. In other words, the A register is the most commonly
used register and it is impossible to imagine a microcontroller
without it. More than half instructions used by the 8051
microcontroller use somehow the accumulator.B
RegisterMultiplication and division can be performed only upon
numbers stored in the A and B registers. All other instructions in
the program can use this register as a spare accumulator (A).
During the process of writing a program, each register is called
by its name so that their exact addresses are not of importance for
the user. During compilation, their names will be automatically
replaced by appropriate addresses.R Registers (R0-R7)
This is a common name for 8 general-purpose registers (R0, R1,
R2 ...R7). Even though they are not true SFRs, they deserve to be
discussed here because of their purpose. They occupy 4 banks within
RAM. Similar to the accumulator, they are used for temporary
storing variables and intermediate results during operation. Which
one of these banks is to be active depends on two bits of the PSW
Register. Active bank is a bank the registers of which are
currently used.The following example best illustrates the purpose
of these registers. Suppose it is necessary to perform some
arithmetical operations upon numbers previously stored in the R
registers: (R1+R2) - (R3+R4). Obviously, a register for temporary
storing results of addition is needed. This is how it looks in the
program:MOV A,R3; Means: move number from R3 into accumulatorADD
A,R4; Means: add number from R4 to accumulator (result remains in
accumulator)MOV R5,A; Means: temporarily move the result from
accumulator into R5MOV A,R1; Means: move number from R1 to
accumulatorADD A,R2; Means: add number from R2 to accumulatorSUBB
A,R5; Means: subtract number from R5 (there are R3+R4)Program
Status Word (PSW) Register
PSW register is one of the most important SFRs. It contains
several status bits that reflect the current state of the CPU.
Besides, this register contains Carry bit, Auxiliary Carry, two
register bank select bits, Overflow flag, parity bit and
user-definable status flag.P - Parity bit.If a number stored in the
accumulator is even then this bit will be automatically set (1),
otherwise it will be cleared (0). It is mainly used during data
transmit and receive via serial communication.- Bit 1.This bit is
intended to be used in the future versions of microcontrollers.OV
Overflowoccurs when the result of an arithmetical operation is
larger than 255 and cannot be stored in one register. Overflow
condition causes the OV bit to be set (1). Otherwise, it will be
cleared (0).RS0, RS1 - Register bank select bits.These two bits are
used to select one of four register banks of RAM. By setting and
clearing these bits, registers R0-R7 are stored in one of four
banks of RAM.RS1RS2SPACE IN RAM
00Bank0 00h-07h
01Bank1 08h-0Fh
10Bank2 10h-17h
11Bank3 18h-1Fh
F0 - Flag 0.This is a general-purpose bit available for use.AC -
Auxiliary Carry Flagis used for BCD operations only.CY - Carry
Flagis the (ninth) auxiliary bit used for all arithmetical
operations and shift instructions.Data Pointer Register (DPTR)DPTR
register is not a true one because it doesn't physically exist. It
consists of two separate registers: DPH (Data Pointer High) and
(Data Pointer Low). For this reason it may be treated as a 16-bit
register or as two independent 8-bit registers. Their 16 bits are
primarly used for external memory addressing. Besides, the DPTR
Register is usually used for storing data and intermediate
results.
Stack Pointer (SP) Register
A value stored in the Stack Pointer points to the first free
stack address and permits stack availability. Stack pushes
increment the value in the Stack Pointer by 1. Likewise, stack pops
decrement its value by 1. Upon any reset and power-on, the value 7
is stored in the Stack Pointer, which means that the space of RAM
reserved for the stack starts at this location. If another value is
written to this register, the entire Stack is moved to the new
memory location.P0, P1, P2, P3 - Input/Output Registers
If neither external memory nor serial communication system are
used then 4 ports with in total of 32 input/output pins are
available for connection to peripheral environment. Each bit within
these ports affects the state and performance of appropriate pin of
the microcontroller. Thus, bit logic state is reflected on
appropriate pin as a voltage (0 or 5 V) and vice versa, voltage on
a pin reflects the state of appropriate port bit.As mentioned, port
bit state affects performance of port pins, i.e. whether they will
be configured as inputs or outputs. If a bit is cleared (0), the
appropriate pin will be configured as an output, while if it is set
(1), the appropriate pin will be configured as an input. Upon reset
and power-on, all port bits are set (1), which means that all
appropriate pins will be configured as inputs.
I/O ports are directly connected to the microcontroller pins.
Accordingly, logic state of these registers can be checked by
voltmeter and vice versa, voltage on the pins can be checked by
inspecting their bits!2.6 Counters and TimersAs you already know,
the microcontroller oscillator uses quartz crystal for its
operation. As the frequency of this oscillator is precisely defined
and very stable, pulses it generates are always of the same width,
which makes them ideal for time measurement. Such crystals are also
used in quartz watches. In order to measure time between two events
it is sufficient to count up pulses coming from this oscillator.
That is exactly what the timer does. If the timer is properly
programmed, the value stored in its register will be incremented
(or decremented) with each coming pulse, i.e. once per each machine
cycle. A single machine-cycle instruction lasts for 12 quartz
oscillator periods, which means that by embedding quartz with
oscillator frequency of 12MHz, a number stored in the timer
register will be changed million times per second, i.e. each
microsecond.The 8051 microcontroller has 2 timers/counters called
T0 and T1. As their names suggest, their main purpose is to measure
time and count external events. Besides, they can be used for
generating clock pulses to be used in serial communication, so
called Baud Rate.Timer T0As seen in figure below, the timer T0
consists of two registers TH0 and TL0 representing a low and a high
byte of one 16-digit binary number.
