About STM32L486 Reset Pin problem

[Kevin Zhang]

Posted on February 07, 2018 at 08:21

Hi All,

   I had found a problem with STM32L486, That I connect this pin with 100K pull up to 3V3 and a 0.1uF cap to GND. It won't work when I drag&drop through SWDIO/SWDCLK .hex files in to it. But if I connect this 'Reset' pin just with 0.1uF cap to GND, or connect this pin with 10K pull up to 3V3 and 0.1uF cap to GND. it works fine(can drag&drop .hex file in to it)

   Any one have ideas about this?

[Szymon PANECKI]

Posted on February 07, 2018 at 09:36

Hello Kevin,

There is no need to connect external pull-up resistor to NRST pin of STM32L4 MCU. Pull-up resistor is already embedded in internal reset circuitry (it is called Rpu). Please find below a relevant block diagram. Rpu has typically value 40kOhm.

It could be that external pull-up 100kOhm is too strong and together with internal pull-up they don't allow reset signal to reach a threshod needed to reset the MCU. 

External pull-up 

10kOhm is normally ok, but again please note, that this is not mandatory. Only 100nF capacitor between NRST and ground is needed.

I would suggest either to use external pull-up 

10kOhm or to remove completely external pull-up resistor.

Regards

Szymon