STM32F411 - failing to read backup battery voltage (VBAT)

Question

I am doing some development on a Weact STM32F411 black pill. Most of it has gone fine. However I am trying to read the voltage on the VBAT pin (backup 3V button cell) using the internal ADC, but I cannot get sensible readings.

It is only read once after reset, so register default values are assumed, and from the manual the STM32F411CE can put VBAT (/4) directly on to ADC #18 by setting VBATE. From the schematic, the ADC ref +/- are 3V3/0V on the board. The code is as follows (the peripheral clock for ADC1 on APB2 is already enabled).

... //  peripheral clock already enabled
ADC->CCR |= 0b01 << 16; //  ADC clock prescaler to /4 (should give 24MHz)

ADC1->CR1 |= 0b00 << 24;    //  12 bit resolution
ADC1->CR2 |= 0b0 << 11; //  right aligned
ADC1->SMPR1 |= 0b111 << 24; //  ADC #18 sampling time 480 cycles
ADC1->SQR1 |= 0 << 20;  //  1 conversion (n = L + 1)
ADC1->SQR3 |= 18 << 0;  //  conversion[0] is ADC #18
ADC1->CR2 |= 0b0 << 1;  //  continuous mode is off
ADC->CCR |= 0b1 << 22;  //  VBATE - backup battery voltage connected to #18

ADC1->CR2 |= 0b1 << 0;  //  ADC module is on

delayCycles(100);   //  short delay (100x NOP)

ADC1->CR2 |= ADC_CR2_SWSTART;   //  start conversion

while ((ADC1->SR & ADC_SR_EOC) != ADC_SR_EOC) {}    //  wait for conversion finish

U16     rawCounts = ADC1->DR;
...

(I'm aware that things like << 0 and 0 << n do nothing, it's just for readability).

The code runs once soon after start up and what I find is that whatever voltage (0 to 3.3V) is on the VBAT pin, I tend to get an ADC count reading of ~1010 give or take a few counts, which with a 12 bit ADC and /4 divider internally, gives around 3.2-3.3V.

Any suggestions appreciated.

nmw01223 · Accepted Answer · 2024-08-27 15:48:50Z

0

After some investigation, think the reason for this is clear. On the Weact schematic the CPU VBAT pin is fed via two Schottky diodes from the external module VB pin and CPU 3V3 respectively. That therefore says that whenever the CPU is powered, it is going to have 3V3 less a small diode drop on the VBAT CPU pin. So, trying to read the backup battery voltage using the internal connections is never going to work, unfortunately, as the CPU has to be powered to do it, so all it will ever do is read its own 3V3.

answered Aug 27, 2024 at 15:48

nmw01223

1,7454 gold badges26 silver badges44 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

STM32F411 - failing to read backup battery voltage (VBAT)

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related