354 33 Powerpoint-slides CH11

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N. Senthil Kumar,

M. Saravanan &

S. Jeevananthan

© Oxford University Press 2013



HARDWARE FEATURES OF 8051




Introduction• The major benefit of microcontrollers lies in their built-

in parallel ports. The parallel ports can be used tointerface all data converters (ADCs, DACs, etc.) anddisplay devices (LEDs, LCDs, etc.).

• Any microcontroller-based system needs to transferdata between the external peripherals and themicrocontroller. The microcontroller needs to read datafed by the user from the external interface, process it,

and give the output to the peripherals or to the useragain. To communicate data with the external world,the microcontroller needs ports. The ports maysupport either parallel or serial data transfer.




Parallel Ports of 8051• 8051 has 4 I/O ports namely Port0, Port1, Port2 and

Port3. The major constraint -the number of pin count. to

reduce - number of pins of the microcontroller ICs, the

pins allotted for the parallel ports -alternate functionsalso.

• Out of the available four parallel ports of 8051, Port 1 is

used exclusively for input and output functions alone.




Parallel Ports of 8051• The other port pins have -distinct function in addition to -

used for input and output functions. So,all the 24 pins of

Port 0, port 2, port 3 -two different functions based on

the commands or programs running.• All the four ports are bi-directional -programmed to have

input or output operation. All the 8 port pins -connected

through 8 D type port latches. One D type latch connects

the data in it to a port pin when the port is used asoutput port.




Parallel Ports of 8051

• The user can access all the four ports using -addresses

mapped in the special function register area.

• Note that it is possible to address individual bits of all

the four ports by - bit addresses. Using this bit address,

individual bits can be read in or changed.





Parallel Ports and Port Pins of 8051




Structure of Port 1• Port 1 -only port in 8051 -used exclusively for input and output

operation.

• The output of the port latch is connected to the port pin

through a transistor driver with internal pull up resistor. The

port can be operated as an input after writing 1 to all the bitsof port 1 latch.

• 8051 ports are organized such that most instructions read the

data from the pin for read operation and some instructions

read the data from the latch.• So, the input buffer consists of the select logic and the related

control signals – ‘Read Latch’, ‘Read Pin’ for discriminating this.




Internal structure of PORT 1




• Pins of ports 0 and 2 can be used as input port pins ifa 1 is written to the corresponding port latches by

the programmer.

•

For using Pins of ports 0 and 2 as output pins, a pullup resistor may have to be connected to the

corresponding port pins.

• Ports 0 and 2 have an alternate function in addition

to being used as input and output ports.

Structures of Port 0 and Port 2




• The Ports 0and 2 are used as address/ data bus when

external memory or I/O devices are accessed. Port 0 -low

order address bus and the port 2 -higher order address

bus.

• The drivers of Port0 and Port2 have an internal

multiplexer to serve this purpose as shown in figures.

Structures of Port 0 and Port 2



Internal Structure of Port 0






Structure of Port 3• Port 3 is different from the other ports in the aspect that

individual port pins can be programmed for input and

output operation.

•Each pin of port 3 can be programmed - for input, outputoperation or alternate functions.

• All the Port3 pins -serve an alternate function according

to the hardware signals and interfacing.

• the alternate functions can be activated only if the port3bits are written with 1s in their position.






Alternate functions of Port 3• Regarding the port access-two possibilities for read operation.

The read instruction for a port can either read the port latch

or the port pins.

• This difference is made in the internal hardware of 8051 in

order to avoid misinterpretation of the voltage level at the

pins.

• The instructions, which have the operations – Read, Modify

and write to the port – read the Port latches and not the pins.



Alternate Functions of Port 3

• All other instructions accessing the ports read the data from theport pins only. The example instructions that read the port latch are

– ANL P1,A;

– ORL P2,A;

–XRL P3,A;

– INC P0;

– DEC P1;

– JBC P1.1,DELAY;

•Note that these instructions read the port, modify the contents andthen write the data back to the port. So, the port latches are read

and not the pins.

– CPL P3.0;

– DJNZ P3,LABEL

–

CLR P3.1; – SETB P1.2;

– MOV P2.2,C;



External Memory Interfacing in 8051

• External memory interfaced to 8051 can be of two types-external program memory and external data memory.

• The external memory accesses are accomplished withthe Ports 0 and 2 of 8051 as they serve as the

multiplexed address/ data buses. The external memory in8051 is always accessed with 16 bit addresses.

• The 8051 outputs the signal ALE (Address Latch enable)in order to demultiplex the lower order address and databus.

• In addition, the micro-controller sends the control signalson the Port3 lines.



Program memory interfacing

• The program memory can be placed outside the chip inaddition to the internal program memory. The completeprogram memory can be placed outside the chipneglecting the internal program memory.