Accordingly, if the content of the timer T0 is equal to 0 (T0=0)
then both registers it consists of will contain 0. If the timer
contains for example number 1000 (decimal), then the TH0 register
(high byte) will contain the number 3, while the TL0 register (low
byte) will contain decimal number 232.
Formula used to calculate values in these two registers is very
simple:TH0 256 + TL0 = TMatching the previous example it would be
as follows:3 256 + 232 = 1000
Since the timer T0 is virtually 16-bit register, the largest
value it can store is 65 535. In case of exceeding this value, the
timer will be automatically cleared and counting starts from 0.
This condition is called an overflow. Two registers TMOD and TCON
are closely connected to this timer and control its operation.TMOD
Register (Timer Mode)The TMOD register selects the operational mode
of the timers T0 and T1. As seen in figure below, the low 4 bits
(bit0 - bit3) refer to the timer 0, while the high 4 bits (bit4 -
bit7) refer to the timer 1. There are 4 operational modes and each
of them is described herein.
Bits of this register have the following function: GATE1enables
and disables Timer 1 by means of a signal brought to the INT1 pin
(P3.3): 1- Timer 1 operates only if the INT1 bit is set. 0- Timer 1
operates regardless of the logic state of the INT1 bit. C/T1selects
pulses to be counted up by the timer/counter 1: 1- Timer counts
pulses brought to the T1 pin (P3.5). 0- Timer counts pulses from
internal oscillator. T1M1,T1M0These two bits select the operational
mode of the Timer 1.T1M1T1M0MODEDESCRIPTION
00013-bit timer
01116-bit timer
1028-bit auto-reload
113Split mode
GATE0enables and disables Timer 1 using a signal brought to the
INT0 pin (P3.2): 1- Timer 0 operates only if the INT0 bit is set.
0- Timer 0 operates regardless of the logic state of the INT0 bit.
C/T0selects pulses to be counted up by the timer/counter 0: 1-
Timer counts pulses brought to the T0 pin (P3.4). 0- Timer counts
pulses from internal oscillator. T0M1,T0M0These two bits select the
oprtaional mode of the Timer 0.T0M1T0M0MODEDESCRIPTION
00013-bit timer
01116-bit timer
1028-bit auto-reload
113Split mode
Timer 0 in mode 0 (13-bit timer)This is one of the rarities
being kept only for the purpose of compatibility with the previuos
versions of microcontrollers. This mode configures timer 0 as a
13-bit timer which consists of all 8 bits of TH0 and the lower 5
bits of TL0. As a result, the Timer 0 uses only 13 of 16 bits. How
does it operate? Each coming pulse causes the lower register bits
to change their states. After receiving 32 pulses, this register is
loaded and automatically cleared, while the higher byte (TH0) is
incremented by 1. This process is repeated until registers count up
8192 pulses. After that, both registers are cleared and counting
starts from 0.
Timer 0 in mode 1 (16-bit timer)Mode 1 configures timer 0 as a
16-bit timer comprising all the bits of both registers TH0 and TL0.
That's why this is one of the most commonly used modes. Timer
operates in the same way as in mode 0, with difference that the
registers count up to 65 536 as allowable by the 16 bits.
Timer 0 in mode 2 (Auto-Reload Timer)Mode 2 configures timer 0
as an 8-bit timer. Actually, timer 0 uses only one 8-bit register
for counting and never counts from 0, but from an arbitrary value
(0-255) stored in another (TH0) register.The following example
shows the advantages of this mode. Suppose it is necessary to
constantly count up 55 pulses generated by the clock.If mode 1 or
mode 0 is used, It is necessary to write the number 200 to the
timer registers and constantly check whether an overflow has
occured, i.e. whether they reached the value 255. When it happens,
it is necessary to rewrite the number 200 and repeat the whole
procedure. The same procedure is automatically performed by the
microcontroller if set in mode 2. In fact, only the TL0 register
operates as a timer, while another (TH0) register stores the value
from which the counting starts. When the TL0 register is loaded,
instead of being cleared, the contents of TH0 will be reloaded to
it. Referring to the previous example, in order to register each
55th pulse, the best solution is to write the number 200 to the TH0
register and configure the timer to operate in mode 2.
Timer 0 in Mode 3 (Split Timer)Mode 3 configures timer 0 so that
registers TL0 and TH0 operate as separate 8-bit timers. In other
words, the 16-bit timer consisting of two registers TH0 and TL0 is
split into two independent 8-bit timers. This mode is provided for
applications requiring an additional 8-bit timer or counter. The
TL0 timer turns into timer 0, while the TH0 timer turns into timer
1. In addition, all the control bits of 16-bit Timer 1 (consisting
of the TH1 and TL1 register), now control the 8-bit Timer 1. Even
though the 16-bit Timer 1 can still be configured to operate in any
of modes (mode 1, 2 or 3), it is no longer possible to disable it
as there is no control bit to do it. Thus, its operation is
restricted when timer 0 is in mode 3.
The only application of this mode is when two timers are used
and the 16-bit Timer 1 the operation of which is out of control is
used as a baud rate generator.Timer Control (TCON) RegisterTCON
register is also one of the registers whose bits are directly in
control of timer operation.Only 4 bits of this register are used
for this purpose, while rest of them is used for interrupt control
to be discussed later.
TF1bit is automatically set on the Timer 1 overflow. TR1bit
enables the Timer 1. 1- Timer 1 is enabled. 0- Timer 1 is disabled.
