NPOR in STM32F098

[Lukasz Przenioslo]

Posted on February 19, 2017 at 17:17

Hello there,

I am working with STM32F098 mcu, it has NPOR pin. Reading the documentation I understood that I have to externally pull this pin low until a stable VCC will be present. When it is present, I have to leave it high impedance as it has internal pull-up.

For this I use TPS3825:

The TPS3825 helds the &sharpRST line low when &sharpMR is low or until VCC reaches more than 3V. My problem is that in my circuit the NPOR pin acts like an output pulling the &sharpRST line low. After I cut off the NPOR line from the &sharpRST net the mcu started to work. When measuring voltage on NPOR pin then it is 0V.

How does this NPOR pin work really? Did I understand something wrong? I would appreciate all help.

#hardware #stm32 #npor

[T J]

Posted on February 19, 2017 at 17:46

Are you using the TPS3825-33 ?

Are you sure it is a TPS3825 ?  and not any of these..  TPS3820 TPS3823 TPS3823A or TPS3828 or TPS3824

On all these other packages, pin 4 is not /MR  it is WDI

Watchdog timer input. (TPS3820 TPS3823 TPS3823A or TPS3828 or TPS3824)

If WDI remains high or low longer than the time- out period, then reset is triggered.The timer clears when reset is asserted or when WDI sees a rising edge or a falling edge.If unused, the WDI connection must be high impedance to prevent it from causing a reset event.

[Lukasz Przenioslo]

Posted on February 19, 2017 at 17:49

TPS3825-33DBV

I am certain.

[Tesla DeLorean]

Posted on February 19, 2017 at 18:17

http://www.ti.com/product/TPS3825-Q1

 

A push-pull driver for the reset output hardly seems appropriate.

♯ POR wants an open-collector, other ARM systems such a pin is bidirectional

[Lukasz Przenioslo]

Posted on February 19, 2017 at 18:21

I know that &sharpRST is somewhat bidirectional thats why I added a 100R resistor, but is NPOR bidirectional as well? I didnt find suitable TPS3825 with open drain output and thats why I stick with push-pull.