Simple asm delay ?

[Jerson]

Assuming you have TMR0 (8 bit timer) free, you can try this code

AsmDelay macro  delayus
      movlw  0FFh-delayus    ; delayus to overflow                          1T
      movwf  TMR0                                                                            1T
      bcf        INTCON, T0IF    ; clear the timer0 Overflow flag          1T
      btfss     INTCON, T0IF    ; wait till it overflows                          1T/2T
      goto      $-1                   ; back to the btfss                              2T
  
      endm

The minimum delay you will achieve will be 4T cycles or roughly 2uS at 8MHz.


The way to use it would be

      GPIO = %01
      @ AsmDelay 12
      GPIO = %10
      @ AsmDelay 24

I hope this takes you closer to your goal. The code is just a concept, you may have to correct it. One important point you should note - the maximum delay will be 255uS

[Bruce]

Try this.

asm
DelayCycles macro ncy ; ncy can be from 1 up to 256
  local n
n set   (ncy)   ; assign ncy value to label n

  while n > 2   ; inserts goto $+1 while n > 2
    goto $+1    ; burn 2 cycles
n   set   n - 2 ; n = n - 2
  endw
                ; handle left-overs
  while n > 0   ; 1 nop for odd numbers
    nop         ; burn 1 cycle
n   set   n - 1 ; n = n - 1
  endw
  endm
endasm

Main:
  HIGH 0
  @ DelayCycles 1  ; adds 1 nop
  LOW 0
  @ DelayCycles 5  ; adds 2 x goto $+1 and 1 x nop
  GOTO Main

[Jerson]

Brilliant work.

PS. It has a catch though. Te macro expansion occurs at compile time. So, you will not be able to "feed" the delay value at runtime

[ultiblade]

Dear Jerson & Bruce,

I could not get here earlier, sorry. I appreciate your efforts !

Jerson, if i declare the delay variable as a byte, it takes even longer.
I just don't understand .. The delay macro i found works perfect if given a number straight from the call, if it is fed by a variable the delay jumps to 20 or 30us. It also seems that the delay is fixed after that, if i compile @ wait 1000, it delays a perfect 1000 cycles, if i compile @ wait ledred (i.e. ledred = 100) it delays 20uS no matter what ledred is ?

Bruce, your asm code also does the trick, perfect 1us or 50us delays, pretty neat. But it also suffers from the same problem. If i compile @ Delaycycles 200 if works perfect, if i compile ledred var byte, ledred = 200, @ delaycycles _ledred it gives a delay of 33uS no matter what the value of the leded var is ??

Apart from that, i need to do a delay between 1 and 1000uS, using tmr0 the max delay will be 256, and i can use the Bruce asm up to 512, the ASM code i posted earlier can go up to about 1024.

I would use the 16-bit TMR1 if it was available but i need that to do a 100Hz sync (count) interrupt...

Any ideas ?

[Bruce]

@ delaycycles _ledred it gives a delay of 33uS no matter what the value of the leded var is ??

That's because when you pass _ledred to the macro as the argument, it uses the 'address'
of ledred, and not the actual value in RAM. Look at the address PBP places ledred at to see
why that happens.

You can't put a macro inside a loop with a variable as the argument. What you're looking to do just
isn't possible (as far as I know) with a macro inside the loop, and calls to any .asm routine, timer, etc,
just aren't going to give you from 1uS to 1000uS delay periods you need immediately after
placing a value on GPIO.

I would re-think the whole process if I'm stuck at 8MHz, or switch osc speed & PIC to support using
the pauseus commands.

[ultiblade]

Hi Bruce,

Thank you for your detailed answer ! All is clear now.

I have adapted the code in a way that the led's never go below 12uS, that way the 'color' switch on and off effect (color switches on for 12uS instead of 1uS) does not occur. This makes the colors somewhat more pastel, but it is still a smooth fade and a cool effect.

And, yes, if i'm going to make a new design i will certainly use a larger external osc ..

Thanks for all your answers and information !

Best regards,
UB

[Bruce]

Grab a new 8-pin PIC12F1822 with 32MHz internal osc when they come out. Then you can
get PAUSEUS down to 2uS. These even have an onboard EUSART.