TF0bit is automatically set on the Timer 0 overflow. TR0bit enables
the timer 0. 1- Timer 0 is enabled. 0- Timer 0 is disabled.How to
use the Timer 0 ?In order to use timer 0, it is first necessary to
select it and configure the mode of its operation. Bits of the TMOD
register are in control of it:
Referring to figure above, the timer 0 operates in mode 1 and
counts pulses generated by internal clock the frequency of which is
equal to 1/12 the quartz frequency.Turn on the timer:
The TR0 bit is set and the timer starts operation. If the quartz
crystal with frequency of 12MHz is embedded then its contents will
be incremented every microsecond. After 65.536 microseconds, the
both registers the timer consists of will be loaded. The
microcontroller automatically clears them and the timer keeps on
repeating procedure from the beginning until the TR0 bit value is
logic zero (0).How to 'read' a timer?Depending on application, it
is necessary either to read a number stored in the timer registers
or to register the moment they have been cleared.- It is extremely
simple to read a timer by using only one register configured in
mode 2 or 3. It is sufficient to read its state at any moment.
That's all!- It is somehow complicated to read a timer configured
to operate in mode 2. Suppose the lower byte is read first (TL0),
then the higher byte (TH0). The result is:TH0 = 15 TL0 =
255Everything seems to be ok, but the current state of the register
at the moment of reading was:TH0 = 14 TL0 = 255In case of
negligence, such an error in counting (255 pulses) may occur for
not so obvious but quite logical reason. The lower byte is
correctly read (255), but at the moment the program counter was
about to read the higher byte TH0, an overflow occurred and the
contents of both registers have been changed (TH0: 1415, TL0:
2550). This problem has a simple solution. The higher byte should
be read first, then the lower byte and once again the higher byte.
If the number stored in the higher byte is different then this
sequence should be repeated. It's about a short loop consisting of
only 3 instructions in the program.There is another solution as
well. It is sufficient to simply turn the timer off while reading
is going on (the TR0 bit of the TCON register should be cleared),
and turn it on again after reading is finished.Timer 0 Overflow
DetectionUsually, there is no need to constantly read timer
registers. It is sufficient to register the moment they are
cleared, i.e. when counting starts from 0. This condition is called
an overflow. When it occurrs, the TF0 bit of the TCON register will
be automatically set. The state of this bit can be constantly
checked from within the program or by enabling an interrupt which
will stop the main program execution when this bit is set. Suppose
it is necessary to provide a program delay of 0.05 seconds (50 000
machine cycles), i.e. time when the program seems to be
stopped:First a number to be written to the timer registers should
be calculated:
Then it should be written to the timer registers TH0 and
TL0:
When enabled, the timer will resume counting from this number.
The state of the TF0 bit, i.e. whether it is set, is checked from
within the program. It happens at the moment of overflow, i.e.
after exactly 50.000 machine cycles or 0.05 seconds.How to measure
pulse duration?
Suppose it is necessary to measure the duration of an operation,
for example how long a device has been turned on? Look again at the
figure illustrating the timer and pay attention to the function of
the GATE0 bit of the TMOD register. If it is cleared then the state
of the P3.2 pin doesn't affect timer operation. If GATE0 = 1 the
timer will operate until the pin P3.2 is cleared. Accordingly, if
this pin is supplied with 5V through some external switch at the
moment the device is being turned on, the timer will measure
duration of its operation, which actually was the objective.How to
count up pulses?Similarly to the previous example, the answer to
this question again lies in the TCON register. This time it's about
the C/T0 bit. If the bit is cleared the timer counts pulses
generated by the internal oscillator, i.e. measures the time
passed. If the bit is set, the timer input is provided with pulses
from the P3.4 pin (T0). Since these pulses are not always of the
same width, the timer cannot be used for time measurement and is
turned into a counter, therefore. The highest frequency that could
be measured by such a counter is 1/24 frequency of used
quartz-crystal.Timer 1Timer 1 is identical to timer 0, except for
mode 3 which is a hold-count mode. It means that they have the same
function, their operation is controlled by the same registers TMOD
and TCON and both of them can operate in one out of 4 different
modes.
2.7 UART (Universal Asynchronous Receiver and Transmitter)One of
the microcontroller features making it so powerful is an integrated
UART, better known as a serial port. It is a full-duplex port, thus
being able to transmit and receive data simultaneously and at
different baud rates. Without it, serial data send and receive
would be an enormously complicated part of the program in which the
pin state is constantly changed and checked at regular intervals.
When using UART, all the programmer has to do is to simply select
serial port mode and baud rate. When it's done, serial data
transmit is nothing but writing to the SBUF register, while data
receive represents reading the same register. The microcontroller
takes care of not making any error during data transmission.
Serial port must be configured prior to being used. In other
words, it is necessary to determine how many bits is contained in
one serial word, baud rate and synchronization clock source. The
whole process is in control of the bits of the SCON register
(Serial Control).Serial Port Control (SCON) Register
SM0- Serial port mode bit 0 is used for serial port mode
selection. SM1- Serial port mode bit 1. SM2- Serial port mode 2
bit, also known as multiprocessor communication enable bit. When
set, it enables multiprocessor communication in mode 2 and 3, and
eventually mode 1. It should be cleared in mode 0. REN- Reception
Enable bit enables serial reception when set. When cleared, serial
reception is disabled. TB8- Transmitter bit 8. Since all registers
are 8-bit wide, this bit solves the problem of transmiting the 9th
bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th
bit. RB8- Receiver bit 8 or the 9th bit received in modes 2 and 3.
