VREF+ pin on P-NUCLEO-WB55 board

WQ · ‎2023-03-09

Hi, on P-NUCLEO-WB55 board, the VREF+ pin is connected to VDDA which is the analog power supply. See the picture.

My understanding is that this pin is an output when the internal VREF (2.048V or 2.50V) is used. Does this connection mean the use of internal VREF is disabled or the pin/device might be damaged due to contention? Thanks. -wq

Peter BENSCH · ‎2023-03-09

VDDA is not an output, but a supply voltage pin for the analogue circuit functions of the STM32, thus to be understood as an input. In the concrete case, you have three pins:

VDD for supplying the digital part
VDDA to supply the analogue part, here connected to VDD via a filter
VREF+ either as output of VREFBUF or as input for the reference voltage, which is then used internally for comparison

VREFBUF should of course only be activated if VREF+ is used as output.

Details on all pins can be found in the reference manual and the data sheet.

Does it answer your question?

Regards

/Peter

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WQ · ‎2023-03-10

Thanks Peter. But my question is not answered. My question is that, with the VREF+ and VDDA pins are tied together, can't we use the internal reference voltage (either 2.048V or 2.5V)? The VREF+ pin is the output of the internal VREF (either 2.048V or 2.5V) when the internal VREF is turned on. Will it fight against VDDA and cause damage?

Peter BENSCH · ‎2023-03-13

VREF+ has the two functions input and output.

When VREFBUF is switched off, VREF+ can be used as an input, e.g. as a reference voltage for the ADC, whereby the permissible values of the data sheet must be observed. However, VREF+ can also be connected to VDDA and thus used for ratiometric measurements, whereby e.g. resistance sensors can be evaluated without the influence of the supply voltage.

With VREFBUF switched on, VREF+ can serve as an output of the internal reference voltage.

Of course, you should only use one of the two modes and not activate VREFBUF if you apply an external reference voltage- to VREF+, even if this does not destroy the STM32WB55.

Clear now?

Regards

/Peter

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