• Applying proper the voltage level on the input line EA of

8051 can do the selection of any of the above twomethods.

• Connecting EA to Ground will disable the internalprogram memory and all program memory accesses aredone to external memory. The Read strobe signal givenby the micro-controller is PSEN.

• This active low signal is connected to the Read selectionline of the memory chips.



Connecting External Program

Memory to 8051



Connecting External ProgramMemory to 8051

• Connecting EA pin of 8051 to the logic 1 or +5V willprogram the microcontroller to use the internalprogram memory for the addresses starting from0000. After the available internal memory, the

external memory is accessed.• If = 0; The internal program memory is not

accessed.

• If = 1; Then internal program memory is accessedfor address 0000-0FFF (or the available range) andexternal program memory is accessed for addressesgreater than 0FFF.



Connecting EPROM IC 27128 to 8051



Accessing External Data Memory

• The data memory in the system - be Random Access

memory as it should facilitate both read and write

operation of data. The external data memory is

interfaced in the same way as the program memoryis interfaced.

• The major difference is that the read and writes

operations can be done in Read /Write Random

Access Memory.



Accessing External Data Memory

• The control signals for reading and writing to data

memory are available from the port3 pins. P3.6 pin

gives data memory write enable signal and P3.7 pin

gives out the RAM read enable signal.

• A decoder logic circuit is necessary to select the RAM

chip based on the higher order address lines.



Connection External Data Memory to8051



Connecting RAM Chip 6264 to8051



8051 Timers• The 8051 comes -with two 16 bit timers, both of

which may be controlled, set, read, and configured

individually. The 8051 timers have three general

functions:

• Programming predefined length of time, and then

issuing an interrupt request.

• Counting the transitions on an external pin,

• Generating baud rates for the serial port.



8051 Timers• Basically the timers are the digital counters which are

incremented at the pulses given to it. The timers can becontrolled to -function through four SFRs namely, TMOD,TCON, TH0/TL0 and TH1/TL1.

• The timers will have overflow when it counts to full valueand resets to 0 upon next count.

• The overflow in the timers will set the two bits in theTCON SFR. This overflow can be programmed to interrupt

the microcontroller execution and execute a predefinedsubroutine



8051 Timers• If the timer registers are incremented by the internal

clock pulses from the microcontroller, then the operation

is termed as ‘Timing’ operation.

•

Meanwhile if the timer registers get their clock pulsesfrom an external device through the port 3 pins of 8051,

then the operation is termed as ‘Counting’.

• Timer 0 external input pin P3.4 (T0) is used give clock

input to timer 0 to act as counter.• Timer 1 external input pin P3.5 (T1) is used give clock

input to timer 1.



Timer SFRS• The 8051 has two timers and each of them will have

similar operations and functions. The timer in 8051 is

basically a 16-bit register which can be incremented

depending upon the clock pulses applied to it.• These timer registers are configured as the Special

Function Registers.



Timer SFRS• These SFRs at any time has the timer/counter register

content. So, the timers can be stopped at any time and thecontents can be read from these registers .Since there are

only two bytes devoted to the value of each timer it is

apparent that the maximum value a timer may have is

65,535.• If a timer contains the value 65,535 and is subsequently

incremented, it will reset to 0 with an indication of overflow.

• One timer is TIMER0 and the other is TIMER1. Each timer

also has two 8 bit SFRs namely TH0 and TL0 forming thehigher and lower order bytes of Timer0 and TH1 and TL1

forming the higher and lower bytes of Timer1.



Timer SFRS• These SFRs at any time has the timer/counter register

content. So, the timers can be stopped at any time and thecontents can be read from these registers .Since there are

only two bytes devoted to the value of each timer it is

apparent that the maximum value a timer may have is

65,535.• If a timer contains the value 65,535 and is subsequently

incremented, it will reset to 0 with an indication of overflow.

• One timer is TIMER0 and the other is TIMER1. Each timer

also has two 8 bit SFRs namely TH0 and TL0 forming thehigher and lower order bytes of Timer0 and TH1 and TL1

forming the higher and lower bytes of Timer1.



TMOD SFR• The TMOD SFR -used to control the mode of operation of

both timers. Each bit of the SFR gives the micro controller

specific information -how to run a timer. The higher order

four bits (bits 4 through 7) relate to Timer 1 whereas the

low four bits (bits 0 through 3) perform the samefunctions for timer 0.



BIT Patterns for TMOD (89h) SFR



TCON SFR• SFR that controls the two timers and provides

valuable information about them is TCON.



BIT Patterns for TCON (88h) SFR

Bit Name Bit Address Explanation of Function Timer

7 TF1 8Fh Timer 1 Overflow. The micro

controller sets this bit when Timer

1 overflows.

1

6 TR1 8Eh Timer 1 Run. When this bit is set

Timer 1 is turned on. When this bit

is cleared Timer 1 is off.

