STM32F446ZE not booting

[CO_it]

Hello,

some of our boards do not boot after powering them. We realised that most of the time this happens, the 1.2V at VCAP1 and VCAP2 == 0V. At our boards the BYPASS_REG pin is floating. So there are two questions for us now:

1. Could the floating BYPASS_REG  pin cause our Problems?

2. does the BYPASS_REG pin have an internal pulldown resistor?

We also realised, that if we add a 10µF capacitor to the NRST pin, the board will boot everytime. (Normaly there is a 100nF capacitor)

Thank you in advance :)

[Peter BENSCH]

You are obviously using one of the packages WLCSP or UFBGA and therefore have the pin BYPASS_REG. However, this pin must have a defined level and must not float. I assume that you want to use the internal voltage regulator, so you should connect BYPASS_REG to GND via a pull-down.

In this case, it is also mandatory to connect a 2.2µF capacitor to GND on each of the pins VCAP1 and VCAP2 (see data sheet). You should reduce to 100nF at NRST again.

Hope that helps?

Regards
/Peter

[CO_it]

Thank you for the feedback. We already realised that we should had connected this pin.

Can I correctly understand from your answer that there is no internal pulldown resistor installed on the pin?

[Peter BENSCH]

Correct. It would be mentioned in the data sheet if there was an internal pull-down at the pin BYPASS_REG. This is corroborated by the statement in section 3.17.1: On packages embedding the BYPASS_REG pin, the regulator is enabled by holding BYPASS_REG low. ...so you have to actively pull it to GND and not let it float (for low).

[Issamos]

Hello @CO_it 

This faq and this AN2606 may help you.

Best regards.

II

[CO_it]

Tnak you