8051 Interrupts & Timers

Abdul Qadir JavaidE204

abdulqadir_17@hotmail.com

abdul.q.javaid@theiet.org

Diagram taken from‘The 8051 Microcontroller Architecture,Programming & Applications’ by Kenneth J. Ayala

•

Interrupts

• Two ways to make the computer program make aware of the internal and external conditions of the circuit

• Software instructions and the hardware signals

• 5 interrupts in 8051• 3 internal interrupts: timer flag 0, timer flag1

& Serial port interrupt• 2 external at pins INT0 & INT1

• Timer flag interrupt: The timer/counter overflow flag TF0 & TF1 is set to 1.

• Serial Port interrupt• External interrupt: INT0 & INT1. • 2 ways to set the interrupt flags IE0 & IE1 in

TCON to 1.

Interrupt Registers

Bit Symbol Function7 EA Enable Interrupt bit. If logic 0 clears the

interrupts. Set to 1 enables.6 -- Not implemented.5 ET2 Reserved for future use.4 ES Enable Serial port. If 1 then enables. If 0 then

disables.3 ET1 Enable timer 1 overflow. If 1 then enables. If 0

then disables.2 EX1 Enable external interrupt 1 if set to 1 to

enable INT1 1 ET0 Enable timer 0 overflow if set to 1. 0 EX0 Enable external interrupt 0 if set to 1 to

enable INT0

oInterrupt causes a hardware call to one of the memory locationsoProgrammer should place a service routine at that addressoInterrupt saves the PC of the program on the stack

Oscillator

‘f’ is smallest interval of time

1 instruction takes 1 machine cycle

12MHz crystal 1sec for one instruction

Bit 7/5(TF1/TF0): Timer1/0 overflow flag. Set when timer rolls from all ones to zeros. Cleared when processor executes the interrupt routine.

Bit 6/4(TR1/TR0): Timer1/0 run control bit. Set to one to enable timer to count. Cleared to zero to stop. Does not reset.

Bit 3/1(IE1/IE0): External Interrupt 1/0 Edge flag. Set to 1 when high-low signal at port 3.3(INT1) & port 3.2(INT0). Cleared by interrupt service routine.

Bit 2/0(IT1/IT0): External Interrupt 1/0 type control bit.

TCON Register

THX THL

Bit 7/3 (Gate) : OR gate enable bit. Controls RUN/STOP of timer 1/0. If TRx = 1, Gate = 1, timer runs when INT is high.If Gate = 0, timer runs when TRx = 1.Bit 6/2 (C/T) : Set to 1 for timer 1/0 to act as Counters for external input pulses at pins 3.5(T1) or 3.4(T0). Set to 0 for timer.Bit 5/1 (M1) : Timer/Counter Operating mode select bit 1.Bit 4/0 (M0) : Timer/Counter Operating mode select bit 0.

M1 M0 Mode

0 0 0

0 1 1

1 0 2

1 1 3

Not Bit Addressable

TIMER MODE 0

• THX as 8-bit counter• TLX as 5-bit counter

TIMER MODE 1

• Similar to Mode 0• Behaves as a 16 bit timer• E-g If its required for timer to overflow after

1000 counts then load FCh in THX and 18h in TLX.

• 16 bit counter max value is (65535)• If 6MHz crystal used then timer flag sets in

0.1311 sec

TIMER MODE 2

• Only TLX used as counter• THX to hold a value• Auto-reload mode• TLX counts up from number in THX• Overflows & Initialised to the number in THX

TIMER MODE 3

• So far Timer 0 and Timer 1 has been shown to be behaving independently

• Not true for Mode 3• If mode 3 chosen for timer 0, then placing

timer1 in mode 3 causes it to stop counting• TR1 & TF1 used by timer 0• Timer 0 becomes 2 separate 8-bit counters

Contd....

• TL0 overflows(00h-FFh) & sets timer flag TF0 • TH0 behaves under the control of TR1 & sets

TF1 when overflows• If timer 0 in mode 3: Timer 1 can be used in

mode 0, 1 & 2 with 1 exception: No interrupts will be generated

• Timer1 can be used for baud rate generation of serial port

Difference between Timer & Counter

• Source of clock pulses• Timer : Clock pulses from oscillator• Counter: Pin T0(P3.4) for counter 0

Pin T1(P3.5) for counter 1 C/T bit must be set for counter

• Maximum input frequency is Oscillator frequency/24

ExampleSTART

Turn off the LEDIF push button is On THEN

Initialize timer 1 to genrate interruptsWait for timer interrupt

ENDIFEND

Timer 1 initializationSTART

Enable timer 1 interruptsSet timer 1 to 8-bit auto reload modeLoad timer valueEnable microcontroller interruptsTurn on timer 1

END

Timer 1 Interrupt service routineSTART

IF the required time has elapsed THENTurn LED on or offENDIF

END

STARTTurn off the LEDIF push button is On THEN

Initialize timer 1 to genrate interruptsWait for timer interrupt

ENDIFEND

Timer 1 initializationSTART

Enable timer 1 interruptsSet timer 1 to 8-bit auto reload modeLoad timer valueEnable microcontroller interruptsTurn on timer 1

END

Timer 1 Interrupt service routineSTART

IF the required time has elapsed THENTurn LED on or offENDIF

END

CODE#include<reg51.h>

sbit LED = P1^7;sbit push_button = P3^2;int count;void init_timer();/*Main Program*/main(){

count = 0; //Initialize countLED = 1; //LED is OFFwhile(push_button == 1) //Scan for the push button{}

init_timer(); //Initialize timer1for(;;) //Endless loop{}} //End of main program

/*Timer 1 Initialization routine*/void init_timer(){

ET1=1 ; //Enable timer 1TMOD = 0x20; //Timer 1 in mode 2TH1 = 0x6; //For 250sec pulseEA = 1; //Enable interruptsTR1 = 1; //Turn on timer 1

}

#include<reg51.h>

sbit LED = P1^7;sbit push_button = P3^2;int count;void init_timer();/*Main Program*/main(){

count = 0; //Initialize countLED = 1; //LED is OFFwhile(push_button == 1) //Scan for the push button{}

init_timer(); //Initialize timer1for(;;) //Endless loop{}} //End of main program

/*Timer 1 Initialization routine*/void init_timer(){

ET1=1 ; //Enable timer 1TMOD = 0x20; //Timer 1 in mode 2TH1 = 0x6; //For 250sec pulseEA = 1; //Enable interruptsTR1 = 1; //Turn on timer 1

}

Contd...

/*Timer 1 interrupt service routine*/timer1() interrupt 3{

count++; //increment countif(count ==100){

count = 0; //Reset countLED = ~LED;

}}

/*Timer 1 interrupt service routine*/timer1() interrupt 3{

count++; //increment countif(count ==100){

count = 0; //Reset countLED = ~LED;

}}