1

5 TF0 8Dh Timer 0 Overflow. The micro

controller sets this bit when Timer

0 overflows.

0

4 TR0 8Ch Timer 0 Run. When this bit is setTimer 0 is turned on. When this bit

is cleared Timer 0 is off.

0



TCON SFR

• Only 4 of the 8 bits of the TCON SFR is defined. the other 4

bits of the SFR don’t have anything to do with timers. They

are related with Interrupts and they will be discussed in

the chapter that addresses interrupts.

• Note that the individual bits of TCON register can be

addressed separately by their bit addresses. This allows

the programmer to run the timers using bit addressable

instructions and check the overflow independently.



Timer Operating Modes

• The two 16 bit timers of 8051 can be operated in any

one of the four modes. The mode selection can be

done by the setting the proper bits in the TMOD SFR.



Operating Modes of 8051



Mode 0 - 13-bit Timer Mode• Timer mode "0" is a 13-bit timer. Out of 16 bits of Timers,

only 13bits are used. The 5 bits of lower order byte is used

and 8 bits of the higher order byte of the timers are used

in Mode0. Lower order byte TL0/1 will count from 0 to 31.

• When TL0/1 is incremented from 31, it will "reset" to 0

and increment TH0/1. So, the timer can only contain 8192

values from 0 to 8192.

• The timer can be operated as timer with internal clock

pulses or as a counter with external clock pulses.

• This selection is done by D2 bits of TMOD for Timer 0 and

D6 bit of TMOD for Timer1



TCON SFR

• The clock pulses selected by D2 and D6 bits of TMOD

is then controlled by programmer setting and

connected to the Timer registers. The control is by

three different means.

• First is the Timer Run control bits D4 and D6 of TCON

register. The timer will run only when Timer run

control bits are set to 1.



TCON SFR

• The other controls for the timers are through theGATE control bits D4 and D7 of TMOD and theExternal inputs for timer. Setting GATE to 1 allows thetimer to count only if the external control input INT0

or INT1 is set to 1. Setting Gate to 0 will disable thecorresponding external timer control inputs INT0 andINT1.

• Setting Timer to mode 0 will overflow back to zero

after 8192 counts. This will set the TF1 and TF0 bitsfor timer 1 and timer 0 respectively.



Mode 0 Operation of Timer 1 of

8051



Mode 1 – 16-bit Timer Mode• In timer Mode 1 of 8051, each timer is operated as a

16-bit timer. It functions just like mode 0 except thatall 16 bits are used.

• TL0/1 bits are incremented from 0 to 255. When

TL0/1 is incremented from 255, it resets to 0 andcauses TH0/1 to be incremented by 1. Since this is afull 16-bit timer, the timer may contain -65536 distinctvalues.

• The control of gating and running the timer in mode 1is similar to that of mode 0




8051



Mode 2 – 8-bit Timer Auto

Reload Mode• Only 4 of the 8 bits of the TCON SFR is defined. the other 4

bits of the SFR don’t have anything to do with timers. They

are related with Interrupts and they will be discussed in

the chapter that addresses interrupts.• Note that the individual bits of TCON register can be

addressed separately by their bit addresses. This allows

the programmer to run the timers using bit addressable

instructions and check the overflow independently.




Reload Mode• For example, let’s say TH0 holds the value FDh and

TL0 holds the value FEh. Then at the next counting

pulse, TL0 will be incremented to FFh. Then for the

next pulse, the TL0 will overflow and should havebecome 00.

• But, as it is reload mode, the TL0 will be loaded with

TH0 i.e., FDh. The value of TH0 will never be changed.TH0/1 is set to a known value and TL0/1 is the SFR

that is constantly incremented.




Reload Mode• The auto-reload mode is very commonly used for

establishing a baud rate for Serial Communications.

• The control of gating and running the timer in mode 2

is similar to that of mode 0



Mode 2 Operating of Timer 1 of

8051



Mode 3 – Split Timer Mode

• Mode "3" of 8051 timer is a split-timer mode and is

applicable only for Timer 0. When Timer 0 is placed in

mode 3, it essentially becomes two separate 8-bit

timers. That is, Timer 0 is TL0 and Timer 1 is TH0.• Both timers count from 0 to 255 and overflow back to

0.

• In mode 3, all the bits that are related to Real Timer 1

will simply hold its count and will not run and the

situation is similar to keeping TR1=0.



Mode 3 – Split Timer Mode

• In Split Timer mode of Timer 0, the real Timer 1 (i.e.TH1 and TL1) can not be started or stopped since thebits that do that are now linked to TH0. The real timer1, in this case, will be incremented every machine

cycle no matter what.

