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VREFBUF and STM32H563VI

mccabehm
Associate III

I have VREFBUF configured for output to comparator on my board. I see this on MCU Vref+ pin...

scope_39.png

  1. Is 1.2MHz waveform typical? Are capacitors on Vref+ expected to smooth?
  2.  I looked for avg closer to 2.5v (VREFBUF_VOLTAGE_SCALE0). How close to 2.5v should this be?

VREFBUF_CSR = 0x09.

1 ACCEPTED SOLUTION

Accepted Solutions
Foued_KH
ST Employee

Hello,

You can check the Errata sheet :  
The pull-down resistor on VREFBUF output is missing, thus causing a slow discharge of the capacitor on the
VREF+ pin.
Workaround : 
When switching down the VREFBUF output, wait for a sufficient time to discharge of the capacitor connected to
VREF+ pin.

Foued

To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

View solution in original post

4 REPLIES 4
mccabehm
Associate III

The waveform above does not appear until I set ENVR in VREFBUF_CCR. No GPIOs active.

There is no 1.2MHz coming from the board.

Does this waveform mean that Vrefint is not steady? If yes, this would seem bad news for ADC accuracy.

Bob S
Principal

I don't know specifically about the H5 series chips, but generally, yes, you need external caps on the VREF+ pin for stability.  For the F4xx ST recommends 1uF and 100nF in parallel.

Foued_KH
ST Employee

Hello,

You can check the Errata sheet :  
The pull-down resistor on VREFBUF output is missing, thus causing a slow discharge of the capacitor on the
VREF+ pin.
Workaround : 
When switching down the VREFBUF output, wait for a sufficient time to discharge of the capacitor connected to
VREF+ pin.

Foued

To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

Moving caps as close as possible to MCU (mentioned in DS14258 under General PCB design guidelines) fixed this for me.