Cleared by hardware if 9th bit received is a logic 0. Set by
hardware if 9th bit received is a logic 1. TI- Transmit Interrupt
flag is automatically set at the moment the last bit of one byte is
sent. It's a signal to the processor that the line is available for
a new byte transmite. It must be cleared from within the software.
RI- Receive Interrupt flag is automatically set upon one byte
receive. It signals that byte is received and should be read
quickly prior to being replaced by a new data. This bit is also
cleared from within the software.As seen, serial port mode is
selected by combining the SM0 and SM2
bits:SM0SM1MODEDESCRIPTIONBAUD RATE
0008-bit Shift Register1/12 the quartz frequency
0118-bit UARTDetermined by the timer 1
1029-bit UART1/32 the quartz frequency (1/64 the quartz
frequency)
1139-bit UARTDetermined by the timer 1
In mode 0, serial data are transmitted and received through the
RXD pin, while the TXD pin output clocks. The bout rate is fixed at
1/12 the oscillator frequency. On transmit, the least significant
bit (LSB bit) is sent/received first.TRANSMIT- Data transmit is
initiated by writing data to the SBUF register. In fact, this
process starts after any instruction being performed upon this
register. When all 8 bits have been sent, the TI bit of the SCON
register is automatically set.
RECEIVE- Data receive through the RXD pin starts upon the two
following conditions are met: bit REN=1 and RI=0 (both of them are
stored in the SCON register). When all 8 bits have been received,
the RI bit of the SCON register is automatically set indicating
that one byte receive is complete.
Since there are no START and STOP bits or any other bit except
data sent from the SBUF register in the pulse sequence, this mode
is mainly used when the distance between devices is short, noise is
minimized and operating speed is of importance. A typical example
is I/O port expansion by adding a cheap IC (shift registers
74HC595, 74HC597 and similar).Mode 1
In mode 1, 10 bits are transmitted through the TXD pin or
received through the RXD pin in the following manner: a START bit
(always 0), 8 data bits (LSB first) and a STOP bit (always 1). The
START bit is only used to initiate data receive, while the STOP bit
is automatically written to the RB8 bit of the SCON
register.TRANSMIT- Data transmit is initiated by writing data to
the SBUF register. End of data transmission is indicated by setting
the TI bit of the SCON register.
RECEIVE- The START bit (logic zero (0)) on the RXD pin initiates
data receive. The following two conditions must be met: bit REN=1
and bit RI=0. Both of them are stored in the SCON register. The RI
bit is automatically set upon data reception is complete.
The Baud rate in this mode is determined by the timer 1
overflow.Mode 2
In mode 2, 11 bits are transmitted through the TXD pin or
received through the RXD pin: a START bit (always 0), 8 data bits
(LSB first), a programmable 9th data bit and a STOP bit (always 1).
On transmit, the 9th data bit is actually the TB8 bit of the SCON
register. This bit usually has a function of parity bit. On
receive, the 9th data bit goes into the RB8 bit of the same
register (SCON).The baud rate is either 1/32 or 1/64 the oscillator
frequency.TRANSMIT- Data transmit is initiated by writing data to
the SBUF register. End of data transmission is indicated by setting
the TI bit of the SCON register.
RECEIVE- The START bit (logic zero (0)) on the RXD pin initiates
data receive. The following two conditions must be met: bit REN=1
and bit RI=0. Both of them are stored in the SCON register. The RI
bit is automatically set upon data reception is complete.
Mode 3Mode 3 is the same as Mode 2 in all respects except the
baud rate. The baud rate in Mode 3 is variable.
The parity bit is the P bit of the PSW register. The simplest
way to check correctness of the received byte is to add a parity
bit to it. Simply, before initiating data transmit, the byte to
transmit is stored in the accumulator and the P bit goes into the
TB8 bit in order to be a part of the message. The procedure is
opposite on receive, received byte is stored in the accumulator and
the P bit is compared with the RB8 bit. If they are the same-
everything is OK!Baud RateBaud Rate is a number of sent/received
bits per second. In case the UART is used, baud rate depends on:
selected mode, oscillator frequency and in some cases on the state
of the SMOD bit of the SCON register. All the necessary formulas
are specified in the table:BAUD RATEBITSMOD
Mode 0Fosc. / 12
Mode 11 Fosc.16 12 (256-TH1)BitSMOD
Mode 2Fosc. / 32Fosc. / 6410
Mode 31 Fosc.16 12 (256-TH1)
Timer 1 as a clock generatorTimer 1 is usually used as a clock
generator as it enables various baud rates to be easily set. The
whole procedure is simple and is as follows: First, enable Timer 1
overflow interrupt. Configure Timer T1 to operate in auto-reload
mode. Depending on needs, select one of the standard values from
the table and write it to the TH1 register. That's all.BAUD
RATEFOSC. (MHZ)BIT SMOD
11.05921214.74561620
15040 h30 h00 h0
300A0 h98 h80 h75 h52 h0
600D0 hCC hC0 hBB hA9 h0
1200E8 hE6 hE0 hDE hD5 h0
2400F4 hF3 hF0 hEF hEA h0
4800F3 hEF hEF h1
4800FA hF8 hF5 h0
9600FD hFC h0
9600F5 h1
19200FD hFC h1
38400FE h1
76800FF h1
Multiprocessor CommunicationAs you may know, additional 9th data
bit is a part of message in mode 2 and 3. It can be used for
checking data via parity bit. Another useful application of this
bit is in communication between two or more microcontrollers, i.e.
multiprocessor communication. This feature is enabled by setting
the SM2 bit of the SCON register. As a result, after receiving the
STOP bit, indicating end of the message, the serial port interrupt
will be generated only if the bit RB8 = 1 (the 9th bit).This is how
it looks like in practice:Suppose there are several
microcontrollers sharing the same interface. Each of them has its
own address. An address byte differs from a data byte because it
has the 9th bit set (1), while this bit is cleared (0) in a data
byte. When the microcontroller A (master) wants to transmit a block
of data to one of several slaves, it first sends out an address
byte which identifies the target slave. An address byte will
generate an interrupt in all slaves so that they can examine the
received byte and check whether it matches their address.
