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pferd
pferd
Associate II
August 10, 2018
Question

STM32 USART half-duplex mode. External, internal resistor.

  • August 10, 2018
  • 5 replies
  • 1229 views

Hello! I have a problem with half-duplex in USART. I connected two boards in this mode. Gpio

mode is open drain in each boards.

In the first case I use internal pullup resistor. The transmission doesn't work.

In the second case I use external pullup resister. The transmission works.

could you explain Why ?

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    5 replies

    Tesla DeLorean
    Tesla DeLorean
    Guru
    August 10, 2018

    Some random external resistor value, or something specific? Which STM32 part?

    Nominally they are 30-50K on the datasheet of the part I looked at (STM32H7), very low wattage.

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    pferd
    pferdAuthor
    Associate II
    August 10, 2018

    External resistor is 10K.

    STM32F091CC and STM32F417 are connected.

    Bob S
    Bob S
    Super User
    August 10, 2018

    More importantly, what baud rate? At higher baud rates (>= 115K) even 10K may be marginal. The issue is rise times when the open-drain releases. Depends on board capacitance, etc. The smaller the pull-up value, the faster the rise time and the higher the baud rate you can handle. Look on a scope if you have one and check rise time vs. bit period.

    pferd
    pferdAuthor
    Associate II
    August 10, 2018

    Baud rate is 115200bps.

    pferd
    pferdAuthor
    Associate II
    August 11, 2018

    Bob S thank you for help. You are right. The problem was in the connector between boards. It has a big capacitance and the rise time is slow.

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