STM32F40G UART, unexpected pulse on Rx Line

[IAhma.1]

I am sending some data on UART, When I enable Transmitter TE bit in USART_CR1 register, a pulse is observed on Tx line ( as shown in figure). That's fine but I am observing some 190nsec pulses on Rx lines though I am not sending anything to it! I have simply connected Tx/Rx lines to logic analyzer.

What is possibly causing these pulses?

[TDK]

How is the RX line connected? It is high impedance on the STM32 side, so would need an external resistor or be driven externally. Probably what you are seeing is noise crosstalk between lines. An analog capture would likely show better info.

UART is typically idle high. Is yours intentionally idle low?

[IAhma.1]

I have simply connected Rx/Tx lines to saleae logic analyzer via jumper wires. Nothing else.

 

UART is typically idle high. Is yours intentionally idle low? => I don't know what is means and how to make lines low. Please guide me on this as well.

[IAhma.1]

Yes, You are probably right. I reduced jump wires and unexpected pulses are gone. thanks you.

Just needs further information on: UART is typically idle high. Is yours intentionally idle low? 

[TDK]

> I have simply connected Rx/Tx lines to saleae logic analyzer via jumper wires. Nothing else.

Well then the line is floating and any measurement on it is meaningless. You need an external pullup or pulldown. Typically this would be the job of the transmitter, but you don't have one connected to this line.

> I don't know what is means and how to make lines low. Please guide me on this as well.

Since you're connected to the RS232 connector and not directly to the pin, this is likely being done on the board.

[IAhma.1]

Thanks for you kind response... Just wanna know one thing...

If we disable Tx / Rx (not complete UART), will it save any power?

[TDK · Accepted solution]

That is not something that is specified in the datasheet. I would not expect it to have a noticeable effect on power consumption.

[IAhma.1]

Thank you so much @TDK​ ..:grinning_face:

I wish there was button to mark your reply as accepted answer.

[TDK]

You're welcome. You can mark a reply as the accepted answer by clicking on "Select as Best".

Edit: I am guessing on the power consumption issue. If it's something that is important for your application, you should measure it.