Of course, only one of them will match the address and
immediately clear the SM2 bit of the SCON register and prepare to
receive the data byte to come. Other slaves not being addressed
leave their SM2 bit set ignoring the coming data bytes.
2.8 8051 Microcontroller InterruptsThere are five interrupt
sources for the 8051, which means that they can recognize 5
different events that can interrupt regular program execution. Each
interrupt can be enabled or disabled by setting bits of the IE
register. Likewise, the whole interrupt system can be disabled by
clearing the EA bit of the same register. Refer to figure
below.Now, it is necessary to explain a few details referring to
external interrupts- INT0 and INT1. If the IT0 and IT1 bits of the
TCON register are set, an interrupt will be generated on high to
low transition, i.e. on the falling pulse edge (only in that
moment). If these bits are cleared, an interrupt will be
continuously executed as far as the pins are held low.IE Register
(Interrupt Enable) EA- global interrupt enable/disable: 0 -
disables all interrupt requests. 1 - enables all individual
interrupt requests. ES- enables or disables serial interrupt: 0 -
UART system cannot generate an interrupt. 1 - UART system enables
an interrupt. ET1- bit enables or disables Timer 1 interrupt: 0 -
Timer 1 cannot generate an interrupt. 1 - Timer 1 enables an
interrupt. EX1- bit enables or disables external 1 interrupt: 0 -
change of the pin INT0 logic state cannot generate an interrupt. 1
- enables an external interrupt on the pin INT0 state change. ET0-
bit enables or disables timer 0 interrupt: 0 - Timer 0 cannot
generate an interrupt. 1 - enables timer 0 interrupt. EX0- bit
enables or disables external 0 interrupt: 0 - change of the INT1
pin logic state cannot generate an interrupt. 1 - enables an
external interrupt on the pin INT1 state change.Interrupt
PrioritiesIt is not possible to forseen when an interrupt request
will arrive. If several interrupts are enabled, it may happen that
while one of them is in progress, another one is requested. In
order that the microcontroller knows whether to continue operation
or meet a new interrupt request, there is a priority list
instructing it what to do.The priority list offers 3 levels of
interrupt priority:1. Reset! The apsolute master. When a reset
request arrives, everything is stopped and the microcontroller
restarts.2. Interrupt priority 1 can be disabled by Reset only.3.
Interrupt priority 0 can be disabled by both Reset and interrupt
priority 1.The IP Register (Interrupt Priority Register) specifies
which one of existing interrupt sources have higher and which one
has lower priority. Interrupt priority is usually specified at the
beginning of the program. According to that, there are several
possibilities: If an interrupt of higher priority arrives while an
interrupt is in progress, it will be immediately stopped and the
higher priority interrupt will be executed first. If two interrupt
requests, at different priority levels, arrive at the same time
then the higher priority interrupt is serviced first. If the both
interrupt requests, at the same priority level, occur one after
another, the one which came later has to wait until routine being
in progress ends. If two interrupt requests of equal priority
arrive at the same time then the interrupt to be serviced is
selected according to the following priority list:1. External
interrupt INT02. Timer 0 interrupt3. External Interrupt INT14.
Timer 1 interrupt5. Serial Communication InterruptIP Register
(Interrupt Priority)The IP register bits specify the priority level
of each interrupt (high or low priority).
PS- Serial Port Interrupt priority bit Priority 0 Priority 1
PT1- Timer 1 interrupt priority Priority 0 Priority 1 PX1- External
Interrupt INT1 priority Priority 0 Priority 1 PT0- Timer 0
Interrupt Priority Priority 0 Priority 1 PX0- External Interrupt
INT0 Priority Priority 0 Priority 1Handling InterruptWhen an
interrupt request arrives the following occurs:1. Instruction in
progress is ended.2. The address of the next instruction to execute
is pushed on the stack.3. Depending on which interrupt is
requested, one of 5 vectors (addresses) is written to the program
counter in accordance to the table below:4. INTERRUPT SOURCEVECTOR
(ADDRESS)
IE03 h
TF0B h
TF11B h
RI, TI23 h
All addresses are in hexadecimal format
5. These addresses store appropriate subroutines processing
interrupts. Instead of them, there are usually jump instructions
specifying locations on which these subroutines reside.6. When an
interrupt routine is executed, the address of the next instruction
to execute is poped from the stack to the program counter and
interrupted program resumes operation from where it left off.
From the moment an interrupt is enabled, the microcontroller is
on alert all the time. When an interrupt request arrives, the
program execution is stopped, electronics recognizes the source and
the program jumps to the appropriate address (see the table above).
This address usually stores a jump instruction specifying the start
of appropriate subroutine. Upon its execution, the program resumes
operation from where it left off.ResetReset occurs when the RS pin
is supplied with a positive pulse in duration of at least 2 machine
cycles (24 clock cycles of crystal oscillator). After that, the
microcontroller generates an internal reset signal which clears all
SFRs, except SBUF registers, Stack Pointer and ports (the state of
the first two ports is not defined, while FF value is written to
the ports configuring all their pins as inputs). Depending on
surrounding and purpose of device, the RS pin is usually connected
to a power-on reset push button or circuit or to both of them.
