Create Delay in Arduino Uno using Assembly language without using timer

Question

I just started learning about micro controllers and I was not able to understand how we could introduce delays in the code without using timers. My board has a clock of 16MHZ. Let's say I want to introduce 5ms delay before I check if a button is pressed. How would I identify how many instructions I need to execute to get 5 ms delay and how would I program it? Is there a standardized way of doing this? It looks like a very standard thing but I am not able to understand how it is done.

I am programming using Assembly language on Atmega 328 Arduino uno.

Answer 1

Generally you figure out how many clock cycles you need to burn, then write a loop. Consult your datasheet to determine how many cycles your loop takes and calculate how many iterations you need.

       ldi r16, x ; 1 cycle
loop:  nop        ; 1 cycle
       dec r16    ; 1 cycle
       brne loop1 ; 2 cycles when jumping, 1 otherwise

Depending on the value of x, this loop will take x * 4 cycles. With a 16MHz board 1ms is 16000 cycles, so 5ms would be 80000 cycles. That's more than this 8 bit loop can manage so we need to make a 16 bit counter.

       ldi r16, x ; 1 cycle
       ldi r17, y ; 1 cycle
loop:  nop        ; 1 cycle
       dec r16    ; 1 cycle
       brne skip  ; 2 cycles when jumping, 1 otherwise
       dec r17    ; 1 cycle
skip:  brne loop  ; 2 cycles when jumping, 1 otherwise

Okay so our loop body now takes 6 cycles per iteration. Notice that it's 6 cycles no matter if r16 is wrapping or not. The setup takes 2 cycles but the final brne gives us 1 cycle back so we got 1 cycle overhead. That means we need 79999 cycles which is 13333 iterations and one more cycle to waste. Thus x=low(13333)=21 and y=high(13333)=52 and add a nop.

That's the general idea, I hope I have not miscalculated anything. If you intend to make a function of this, factor in the overhead of the call and return. Also, you can make it parametrized.