Timing inaccuracy - any ideas?

[Optech]

Thanks for the info I didn't realise the instruction times were for assembly not pbp.

I set up the program as suggested and got a 6.2 KHz frequency readout +/-0.1 Khz.
The duty cycle is 50% +/-2%. (suggests about 27 us per command?)

The trace on the scope is not square; doen't even acheive the full amplitude - almost as though there is a stray capacitance somewhere.

I then tried the same program with the internal RC oscillator with similar results. I've checked and rechecked the circuitboard and all is as it should be.

I returned to the external resonator, and added a 'pause 5' to each half of the main loop and got the following readout on the scope:

Frequency 100.0 Hz
High pulse width 5000 us
Low pulse width 5000 us
Cycle time 10000 us
Duty cycle 50.00%

Not a flicker. Perfect squarewave form. Totally mystified.
Reprogrammed original program (cut down version as above). Same result of flickering up and down +/- 0.1% and being out by 4 ms.

I can't progress this any further due to other commitments for now so I'll wait until I can try another resonator on Monday. Any further suggestions welcome of course.

Many thanks

[skimask]

I set up the program as suggested and got a 6.2 KHz frequency readout +/-0.1 Khz.
The duty cycle is 50% +/-2%. (suggests about 27 us per command?)

Well, if you would've used that program EXACTLY as I wrote it, you would've seen nothing, since there were no port assignments, no TRISIO statements, no nothing.
So, let's see what you actually did write.
And while you're at it, open up your program's .lst file and look thru the actual PIC assembly code. There you'll see all the instructions that the PIC is actually executing. Take a look at the datasheet, and you'll see that most ASSEMBLY instructions take one cycle, gotos/branches/calls take 2 cycles, and conditional instructions can either take one or two cycles depending on results. That's one way to figure out how fast a loop should be.

The trace on the scope is not square; doen't even acheive the full amplitude - almost as though there is a stray capacitance somewhere.

Then that would tell me that maybe your 'scope's probes and/or the 'scope itself isn't capable of reading higher frequencies, or maybe they need to be cal'd.

Also, are you SURE that your CONFIG bits are being set correctly? You may think you're running 8Mhz external when you're actually running 125Khz on the internal clock, or worse, running on the 37Khz internal R/C clock.

[Optech]

Replacement of the crystal has eliminated the instability, and with a minor adjustment of the pause values I have a sufficiently accurate cycle time for the application, so it looks like a dodgy oscillator was the cause of the problem.

Many thanks for all the advice - it's been an education.