Figure below illustrates one of the simplest circuit providing safe
power-on reset.
Basically, everything is very simple: after turning the power
on, electrical capacitor is being charged for several milliseconds
throgh a resistor connected to the ground. The pin is driven high
during this process. When the capacitor is charged, power supply
voltage is already stable and the pin remains connected to the
ground, thus providing normal operation of the microcontroller.
Pressing the reset button causes the capacitor to be temporarily
discharged and the microcontroller is reset. When released, the
whole process is repeatedThrough the program- step by
step...Microcontrollers normally operate at very high speed. The
use of 12 Mhz quartz crystal enables 1.000.000 instructions to be
executed per second. Basically, there is no need for higher
operating rate. In case it is needed, it is easy to built in a
crystal for high frequency. The problem arises when it is necessary
to slow down the operation of the microcontroller. For example
during testing in real environment when it is necessary to execute
several instructions step by step in order to check I/O pins' logic
state.Interrupt system of the 8051 microcontroller practically
stops operation of the microcontroller and enables instructions to
be executed one after another by pressing the button. Two interrupt
features enable that: Interrupt request is ignored if an interrupt
of the same priority level is in progress. Upon interrupt routine
execution, a new interrupt is not executed until at least one
instruction from the main program is executed.In order to use this
in practice, the following steps should be done:1. External
interrupt sensitive to the signal level should be enabled (for
example INT0).2. Three following instructions should be inserted
into the program (at the 03hex. address):
What is going on? As soon as the P3.2 pin is cleared (for
example, by pressing the button), the microcontroller will stop
program execution and jump to the 03hex address will be executed.
This address stores a short interrupt routine consisting of 3
instructions.The first instruction is executed until the push
button is realised (logic one (1) on the P3.2 pin). The second
instruction is executed until the push button is pressed again.
Immediately after that, the RETI instruction is executed and the
processor resumes operation of the main program. Upon execution of
any program instruction, the interrupt INT0 is generated and the
whole procedure is repeated (push button is still pressed). In
other words, one button press - one instruction.2.9 8051
Microcontroller Power Consumption ControlGenerally speaking, the
microcontroller is inactive for the most part and just waits for
some external signal in order to takes its role in a show. This can
cause some problems in case batteries are used for power supply. In
extreme cases, the only solution is to set the whole electronics in
sleep mode in order to minimize consumption. A typical example is a
TV remote controller: it can be out of use for months but when used
again it takes less than a second to send a command to TV receiver.
The AT89S53 uses approximately 25mA for regular operation, which
doesn't make it a pover-saving microcontroller. Anyway, it doesnt
have to be always like that, it can easily switch the operating
mode in order to reduce its total consumption to approximately
40uA. Actually, there are two power-saving modes of
operation:IdleandPower Down.
Idle modeUpon the IDL bit of the PCON register is set, the
microcontroller turns off the greatest power consumer- CPU unit
while peripheral units such as serial port, timers and interrupt
system continue operating normally consuming 6.5mA. In Idle mode,
the state of all registers and I/O ports remains unchanged.In order
to exit the Idle mode and make the microcontroller operate
normally, it is necessary to enable and execute any interrupt or
reset. It will cause the IDL bit to be automatically cleared and
the program resumes operation from instruction having set the IDL
bit. It is recommended that first three instructions to execute now
are NOP instructions. They don't perform any operation but provide
some time for the microcontroller to stabilize and prevents
undesired changes on the I/O ports.Power Down modeBy setting the PD
bit of the PCON register from within the program, the
microcontroller is set to Power down mode, thus turning off its
internal oscillator and reduces power consumption enormously. The
microcontroller can operate using only 2V power supply in power-
down mode, while a total power consumption is less than 40uA. The
only way to get the microcontroller back to normal mode is by
reset.While the microcontroller is in Power Down mode, the state of
all SFR registers and I/O ports remains unchanged. By setting it
back into the normal mode, the contents of the SFR register is
lost, but the content of internal RAM is saved. Reset signal must
be long enough, approximately 10mS, to enable stable operation of
the quartz oscillator.PCON register
The purpose of the Register PCON bits is: SMOD Baud rate is
twice as much higher by setting this bit. GF1 General-purpose bit
(available for use). GF1 General-purpose bit (available for use).
GF0 General-purpose bit (available for use). PD By setting this bit
the microcontroller enters thePower Downmode. IDL By setting this
bit the microcontroller enters theIdlemode.Chapter 3 : The 8051
Instruction Set 3.1 Types of instructions 3.2 Description of the
8051 instructionsIntroductionThe process of writing program for the
microcontroller mainly consists of giving instructions (commands)
in the specific order in which they should be executed in order to
carry out a specific task. As electronics cannot understand what
for example an instruction if the push button is pressed- turn the
light on means, then a certain number of simpler and precisely
defined orders that decoder can recognise must be used. All
commands are known as INSTRUCTION SET. All microcontrollers
compatibile with the 8051 have in total of 255 instructions, i.e.
255 different words available for program writing.At first sight,
it is imposing number of odd signs that must be known by heart.
