[mluckham]

Glitches Eliminated using Interrupt Context Saving, BUT ...


Can someone give me some advice?

I have written some pulse-measurement code for last years' wheel encoders, and along the way I discovered some things.

1) the CCP (Capture/Compare) feature that would have been the ideal solution cannot be used because there are no CCP input pins available on the full-size controller.

2) therefore I had to use pulse edge detection - interrupt on the leading edge of the pulse, capture the time, reprogram to interrupt on trailing edge, interrupt on trailing edge, capture the time again - subtracting the two times to get pulse width. To get the times I use TIMER3 programmed at 1/10 microsecond resolution.

3) all along (even before I had started working on the pulse measurement code) I had been seeing glitches on printf() output and also servo chattering. I'm using Kevin's interrupt-driven serial port code, but the printf() glitches were happening using the standard IFI serial port code also.

Suspecting some registers were getting clobbered by the interrupt service routines, I played around with the interrupt #pragma in user_routines_fast and managed to fix both problems ... but at a cost of substantial additional overhead - that affects both the minimum length of pulse I can measure, and the accuracy in measuring them.

I have some other interrupt-driven code that implements a wallclock timer using TIMER0, but disabling those interrupts does not fix the glitches - they are coming from elsewhere.

Here is the #pragma along with comments:

#pragma interruptlow InterruptHandlerLow save=PROD, section(".tmpdata"), section("MATH_DATA") /* You may want to save additional symbols. */
			// Dec2006 - failure to save section .tmpdata results in glitchy servos
			//         - failure to save section MATH_DATA results in glitchy printf
			//         - COST OF SAVING SECTIONS IS ABOUT 14 MICROSECONDS PER SECTION!!!
			//           WHICH AFFECTS MEASURING ENCODER PULSE LENGTHS
			//              minimum pulse measurable with save=PROD  is 14 usec
			//              adding section (".tmpdata")              is 27 usec
			//              adding section ("MATH_DATA")             is 40 usec

I'm happy to be rid of the glitches, but if I wasn't getting them in the first place I wouldn't need the extra #pragma sections and could measure smaller pulses.

Can anyone shed some light on this?