1
Micro processor And Interfacing
What are microcontrollers and what are they used for?
As all other good things, this powerful component is basically
very simple and is obtained by uniting tested and high- quality
"ingredients" (components) as per following receipt:
1. The simplest computers processor is used as a "brain" of the
future system.
2. Depending on the taste of the producer, it is added : a bit
of memory, a few A/D converters, timers, input/output lines
etc.
3. It is all placed in one of standard packages.
4. A simple software that will be able to control it all and
about which everyone will be able to learn has been developed.
Three things have had a crucial impact on such a success of the
microcontrollers:
Powerful and intelligently chosen electronics embedded in the
microcontrollers can via input/output devices ( switches, push
buttons, sensors, LCD displays, relays) control various processes
and devices such as: industrial automatics, electric current,
temperature, engine performance etc.
A very low price enables them to be embedded in such devices in
which, until recent time it was not worth embedding anything.
Thanks to that, the world is overwhelmed today with cheap automatic
devices and various intelligent appliences.
Prior knowledge is hardly needed for programming. It is
sufficient to have any kind of PC (software in use is not demanding
at all and it is easy to learn to work on it) and one simple device
(programmer) used for transffering completed programs into the
microcontroller.
Therefore, if you are infected with a virus called electronics,
there is nothing left for you to do but to learn how to control its
power and how to direct it at the right course.
How does microcontroller operate?Even though there is a great
number of various microcontrollers and even greater number of
programs designed for the microcontrollers use only, all of them
have many things in common. That means that if you learn to handle
one of them you will be able to handle them all. A typical scenario
on whose basis it all functions is as follows:
1. Power supply is turned off and everything is so stillchip is
programmed, every thing is in place, nothing indicates what is to
come
2. Power supply connectors are connected to the power supply
source and every thing starts to happen at high speed! The control
logic registers what is going on first. It enables only quartz
oscillator to operate. While the first preparations are in progress
and parasite capacities are being charged, the first milliseconds
go by.
3. Power supply connectors are connected to the power supply
source and every thing starts to happen at high speed! The control
logic registers what is going on first. It enables only quartz
oscillator to work. While the first preparations are in progress
and parasite capacities are being charged, the first milliseconds
go by.
4. Voltage level has reached its full value and frequency of
oscillator has became stable. The bits are being written to the
SFRs, showing the state of all periph erals and all pins are
configured as outputs. Everything occurs in harmony to the pulses
rhythm and the overall electronis starts operating. Since this
moment the time is measured in micro and nanoseconds.
5. Program Counter is reset to zero address of the program
memory. Instruction from that address is sent to instruction
decoder where its meaning is recognised and it is executed with
immediate effect.
The value of the Program Counter is being incremented by 1 and
the whole process is being repeated...several million times per
second.
Difference
Where ever
Small size
Low cost
Low power Architecture:Harvard university
The Architecture given by Harvard University has the following
advantages:
1: Data Space and Program Space are distinct
2: There is no Data corruption or loss of data
Disadvantage is:
1: The circuitry is very complex.Features of 8051
8 bit cpu
4k on chip ROM 128 Bytes on chip RAM
32 I/O
Two 16 bit timers
Full duplex UART
6 Source/5 Vector interrupts with two level priority levels
On chip clock Oscillator. Block Diagram:
The 8051 microcontroller has nothing impressive at first
sight:
4 Kb program memory is not much at all.
128Kb RAM (including SFRs as well) satisfies basic needs, but it
is not imposing amount.
4 ports having in total of 32 input/output lines are mostly
enough to make connection to peripheral environment and are not
luxury at all.
The whole configuration is obviously envisaged as such to
satisfy the needs of most programmers who work on development of
automation devices. One of advantages of this microcontroller 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. The other advantage is the way RAM is organized, the way
Central Processor Unit (CPU) operates and ports which maximally use
all recourses and enable further upgrading.
8051 Microcontroller's pins
Pins 1-8: Port 1 Each of these pins can be configured as input
or output.
Pin 9: RS Logical one on this pin stops microcontrollers
operating and erases the contents of most registers. By applying
logical zero to this pin, the program starts execution from the
beginning. In other words, a positive voltage pulse on this pin
resets the microcontroller.
Pins10-17: Port 3 Similar to port 1, each of these pins can
serve as universal input or output . Besides, all of them have
alternative functions:
Pin 10: RXD Serial asynchronous communication input or Serial
synchronous communication output.
Pin 11: TXD Serial asynchronous communication output or Serial
synchronous communication clock output.
Pin 12: INT0 Interrupt 0 input
Pin 13: INT1 Interrupt 1 input
Pin 14: T0 Counter 0 clock input
Pin 15: T1 Counter 1 clock input
Pin 16: WR Signal for writing to external (additional) RAM
Pin 17: RD Signal for reading from external RAM
Pin 18, 19: X2, X1 Internal oscillator input and output. A
quartz crystal which determines operating frequency is usually
connected to these pins. Instead of quartz crystal, the miniature
ceramics resonators can be also used for frequency stabilization.
Later versions of the microcontrollers operate at a frequency of 0
Hz up to over 50 Hz.
Pin 20: GND Ground
Pin 21-28: Port 2 If there is no intention to use external
memory then these port pins are configured as universal
inputs/outputs. In case external memory is used then the higher
address byte, i.e. addresses A8-A15 will appear on this port. It is
important to know that even memory with capacity of 64Kb is not
used ( i.e. note all bits on port are used for memory addressing)
the rest of bits are not available as inputs or outputs.