However, It is not so complicated as it looks like. Many
instructions are considered to be different, even though they
perform the same operation, so there are only 111 truly different
commands. For example: ADD A,R0, ADD A,R1, ... ADD A,R7 are
instructions that perform the same operation (additon of the
accumulator and register). Since there are 8 such registers, each
instruction is counted separately. Taking into account that all
instructions perform only 53 operations (addition, subtraction,
copy etc.) and most of them are rarely used in practice, there are
actually 20-30 abbreviations to be learned, which is acceptable.3.1
Types of instructionsDepending on operation they perform, all
instructions are divided in several groups: Arithmetic Instructions
Branch Instructions Data Transfer Instructions Logic Instructions
Bit-oriented InstructionsThe first part of each instruction, called
MNEMONIC refers to the operation an instruction performs (copy,
addition, logic operation etc.). Mnemonics are abbreviations of the
name of operation being executed. For example: INC R1- Means:
Increment register R1 (increment register R1); LJMP LAB5- Means:
Long Jump LAB5 (long jump to the address marked as LAB5); JNZ LOOP-
Means: Jump if Not Zero LOOP (if the number in the accumulator is
not 0, jump to the address marked as LOOP);The other part of
instruction, called OPERAND is separated from mnemonic by at least
one whitespace and defines data being processed by instructions.
Some of the instructions have no operand, while some of them have
one, two or three. If there is more than one operand in an
instruction, they are separated by a comma. For example: RET-
return from a subroutine; JZ TEMP- if the number in the accumulator
is not 0, jump to the address marked as TEMP; ADD A,R3- add R3 and
accumulator; CJNE A,#20,LOOP- compare accumulator with 20. If they
are not equal, jump to the address marked as LOOP;Arithmetic
instructionsArithmetic instructions perform several basic
operations such as addition, subtraction, division, multiplication
etc. After execution, the result is stored in the first operand.
For example:ADD A,R1- The result of addition (A+R1) will be stored
in the accumulator.ARITHMETIC INSTRUCTIONS
MnemonicDescriptionByteCycle
ADD A,RnAdds the register to the accumulator11
ADD A,directAdds the direct byte to the accumulator22
ADD A,@RiAdds the indirect RAM to the accumulator12
ADD A,#dataAdds the immediate data to the accumulator22
ADDC A,RnAdds the register to the accumulator with a carry
flag11
ADDC A,directAdds the direct byte to the accumulator with a
carry flag22
ADDC A,@RiAdds the indirect RAM to the accumulator with a carry
flag12
ADDC A,#dataAdds the immediate data to the accumulator with a
carry flag22
SUBB A,RnSubtracts the register from the accumulator with a
borrow11
SUBB A,directSubtracts the direct byte from the accumulator with
a borrow22
SUBB A,@RiSubtracts the indirect RAM from the accumulator with a
borrow12
SUBB A,#dataSubtracts the immediate data from the accumulator
with a borrow22
INC AIncrements the accumulator by 111
INC RnIncrements the register by 112
INC RxIncrements the direct byte by 123
INC @RiIncrements the indirect RAM by 113
DEC ADecrements the accumulator by 111
DEC RnDecrements the register by 111
DEC RxDecrements the direct byte by 112
DEC @RiDecrements the indirect RAM by 123
INC DPTRIncrements the Data Pointer by 113
MUL ABMultiplies A and B15
DIV ABDivides A by B15
DA ADecimal adjustment of the accumulator according to BCD
code11
Branch InstructionsThere are two kinds of branch
instructions:Unconditional jump instructions: upon their execution
a jump to a new location from where the program continues execution
is executed.Conditional jump instructions: a jump to a new program
location is executed only if a specified condition is met.
Otherwise, the program normally proceeds with the next
instruction.BRANCH INSTRUCTIONS
MnemonicDescriptionByteCycle
ACALL addr11Absolute subroutine call26
LCALL addr16Long subroutine call36
RETReturns from subroutine14
RETIReturns from interrupt subroutine14
AJMP addr11Absolute jump23
LJMP addr16Long jump34
SJMP relShort jump (from 128 to +127 locations relative to the
following instruction)23
JC relJump if carry flag is set. Short jump.23
JNC relJump if carry flag is not set. Short jump.23
JB bit,relJump if direct bit is set. Short jump.34
JBC bit,relJump if direct bit is set and clears bit. Short
jump.34
JMP @A+DPTRJump indirect relative to the DPTR12
JZ relJump if the accumulator is zero. Short jump.23
JNZ relJump if the accumulator is not zero. Short jump.23
CJNE A,direct,relCompares direct byte to the accumulator and
jumps if not equal. Short jump.34
CJNE A,#data,relCompares immediate data to the accumulator and
jumps if not equal. Short jump.34
CJNE Rn,#data,relCompares immediate data to the register and
jumps if not equal. Short jump.34
CJNE @Ri,#data,relCompares immediate data to indirect register
and jumps if not equal. Short jump.34
DJNZ Rn,relDecrements register and jumps if not 0. Short
jump.23
DJNZ Rx,relDecrements direct byte and jump if not 0. Short
jump.34
NOPNo operation11
Data Transfer InstructionsData transfer instructions move the
content of one register to another. The register the content of
which is moved remains unchanged. If they have the suffix X (MOVX),
the data is exchanged with external memory.DATA TRANSFER
INSTRUCTIONS
MnemonicDescriptionByteCycle
MOV A,RnMoves the register to the accumulator11
MOV A,directMoves the direct byte to the accumulator22
MOV A,@RiMoves the indirect RAM to the accumulator12
MOV A,#dataMoves the immediate data to the accumulator22
MOV Rn,AMoves the accumulator to the register12
