STM32G0B1RCT6 HSE Bypass Mode - Intermittent Startup Issues

flawlessAuthor

Graduate II

I'll answer because my colleague @Satty has issues login in, it says he's banned for some reason.
@TDK @waclawek.jan We tried different BOR levels but it doesn't help.
We think this isn't solely a power supply issue. Connecting it directly to MCU's 3.3V rail didn't resolve the HSE startup problem. But in case of using wires to close the circuit instead of power supply channel output enable button, HSE starts reliably. This approach doesn't help if used on main power rail (12V) which implies that it actually is some kind of power timing issue.

Super User

Make sure *all* VSS and VDD (including VSSA/VDDA) pins are connected properly. Measure continuity directly at the pins (think of bad solder joints).

How is VDDA connected? And what about VBAT?

JW

flawlessAuthor

Graduate II

This MCU has single VDD and VSS pins, VSS is just connected to the common ground directly. VBAT is currently floating.
We didn't tried measuring it directly at the pins, but the fact that it reproduces on multiple boards, including resoldered one with MCU from dev board, so we will check it, but we think this is not the issue.

Update: connecting VBAT to VDD make it works stable

A

Andrew Neil

Super User

@flawless wrote:
connecting VBAT to VDD make it works stable

AN5096: Getting started with STM32G0 Series hardware development does recommend that you should do that:

Super User

> connecting VBAT to VDD make it works stable

Oh.

Maybe another effect of the VBAT brownout erratum? Although I don't see, how would HSE be related to the backup domain.

JW

flawlessAuthor

Graduate II

This actually makes us a bit confused because we didn't find any traces of this issue from other developers. HSE doesn't look like super rare thing to use, maybe many solutions just have VBAT connected to VDD by default, but anyway it's strange.
Btw, we tried to connect VBAT to VSS and it also won't help to make HSE start, to prevent it from floating. Other strange thing is how VDD rise time affects that (with floating VBAT), as we tested it's always start if we just connect it to already running power supply.
Also it doesn't look like this issue is closely related to erratum at least because we found how to stable reproduce both behaviours, when it starts or not, for instance it always depends just on VDD rise time, but erratum describes more constrained scenario that would be hard to reproduce by hand consistently.

Super User

Thanks for the detailed info.

I would like to ask you to do one more experiment: in the "failed" state, i.e. when HSE does not work, can you please read out content of RCC_BDCR register and post it.

Thanks,

JW

flawlessAuthor

Graduate II

Yes, sure, here it is

It actually stays the same when HSE is ok, changing VBAT leads to the same content of BDCR

Super User

OK I see, thanks.

JW

Super User

At this point, no more ideas, sorry.

JW

flawlessAuthor

Graduate II

Anyway, thank you for your suggestions, we already found many interesting cues with your help. We will play a bit more with what we have so far.

A

Amel NASRI

Technical Moderator

Hi @flawless ,

Please note that it is explicitly mentioned in the reference manual RM0444 that is is required to connect VBAT to VDD when no external battery is used:

Important to pay attention to this warning when this condition isn't respected and that may explain the behavior you see:

You have the same in the he AN5096: Getting started with STM32G0 Series hardware development.

-Amel

A

Anonymous

Yes thank you, but I read a few discussions where people say, you can leave VBAT floating in case you don't use anything from VBAT power domain. I just assumed, HSE isn't part of it, so it shouldn't affect it in any way. I hope, VBAT is the real reason behind HSE problems because it is somehow fixable. Though as I said, the fact that in case of battery discharge, HSE could just stop working is a bit worrying.

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Andrew Neil

Super User

@Sattys wrote:
I read a few discussions where people say, you can leave VBAT floating in case you don't use anything from VBAT power domain.

Do you have (a) link(s) to any such thread(s) ?

@Sattys wrote:
the fact that in case of battery discharge, HSE could just stop working is a bit worrying.

You should be able to fall-back to an internal oscillator in that case?

Presumably, you'd want to detect low battery & act on that before it got to this stage?

A

Anonymous

To finally confirm described behavior we bought NUCLEO-G0B1RE board.
We swapped board's MCU with one of our board's one to check all the possibilities. Also, we unsoldered NUCLEO board's SB26 bridge (it connects VBAT to VDD). Both MCUs were then programmed with same basic software that enables HSE & MCO to output HSE.

Our board behaved exactly the same as was previously described.

NUCLEO board was powered by Vin using 12V from my lab PSU. It also behaved exactly the same as our board: HSE didn't start without power cycling in a "special" manner.