Pin 29: PSEN If external ROM is used for storing program then it
has a logic-0 value every time the microcontroller reads a byte
from memory.
Pin 30: ALE Prior to each reading from external memory, the
microcontroller will set the lower address byte (A0-A7) on P0 and
immediately after that activates the output ALE. Upon receiving
signal from the ALE pin, the external register (74HCT373 or
74HCT375 circuit is usually embedded ) memorizes the state of P0
and uses it as an address for memory chip. In the second part of
the microcontrollers machine cycle, a signal on this pin stops
being emitted and P0 is used now for data transmission (Data Bus).
In this way, by means of only one additional (and cheap) integrated
circuit, data multiplexing from the port is performed. This port at
the same time used for data and address transmission.
Pin 31: EA By 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. That means that even there is a
program written to the microcontroller, it will not be executed,
the program written to external ROM will be used instead.
Otherwise, by applying logic one to the EA pin, the microcontroller
will use both memories, first internal and afterwards external (if
it exists), up to end of address space.
Pin 32-39: Port 0 Similar to port 2, if external memory is not
used, these pins can be used as universal inputs or outputs.
Otherwise, P0 is configured as address output (A0-A7) when the ALE
pin is at high level (1) and as data output (Data Bus), when logic
zero (0) is applied to the ALE pin.
Pin 40: VCC Power supply +5V
Register set of 8051The AccumulatorIf youve worked with any
other assembly languages you will be familiar with the concept of
an Accumulator register.
The Accumulator, as its name suggests, is used as a general
register to accumulate the results of a large number of
instructions. It can hold an 8-bit (1-byte) value and is the most
versatile register the 8051 has due to the shear number of
instructions that make use of the accumulator. More than half of
the 8051s 255 instructions manipulate or use the accumulator in
some way.
For example, if you want to add the number 10 and 20, the
resulting 30 will be stored in the Accumulator. Once you have a
value in the Accumulator you may continue processing the value or
you may store it in another register or in memory.
The "R" registersThe "R" registers are a set of eight registers
that are named R0, R1, etc. up to and including R7.
These registers are used as auxillary registers in many
operations. To continue with the above example, perhaps you are
adding 10 and 20. The original number 10 may be stored in the
Accumulator whereas the value 20 may be stored in, say, register
R4. To process the addition you would execute the command:
ADD A,R4
After executing this instruction the Accumulator will contain
the value 30.
You may think of the "R" registers as very important auxillary,
or "helper", registers. The Accumulator alone would not be very
useful if it were not for these "R" registers.
The "R" registers are also used to temporarily store values. For
example, lets say you want to add the values in R1 and R2 together
and then subtract the values of R3 and R4. One way to do this would
be:
MOV A,R3 ;Move the value of R3 into the accumulatorADD A,R4 ;Add
the value of R4MOV R5,A ;Store the resulting value temporarily in
R5MOV A,R1 ;Move the value of R1 into the accumulatorADD A,R2 ;Add
the value of R2SUBB A,R5 ;Subtract the value of R5 (which now
contains R3 + R4)
As you can see, we used R5 to temporarily hold the sum of R3 and
R4. Of course, this isnt the most efficient way to calculate
(R1+R2) - (R3 +R4) but it does illustrate the use of the "R"
registers as a way to store values temporarily.
The "B" RegisterThe "B" register is very similar to the
Accumulator in the sense that it may hold an 8-bit (1-byte)
value.
The "B" register is only used by two 8051 instructions: MUL AB
and DIV AB. Thus, if you want to quickly and easily multiply or
divide A by another number, you may store the other number in "B"
and make use of these two instructions.
Aside from the MUL and DIV instructions, the "B" register is
often used as yet another temporary storage register much like a
ninth "R" register.
The Data Pointer (DPTR)The Data Pointer (DPTR) is the 8051s only
user-accessable 16-bit (2-byte) register. The Accumulator, "R"
registers, and "B" register are all 1-byte values.
DPTR, as the name suggests, is used to point to data. It is used
by a number of commands which allow the 8051 to access external
memory. When the 8051 accesses external memory it will access
external memory at the address indicated by DPTR.
While DPTR is most often used to point to data in external
memory, many programmers often take advantge of the fact that its
the only true 16-bit register available. It is often used to store
2-byte values which have nothing to do with memory locations.
The Program Counter (PC)The Program Counter (PC) is a 2-byte
address which tells the 8051 where the next instruction to execute
is found in memory. When the 8051 is initialized PC always starts
at 0000h and is incremented each time an instruction is executed.
It is important to note that PC isnt always incremented by one.
Since some instructions require 2 or 3 bytes the PC will be
incremented by 2 or 3 in these cases.
The Program Counter is special in that there is no way to
directly modify its value. That is to say, you cant do something
like PC=2430h. On the other hand, if you execute LJMP 2430h youve
effectively accomplished the same thing.
The Stack Pointer (SPThe Stack Pointer, like all registers
except DPTR and PC, may hold an 8-bit (1-byte) value. The Stack
Pointer is used to indicate where the next value to be removed from
the stack should be taken from.
When you push a value onto the stack, the 8051 first increments
the value of SP and then stores the value at the resulting memory
location.
When you pop a value off the stack, the 8051 returns the value
from the memory location indicated by SP, and then decrements the
value of SP.
This order of operation is important. When the 8051 is
initialized SP will be initialized to 07h. If you immediately push
a value onto the stack, the value will be stored in Internal RAM
address 08h. This makes sense taking into account what was
mentioned two paragraphs above: First the 8051 will increment the
value of SP (from 07h to 08h) and then will store the pushed value
at that memory address (08h).