MOV Rn,directMoves the direct byte to the register24
MOV Rn,#dataMoves the immediate data to the register22
MOV direct,AMoves the accumulator to the direct byte23
MOV direct,RnMoves the register to the direct byte23
MOV direct,directMoves the direct byte to the direct byte34
MOV direct,@RiMoves the indirect RAM to the direct byte24
MOV direct,#dataMoves the immediate data to the direct
byte33
MOV @Ri,AMoves the accumulator to the indirect RAM13
MOV @Ri,directMoves the direct byte to the indirect RAM25
MOV @Ri,#dataMoves the immediate data to the indirect RAM23
MOV DPTR,#dataMoves a 16-bit data to the data pointer33
MOVC A,@A+DPTRMoves the code byte relative to the DPTR to the
accumulator (address=A+DPTR)13
MOVC A,@A+PCMoves the code byte relative to the PC to the
accumulator (address=A+PC)13
MOVX A,@RiMoves the external RAM (8-bit address) to the
accumulator13-10
MOVX A,@DPTRMoves the external RAM (16-bit address) to the
accumulator13-10
MOVX @Ri,AMoves the accumulator to the external RAM (8-bit
address)14-11
MOVX @DPTR,AMoves the accumulator to the external RAM (16-bit
address)14-11
PUSH directPushes the direct byte onto the stack24
POP directPops the direct byte from the stack/td>23
XCH A,RnExchanges the register with the accumulator12
XCH A,directExchanges the direct byte with the accumulator23
XCH A,@RiExchanges the indirect RAM with the accumulator13
XCHD A,@RiExchanges the low-order nibble indirect RAM with the
accumulator13
Logic InstructionsLogic instructions perform logic operations
upon corresponding bits of two registers. After execution, the
result is stored in the first operand.LOGIC INSTRUCTIONS
MnemonicDescriptionByteCycle
ANL A,RnAND register to accumulator11
ANL A,directAND direct byte to accumulator22
ANL A,@RiAND indirect RAM to accumulator12
ANL A,#dataAND immediate data to accumulator22
ANL direct,AAND accumulator to direct byte23
ANL direct,#dataAND immediae data to direct register34
ORL A,RnOR register to accumulator11
ORL A,directOR direct byte to accumulator22
ORL A,@RiOR indirect RAM to accumulator12
ORL direct,AOR accumulator to direct byte23
ORL direct,#dataOR immediate data to direct byte34
XRL A,RnExclusive OR register to accumulator11
XRL A,directExclusive OR direct byte to accumulator22
XRL A,@RiExclusive OR indirect RAM to accumulator12
XRL A,#dataExclusive OR immediate data to accumulator22
XRL direct,AExclusive OR accumulator to direct byte23
XORL direct,#dataExclusive OR immediate data to direct
byte34
CLR AClears the accumulator11
CPL AComplements the accumulator (1=0, 0=1)11
SWAP ASwaps nibbles within the accumulator11
RL ARotates bits in the accumulator left11
RLC ARotates bits in the accumulator left through carry11
RR ARotates bits in the accumulator right11
RRC ARotates bits in the accumulator right through carry11
Bit-oriented InstructionsSimilar to logic instructions,
bit-oriented instructions perform logic operations. The difference
is that these are performed upon single bits.BIT-ORIENTED
INSTRUCTIONS
MnemonicDescriptionByteCycle
CLR CClears the carry flag11
CLR bitClears the direct bit23
SETB CSets the carry flag11
SETB bitSets the direct bit23
CPL CComplements the carry flag11
CPL bitComplements the direct bit23
ANL C,bitAND direct bit to the carry flag22
ANL C,/bitAND complements of direct bit to the carry flag22
ORL C,bitOR direct bit to the carry flag22
ORL C,/bitOR complements of direct bit to the carry flag22
MOV C,bitMoves the direct bit to the carry flag22
MOV bit,CMoves the carry flag to the direct bit23
3.2 Description of all 8051 instructionsHere is a list of the
operands and their meanings: A- accumulator;Rn- is one of working
registers (R0-R7) in the currently active RAM memory bank; Direct-
is any 8-bit address register of RAM. It can be any general-purpose
register or a SFR (I/O port, control register etc.); @Ri- is
indirect internal or external RAM location addressed by register R0
or R1; #data- is an 8-bit constant included in instruction (0-255);
#data16- is a 16-bit constant included as bytes 2 and 3 in
instruction (0-65535); addr16- is a 16-bit address. May be anywhere
within 64KB of program memory; addr11- is an 11-bit address. May be
within the same 2KB page of program memory as the first byte of the
following instruction; rel- is the address of a close memory
location (from -128 to +127 relative to the first byte of the
following instruction). On the basis of it, assembler computes the
value to add or subtract from the number currently stored in the
program counter; bit- is any bit-addressable I/O pin, control or
status bit; and C- is carry flag of the status register (register
PSW).ACALL addr11- Absolute subroutine calladdr11:Subroutine
addressDescription:Instruction unconditionally calls a subroutine
located at the specified code address. Therefore, the current
address and the address of called subroutine must be within the
same 2K byte block of the program memory, starting from the first
byte of the instruction following ACALL.Syntax: ACALL [subroutine
name];Bytes: 2 (instruction code, subroutine address);STATUS
register flags: No flags are affected.EXAMPLE:
Before execution: PC=0123hAfter execution: PC=0345hADD A,Rn-
Adds the register Rn to the accumulatorA: accumulatorRn: any R
register (R0-R7)Description: Instruction adds the register Rn
(R0-R7) to the accumulator. After addition, the result is stored in
the accumulator.Syntax: ADD A,Rn;Byte: 1 (instruction code);STATUS
register flags: C, OV and AC;EXAMPLE:
Before execution: A=2Eh (46 dec.) R4=12h (18 dec.)After
execution: A=40h (64 dec.) R4=12hADD A,@Ri- Adds the indirect RAM
to the accumulatorA: accumulatorRi: Register R0 or
R1Description: