Boot0 pin on STM32L011 has internal pull resistor?

[Paolo Andreuzza]

Hello all.

I'm using BOOT0-PB9 pin on a STM32L011. In the reference manual I've found that "The pin logic level can then be read as an input value on the shared GPIO pin.", but nothing about internal pull resistor.

The question is: is this pin provided of internal pull up - pull down resistor? Even trying enabling them (LL_GPIO_SetPinPull(GPIOB, LL_GPIO_PIN_9, LL_GPIO_PULL_UP); ) nothing happened.

If I apply an external pull resistor (after the MCU boot up) I can read the port status correctly.

Thank you in advance.

Paolo

[MikeDB]

I don't think any ST MCUs have a pull down on Boot0. I've always added one externally anyway.

[Tesla DeLorean]

In STM32 designs you want an external pull-down on BOOT0 pins to ensure there is no confusion about wanting to run your code from flash, and what the VIH level is as the power supply ramps vs the point the micro-controller starts to function. For a robust design you might also want to consider a properly thresholded POR circuit.

[Paolo Andreuzza]

Thank you for your answers.

I realize that my question was not so clear, I'll try to make it better.

In my board I've an external pull down resistor, to make the MCU start from flash.

After the MCU has started up, I would program the PB9-BOOT0 pin as input, and enable its pull up resistor, to polarize a bit this input, to be able to read an open drain output connected to it.

The problem is that even if I enable the PB9 internal pull up, the pin level is always 0 volt, while I expected to find a voltage value determined by the voltage divider made by my external pull down resistor and the internal pull up resistor.

It seem as if the internal pull up is missing in this particular port.

Can anybody confirm that?

Thank you.

Paolo

[MikeDB]

Ok I've just read the datasheet and I think it's quite clear from the frequency and leakage current statements on pages 40, 75 and 78 that this is a different input configuration to normal and thus it's unlikely there is a pullup resistor available.

[Paolo Andreuzza]

Thank you, Mike, it's even my thought, I completely agree to you.

Anyway, I was wondering if our though could have an official confirmation from ST, since in the manual is also written that after the boot up, this pin can be used as PB9, not specifying anything about the lack of pull up resistor.

[Igor Cesko]

Hi Paolo Andreuzza ,

Answer to this question is here.

Regards

Igor