What to do with unused MCU I/Os

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Guido Tent

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TentLabs.com
maczrool said:
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
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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
 
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maczrool

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Joined 2002
eutronix.com
Guido Tent said:



Stu

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

regards
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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

seangoesbonk

Member
Joined 2002
maczrool said:
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).
Click to expand...

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/suppdoc/design/lowpwr/index.htm
 
M

maczrool

Member
Joined 2002
eutronix.com
seangoesbonk said:


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/suppdoc/design/lowpwr/index.htm
Click to expand...


Thanks for the suggestions and the link - very helpful.

Stu
 
G

Guido Tent

Member
Joined 2001
TentLabs.com
maczrool said:



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
Click to expand...

Do not let them float

1 - Connect them to ground if you want to reduce induced nois

2 - Do above and configure them low to reduce the groundbounce

3 - put them to sleep to do the best you can

regards
 
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