CPU frozen on power up, BOR not working

[syn]

Hello,

I have a custom board with a 4.2V battery followed by a linear voltage regulator at 3.0V powering a STM32WL33.

When battery is inserted sometimes the power up is not clean and the cpu freezes, no code is executed. (the code is just pulling down a GPIO, no other initialization)

BOOT pin A10 has a 560 Ohm pull down.

Pulling down NRST later doesn’t have any effect. 

Freeze happens when the VDDIO goes down to 500mV and then rise up again.

As you can see in the following images in clean startup the core VCAP voltage rise up from 0 to 1.2V very fast, while in bad case the VCAP voltage follows the VDDIO ramp from 0.5V to 1.2V and the core freeze.

BOR is not working!!! NRST is not pulled down.

Schematic is as suggested.

Any workaround?

Thanks!

Fabio

 

Good power-up:

Green: VDDIO;   Yellow: SMPS output on VFBSD;   Cyan: VCAP;   Purple: NRST

 

Bad power-up:

Schematic:

 

[syn]

Problem found:

in my circuit a big load (about 30 Ohm) is connected to VDDIO and when battery is loosing contact the VDDIO voltage goes down very fast and BOR circuit doesn’t have enough time to pull down NRST.

 

 

[David Littell]

Thanks very much for reporting back!