What to do with unused MCU I/Os

[maczrool]

I'm building a DAC which uses a PIC18F448. The design is not going to require the use of all the I/Os, so I was wondering what the best practice would be with regards to them. Should they be left floating or grounded? Seems to me it would be far wiser to ground them, but maybe I'm missing something.

Stu

[bocka]

Use a pull-up resistor or better a resistor-network to +Vdd. About 10k, value is not critical. Never left an input port open untill you're shure it got an internal pull-up or -down

[MWP]

Read the PIC manuals... they normally state how unused pins should be connected.

Most newer uC's dont require any pull up/downs.

[daatkins]

I'd connect the spare IO lines to LEDs to help with code debugging: light them when you reach a breakpoint, that kind of thing.

Nice one,
David.

[Guido Tent]

Originally posted by maczrool
I'm building a DAC which uses a PIC18F448. The design is not going to require the use of all the I/Os, so I was wondering what the best practice would be with regards to them. Should they be left floating or grounded? Seems to me it would be far wiser to ground them, but maybe I'm missing something.

Stu

Hi,

Select a PIC that likes to sleep and power it down to sleep mode when unused.

If you want to terminate the pins: Make sure the are logically low, and connect them to ground, as much of them as possible.

regards

[maczrool]

Thanks everybody! I think I'll ground them then as originally planned.

Stu

[Guido Tent]

Originally posted by maczrool
Thanks everybody! I think I'll ground them then as originally planned.

Stu

Stu

OK, but realize yourself that it only makes sense once you make them actively low

regards

[maczrool]

Originally posted by Guido Tent



Stu

OK, but realize yourself that it only makes sense once you make them actively low

regards

Hi Guido,

So are you saying it would be just as good to leave it floating if the pins are not used in the code? I just don't want them wandering all over the place, spitting noise into the system and possibly wasting power (this is a battery powered application).

Stu

[seangoesbonk]

Originally posted by maczrool
So are you saying it would be just as good to leave it floating if the pins are not used in the code? I just don't want them wandering all over the place, spitting noise into the system and possibly wasting power (this is a battery powered application).

You can do one of two things with unused I/O pins:

1. Configure them as inputs and connect them to Vdd or Vss via pull up (or down) resistors, or

2. Configure them as outputs, drive them high or low, (in your software) and do not connect them to anything. They will not "wander".

If you make them outputs and connect them to ground (or Vdd), you could have a big problem if the pin is driven high. (on startup or otherwise)

Edit: Check out the "Low Power Solutions Design Center" at Microchip's web site: http://www.microchip.com/1010/suppdo...wpwr/index.htm

[maczrool]

Originally posted by seangoesbonk


You can do one of two things with unused I/O pins:

1. Configure them as inputs and connect them to Vdd or Vss via pull up (or down) resistors, or

2. Configure them as outputs, drive them high or low, (in your software) and do not connect them to anything. They will not "wander".

If you make them outputs and connect them to ground (or Vdd), you could have a big problem if the pin is driven high. (on startup or otherwise)

Edit: Check out the "Low Power Solutions Design Center" at Microchip's web site: http://www.microchip.com/1010/suppdo...wpwr/index.htm

Thanks for the suggestions and the link - very helpful.

Stu