ADC voltage reference

[lolloz]

Hi to all,

I'm using a gpy with a pytrack 2 and I'm trying to calibrate it getting the voltage reference to P22. So, I run these commands:

from machine import ADC
adc = ADC()
adc.vref_to_pin('P22')

but when I go measure the voltage on P22 with a voltmeter I get almost 3V. That is not I expected, should be around 1,1V.

So, anyone can help me about where I'm wrong?

[lolloz]

@robert-hh thank you so much for your quickly answer.

[robert-hh]

@lolloz Pytrack 2 uses P21 and P22 as SDA and SCL for the sensors, which means that the have a pull-up resistor connected. And P6 is used by the GPY LTE modem.
So you cannot use vref_to_pin() in that set-up. Since you need a multimeter anyhow, you can get the calibration value by measuring a known voltage with multimeter and the internal adc, get the default vref and calculate the vref value from ratio of externally to internally measured voltage.