SP is modified directly by the 8051 by six instructions: PUSH,
POP, ACALL, LCALL, RET, and RETI. It is also used intrinsically
whenever an interrupt is triggered (more on interrupts later. Dont
worry about them for now!).
MEMORY ORGANISATION:
Counters and Timers
As explained in the previous chapter, the main oscillator of the
microcontroller uses quartz crystal for its operating. As the
frequency of this oscillator is precisely defined and very stable,
these pulses are the most suitable for time measuring (such
oscillators are used in quartz clocks as well). In order to measure
time between two events it is only needed to count up pulses from
this oscillator. That is exactly what the timer is doing. Namely,
if the timer is properly programmed, the value written to the timer
register will be incremented or decremented after each coming
pulse, i.e. once per each machine cycle cycle. Taking into account
that one instruction lasts 12 quartz oscillator periods (one
machine cycle), by embedding quartz with oscillator frequency of
12MHz, a number in the timer register will be changed million times
per second, i.e. each microsecond.
The 8051 microcontrollers have 2 timer counters called T0 and
T1. As their names tell, their main purpose is to measure time and
count external events. Besides, they can be used for generating
clock pulses used in serial communication, i.e. Baud Rate.
Timer T0
As it is shown in the picture below, this timer consists of two
registers TH0 and TL0. The numbers these registers include
represent a lower and a higher byte of one 16-digit binary
number.
This means that if the content of the timer 0 is equal to 0
(T0=0) then both registers it includes will include 0. If the same
timer contains for example number 1000 (decimal) then the register
TH0 (higher byte) will contain number 3, while TL0 (lower byte)
will contain decimal number 232.
Formula used to calculate values in registers is very simple:TH0
256 + TL0 = TMatching the previous example it would be as follows
:3 256 + 232 = 1000
Since the timers are virtually 16-bit registers, the greatest
value that could be written to them is 65 535. In case of exceeding
this value, the timer will be automatically reset and afterwords
that counting starts from 0. It is called overflow. Two registers
TMOD and TCON are closely connected to this timer and control how
it operates.
TMOD Register (Timer Mode)
This register selects mode of the timers T0 and T1. As
illustrated in the following picture, the lower 4 bits (bit0 -
bit3) refer to the timer 0, while the higher 4 bits (bit4 - bit7)
refer to the timer 1. There are in total of 4 modes and each of
them is described here in this book.
Bits of this register have the following purpose:
GATE1 starts and stops Timer 1 by means of a signal provided to
the pin INT1 (P3.3):
1 - Timer 1 operates only if the bit INT1 is set
0 - Timer 1 operates regardless of the state of the bit INT
1
C/T1 selects which pulses are to be counted up by the
timer/counter 1:
1 - Timer counts pulses provided to the pin T1 (P3.5)
0 - Timer counts pulses from internal oscillator
T1M1,T1M0 These two bits selects the Timer 1 operating mode.
T1M1T1M0ModeDescription
00013-bit timer
01116-bit timer
1028-bit auto-reload
113Split mode
GATE0 starts and stops Timer 1, using a signal provided to the
pin INT0 (P3.2):
1 - Timer 0 operates only if the bit INT0 is set
0 - Timer 0 operates regardless of the state of the bit INT0
C/T0 selects which pulses are to be counted up by the
timer/counter 0:
1 - Timer counts pulses provided to the pin T0(P3.4)
0 - Timer counts pulses from internal oscillator
T0M1,T0M0 These two bits select the Timer 0 operating mode.
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 compatibility
with the previuos versions of the microcontrollers. When using this
mode, the higher byte TH0 and only the first 5 bits of the lower
byte TL0 are in use. Being configured in this way, the Timer 0 uses
only 13 of all 16 bits. How does it operate? With each new pulse
coming, the state of the lower register (that one with 5 bits) is
changed. After 32 pulses received it becomes full and automatically
is reset, while the higher byte TH0 is incremented by 1. This
action will be repeated until registers count up 8192 pulses. After
that, both registers are reset and counting starts from 0.
Timer 0 in mode 1 (16-bit timer)
All bits from the registers TH0 and TL0 are used in this mode.
That is why for this mode is being more commonly used. Counting is
performed in the same way as in mode 0, with difference that the
timer counts up to 65 536, i.e. as far as the use of 16 bits
allows.
Timer 0 in mode 2 (Auto-Reload Timer)
What does auto-reload mean? Simply, it means that such timer
uses only one 8-bit register for counting, but it never counts from
0 but from an arbitrary chosen value (0- 255) saved in another
register.
The advantages of this way of counting are described in the
following example: suppose that for any reason it is continuously
needed to count up 55 pulses at a time from the clock
generator.
When using mode 1 or mode 0, It is needed to write number 200 to
the timer registers and check constantly afterwards whether
overflow occured, i.e. whether the value 255 is reached by counting
. When it has occurred, it is needed to rewrite number 200 and
repeat the whole procedure. The microcontroller performs the same
procedure in mode 2 automatically. Namely, in this mode it is only
register TL0 operating as a timer ( normally 8-bit), while the
value from which counting should start is saved in the TH0
register. Referring to the previous example, in order to register
each 55th pulse, it is needed to write the number 200 to the
register and configure the timer to operate in mode 2.