• When two separate timers in addition to a baud rategenerator is required in an application, then realTimer 1 can be used as a baud rate generator andTH0/TL0 can be used as two separate timers in mode3.




8051



Timer Control and Operation

• For timer operation (C/T = 0 in TMOD), the timerregister counts the divided-down clock. The timerregister is incremented once every (FOSC / 12) instandard mode. If the clock frequency is

11,059,000KHz, then the counter will be incrementedat the rate of (11,059,000KHz/12) = 921,583 KHz.

• This means the counter will be incremented 921,583times in a second. Thus to have delay of say 0.1seconds, then the counter must be initialized to thecount value of (0.1*921,583) = 92158.




Following steps are the program steps to initialize and use a

timer in 8051

• Decide what mode the timer to be in.

•Initialize the TMOD SFR.

• Write the timer value to Timer register

• Start the timer running by setting the TR0/1 bit in TCON

register

• Check for TF0/1 bit or program to handle timer overflow

as interrupt and execute interrupt subroutine.



Steps in Timer Control

Write TMOD

Write TCON

Write timer register

Start timer

Program Required Operation

Continue




• To set the bit TR1 of TCON (D6 bit), any one of the

following two commands can be used -MOV TCON,

#40h OR SETB TR1

• As, TR1 is a bit addressable location, SETB instruction

is used and this has the benefit of setting the TR1 bit

of TCON without changing the value of any of the

other bits of the SFR.



Reading Value of 16-bit Timer

• There are two common ways of reading the value of a16-bit timer;

1. Read the actual value of the timer as a 16-bitnumber from TH0/TL0 or TH1/TL1

2. Detect when the timer has overflowed from theTF0/1 bits of TCON. If TF0 is set, it means thattimer 0 has overflowed; if TF1 is set, it means that

timer 1 has overflowed. This overflow can act asan interrupt and can directly run an Interruptservice routine, if enabled properly.



Using Timers and Counters

• Timers in 8051 can be programmed to count pulses

from external devices. For example, a sensor placed

along a conveyor belt can give one pulse for every

object moving on the conveyor.• This pulse can be counted by the 8051 timer in

counter mode. The pulses from the sensor can be

given as an external input to a timer and programmed

to count.




• The timer can be programmed to give an interrupt after apredefined count value. For example, after counting 12objects in the conveyor or after counting 12 pulses in thetimer, an interrupt may be given to the 8051 system togive signal to some other action like packing the 12 items

• It is important to note that the 8051 checks the P3.4 lineeach instruction cycle (12 clock cycles). This means that ifP3.4 is low, goes high, and goes back low in 6 clock cyclesit will not be detected by the 8051.

•

This also means the 8051 event counter is only capable ofcounting events that occur at a maximum of 1/24th therate of the crystal frequency.




• That is to say, if the crystal frequency is 12 MHz, it can

count a maximum of 500,000 events per second

(12.000 MHz * 1/24 = 500,000).

• If the event being counted occurs more than 500,000times per second, it will not be able accurately

counted by the 8051.

I t t S d I t t



Interrupt Sources and Interrupt

Vector Addresses• 8051 basically has following five interrupt sources so

that any of the following events will make 8051 to

execute an interrupt service routine.

• Timer 0 Overflow.• Timer 1 Overflow.

• Reception/Transmission of Serial Character.

•

External hardware interrupt 0.• External hardware interrupt 1.

I t t S d I t t




Vector Addresses• Different interrupt sources have to be distinguished

and 8051 must execute different subroutines

depending –interrupt triggered. This is accomplished

by jumping or calling to a fixed address wheninterrupt occurs.

• These addresses are called interrupt vector addresses

or interrupt handler addresses.

I S d I




Vector Addresses

Interrupt Flag Interrupt Vector Address

External 0 IE0 0003h

Timer 0 TF0 000Bh

External 1 IE1 0013h

Timer 1 TF1 001Bh

Serial RI/TI 0023h





Vector Addresses• Whenever Timer 0 overflows (i.e., the TF0 bit is set),

the main program will be temporarily suspended and

control will jump to 000BH.

• It is assumed that service routine at address 0003Hhandles the situation of Timer 0 overflowing.

Enabling and Disabling



Enabling and Disabling

Interrupts

• By default at power up, all interrupts are disabled. This

means that even if, for example, the TF0 bit is set, the

8051 will not execute the interrupt. Programming must be

done specifically to enable interrupts.• Interrupt Enable Special Function Register IE SFR at the

address A8h is used to enable and disable interrupts by

modifying its bits

• The interrupts enabling can be handled individually by -bit addresses for the individual bits of IE register.



