Termination of Rx pin?

Guest · Posted: Mon Apr 20, 2009 2:29 pm

I'm working on a desgn that implements a serial port for configuration and parameter updating. Since this feature is seldom used, the board will not have a MAX232 interface IC on it, but simply a header with the required signals. An adapter cable with a MAX232 will be used with the board when required.

My question is what, if any, termination do I need on the Rx input to the PIC? Should I include a 10K pulldown to avoid noise pickup?

Chas

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Guest · Posted: Mon Apr 20, 2009 3:09 pm

Hi PCM,

Meaning what then? It's OK to leave floating, or I need to add a pull-up instead of a pull-down?

Chas

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Put a 10K pull-up on it.

Guest · Posted: Mon Apr 20, 2009 5:42 pm

Hi PCM,

You made the distinction of a hardware UART, so does that mean that a software UART input would require the pull-down?

Chas

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The software UART can be configured for normal or inverted mode
(with the 'INVERT' paramter in the #use rs232 statement).
If the inverted mode is used, the idle state is a low level and a
pull-down resistor would be used on the Rx pin.