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STM32F401 UART pull up to 5V on RX

Question asked by Danilotto Runi on Apr 10, 2018
Latest reply on Apr 12, 2018 by Danilotto Runi

Hi all,

 

Checking a schematic of an RX485 transceiver connected to STM32F401 UART, I got into this detail that captured my attention:

 

 

The RX line is directly connected to the RX pin of UART1 of the the MCU and it is pulled up to 5V, there is not level adapter between.

The UART pins on the MCU are 5V tolerant but, checking into the configuration settings on STMCube configurator I found out that RX and TX pin have the pull-up settings activated and it is not possible to modify this setting (at least in the configurator...

 

pulling up to 5V externally the line could be ok because the pin is 5V tolerant, I did it previously on I2C lines but they were configured as open drain.

 

What does it happen if I apply 5V to a PIN that is internally pulled up to 3.3V? Does the MCU is damaged?

Is it possible to configure the UART pins as open drain or disable the pull-up?

 

Thanks

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