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STM32WB55RGV6 is not starting up.


We have a product that uses the STM32WB55RGV6 controller. Some of the boards start up without issue, some of them occasionally fail with the STM32WB55RGV6 processor not starting up, and others never start.

We don't know if it is a grounding issue with the processor or perhaps the layout/capacitance of the external crystal, or another reason. There is a full ground plane on an inner layer, but I wonder if the ground plane under the processor should have more vias. Any help would be appreciated.



ST Employee

Hello @BBoos.1​ and welcome o ST Community,

In fact, there are some general guidelines you can follow, please check AN5165 paragraph 4.3 Ground planes.

The ground vias should be placed in a grid pattern across the ground plane under the processor.

Also, as it is stated in the application note, the spacing between vias should be less than 1/10th of the wavelength

Hope that helps!

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.


Thanks Sarra. Yes I did see that in the AN5165 and I will implement that in the next version of the board. Until then I still need to learn if my problem is grounding or something else. The layout for the HSE oscillator seems to be similar to the recommended layout and utilizes no capacitors (same as recommended).

Does conformal coating cause issues with start up? I'm sure it adds some capacitance to the system.