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'How to Setting Unused Pins ver VERY low power apps'
1999\06\11@152543 by William K. Borsum

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Greetings All:
Today's Question
What is the best way to treat unused pins on a PIC for absolutely lowest
possible power consumption? (and the theory behind your choice please).

Options are:    output, drive hi, no termination
               output, drive lo, no termination
               input with pullup
               input with pulldown
               input floating (not really an option--pin will oscillate sometim
es)

Some thoughts from my end:
1. Inputs require a pull-up or -down resistor, possibly in the megohm
range, but still some current flow, plus an extra part.
2. Outputs don't require external parts, but with big ground planes and
potential board leakage, driving low might be the best.  However, most of
the programmers I have used tend to drive the pins hi.

So What is the consensus?
kelly
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All legitimate attachments to this email will be clearly identified in the
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William K. Borsum, P.E.
OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>

1999\06\11@153210 by Jamil J. Weatherbee

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I talked to microchip tech support at one time about this, there really is
no concensus, because if your device is held in reset all the pins are in
tristate so i suppose it could draw lots of power.

The most economical, and safe from a software perspective seems to be just
configuring them as outputs.  Next to that probably putting a 100k
resistor to ground and configuring as outputs with latches low.  The 100k
resistors take care of having a tristate input during reset/input during
startup.



On Fri, 11 Jun 1999, William K. Borsum wrote:

> Greetings All:
> Today's Question
> What is the best way to treat unused pins on a PIC for absolutely lowest
> possible power consumption? (and the theory behind your choice please).
>
> Options are:    output, drive hi, no termination
>                 output, drive lo, no termination
>                 input with pullup
>                 input with pulldown
>                 input floating (not really an option--pin will oscillate somet
imes)
{Quote hidden}

1999\06\12@022030 by Ernie Murphy

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Another 2 cents worth...

 The project currently on my bench will have such an unused input in some
configurations, hence I've been musing about this. Here are the 6 (well, 8)
choices:

1) input, no resistor:  Bad, bad, very bad. Oscillation isn't really the
problem here (who cares how an unused pin changes), but the possibility it
will drift to Vcc/2 and hence turn on 2 fets from rail to rail is the scary
thing.

2) input with pull-up: OK, make the pullup the largest value you dare. Megs
would be OK in most enviroments. Dissipation will be practically zero.

3) input with pull-down: same as pull-up.

4) output no resistor: zero component count. Again no disipation for either
output state. Only danger is if the board is somehow shorted pin to pin. If
this is possible, perhaps one would use the same hi or lo state as the
adjacent pins to minimize uncontrolled short current.

5) output with pulldown: Adds a component for no reason. A high value
resistor will of cource be chosen... which is in parallel with the low
impeadance output stage... in the equivalent circuit the resistor drops
out. So why use it?

6) output with pullup: same as #5

I'm of course assuming a pulled-up output will be driven high, and
pulled-down driven low (ah, there are numbers 7 & 8). To cross these is to
just waste power. Incidentally, pulling up a high output will disipate zero
current, as the output only has its .7 drop from the rail IF there is
current being pulled out of it. No current, no drop. High outputs source
current, not sink it.

My choice is #4, output with no resistor. Since my free pin is next to the
ground pin, thats where I nailed it... output in the zero state.




At 12:24 PM 6/11/99 -0700, you wrote:
>Greetings All:
>Today's Question
>What is the best way to treat unused pins on a PIC for absolutely lowest
>possible power consumption? (and the theory behind your choice please).
>
>Options are:    output, drive hi, no termination
>                output, drive lo, no termination
>                input with pullup
>                input with pulldown
>                input floating (not really an option--pin will oscillate
sometimes)
{Quote hidden}


'How to Setting Unused Pins ver VERY low power apps'
1999\09\30@103321 by Larry G. Nelson Sr.
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Output drive High or Low depending on what they are attached to. I just did
a project where we were using the pin as an input to look for a slow
transition (analog ramp) and the power consumption was much higher when the
port was an input. I set it to output for lower power and switched to input
to test then back to output to reduce power.


At 12:24 PM 6/11/99 -0700, you wrote:
>Greetings All:
>Today's Question
>What is the best way to treat unused pins on a PIC for absolutely lowest
>possible power consumption? (and the theory behind your choice please).
>
>Options are:    output, drive hi, no termination
>                output, drive lo, no termination
>                input with pullup
>                input with pulldown
>                input floating (not really an option--pin will oscillate
sometimes)
{Quote hidden}

Larry G. Nelson Sr.
EraseMEL.Nelsonspam_OUTspamTakeThisOuTieee.org
http://www.ultranet.com/~nr

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