Timer 0 in Mode 3 (Split Timer)
By configuring Timer 0 to operate in Mode 3, the 16-bit counter
consisting of two registers TH0 and TL0 is split into two
independent 8-bit timers. In addition, all control bits which
belonged to the initial Timer 1 (consisting of the registers TH1
and TL1), now control newly created Timer 1. This means that even
though the initial Timer 1 still can be configured to operate in
any mode ( mode 1, 2 or 3 ), it is no longer able to stop, simply
because there is no bit to do that. Therefore, in this mode, it
will uninterruptedly operate in the background .
The only application of this mode is in case two independent
'quick' timers are used and the initial Timer 1 whose operating is
out of control is used as baud rate generator.
TCON - Timer Control Register
This is also one of the registers whose bits directly control
timer operating.Only 4 of all 8 bits this register has are used for
timer control, while others are used for interrupt control which
will be discussed later.
TF1 This bit is automatically set with the Timer 1 overflow
TR1 This bit turns the Timer 1 on
1 - Timer 1 is turned on
0 - Timer 1 is turned off
TF0 This bit is automatically set with the Timer 0 overflow.
TR0 This bit turns the timer 0 on
1 - Timer 0 is turned on
0 - Timer 0 is turned off
How to start Timer 0 ?
Normally, first this timer and afterwards its mode should be
selected. Bits which control that are resided in the register
TMOD:
This means that timer 0 operates in mode 1 and counts pulses
from internal source whose frequency is equal to 1/12 the quartz
frequency.In order to enable the timer, turn it on:
Immediately upon the bit TR0 is set, the timer starts operating.
Assuming that a quartz crystal with frequency of 12MHz is embedded,
a number it contains will be incremented every microsecond. By
counting up to 65.536 microseconds, the both registers that timer
consists of will be set. The microcontroller automatically reset
them and the timer keeps on repeating counting from the beginning
as far as the bits value is logic one (1).
How to 'read' a timer ?
Depending on the timers application, it is needed to read a
number in the timer registers or to register a moment they have
been reset.
- Everything is extremely simple when it is needed to read a
value of the timer which uses only one register for counting (mode
2 or Mode 3) . It is sufficient to read its state at any moment and
it is it!
- It is a bit complicated to read a timers value when it
operates in mode 2. Assuming that the state of the lower byte is
read first (TL0) and the state of the higher byte (TH0) afterwards,
the result is:
TH0 = 15 TL0 = 255
Everything seems to be in order at first sight, but the current
state of register at the moment of reading was:
TH0 = 14 TL0 = 255
In case of negligence, this error in counting ( 255 pulses ) may
occur for not so obvious but quite logical reason. Reading the
lower byte is correct ( 255 ), but at the same time the program
counter was taking a new instruction for the TH0 state reading, an
overflow occurred and both registers have changed their contents (
TH0: 1415, TL0: 2550). The problem has simple solution: the state
of the higher byte should be read first, then the state of the
lower byte and once again the state of the higher byte. If the
number stored in the higher byte is not the same both times it has
been read then this sequence should be repeated ( this is a mini-
loop consisting of only 3 instructions in a program).
There is another solution too. It is sufficient to simply turn
timer off while reading ( the bit TR0 in the register TCON should
be 0), and turn it on after that.
Detecting Timer 0 Overflow
Usually, there is no need to continuously read timer registers
contents. It is sufficient to register the moment they are reset,
i.e. when counting starts from 0. It is called overflow. When this
has occurred, the bit TF0 from the register TCON will be
automatically set. The microcontroller is waiting for that moment
in a way that program will constantly check the state of this bit.
Furthermore, an interrupt to stop the main program execution can be
enabled. Assuming that it is needed to provide a program pause (
time the program appeared to be stopped) in duration of for example
0.05 seconds ( 50 000 machine cycles ):
First, it is needed to calculate a number that should be written
to the timer registers:
This number should be written to the timer registers TH0 and
TL0:
Once the timer is started it will continue counting from the
written number. Program instruction checks if the bit TF0 is set,
which happens at the moment of overflow, i.e. after exactly 50.000
machine cycles and 0.05 seconds respectively.
UART (Universal Asynchronous Receiver and Transmitter)
One of the features that makes this microcontroller so powerful
is an integrated UART, better known as a serial port. It is a
duplex port, which means that it can transmit and receive data
simultaneously. Without it, serial data sending and receiving would
be endlessly complicated part of the program where the pin state
continuously is being changed and checked according to strictly
determined rhythm. Naturally, it does not happen here because the
UART resolves it in a very elegant manner. All the programmer needs
to do is to simply select serial port mode and baud rate. When the
programmer is such configured, serial data sending is done by
writing to the register SBUF while data receiving is done by
reading the same register. The microcontroller takes care of all
issues necessary for not making any error during data exchange.
Serial port should be configured prior to being used. That
determines how many bits one serial word contains, what the baud
rate is and what the pulse source for synchronization is. All bits
controlling this are stored in the SFR Register SCON (Serial
Control).
SCON Register (Serial Port Control Register) SM0 - bit selects
mode
SM1 - bit selects mode
SM2 - bit is used in case that several microcontrollers share
the same interface. In normal circumstances this bit must be
cleared in order to enable connection to function normally.
REN - bit enables data receiving via serial communication and
must be set in order to enable it.
TB8 - Since all registers in microcontroller are 8-bit
registers, this bit solves the problem of sending the 9th bit in
modes 2 and 3. Simply, bits content is sent as the 9th bit.
RB8 - bit has the same purpose as the bit TB8 but this time on
the receiver side. This means that on receiving data in 9-bit
format , the value of the last ( ninth) appears on its
location.