Bit Patterns for the IE SFR (A8H)Bit

position

D7 D6 D5 D4 D3 D2 D1 D0

Bit

Address

AF AC AB AA A9 A8

Name EA - - ES ET1 EX1 ET0 EX0

Explana

tion Global

Interrupt

Enable/

Disable

Undefi

ned

Undefin

ed

Enable

Serial

Interrupt

Enable

Timer 1

Interrupt

Enable

External 1

Interrupt

Enable

Timer 0

Interrupt

Enable

External 0

Interrupt



Bit Patterns for the IE SFR (A8H)

• Each of the 8051’s interrupts has its own bit in the IE

SFR. A particular interrupt can be enabled by -

corresponding bit. For example, to enable Timer 1

Interrupt, the one of the following instructions can beexecuted.

•

MOV IE, #08h OR SETB ET1



Bit Patterns for the IE SFR (A8H)

• However, before Timer 1 Interrupt (or any other

interrupt) is truly enabled, bit 7 of IE SFR must also be

set. Bit 7, the Global Interrupt Enable/Disable, enables

or disables all interrupts simultaneously.• That is, if bit 7 is cleared then no interrupts will occur,

even if all the other bits of IE are set. Setting bit 7 will

enable all the interrupts -selected by setting other bits

in IE.

Interrupt Priorities and Polling




Sequence• The 8051 automatically evaluates whether an

interrupt occurs after every instruction. Whenchecking for interrupt conditions, it checks them inthe following order:

• External 0 Interrupt

• Timer 0 Interrupt

• External 1 Interrupt

• Timer 1 Interrupt• Serial Interrupt





Sequence• The above list -gives the interrupt priority.

• So, whenever the External 0 interrupt and Timer 1

interrupt occurs at the same instant, then 8051

microcontroller executes the interrupt service routinecorresponding to External 0 interrupt first.

• Then 8051 microcontroller will return to the main

program, execute one instruction and then execute

the interrupt service routine corresponding to Timer 1

Interrupt.

I t t P i iti d P lli




Sequence• If a Serial Interrupt occurs at the exact same instant that an External 0

Interrupt occurs, the External 0 Interrupt will be executed first and the

Serial Interrupt will be executed once the External 0 Interrupt has

completed.

• The 8051 offers two levels of interrupt priority: high and low. By using

interrupt priorities, the above interrupts can be divided into two

separate interrupt priorities. So, the five interrupts can be again

prioritized.

• Interrupt priorities are controlled by the IP SFR (B8h). For example, ifthe Serial Interrupt is much more important than the Timer 0 interrupt,

then the Interrupt Priority register IP SFR at the address B8h can be

properly programmed to set the priority.





Sequence• This is done by assigning a high priority to the Serial

Interrupt and a low priority to the Timer 0 Interrupt. By

setting the D4 bit to 1, the serial interrupt will be set to

higher priority and making D1 bit to 0, the Timer 0 interrupt

will be set to lower priority.

• Note that the priority can be set individually by using the bit

addresses of the IP register. For example, the timer 0

interrupt priority can be made high by setting the D1 bit of

IP SFR. So, the following instructions can be used for thesame.

• SETB PT0 (or) SETB B9H (or) MOV IP, #82H



Bit Patterns for the IP SFR

Bit position D7 D6 D5 D4 D3 D2 D1 D0

Bit Address BC BB BA B9 B8

Name EA - - PS PT1 PX1 PT0 PX0

Explanation

EnableInterrupts

-

Made 0 to

disable all

interrupts

Undef

ined

Undefin

ed

Serial

Interrupt

Priority

Timer 1

Interrupt

Priority

External 1

Interrupt

Priority

Timer 0

Interrupt

Priority

External 0

Interrupt

Priority





SequenceWhen considering interrupt priorities, the following rules apply:

• Nothing can interrupt a high-priority interrupt--not evenanother high priority interrupt.

• A high-priority interrupt may interrupt a low-priority interrupt.

• A low-priority interrupt may only occur if no other interrupt isalready executing.

• If two interrupts occur at the same time, the interrupt withhigher priority will execute first. If both interrupts are of thesame priority the interrupt which is serviced first by pollingsequence will be executed first.





Sequence• The complete structure of the 8051 interrupts can be well

understood -figure 11.16. The five interrupt sources arepassed first - IE register, which decides the enabling anddisabling of interrupts. The global interrupt enable -shown

-figure.• The IP register - set two priority levels among the available

interrupts. This is shown in the figure as high priority andlow priority blocks. The bits IT0 and IT1 can be set by

TCON special function register and this is used to selectwhether the hardware interrupt is level triggered or edgetriggered.

f 5



Structure of 8051 Interrupts

IE1 IT1 1

0

IE0 IT0 1

0

TF0

TF1

RI

TI

Interrupt

enables

Global

Enable Interrupt

enabled

High

priority

interrupt

Low

priority

interrupt

Interrupt polling

sequence

IP

register

IE

register

INT1

INT0



Timing of Interrupts• 8051 micro-controller samples the hardware signal level on its pins

once in every machine cycle. A machine cycle is the time taken by

the controller to access one memory location or I/O device.

