ChibiOS Free Embedded RTOS

This is a peculiar situation: I have a stm32f407 disco board, interfacing with a bunch of different external devices. Two in particular are an adc sampling voltage, and the ext driver being used to count pulses from a tach. Independently, these work perfectly. In fact, if the voltage being sampled by the ADC is reasonably constant, I have no issues. However, if the voltage being sampled is constantly changing, my tach pulses show lots of variation. I have hooked a scope to the tach pulse input line, and I see there is no change, irrespective of the voltage. I changed the tach input to a controlled 100hz HW pulse, and still see the same pattern. So there is some relationship between the ADC sampling a variant voltage, and the tach.

I know this could have many SW possibilities, but Im curious if anyone has any obvious things to chase. Incidentally, the problem seems to improve if I hook the scope up to the tach input line - but doesnt get perfect. I tried changing from the onboard pulldown resistor, to a hardwired 10K pulldown - no change...

Hi,

The impedance driving an ADC input determines the minimum sampling time, there are instructions about this in the STM32 Reference Manual. Lower impedances allow faster conversions.

Giovanni

The voltage value is not my issue. Im seeing some kind of weirdness with the tach/EXT. Only when the voltage being sampled on the ADC, is varying. If that voltage is steady, the tach/ext input counts correctly. Again, Im driving the tach with a fixed freq 100hz clock. There is no variation of that signal. I will debug, just wanted to know if anyone ever found a connection between ADC and the EXT drivers. Something silly Im missing...

Hmm. I think we can delete this thread. I removed some custom HW, and the problem went away. There is clearly a ground/noise issue, with my HW.