TI - bit is automatically set at the moment the last bit of one
byte is sent when the USART operates as a transmitter. In that way
processor knows that the line is available for sending a new byte.
Bit must be clear from within the program!
RI - bit is automatically set once one byte has been received.
Everything functions in the similar way as in the previous case but
on the receive side. This is line a doorbell which announces that a
byte has been received via serial communication. It should be read
quickly prior to a new data takes its place. This bit must also be
also cleared from within the program!
As seen, serial port mode is selected by combining the bits SM0
and SM2 :
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, the data are transferred through the RXD pin, while
clock pulses appear on the TXD pin. The bout rate is fixed at 1/12
the quartz oscillator frequency. On transmit, the least significant
bit (LSB bit) is being sent/received first. (received).
TRANSMIT - Data transmission in form of pulse train
automatically starts on the pin RXD at the moment the data has been
written to the SBUF register.In fact, this process starts after any
instruction being performed on this register. Upon all 8 bits have
been sent, the bit TI in the SCON register is automatically
set.
RECEIVE - Starts data receiving through the pin RXD once two
necessary conditions are met: bit REN=1 and RI=0 (both bits reside
in the SCON register). Upon 8 bits have been received, the bit RI
(register SCON) is automatically set, which indicates that one byte
is received.
Since, there are no START and STOP bits or any other bit except
data from the SBUF register, this mode is mainly used on shorter
distance where the noise level is minimal and where operating rate
is important. A typical example for this is I/O port extension by
adding cheap IC circuit ( shift registers 74HC595, 74HC597 and
similar).
Mode 1
In Mode1 10 bits are transmitted through TXD or received through
RXD in the following manner: a START bit (always 0), 8 data bits
(LSB first) and a STOP bit (always 1) last. The START bit is not
registered in this pulse train. Its purpose is to start data
receiving mechanism. On receive the STOP bit is automatically
written to the RB8 bit in the SCON register.
TRANSMIT - A sequence for data transmission via serial
communication is automatically started upon the data has been
written to the SBUF register. End of 1 byte transmission is
indicated by setting the TI bit in the SCON register.
RECEIVE - Receiving starts as soon as the START bit (logic zero
(0)) appears on the pin RXD. The condition is that bit REN=1and bit
RI=0. Both of them are stored in the SCON register. The RI bit is
automatically set upon receiving has been completed.
The Baud rate in this mode is determined by the timer 1 overflow
time.
Mode 2
In mode 2, 11 bits are sent through TXD or received through RXD:
a START bit (always 0), 8 data bits (LSB first), additional 9th
data bit and a STOP bit (always 1) last. On transmit, the 9th data
bit is actually the TB8 bit from the SCON register. This bit
commonly has the purpose of parity bit. Upon transmission, the 9th
data bit is copied to the RB8 bit in the same register ( SCON).The
baud rate is either 1/32 or 1/64 the quartz oscillator
frequency.
TRANSMIT - A sequence for data transmission via serial
communication is automatically started upon the data has been
written to the SBUF register. End of 1 byte transmission is
indicated by setting the TI bit in the SCON register.
RECEIVE - Receiving starts as soon as the START bit (logic zero
(0)) appears on the pin RXD. The condition is that bit REN=1and bit
RI=0. Both of them are stored in the SCON register. The RI bit is
automatically set upon receiving has been completed.
Mode 3
Mode 3 is the same as Mode 2 except the baud rate. In Mode 3 is
variable and can be selected.
The parity bit is the bit P in the PSW register. The simplest
way to check correctness of the received byte is to add this parity
bit to the transmit side as additional bit. Simply, immediately
before transmit, the message is stored in the accumulator and the
bit P goes into the TB8 bit in order to be a part of the message.
On the receive side is the opposite : received byte is stored in
the accumulator and the bit P is compared with the bit RB8 (
additional bit in the message). If they are the same- everything is
OK!
Baud Rate
Baud Rate is defined as a number of send/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
bit SMOD stored in the SCON register. All 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 baud rate generator
Timer 1 is usually used as a baud rate generator because it is
easy to adjust various baud rate by the means of this timer. The
whole procedure is simple:
First, Timer 1 overflow interrupt should be disabled
Timer T1 should be set in auto-reload mode
Depending on necessary baud rate, in order to obtain some of the
standard values one of the numbers from the table should be
selected. That number should be written 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 Communication
As described in the previous text, modes 2 and 3 enable the
additional 9th data bit to be part of message. It can be used for
checking data via parity bit. Another useful application of this
bit is in communication between two microcontrollers, i.e.
multiprocessor communication. This feature is enabled by setting
the SM2 bit in the SCON register. The consequence is the following:
when the STOP bit is ready, indicating end of message, the serial
port interrupt will be requested only in case the bit RB8 = 1 (the
9th bit).
The whole procedure will be performed as follows:
Suppose that there are several connected microcontrollers having
to exchange data. That means that each of them must have its
address. The point is that each address sent via serial
communication has the 9th bit set (1), while data has it cleared
(0). If the microcontroller A should send data to the
microcontroller C then it at will place first send address of C and
the 9th bit set to 1. That will generate interrupt and all
microcontrollers will check whether they are called.
Of course, only one of them will recognize this address and
immediately clear the bit SM2 in the SCON register. All following
data will be normally received by that microcontroller and ignored
by other microcontrollers.
