> I am trying to generate a delay for the 20 MHz PIC16F877 that I
> am using to interface it to the LCD.  I think I don't understand
> how to generate the delay because I know that my hardware is
> working perfectly.  Can some one please tell me how to calculate
> the delay in seconds for the following code.
> Thank you
> Sarah
>
> LOOPD   equ   2Ah
> .
> .
> .
>
> delay   movlw   .90
>         movwf   LOOPD
> repeat  nop
>         decfsz  LOOPD
>         goto    repeat
>         return

I personally don't like busy-waits much.  The timing can't be relied on when
there are interrupts, and the processor is completely out to lunch during
the wait.

Especially when the wait is more than a few milliseconds, I prefer to set up
a periodic interrupt.  1mS is often a convenient interrupt period.  The
interrupt routine decrements a counter if it is not already zero.  The
foreground code sets the counter then checks for it to become zero.
Meanwhile, it can be doing other tasks, as long as it gets back to checking
the wait counter fast enough.

Note that you can support more than one simultaneous wait counter this way,
and they can each be clocked at a separate multiple of the interrupt period.
I've had projects that simultaneously used 1mS, 10mS, and 100mS wait
counters, all derived from the basic 1mS interrupt.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@embedinc.com, http://www.embedinc.com

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