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Lukasz Przenioslo
Lukasz Przenioslo
Associate III
February 19, 2017
Question

NPOR in STM32F098

  • February 19, 2017
  • 1 reply
  • 1058 views
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:

0690X0000060MqGQAU.jpg

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
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    1 reply

    T J
    T J
    Senior III
    February 19, 2017
    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
    Lukasz PrzeniosloAuthor
    Associate III
    February 19, 2017
    Posted on February 19, 2017 at 17:49

    TPS3825-33DBV

    I am certain.

    Tesla DeLorean
    Tesla DeLorean
    Guru
    February 19, 2017
    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

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