[WizardOfAz]

I want to sample some analog inputs at 4ms.

The first way I tried this was to set up a timer interrupt at 4ms and in the ISR, set a tic4ms flag. Then in the polling loop, whenever the tic4ms flag was set, I'd clear it and run the sampling code. The problem with this is that my main processing, which runs every 26ms, uses about 9ms and I miss some of the 4ms tics.

Next try was to read the analog inputs in the ISR itself. I hoped that doing
#pragma interruptlow InterruptHandlerLow save=PROD, section(".tmpdata")
would save enough context, but it evidently does not - I am getting a smattering of wild data now in the main polling process. First I thought this was because I was calling Get_Analog_Value in the ISR as well as in the main process. Get_Analog_Value uses some SFRs, and they are not saved across the interrupt, so this would be expected to at least give some bad samples if not worse data mangling. But I moved all the analog sampling to the ISR so Get_Analog_Value would be called from only one thread. This did not eliminate the problem.

Anybody have suggestions?

Also, my assumption is that Get_Analog_Value only runs a few tens of microseconds (15-30 or so) so I can do many of them in a 4ms ISR w/o an overrun problem. This is what it looks like in the pic datasheet. Can anyone confirm this?

Thanks for any help.

Bill