How to measure power supply internally on STM32F107?

[Spaghetto]

Hello community,

I'm working on a STM32F107VCT6 where I have practically (by means of very basic filtering) shorted on the PCB VDD/VDDA/VREF+/VBAT and VSS/VSSA/VREF-.

I've runned out of pins so I was wondering if there were a way to measure the power supply (without using external ADC channels) which supplies the MCU (VDD) for the purpose of evaluating the upstream power supply conversion (done by a DCDC).

An interesting application here on the forum based on STM32L010F4 using VREFINT_CAL solution could have been perfect if only STM32F107 had this register (which actually don't).

As a desperate attempt I was doing Vsupply=3.3V*Vrefint/Vmeas. Where Vrefint=1.20V from datasheet, Vmeas is Vrefint measured by means of ADC1 "Vrefint Channel". But it seems stupid to me because falls in a circular reference thus the Vsupply will always be calculated as 3.30V... Or maybe not?

Any smarter solutions?

Thanks in advance.

[TDK]

Measure VREFINT, assume it's 1.20 V per the datasheet, and extrapolating the reading to determine what VREF+/VDDA/VDD is.

You don't need VREFINT_CAL to be able to do this.

[Spaghetto]

Hi @TDK​ , thanks for you answer (again). This idea led me to the "disperate attempt" above, but my consideration is: if Vrefint is always 1.20V no matter what voltage if I supply to the MCU (in his operating voltage range of course, 2.0 to 3.6 V), how can the two voltages be correlated?

In less word: can you define your "extrapolation" above?

[TDK · Accepted solution]

Sure.

Call VADC the voltage at the ADC pin. We know that's 1.2V because the datasheet says so.

The ADC counts are given by COUNTS = 4096 * VADC / VREF+.

Rearrange that to solve for VREF+:

VREF+ = VADC * 4096 / COUNTS

So if your counts are 1638: VREF+ = 1.2 * 4096 / 1638 = 3.00

If your counts are 1489: VREF+ = 1.2 * 4096 / 1489 = 3.30

[waclawek.jan]

Let's denote the voltage on VREF+ as vrefext , maximum ADC readout which corresponds to this voltage as MAX=4096, voltage on the bandgap as vrefint=1.2V and the ADC readout of this voltage as REFINT. The ADC is proportional thus

vrefint/vrefext = REFINT/MAX

from which

vrefext = MAX/REFINT * vrefint

or

voltage_on_VREF+ = 4096/ADC_readout_for_VREFINT * 1.2V

JW

[Spaghetto]

Ha, that's quite funny...I was implementing that formula modifying the one I got in my first post then I realized:

Vsupply=3.3V*Vrefint/Vmeas <<<mine
=3.3V * 1.2V /Vmeas=
=3.3V * 1.2V / (ADCVAL* 3.3V/4096)=
= 1.2V / (ADCVAL/ 4096)=
= 1.2V * 4096 / ADCVAL <<<suggested

So basically I already had already implemented the suggested formula... 🙂 I just didn't realize why it could have been working. I ignored the idea that the 3.3V in my formula was not the reference value but the max ADCval ...

I used the days where the forum got into maintenance to make some tests at different Vin that I can confirm performed well...

Thanks both!