Measuring floating pin voltage (ADC from just the built-in comparator)

evanh

I've recently explored a soft start for audio output that doesn't "pop" on first use - https://forums.parallax.com/discussion/comment/1535297/#Comment_1535297

One thing that came out of this was that I found a useful reason to, using the internal comparator hardware, implement a SAR mode ADC - https://en.wikipedia.org/wiki/Successive-approximation_ADC - rather than using the dedicated sigma-delta ADC. The dedicated ADC hardware isn't suited to measuring a floating pin since the frontend hardware will always pull the pin voltage to VIO/2.

It's far from a performance ADC - doesn't have the sample and hold circuit and the comparator's R2R DAC is quite slow - but works perfectly for a low-res static level check like I needed.

pub  sar_reading( pinnum ) : level | mask
' This routine makes use of the comparator's unloaded inputs
' to measure the floating voltage of the pin.
' Returns an unsigned 8-bit value in "level", GIO to VIO volts
'
  mask := 128
  level := 0
  pinf( pinnum )
  repeat
    wrpin( pinnum, P_LEVEL_A | ((level | mask) << 8) )    ' set comparator DAC level
    waitus(2)    ' DAC settle time about 1.0 us
    if pinr( pinnum )    ' pin voltage higher than comparator's DAC
      level |= mask
    mask >>= 1
  while mask
  wrpin( pinnum, 0 )

evanh

Another way to write the same procedure:

pub  sar_reading( pinnum ) : level | bit
' This routine makes use of the comparator's unloaded inputs
' to measure the floating voltage of the pin.
' Returns an unsigned 8-bit value in "level", GIO to VIO volts
'
  pinf( pinnum )
  level := 0

  repeat bit from 7 to 0
    wrpin( pinnum, P_LEVEL_A | ((level | (1<<bit)) << 8) )    ' set comparator DAC level
    waitus(2)                         ' DAC settle time is about 1.0 us
    level |= pinr( pinnum ) << bit    ' pin voltage higher than comparator's DAC

  wrpin( pinnum, 0 )    ' clean up