Weird ADC failure STM32F091, any ideas

Boards running continuously/long-term? If yes, does the "self-healing" relate to reset? Possible temperature excurses?

The ADC has calibration for a reason.

JW

I would try replacing the mcu with a new one. This would show whether the problem is in the MCu or in the external measuring channel. It would also be good to check if the MCu's internal measurement channel and the calibration of the converter (against the internal reference source and zeroing) are set correctly.

> Ideally I'd like to know the meaning of the calibration register values

IMO in 'F0 you don't have the calibration register available to the user, i.e. it's internal.

According to the Calibration subchapter of ADC chapter of RM0091:

Calibration should be performed before starting A/D conversion. It removes the offset error

which may vary from chip to chip due to process variation.

According to RM0091 ADC_DR description:

Just after a calibration is complete, DATA[6:0] contains the calibration factor.

JW

Nothing is making much sense :unamused_face:. The DR calibration results are 0, 1, 3 and 63 (-1 perhaps), but so what! If I include the measurement of the internal reference and use that to calculate the ADC VCC reference voltages I get 2.748V, 2.776V, 2.760V and 2.491V, all a long way off my actual 3.3V.

You could check what happens if the sample and hold time for a conversion is changed significantly.

Also, measuring Vref voltage will tell you the Vdd. You could put by SW a range detection every 10 minutes to make sure there is no drift.

If the measurement results are unstable like this, I would look at the quality of the supply voltage (ripple, interference)

And when you measure an internal reference source, are the readings stable and correct? Is the measured reference voltage within the range given in the specification? If not and the supply voltage is fine, the MCU must be faulty.

The internal reference comes back as 2022 +/- a few codes. This isn't what I expect, because it implies an ADC VRef of 2.49V. Apart from being a really weird failure mode, on one of my PCBs the fault 'went away', so I'm not convinced it's a hardware fault.

But have your measured the VDDA/VREF+ voltage when the device is in failure mode? VSSA/VREF- state? Maybe soldering quality problems.

> The internal reference comes back as 2022 +/- a few codes. This isn't what I expect, because it implies an ADC VRef of 2.49V.

But that's then 30% *higher* than expected. In the initial post you've said the measured (external voltages) values are 30% *lower* than expected.

I don't doubt your words, just that this somewhat increases the mystery.

We don't know how exactly do you perform conversions. So, for example, as S.Ma said above, too short sampling time may have impact - in that case, if you convert in sequence, preceding channel may influence measurement of the subsequent one.

JW