8051 Microcontroller Interrupts
There are five interrupt sources for the 8051, which means that
they can recognize 5 different event that can interrupt regular
program execution. Each interrupt can be enabled or disabled by
setting bits in the IE register. Also, as seen from the picture
below the whole interrupt system can be disabled by clearing bit EA
from the same register.
Now, one detail should be explained which is not completely
obvious but refers to external interrupts- INT0 and INT1. Namely,
if the bits IT0 and IT1 stored in the TCON register are set,
program interrupt will occur on changing logic state from 1 to 0,
(only at the moment). If these bits are cleared, the same signal
will generate interrupt request and it will be continuously
executed as far as the pins are held low.
IE Register (Interrupt Enable) EA - bit enables or disables all
other interrupt sources (globally)
0 - (when cleared) any interrupt request is ignored (even if it
is enabled)
1 - (when set to 1) enables all interrupts requests which are
individually enabled
ES - bit enables or disables serial communication interrupt
(UART)
0 - UART System can not generate interrupt
1 - UART System enables interrupt
ET1 - bit enables or disables Timer 1 interrupt
0 - Timer 1 can not generate interrupt
1 - Timer 1 enables interrupt
EX1 - bit enables or disables INT 0 pin external interrupt
0 - change of the pin INT0 logic state can not generate
interrupt
1 - enables external interrupt at the moment of changing the pin
INT0 state
ET0 - bit enables or disables timer 0 interrupt
0 - Timer 0 can not generate interrupt
1 - enables timer 0 interrupt
EX0 - bit enables or disables INT1 pin external interrupt
0 - change of the INT1 pin logic state can not cause
interrupt
1 - enables external interrupt at the moment of changing the pin
INT1 state
Interrupt Priorities
It is not possible to predict when an interrupt will be
required. For that reason, if several interrupts are enabled. It
can easily occur that while one of them is in progress, another one
is requested. In such situation, there is a priority list making
the microcontroller know whether to continue operating or meet a
new interrupt request.
The priority list cosists of 3 levels:
1. Reset! The apsolute master of the situation. If an request
for Reset omits, everything is stopped and the microcontroller
starts operating from the beginning.
2. Interrupt priority 1 can be stopped by Reset only.
3. Interrupt priority 0 can be stopped by both Reset and
interrupt priority 1.
Which one of these existing interrupt sources have higher and
which one has lower priority is defined in the IP Register (
Interrupt Priority Register). It is usually done at the beginning
of the program. According to that, there are several
possibilities:
Once an interrupt service begins. It cannot be interrupted by
another inter rupt at the same or lower priority level, but only by
a higher priority interrupt.
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 who came later has to wait until
routine being in progress ends.
If two interrupts of equal priority requests arrive at the same
time then the interrupt to be serviced is selected according to the
following priority list :
1. External interrupt INT0
2. Timer 0 interrupt
3. External Interrupt INT1
4. Timer 1 interrupt
5. Serial Communication Interrupt
IP 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 1
Handling Interrupt
Once some of interrupt requests arrives, everything occurs
according to the following order:
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
following table:
Interrupt SourceVector (address)
IE03 h
TF0B h
TF11B h
RI, TI23 h
All addresses are in hexadecimal format
4. The appropriate subroutines processing interrupts should be
located at these addresses. Instead of them, there are usually jump
instructions indicating the location where the subroutines
reside.
5. When interrupt routine is executed, the address of the next
instruction to execute is poped from the stack to the program
counter and interrupted program continues operating from where it
left off.
From the moment an interrupt is enabled, the microcontroller is
on alert all the time. When interrupt request arrives, the program
execution is interrupted, electronics recognizes the cause and the
program jumps to the appropriate address (see the table above ).
Usually, there is a jump instruction already prepared subroutine
prepared in advance. The subroutine is executed which exactly the
aim- to do something when something else has happened. After that,
the program continues operating from where it left off
Reset
Reset 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
internal reset signal during which all SFRs, excluding SBUF
registers, Stack Pointer and ports are reset ( the state of the
first two ports is indefinite while FF value is being written to
the ports configuring all pins as inputs). Depending on device
purpose and environment it is in, on power-on reset it is usually
push button or circuit or both connected to the RS pin. One of the
most simple circuit providing secure reset at the moment of turning
power on is shown on the picture.
Everything functions rather simply: upon the power is on,
electrical condenser is being charged for several milliseconds
through resistor connected to the ground and during this process
the pin voltage supply is on. When the condenser is charged, power
supply voltage is stable and the pin keeps being connected to the
ground providing normal operating in that way. If later on, during
the operation, manual reset button is pushed, the condenser is
being temporarily discharged and the microcontroller is being
reset. Upon the button release, the whole process is repeated
Through the program- step by step...
The microcontrollers normally operate at very high speed. The
use of 12 Mhz quartz crystal enables 1.000.000 instructions per
second to be executed! In principle, there is no need for higher
operating rate. In case it is needed, it is easy to built-in
crystal for high frequency. The problem comes up when it is
necessary to slow down. For example, when during testing in real
operating environment, several instructions should be executed step
by step in order to check for logic state of I/O pins.
Interrupt system applied on the 8051 microcontrollers
practically stops operating and enables instructions to be executed
one at a time by pushing button. Two interrupt features enable
that:
Interrupt request is ignored if an interrupt of the same
priority level is being in progress.
Upon interrupt routine has been executed, a new interrupt is not
executed until at least one instruction from the main program is
executed.
In order to apply 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 entered into the
program (start from address 03hex.):
What is going on? Once the pin P3.2 is set to 0 (for example, by
pushing button), the microcontroller will interrupt program
execution jump to the address 03hex, will be executed a
mini-interrupt routine consisting of 3 instructions is located at
that address.