• As 8051 takes 12 clock cycles to complete one machine cycle, the

interrupt signal applied at the pins of 8051 must be available for at

least 12 clock periods.

• External interrupts are applied at the pins INT0 and INT1. The

sensing of voltage level applied to this pin can also programmed in

8051. The interrupts can be either level triggered or edge triggered

as set by the IT0 and IT1 bits of the SFR TCON• A ‘0’ on these bit positions will make both the hardware interrupts to

be level triggered. Level triggered means a low level voltage on the

interrupt pins will activate the interrupts.

B P f h TCON SFR



Bit Patterns for the TCON SFRBit position D7 D6 D5 D4 D3 D2 D1 D0

Bit Address 8F 8E 8D 8C 8B 8A 89 88

Name TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0

Explanation

Timer 1

Overflow

flag

Timer 1

runcontrol

bit. Set to

1 by

software

to run.

Timer

0

Overflo

w flag

Timer 0

run

control

bit. Set

to 1 by

softwar

e to

run.

Extern

alInterru

pt 1

edge

detect

bit

Interrupt 1

type control.

Set to 1 bysoftware for

edge

triggering

and cleared

for level

triggering

ExternalInterrupt

0 edge

detect

bit

Interrupt 0

type

control. Set

to 1 bysoftware

for edge

triggering

and

cleared for

level

triggering

Bit Address 8F 8E 8D 8C 8B 8A 89 88





Sequence• A ‘1’ on the IT0 and IT1 bits of the SFR TCON will program

the hardware interrupts as edge triggered. Edge triggeringmeans the change of voltage from high state to low statewill activate the interrupt.

•

When an interrupt is triggered, the micro controller takesthe following actions automatically:

• The current Program Counter is saved on the stack, low-byte first.

• Interrupts of the same and lower priority are blocked.

• In the case of Timer and External interrupts, thecorresponding interrupt flag is cleared.





Sequence• Program execution transfers to the corresponding

interrupt handler vector address.

• The Interrupt Handler Routine executes.

• An interrupt ends when your program executes the RETI

(Return from Interrupt) instruction. When the RETIinstruction is executed the micro controller takes thefollowing actions:

• Two bytes are popped off the stack into the Program

Counter to restore normal program execution.• Interrupt status is restored to its pre-interrupt status.





Sequence• The internal architecture of 8051 is such that the external

hardware interrupts will be cleared automatically when theinterrupt service routine is executed only if is programmed tobe edge or transition triggered.

• If the interrupts are level triggered, then the programmerwill have to reset the interrupt enable corresponding to thisinterrupt using IP SFR

• If the interrupt being handled is a Timer or External interrupt,the micro controller automatically clears the interrupt flagbefore passing control to interrupt handler routine. Thismeans it is not necessary to clear the bit in the program.


8051 S l P



8051 Serial Ports• One of the 8051’s many powerful features is its integrated

Universal Asynchronous Receiver Transmitter (UART),

otherwise known as a serial port. With integrated serial port of

8051, data can be transmitted and received easily by reading

and writing the data to the serial port registers. The features ofthe 8051 serial ports are

• Full duplex operation.

• Receive Buffered.

•Access using single double buffered Register SBUF.

• Four different modes of operation.

• Option to use fixed baud rate or programmable baud rate.


8051 S i l P



8051 Serial Ports

• Full Duplex serial port means that it can transmit and

receive data simultaneously. It is also receive-buffered,

meaning it can commence reception of a second byte

before a previously received byte has been read from the

SBUF register.• However, if the first byte still hasn’t been read by the time

reception of the second byte is complete, one of the bytes

will be lost. The serial port receive and transmit registers

are both accessed at Special Function Register SBUF.


8051 S i l P



8051 Serial Ports•

Data to be transmitted are written to the register SBUFand the data received by the serial port are read from

the register SBUF. Physically reading and writing SBUF

actually accesses two separate registers.

• This technique of having same address for two different

registers is called double buffering.




SERIAL PORT CONTROL SFRS

•The serial port of 8051 is controlled by two registers inSFR area of 8051 as shown in Table

• The two registers are Serial Port control registers; SCON

and serial port buffer register SBUF.

SFR Name Description SFR Address

SCONSerial port control

register98h

SBUF Serial port bufferregister

99h




SERIAL PORT CONTROL SFRS

• In addition to the above two registers, the MSB of PCONregister named as SMOD bit is used to double the baud

rate of serial transmission and reception.

• If SMOD bit is set to 1, then the baud rate is doubled.

• The individual bits of SCON have the functions as shownin Table . As the SCON register has many individual status

bits, the individual bits of this register are bit addressable.

The bit address is also given in Table .

