VREFBUF and STM32H563VI

[mccabehm]

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

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.

[mccabehm]

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]

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 · Accepted solution]

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

[mccabehm]

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