The first instruction is being executed until the push button is
pressed ( logic one (1) on the pin P3.2). The second instruction is
being executed until the push button is released. Immediately after
that, the instruction RETI is executed and processor continues
executing the main program. After each executed instruction, the
interrupt INT0 is generated and the whole procedure is repeated (
push button is still pressed). Button Press = One Instruction.
8051 Microcontroller Power Consumption Control
Conditionally said microcontroller is the most part of its
lifetime is inactive for some external signal in order to takes its
role in a show. It can make a great problem in case batteries are
used for power supply. In extremely cases, the only solution is to
put the whole electronics to sleep in order to reduce consumption
to the minimum. A typical example of this is remote TV 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. While normally
operating, the AT89S53 uses current of approximately 25mA, which
shows that it is not too sparing microcontroller. Anyway, it doesnt
have to be always like this, it can easily switch the operation
mode in order to reduce its total consumption to approximately
40uA. Actually, there are two power-saving modes of operation: Idle
and Power Down.Idle mode
Immediately upon instruction which sets the bit IDL in the PCON
register, the microcontroller turns off the greatest power
consumer- CPU unit while peripheral units serial port, timers and
interrupt system continue operating normally consuming 6.5mA. In
Idle mode, the state of all registers and I/O ports is remains
unchanged.
In order to terminate the Idle mode and make the microcontroller
operate normally, it is necessary to enable and execute any
interrupt or reset.Then, the IDL bit is automatically cleared and
the program continues executing from instruction following that
instruction which has set the IDL bit. It is recommended that three
first following one which set NOP instructions. They do not perform
any operation but keep the microcontroller from undesired changes
on the I/O ports.
Power Down mode
When the bit PD in the register PCON is set from within the
program, the microcontroller is set to Powerdown mode. It and turns
off its internal oscillator reducing drastically consumption in
that way. In power- down mode the microcontroller can operate using
only 2V power supply while the total power consumption is less than
40uA. The only way to get the microcontroller back to normal mode
is reset.
During Power Down mode, the state of all SFR registers and I/O
ports remains unchanged, and after the microcontroller is put get
into the normal mode, the content of the SFR register is lost, but
the content of internal RAM is saved. Reset signal must be long
enough approximately 10mS in order to stabilize quartz oscillator
operating.
PCON register The purpose of the Register PCON bits :
SMOD By setting this bit baud rate is doubled.
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 is set into Power
Down mode.
IDL By setting this bit the microcontroller is set into Idle
mode.
Data Memory
As already mentioned, Data Memory is used for temporarily
storing and keeping data and intermediate results created and used
during microcontrollers operating. Besides, this microcontroller
family includes many other registers such as: hardware counters and
timers, input/output ports, serial data buffers etc. The previous
versions have the total memory size of 256 locations, while for
later models this number is incremented by additional 128 available
registers. In both cases, these first 256 memory locations
(addresses 0-FFh) are the base of the memory. Common to all types
of the 8051 microcontrollers. Locations available to the user
occupy memory space with addresses from 0 to 7Fh. First 128
registers and this part of RAM is divided in several blocks.
The first block consists of 4 banks each including 8 registers
designated as R0 to R7. Prior to access them, a bank containing
that register must be selected. Next memory block ( in the range of
20h to 2Fh) is bit- addressable, which means that each bit being
there has its own address from 0 to 7Fh. Since there are 16 such
registers, this block contains in total of 128 bits with separate
addresses (The 0th bit of the 20h byte has the bit address 0 and
the 7th bit of th 2Fh byte has the bit address 7Fh). The third
group of registers occupy addresses 2Fh-7Fh ( in total of 80
locations) and does not have any special purpose or
feature.Additional Memory Block of Data Memory
In order to satisfy the programmers permanent hunger for Data
Memory, producers have embedded an additional memory block of 128
locations into the latest versions of the 8051 microcontrollers.
Naturally, its not so simpleThe problem is that electronics
performing addressing has 1 byte (8 bits) on disposal and due to
that it can reach only the first 256 locations. In order to keep
already existing 8-bit architecture and compatibility with other
existing models a little trick has been used.
Using trick in this case means that additional memory block
shares the same addresses with existing locations intended for the
SFRs (80h- FFh). In order to differentiate between these two
physically separated memory spaces, different ways of addressing
are used. A direct addressing is used for all locations in the
SFRs, while the locations from additional RAM are accessible using
indirect addressing.
How to extend memory?
In case on-chip memory is not enough, it is possible to add two
external memory chips with capacity of 64Kb each. I/O ports P2 and
P3 are used for their addressing and data transmission.
From the users perspective, everything functions quite simple if
properly connected because the most operations are performed by the
microcontroller itself. The 8051 microcontroller has two separate
reading signals RD#(P3.7) and PSEN#. The first one is activated
byte from external data memory (RAM) should be read, while another
one is activated to read byte from external program memory (ROM).
These both signals are active at logical zero (0) level. A typical
example of such memory extension using special chips for RAM and
ROM, is shown on the previous picture. It is called Hardward
architecture.
Even though the additional memory is rarely used with the latest
versions of the microcontrollers, it will be described here in
short what happens when memory chips are connected according to the
previous schematic. It is important to know that the whole process
is performed automatically, i.e. with no intervention in the
program.
