1. CSC321 8051 Timers Since this is a microcontroller it mainly finds itself in embedded devices Quite often embedded devices need to synchronize events The way this is done is via time –We could do it by inserting a loop or a bunch of NOP commands Is this accurate? –A better way to do it is to use timers Basically an internal stop watch The 8051 has two timers

2. Timers and Interrupts 8051 implementation CSC321

3. Using the timers Controlled through two special function registers (SFR) –TMOD – configure the timers –TCON – control the timers CSC321

4. TMOD CSC321

5. Timer Modes Modes 1 and 2 are the most common –Mode 1: 16-bit timer, sets overflow when complete –Mode 2: 8-bit timer, reloads and sets overflow when complete CSC321

6. TCON Bits 5 and 7 are the overflow bits for each timer and are set by hardware Bits 4 and 6 turn the timers on/off More on bits 0-3 later CSC321

7. Timer control Setting bits TR0 and TR1 (setb TR0 and setb TR1 instructions) start the timers Clearing bits TR0 and TR1 (clr TR0 and clr TR1 instructions) stop the timers CSC321

8. Reading the timers If the timers are stopped, reading is trivial mov R6, TL0 mov R7, TH0 If the timers are running, it gets tricky try: mov A, TH0 mov R6, TL0 cjne A, TH0, try mov R7, A CSC321 We don’t want TH0 to roll over while we are reading TL0

9. Programming the timer (part 1) CSC321 ; set timer 0 to 8-bit mode mov TMOD, #0x02 ; start value mov TL0, #0x00 ; reset value when ; the timer hits 0xFF mov TH0, #0x00 ; start timer 0 setb TR0 ; kill some time nop

10. Programming the timer (part 2) CSC321 ; stop timer 0 clr TR0 tryagain: ; read the high byte of the timer mov A, TH0 ; read the low byte of the timer mov R6, TL0 ; read the high byte again to make sure it didn't change ; while we were reading the low byte cjne A, TH0, tryagain ; keep the high byte if it didn't change mov R7, A ; the full 16-bit timer is now in R7/R6

11. 8051 Interrupts A signal to let the CPU know that something out of the ordinary flow of instructions has occurred Various sources of interrupts in the 8051 –Two external –Two internal (timers) –I/O (serial port) CSC321

12. Set up by the TCON register The bits we didn’t talk about previously CSC321

13. When an interrupt occurs Current instruction is allowed to complete Program counter (PC register) is saved on the stack (SP) Address of the interrupt service routine (ISR) is loaded to the program counter CSC321

14. Interrupt service routine Also known as an interrupt vector –The 8051 has six of them –Usually the ISR is nothing more than a JMP to the actual subroutine (due to lack of space between interrupt vectors) CSC321

15. Enabling/Disabling interrupts To use an interrupt you must first enable it –They are disabled by default (power up) –Enabling is a 2 step process First enable all interrupts – setb EA Then enable individual interrupts – setb ET0 See next slide for why 2 steps… CSC321

16. Why two steps? Because it makes turning them on and off very easy Consider a critical section of code (more than 1 instruction) –You don’t want it to be interrupted –but you don’t want to turn off a bunch of interrupts knowing you’ll have to turn them back on (that takes time) –Using clr EA and setb EA does the trick in the shortest time possible CSC321

17. There is a little bit more… Since the interrupt vectors are in low address memory (see table) you can’t put your main program there –Address 0 is the default program memory address The 8051 assembler provides a solution org address –Provides a way to force code to a particular address – it’s not an instruction, but a directive CSC321

18. Programming interrupts (part 1) Setting up the ISR CSC321 ; org tells the assembler where to place the code org 0 ; start at the main program label jmp main ; timer 0 ISR is at address 0x000B org 0x000B ; obviously not a good ISR but it makes the point timer0: jmp timer0 reti

19. Programming interrupts (part 1) The main program (almost the same as before) CSC321 ; start the main program at address 0x0030 org 0x0030 main: ; set timer 0 to 8-bit mode mov TMOD, #0x02 ; start value mov TL0, #0xFC ; reset value when ; the timer hits 0xFF mov TH0, #0x00 ; enable interrupts setb EA setb ET0

20. Programming interrupts (part 2) more main program (same as before) CSC321 ; start timer 0 setb TR0 ; kill some time nop ; stop timer 0 ;clr TR0

21. Programming interrupts (part 3) more main program (same as before) CSC321 tryagain: ; read the high byte of the timer mov A, TH0 ; read the low byte of the timer mov R6, TL0 ; read the high byte again to make sure it didn't change ; while we were reading the low byte cjne A, TH0, tryagain ; keep the high byte if it didn't change mov R7, A ; the full 16-bit timer is now in R7/R6