G070CBT6 Strange issue with reset line falling down...

[MKowalski]

Hi, I'm facing a very strange issue with my G070CBT6-based board...

When I start work and power up the board everything works fine for a couple of minutes (let's say 5 minutes ) but later I can observe some disturbances on the reset pin... I connect the oscilloscope and I can see the voltage on the reset pin, after a few minutes of the device operating, starts to fall down slowly and there is some mess on the signal...

Here is an example waveform watched:

Yellow -> 3.3V supply

Blue -> reset line

 

I've tried 3 different pcs of the MCU on the same board and also try soldering bare minimum setup (LDO, MCU, caps, etc.. ) on a totally fresh PCB, and every time the same behavior...

No matter, if the code flashed, is my application code, led blink demo, or totally erased flash.

This is the MCU part of the schematic..  I also tried to pull down the SW_CLK ( there is the BOOT0 also on this pin ) but this ain't help...

When the issue occurs, I'm not able to do anything with the MCU. It is "held under reset" from the SWD perspective. If I leave the board unpowered for some time to "cool down" it starts operating again, but again only for a couple of minutes... The MCU is not getting hot or something... supply is very stable 3.3V...

Any ideas what is going on with this? I have never had similar issues and I doubt that I broke the MCU during soldering...

[SofLit]

Hello,

It could be not related to the behavior but please remove that NRST pull-up resistor as there is one internal to the chip:

[SofLit]

Also there is an issue on VREF+

Vref+ is not connected to an external power source example: VDD!

From NUCLEO schematics:

Please make the nucleo schematics as your reference design.

[MKowalski]

I'm aware of that but this is a 40K resistor so I think that adding 10K externally could only improve the immunity here. 

Anyway, I removed it as You suggested but the problem still exists.

[SofLit · Accepted solution]

+ the LED on PC13:

From the datasheet:

[SofLit]

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@MKowalski wrote:

Anyway, I removed it as You suggested but the problem still exists.

---

Already said: "It could be not related to the behavior"

[waclawek.jan]

Hi @SofLit ,

On 'G0, VREF+ does not need to be connected to VDD/VDDA if analog functionality is not used.

While the 'G070 officially does not have VREFBUF, those 'G0 which do have it must not connect VREF+ to any external source.

JW

[MKowalski]

 This is something I missed. I'll check. Thanks for pointing. 

[SofLit]

Indeed. Only G070 doesn't have an internal VREFBUF. So Vref+ needs to be connected to a power source.

[MKowalski]

I moved the LED to PB9 and connected VREF+ to VDD, but the problem has not been solved.