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

[pferd]

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 ?

[Tesla DeLorean]

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.

[pferd]

External resistor is 10K.

STM32F091CC and STM32F417 are connected.

[Bob S]

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]

Baud rate is 115200bps.

[pferd]

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.