Problems with Analog Comparator

[fijam]

I don't think it's a defect, most likely I misunderstand how this peripheral is supposed to work...

I have attiny212 running at 3.5V Vdd / 4Mhz with core 2.5.6. I have a voltage divider (0.36) on pin PA7 that gives me 1.26V (0.36*3.5).

I set up the comparator like in the example:

  bool flag = false;

  Comparator.input_p = in_p::in0;
  Comparator.input_n = in_n::dacref;
  Comparator.reference = ref::vref_2v5;
  Comparator.dacref = X;  // we'll come back to that later
  Comparator.output = out::disable;

  Comparator.init();
  Comparator.attachInterrupt(checkSwitch, RISING);
  Comparator.start();

checkSwitch() just sets a flag and stops the comparator:

void checkSwitch() {
  // comparator interrupt fired
  flag = true;
  Comparator.stop();
}

In the main loop I just wait for the flag to come up true and turn on a LED if it does. Now I'd expect that dacref of around 128 (1/2 of comparator reference 2v5) would be the value where comparator output changes between high and low. But that's not even close! Dacref needs to be around 213.

Similarly, if I just pull the pin to the ground, the dacref value needs to be around 86 for the analog comparator to trigger.

[fijam]

Well, the good news is I am not completely crazy. I pulled out the scope, and noticed that the pin needs a while to stabilize after UPDI programming:

So this was definitely tripping me up. Once I unhooked the UPDI programmer and started the board off battery I got numbers closer to expected..

Another thing I wonder if my high-impedance voltage divider is causing some problems here (the ground pulldown is 249k). I know that it matters for the ADC, but does it for AC as well?