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Spaghetto
Spaghetto
Associate III
June 13, 2023
Solved

How to measure power supply internally on STM32F107?

  • June 13, 2023
  • 5 replies
  • 4985 views

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.

This topic has been closed for replies.
Best answer by TDK

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

5 replies

TDK
TDK
June 13, 2023

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.

"If you feel a post has answered your question, please click ""Accept as Solution""."
Spaghetto
SpaghettoAuthor
Associate III
June 13, 2023

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
TDKBest answer
June 13, 2023

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

"If you feel a post has answered your question, please click ""Accept as Solution""."
    waclawek.jan
    waclawek.jan
    Super User
    June 13, 2023

    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
      SpaghettoAuthor
      Associate III
      June 26, 2023

      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!

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