UART Question (Pull-up configuration)?

RE: UART Question (Pull-up configuration)?

Jared — Thu, 04 Nov 2021 08:24:14 GMT

The assumption in our examples is that it's indeed connected to something. Makes sense to use a pull in your use case.

RE: UART Question (Pull-up configuration)?

eiadj — Wed, 03 Nov 2021 18:30:49 GMT

If there is no internal pull-up on the serial Rx line, how do you prevent communication errors (breaks and framing errors) on the Rx line when the serial port is left unconnected? In some of our implementations, we do not know if the serial port is connected, not connected, or will be connected at some later time. So, disabling the UART when not in use, is not a viable option. For these situations, what do you recommend to prevent these communication errors from occurring?

RE: UART Question (Pull-up configuration)?

Jared — Tue, 02 Nov 2021 11:03:43 GMT

Hi,

I don't think it's normal to use a pull-up on the RX pin as it's expected that this is driven by the UART instance on the other side of the serial line. You can modify the driver as you would like, setting a pull-up on the RX line should in most cases be fine but there is a couple of consequences:

You will see a higher current draw during UART activity as the current will flow from VDD through the resistor and down to GND.
The rise/fall time might be longer, which can result in errors or unstable communication on very high frequencies. This depends on the TX drive strength of the UART instance on the other line etc.

regards

Jared