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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

Timers and Interrupts

8051 implementation

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Using the timers

• Controlled through two special function registers (SFR)– TMOD – configure the timers– TCON – control the timers

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TMOD

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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

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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

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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

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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, ACSC321

We don’t want TH0 to roll over while we are reading TL0

Programming the timer (part 1)

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; set timer 0 to 8-bit modemov TMOD, #0x02; start value mov TL0, #0x00; reset value when; the timer hits 0xFFmov TH0, #0x00; start timer 0setb TR0

; kill some timenopnopnopnopnop

Programming the timer (part 2)

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; stop timer 0clr TR0

tryagain:; read the high byte of the timermov A, TH0; read the low byte of the timermov R6, TL0; read the high byte again to make sure it didn't change; while we were reading the low bytecjne A, TH0, tryagain

; keep the high byte if it didn't changemov R7, A; the full 16-bit timer is now in R7/R6

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)

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Set up by the TCON register

• The bits we didn’t talk about previously

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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

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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)

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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…

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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

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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

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Programming interrupts (part 1)

• Setting up the ISR

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; org tells the assembler where to place the codeorg 0; start at the main program labeljmp main

; timer 0 ISR is at address 0x000Borg 0x000B; obviously not a good ISR but it makes the pointtimer0:jmp timer0reti

Programming interrupts (part 1)

• The main program (almost the same as before)

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; start the main program at address 0x0030org 0x0030

main:; set timer 0 to 8-bit modemov TMOD, #0x02; start value mov TL0, #0xFC; reset value when; the timer hits 0xFFmov TH0, #0x00

; enable interruptssetb EAsetb ET0

Programming interrupts (part 2)

• more main program (same as before)

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; start timer 0setb TR0

; kill some timenopnopnopnopnop

; stop timer 0;clr TR0

Programming interrupts (part 3)

• more main program (same as before)

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tryagain:; read the high byte of the timermov A, TH0; read the low byte of the timermov R6, TL0; read the high byte again to make sure it didn't change; while we were reading the low bytecjne A, TH0, tryagain

; keep the high byte if it didn't changemov R7, A; the full 16-bit timer is now in R7/R6