• The programmer can use these bit addresses to check thestatus of the serial port and set the mode individually.




Bit patterns for SCON (98h) SFR

Bit Name Bit Address Explanation of Function

D7 SM0 9Fh Serial port mode select bits

D6 SM1 9Eh

D5 SM2 9Dh Multiprocessor Communications Enable bit

D4 REN 9Ch Receiver Enable. This bit must be set in order to receive

characters. D3 TB8 9Bh Transmit bit 8. The 9th bit to transmit in mode 2 and 3.

D2 RB8 9Ah Receive bit 8. The 9th bit received in mode 2 and 3.

D1 TI 99h Transmit Interrupt Flag. Set when a byte has been

completely transmitted.

D0 RI 98h Receive Interrupt Flag. Set when a byte has been completely

received.


Bit P tt D i ti



Bit Pattern - Description•

D7 and D6 bits of the SCON register define the operationmodes of the serial port and the basic operating modes

are given in Table.

• The SM0 and SM1 bits can select any one of the four

operating modes described in the next section.





Definition of Bits SM0 and SM1 in

SCON SFR

Bit P tt D i ti



Bit Pattern - Description

• The next bit, SM2, is a flag used for enabling

"Multiprocessor communication" in modes 2 and 3. If SM2

is set to 1 in modes 2 and 3, the “Receive Interrupt” RI flag

will not be activated if the received 9th data bit is 0. If SM2

is set in Mode1, then “Receive Interrupt” RI flag will not beactivated if valid stop bit is not received.

• This can be useful in certain advanced serial applications. It

can be now assumed that SM2 bit has to be cleared so that

RI flag will be set when any character is received.


Bit P tt D i ti




• The next bit, REN, is "Receiver Enable." This bit is set in

order to receive the characters from the receive data line

of the serial port.

•

The TB8 bit is used in modes 2 and 3. In modes 2 and 3, atotal of nine data bits are transmitted. The first 8 bits are

the 8 bits of the data to be transmitted, and the ninth bit

is taken from TB8. The RB8 also operates in modes 2 and

3 on the reception side.


Bit P tt D i ti




• When a byte is received in modes 2 or 3, a total of ninebits are received. In this case, the first eight bits received

are the data of the serial byte received and the value of

the ninth bit received will be placed in RB8.

• TI means "Transmit Interrupt." When a program writes adata to the serial port buffer SBUF, then the serial port

will start shifting this data in the serial transmit line bit

by bit at the predefined clock speed or baud rate. 8051

will give TI signal to the programmer after sending thedata completely.


Bit P tt D i ti



Bit Pattern - Description• Upon sensing the TI bit set to 1, the programmer can

then write the next data for transmission. When the TIbit is set, the programmer may assume that the serialport is "free" and ready to send the next byte.

• Finally, the RI bit means "Receive Interrupt." Whenever a

data is received on the receive data line of the serialport, this serial data will be shifted in to a buffer andthen stored in the SBUF register.

• Setting of RI bit indicates that a byte has been received.Upon sensing the RI bit set to 1, the programmer may

read the data from the SBUF.


OPERATING MODES



OPERATING MODES

• Bits SM0 and SM1 are used to set the serial mode to a

value between 0 and 3.

• Selecting the Serial Mode selects the mode of operation

(8-bit/9-bit, UART or Shift Register) and also determineshow the baud rate will be calculated.

• In modes 0 and 2 the baud rate is fixed based on the

oscillator’s frequency. In modes 1 and 3 the baud rate is

variable based on Timer 1 overflows.




SBUF



SBUF• SBUF is physically two registers with the same address.

• When data to be transmitted is written to the SBUFregister, then it will be shifted bit by bit into the TXD lineof 8051. The port 3 pin 3.1 acts as the TXD line. Theshifting is done by the transmit clock which determines

the baud rate.• Similarly, when the data bits are received on the RXD line

(Pin 3.0 of port 3), the bits are shifted serially into the shiftregister according to the Receive clock.

• After the reception is complete, the data received will be

placed on SBUF from where it can be read by theprogrammer through the internal bus.


R i / T i i



Reception / Transmission•

Mode 0: In this mode serial data is entering and exitingthrough RxD pin. So, in mode 0, Full duplex is not possible –

meaning that both transmission and reception cannot take

place simultaneously. TxD pin outputs the shift clock. 8 bits

are transmitted/received (LSB first). The baud rate is fixed at

1/12 the oscillator frequency. Transmission is started by

writing a data byte to the SBUF register and once the

transmission is complete, TI flag is set

• The reception is started by enabling REN in SCON register.

