STM32C011J4 phantom pull-up on on PA12 [PA10] in ADC_IN12 mode

[RemigiuszB]

Hi

I use CPU pin 6 as ADC input ADC_IN12. I take measurements every 100us and save them to buffer[100] via DMA.

I have a 1M+10k resistor divider connected to ground to the ADC input.

I measure a rectified sine wave signal within 0..2V.

To my surprise, after a few minutes of measurement, the signal increased by about 400mV and it should go down to zero. It turned out that an internal pull-up was created in the CPU on pin 6 and even 10k to ground is not able to lower it to zero.

Connecting a milliammeter to ground gives about 0.7mA. I thought the program was to responisble for this situatiuon, but I made a test program in which the pin is only an input and the pull-up is permanent.

After replacing the CPU, the problem disappears and appears after another few minutes.

I checked it on 4 pieces. I have no idea what could be the reason for this, an oscilloscope connected to this input shows a rectified sine wave one moment and after a few minutes the same signal but with an additional constant component. Does this fit into errata documents?

I reject the possibility that the input signal or interference exceeded VDD or was below VSS. Additionally, a 10nF capacitor was connected to ground

Maybe if I used a divider with a 1k resistor I wouldn't notice the problem but it has a big impact on the measurement itself. Has anyone had a similar problem? 

I have not encountered such a case on any STM32. Is there any way to fix this? I am not lucky because I also tried to measure on pin 4 (NRST) which I disabled in OB software. The measurement itself worked fine on the same program except that at the start for the first 4 clock cycles the pin is a reset and only the logical 1 signal is able to start the CPU which is very problematic if we want to use this pin as an ADC input  :(

Regards

[TDK]

The chips don't spontaneously generate internal shorts. Something happened. And on 4 chips, it's repeatable. Look at transients at startup or whenever a change happens.

Can you show the signal you're measuring on a scope?

This is your circuit?

[SIGNAL] --- [1 MOhm] -+------------+--- [ADC PIN]
                       |            |
                   [10 kOhm]     [10 nF]
                       |            |
                     [GND]        [GND]

You have 200V on the board. Is everything sufficient isolated? Can you show the board layout/schematic? Note that flux residue or cleaning products can cause high resistance paths between components you don't want.