Thu May 19 23:07:08 CEST 2011

Low power PIC tips

The PIC datasheets clearly state that non-digital signals (around 1/2
Vcc) should never be connected to a digital input to avoid linear
biasing of the CMOS input state which draws a lot of power.  However
[1] mentions on page 2-4 that:

  "Sometimes it is appropriate and possible to configure digital
  inputs as analog inputs when the digital input must go to a low
  power state."

Which vaguely implies that analog inputs consume less power than
digital ones.  If the inputs are logic-level (i.e. not 1/2 Vcc) then I
doubt this is true.  Maybe it's a safety measure in case the input
starts floating?  Or is it about the leak current of a CMOS input
stage vs. that of an analog input stage?

What do uC analog inputs look like?  Are they comparator inputs?
Those would be differential pairs.  What about their bias current?

Ha!  It's not the ADC input that counts, but the analog MUX.  I
suppose the ADC input stage is simply off when not converting, not
drawing any leakage current.

For the PIC input pints, the only difference between analog and
digital mode is that digital mode has power to the digital input
buffer, so always uses more power (input buffer leakage).

[1] http://ww1.microchip.com/downloads/en/DeviceDoc/01146B_chapter%202.pdf