Once the data reception is complete, the RI flag is set

• The baud rate in Mode 0 is fixed at one twelfth of the clock

frequency. Baud rate= (Clock frequency/12)


Signal Transmission / Reception



Signal Transmission / Reception

pattern


R ti / T i i



Reception / Transmission

• Mode 1: In this mode, 10 bits are transmitted through

TxD and simultaneously 10 bits can be received through

RxD. The 10 bits are made up of a start bit (0), 8 data bits

(LSB first), and a stop bit (1). On completion of reception,

the stop bit goes into RB8 in Special Function RegisterSCON. The baud rate is variable and is set by the Timer 1

overflow rate. The baud rate for mode 1 is fixed at the

following rate.




R ti / T i i




• In order to generate the baud rate clock from timer 1,

Timer 1 can be configured to act as timer in auto reload

mode with the timer 1 interrupt disabled.

• As in mode 0, the transmission is initiated by writing a

data to SBUF register. Reception is initiated by a 1 to 0

transition that is the start bit received and also when

REN of SCON SFR is 1.


R ti / T i i



Reception / Transmission• Mode 2: In this mode, 11 bits are transmitted through TxD or

received through RxD. The 11 bits are made up of one start bit

(always 0), 8 data bits (LSB first), a programmable 9th data bit, and a

stop bit (always 1). The 9th data bit transmitted is same as TB8 bit in

SCON special function register. It can be assigned the value of 0 or 1

by the programmer. Or, for example, the parity bit (P, in the PSW)could be moved into TB8.

• On reception, the 9th data bit goes into RB8 in Special Function

Register SCON, while the stop bit is ignored. The baud rate is

programmable to either 1/32 or 1/64 of the oscillator clock

frequency.

• Baud rate= (Clock frequency /32) if SMOD bit in PCON SFR is set to 1.

• Baud rate= (Clock frequency /64) if SMOD bit in PCON SFR is set to 0.





Reception / Transmission• Mode 3: In this mode, 11 bits are transmitted through TxD

and simultaneously 11 bits are received through RxD. The11bits are made up of a start bit (0), 8 data bits (LSB first), a

programmable 9th data bit, and a stop bit (1). In fact, Mode

3 is the same as Mode 2 in all respects except baud rate. The

baud rate in Mode 3 is variable.• The baud rate for mode 3 is fixed at the following rate similar

to mode1.

• Baud rate= (Timer 1 overflow rate /16) if SMOD bit in PCON

SFR is set to 1.• Baud rate= (Timer 1 overflow rate /32) if SMOD bit in PCON

SFR is set to 0.




Programming the Serial Port



Programming the Serial Port




Initializing Serial Port



Initializing Serial Port•

In modes 1 and 3, the baud rate is determined by how frequently timer1 overflows. The most common method is to put timer 1 in 8-bit auto-

reload mode (timer mode 2) and set a reload value (TH1) that causes

Timer 1 to overflow at a frequency appropriate to generate a baud rate.

• To determine the value that must be placed in TH1 to generate a given

baud rate, the following equation is used.

• TH1 = 256 - ((Clock frequency / 384) / Baud) if SMOD in PCON SFR is 0.

• TH1 = 256 - ((Clock frequency / 192) / Baud) if SMOD in PCON SFR is 1.

• The following table gives the commonly used baud rates and the

corresponding reload for the timer in mode 2 assuming the clockfrequency of 11.059 MHz and SMOD is reset.


Commonly used baud rates



Commonly used baud rates

Baud Rate Timer value TH1

300 A0h

1200 D0h

2400 FAh

9600 FDh


Baud Rate



Baud RateFollowing set of instructions will set the timer for the baud

rate of 9600.• MOV TMOD, #00100000B ;timer/counter 1 set for mode 2,

8-bit TIMER

• ;operation

• MOV TH1, #0FDh ; timer/counter 1 is

timed for 9600 baud• SETB TR1 ; timer/counter 1 is

enabled for free run

• For initializing the serial port for mode 3 operation, thefollowing instruction can be used.

• MOV SCON, #11010000B


Transmitting and Receiving Data



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Using Serial Port

• Once the Serial Port has been properly configured as explained

above, the serial port is ready to be used to send data and

receive data.

• To write a byte to the serial port one must simply write the value

to be transmitted to the SBUF (99h) SFR. For example, to send

the letter "A" to the serial port, the following instruction can be

written.

MOV SBUF, #’A’


Transmitting and Receiving Data



• Upon execution of the above instruction the 8051 will begin

transmitting the character via the serial port. Once the

transmission is complete, the serial port transmit interrupt

flag TI is set. Since the 8051 does not have a serial output

buffer, a character can not be written to SBUF before theprevious written character is completely transmitted. This

can be accomplished by checking the TI flag.

• Reading data received by the serial port is equally easy. To

read a byte from the serial port one just needs to read thevalue stored in the SBUF (99h) SFR after the 8051 has

automatically set the RI flag in SCON.

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Using Serial Port

354 33 Powerpoint-slides CH11

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