nucleo_g474: mysterious bytes received on UART Rx

[pasuzw]

Hi Guys

I am doing a project and I notice that if I unplug the UART cable(FTDI) and then connect it again, there is a chance that there would be mysterious bytes received from RDR.

What is more interesting is that the mysterious bytes are actually the messages that was sent out to UART Tx.

During the testing, I am not using any UART serial monitoring tool like Putty or what's so ever, so there should not be any inputs to the UART Rx.  But it just happened, not often, but if I repeat disconnect and reconnect dozens of times, then something from UART Tx got into the UART Rx buffer.

I have confirmed that those mysterious bytes are read out from RDR.

Has any one know what's happening ? 

Thanks.

[pasuzw]

Is it a RX+GND disconnection, or is it just the RX wire ?

-> I disconnect the usb port of FTDI C232 HD cable on the computer side.

[pasuzw]

I am using FTDI C232HD, and it has 3 seperate wires to connect to STM32 pins.   I disconnect UART by unplug the USB connecter on the computer side.

[TDK · Accepted solution]

But unplugging the USB doesn't disconnect the device from the STM32. The RX pin is still connected to whatever is on the adapter board, along with TX and GND.

As its power starts to drop, it is still trying to drive that pin but VDD isn't 3.3 V anymore. That's what you're seeing. Eventually, the chip powers off and the pin goes high impedance.

[pasuzw]

Yes, you are right.   I tried again by directly removing the Rx pin cable with the stm32 board.  The internal pull up is working good, no voltage drop. Thank you guys your insight.