oitar, it is likely that ADC scaling is not working properly. Therefore, I propose several options for further action:
1. Remove capacitor C1 and repeat measurements without it. Does anything change?
2. Compile a new firmware, where the PA6 port will be assigned to measure the zener diodes. Apply external voltage(necessarily through a resistor divider) in the same way as you did before and check the response of this port.
3. You can try to upload firmware 1.13k.
1. Did not produce any noticeable change
2. Done, similar results to the original port. In fact that's what I did earlier, using the "Battery Votage" testing pin. They all have a problem with voltages below 1V(at the pin).
Did the following test by commenting off most lines between 156 and 163 in ADC.c:
/* get voltage of reference used */
//if (Ref == ADC_REF_BANDGAP) /* bandgap reference */
//{
// U = Cfg.Bandgap; /* voltage of bandgap reference */
//}
//else /* Vcc as reference */
//{
U = Cfg.Vcc; /* voltage of Vcc */
//}
i.e. forcing VCC reference all the time.
Result: Both zener, and resistor measurements are now proportional, albeit with much bigger error. Also the "Adjustment" menu item keeps completing with Error.
Looks like I need much more digging, regarding bandgap calculation.
3. Haven't had a chance to check 1.13k yet.