When the program during execution encounters the instruction
which resides in exter nal memory (ROM), the microcontroller will
activate its control output ALE and set the first 8 bits of address
(A0-A7) on P0. In this way, IC circuit 74HCT573 which "lets in" the
first 8 bits to memory address pins is activated.
A signal on the pin ALE closes the IC circuit 74HCT573 and
immediately afterwards 8 higher bits of address (A8-A15) appear on
the port. In this way, a desired location in addtional program
memory is completely addressed. The only thing left over is to read
its content.
Pins on P0 are configured as inputs, the pin PSEN is activated
and the microcon troller reads content from memory chip. The same
connections are used both for data and lower address byte.
Similar occurs when it is a needed to read some location from
external Data Memory. Now, addressing is performed in the same way,
while reading or writing is performed via signals which appear on
the control outputs RD or WR .
Addressing Modes:An "addressing mode" refers to how you are
addressing a given memory location. In summary, the addressing
modes are as follows, with an example of each:
Immediate AddressingMOV A,#20h
Direct AddressingMOV A,30h
Indirect AddressingMOV A,@R0
External DirectMOVX A,@DPTR
Code IndirectMOVC A,@A+DPTR
Each of these addressing modes provides important
flexibility.
Immediate AddressingImmediate addressing is so-named because the
value to be stored in memory immediately follows the operation code
in memory. That is to say, the instruction itself dictates what
value will be stored in memory.
For example, the instruction:
MOV A,#20h
This instruction uses Immediate Addressing because the
Accumulator will be loaded with the value that immediately follows;
in this case 20 (hexidecimal).
Immediate addressing is very fast since the value to be loaded
is included in the instruction. However, since the value to be
loaded is fixed at compile-time it is not very flexible.
Direct AddressingDirect addressing is so-named because the value
to be stored in memory is obtained by directly retrieving it from
another memory location. For example:
MOV A,30h
This instruction will read the data out of Internal RAM address
30 (hexidecimal) and store it in the Accumulator.
Direct addressing is generally fast since, although the value to
be loaded isnt included in the instruction, it is quickly
accessable since it is stored in the 8051s Internal RAM. It is also
much more flexible than Immediate Addressing since the value to be
loaded is whatever is found at the given address--which may be
variable.
Also, it is important to note that when using direct addressing
any instruction which refers to an address between 00h and 7Fh is
referring to Internal Memory. Any instruction which refers to an
address between 80h and FFh is referring to the SFR control
registers that control the 8051 microcontroller itself.
The obvious question that may arise is, "If direct addressing an
address from 80h through FFh refers to SFRs, how can I access the
upper 128 bytes of Internal RAM that are available on the 8052?"
The answer is: You cant access them using direct addressing. As
stated, if you directly refer to an address of 80h through FFh you
will be referring to an SFR. However, you may access the 8052s
upper 128 bytes of RAM by using the next addressing mode, "indirect
addressing."
Indirect AddressingIndirect addressing is a very powerful
addressing mode which in many cases provides an exceptional level
of flexibility. Indirect addressing is also the only way to access
the extra 128 bytes of Internal RAM found on an 8052.
Indirect addressing appears as follows:
MOV A,@R0
This instruction causes the 8051 to analyze the value of the R0
register. The 8051 will then load the accumulator with the value
from Internal RAM which is found at the address indicated by
R0.
For example, lets say R0 holds the value 40h and Internal RAM
address 40h holds the value 67h. When the above instruction is
executed the 8051 will check the value of R0. Since R0 holds 40h
the 8051 will get the value out of Internal RAM address 40h (which
holds 67h) and store it in the Accumulator. Thus, the Accumulator
ends up holding 67h.
Indirect addressing always refers to Internal RAM; it never
refers to an SFR. Thus, in a prior example we mentioned that SFR
99h can be used to write a value to the serial port. Thus one may
think that the following would be a valid solution to write the
value 1 to the serial port:
MOV R0,#99h ;Load the address of the serial portMOV @R0,#01h
;Send 01 to the serial port -- WRONG!!This is not valid. Since
indirect addressing always refers to Internal RAM these two
instructions would write the value 01h to Internal RAM address 99h
on an 8052. On an 8051 these two instructions would produce an
undefined result since the 8051 only has 128 bytes of Internal
RAM.
External DirectExternal Memory is accessed using a suite of
instructions which use what I call "External Direct" addressing. I
call it this because it appears to be direct addressing, but it is
used to access external memory rather than internal memory.
There are only two commands that use External Direct addressing
mode:
MOVX A,@DPTRMOVX @DPTR,A
As you can see, both commands utilize DPTR. In these
instructions, DPTR must first be loaded with the address of
external memory that you wish to read or write. Once DPTR holds the
correct external memory address, the first command will move the
contents of that external memory address into the Accumulator. The
second command will do the opposite: it will allow you to write the
value of the Accumulator to the external memory address pointed to
by DPTR.
External IndirectExternal memory can also be accessed using a
form of indirect addressing which I call External Indirect
addressing. This form of addressing is usually only used in
relatively small projects that have a very small amount of external
RAM. An example of this addressing mode is:
MOVX @R0,A
Once again, the value of R0 is first read and the value of the
Accumulator is written to that address in External RAM. Since the
value of @R0 can only be 00h through FFh the project would
effectively be limited to 256 bytes of External RAM. There are
relatively simple hardware/software tricks that can be implemented
to access more than 256 bytes of memory using External Indirect
addressing; however, it is usually easier to use External Direct
addressing if your project has more than 256 bytes of External
RAM.
Naveen Kumar .Auvusali Asst.Professor
EEE dept-SSR Engineering College,