STM32L071 MCU Bootup fails

[Rashmita Pradhan]

Hi ST team,

We are using STM32L071 for the system bootup application. We are observing that for few of the boards - NRST was asserting due to which ST MCU fails to boot. Our initial suspect was on 3.3V Power rail. We did an experiment as below:
- When the injected residual voltage reaches approximately 0.25V on the 3.3V rail, the U1501 fails to start up.
- When the residual voltage is below 0.20V or above 0.30V, the U1501 starts up normally.

What might be the possible reason for NRST pin getting low? Please find the schematics and probed NRST pin.

 

 

[Rashmita Pradhan]

Hi ST Team, 

Waiting for your respond on this issue.

Thanks,

Rashmita 

[Rashmita Pradhan]

Gentle reminder

[Ejack1979]

As the common sense you’d better add a small delay to nRST when power supply is ramping up. Just like an R-C charging delay, which is generally adopted for 60 years or longer...

 

It’s hard to tell at what point the voltage is enough for internal logic unit to work while it oscillate in logic HIGH or LOW. In the case of microcontrollers which are designed for low-power applications, the situation may get worse.

So never rely on the chip itself. As hardware designers we should always give her a stable condition and make her happy. :-P

 

[Andrew Neil]

As the common sense you’d better add a small delay to nRST when power supply is ramping up. Just like an R-C charging delay, which is generally adopted for 60 years or longer...

 

 

Note that the external pullup is unnecessary.

​@Rashmita Pradhan this is shown in both the datasheet and Application Note AN4467,  Getting started with STM32L0xx hardware development

 

https://www.st.com/resource/en/datasheet/stm32l071cb.pdf#page=97

 

https://www.st.com/resource/en/application_note/an4467-getting-started-with-stm32l0xx-hardware-development-stmicroelectronics.pdf#page=30

[LCE]

Also never ever use a mechanical switch / push button without some debounce (R) C.

For some switches I’d even recommend putting in a small TVS.