Wan Zulhelmi Wan Ahmad Kamar wrote:

> What I meant by exact time means that for example
> I want 5 mS of delay, how would I make sure my
> loop will only delay exactly for 5 mS and no
> more or less than that.

Hi Wan,

It depends on the frequency (Fosc) that the PIC is running at and the number
of clock cycles (always an integer value) your delay routine uses --
sometimes you will nail the time delay perfectly (for all practical
purposes) and sometimes you won't (but it's close enough). If you are using
a subroutine, you might also need to account for the overhead of the CALL
and RETURN instructions -- it all depends on how precise you need things.

Another thing to consider is that if you are using interrupts and they are
enabled, they will preempt your software delay if any interrupts occur
during the software delay routine -- this will extend the time of software
delay beyond what you originally calculated it to be.


Back to your original question:

To generate a 5 mS delay, you need to code a delay to execute the following
number of clock cycles:

 Fosc
 ----  *  5 mS
  4

And like I said before, you might want to account for the 2-cycle overhead
of the CALL and the 2-cycle overhead of the RETURN instruction if you are
using a subroutine for this delay (as opposed to in-line code).

There are utilities that people have written will aid you in this process.
If I remember correctly, Andrew Warren had a QuickBasic program for helping
one with software delays many years ago (pre-PIC18). There's probably a slew
of other utilities as well that others have written. You might want to
search the PICList website (www.PICList.com) -- you should be able to find
some good information there to help you out.


Best regards and wishes,

Ken Pergola



















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