piclist 2001\04\06\175709a >
Thread: precise interrupts
www.piclist.com/techref/microchip/ints.htm?key=interrupt
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face BY : Mike Mansheim email (remove spam text)



Using an F876 with a 10M clock, I'm looking to create a precise 1ms
interrupt using timer 1.  I found a thread in the archives that
covered this, and am using Bob Ammerman's suggestion for my first
try (I'll play with the ccp module later).  It's working, but it
did raise a couple of questions.
The technique is to add a constant to the timer value in the
interrupt:
- Bob say's to add 65536 - 2500 + fudge, where fudge accounts for the
 instructions while the timer is turned off.  Since those instructions
 are doing a 16 bit add, there is a btfsc statement.  Wouldn't this
 introduce the possibility of 1 cycle of jitter, depending on whether
 or not the branch was taken?
- I have nearly the same code as Bob's example, so there are 6
 instructions between turning the timer off and back on (including the
 aforementioned btfsc).  I assumed that the fudge would be 6, maybe 7
 to account for the instruction to start the timer back up.  When
 I check it on a scope, I need a fudge of 10 or 11, depending on the
 clock (one running hairs below 10M, the other hairs above).  Any idea
 why this would be?

I found another entry (by Bob again), where he talks about completely
eliminating jitter by looking at the lsb of TMR2.  Why does this indicate
whether or not a 2 cycle instruction is in progress?  Is it the same for
the other timers?
(this was an article of some sort, not part of a thread, as near as I
could tell, so I couldn't pursue it any further in the archive)
Thanks for any help.

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