What to do with unused serial pins?

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1

Hi,

I am building a project around an Atmega328 which uses THREE modules that only have serial communications. One of the modules is connected to the hardware serial pins and the other two use software serial.

Only one of the modules requires bi-directional communications (RX/TX) while the other two modules only receive data / instructions from the MCU.

The project involves RF at VHF and UHF frequencies so I'm a little concerned about leaving the two unused serial pins just free floating.

I've read around and the info I can find suggests that the unused pins could be pulled high via a pull up resistor which would be classed as in their "idle" state?

As it is, the RF from my circuit has caused me some trouble so I've had to put a 1K series resistor on all the data lines in use and then decouple them at each end (MCU end and Module End) with 0.1uF caps to GND (RC filter style circuit I think it is) to try and keep the stray RF / capacitance under control.

It wasn't too much of an issue when I just had two modules that received data from the MCU but now I've got a third module to add which requires data passing to and from the MCU, I can't afford any glitches or conflicts.

So what should I do with the unused RX pins on the MCU (which are the TX pins on the module)?

Also what about any unused pins on the Atmega? I've read people say pull them low, others say pull them high and another school of thought that says Atmel leave them free floating so I'm not sure there either....

Thanks in advance!

2

The only thing that I know about it is that floating inputs will increase power consumption due to the constant switching. So that does not sound like an option to me.

Internal pull-ups would be my choice.

3

Thank you for replying. I will give that a try and see what happens.

Thanks again!

4

pi_and_chips:
I've read around and the info I can find suggests that the unused pins could be pulled high via a pull up resistor which would be classed as in their "idle" state?

You have the hardware in front of you, so you could just check.

With the softserial to the device initialised, but with no comms active, what is the idle state of the pins ?