Pullup resistor for unused pins of a PIC microcontroller?

[treez]

Any unsed, and unconnected pin of an 8 bit PIC microcontroller should be declared as an input and tied to Vdd with a 10k resistor , is this right?

This thread says a pullup resistor must be used if the MCLR pin is set to an input, but what about other genreal IO pins?
https://www.edaboard.com/showthread.php?366639-RC-circuit-added-to-MCLR-pin-of-PIC18F26K20

 

[betwixt]

No, it's wrong!

MCLR is an 'input only' pin on most PICs and it doesn't have an internal pull-up current source simply because it usually doubles as the programming voltage input. It NEEDS a pull-up OR to be actively driven for it to work

Unused pins should never be left floating. The rule is 'if it can be programmed as an output, drive it low', otherwise pull-it up or down. If you can only program the unused pins to be inputs, tie them together and use a single pull-up or pull-down for them.

Brian.

 

[treez]

Thanks, a writer of the following (answer number 3) says that making it output and driving low is an EMI problem, ..presumably they mean EMI getting into the micro and messing it up?

https://electronics.stackexchange.c...-gnd-or-use-software-to-prevent-floating?rq=1

 

[KlausST]

Hi,

Setting an unused pin as output_low is uncritical.
No EMI problem.

Klaus

 

[betwixt]

Agreed, it's a strange claim to make since presumably, although not essentially, the pin would carry no signal.
You could of course make use of the pin(s) by taking them to test pads and outputting some diagnostic data, for example part of a checksum of the internal program ROM or maybe an indication that some internal process is underway.

I often use 'spare' pins as diagnostic aids, for example toggling them as serial data bytes are been received so I can see the UART is working, even if the rest of the program needs debugging.

Brian.