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'[EE] More On Unused Pins.'
2009\09\22@113526 by Bob Axtell

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Olin and everyone:

Guess what? Microchip recommends resistors to GND on unused outputs in the
DSPIC Datasheets (rummaged thru DsPIC33FJ64/710 or something like it).

First time I ever saw that. I think I owe the original OP an apology.

--Bob A

2009\09\23@085304 by olin piclist

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Bob Axtell wrote:
> Guess what? Microchip recommends resistors to GND on unused outputs
> in the DSPIC Datasheets (rummaged thru DsPIC33FJ64/710 or something
> like it).

Hmm.  These are the same people that once published a app note for sensing
AC line voltage by tying it to a PIC pin via a large resistor.  Microchip
datasheets are very good and the limits listed in them should be carefully
followed.  However, I wouldn't look to Microchip for good design advice
beyond that.  I've seen too many irresponsible or downright bad examples.

If you can find a reason for tying the resistors to ground via a resistor
from the datasheet specs, then you've got my attention.  Otherwise, this is
likely mere superstition by whoever wrote that document.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\09\23@155354 by cdb

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face


:: If you can find a reason for tying the resistors to ground via a
:: resistor
:: from the datasheet specs, then you've got my attention.  
:: Otherwise, this is
:: likely mere superstition by whoever wrote that document

I don't have it to hand at the moment, but I do recall uChip produced
a low power consumption application note that required all unused pins
to be tied to a supply node, and IIRC correctly, there was a power
consumption difference between tying high and tying low, don't think
resistors were mentioned though.

Colin
--
cdb, spam_OUTcolinTakeThisOuTspambtech-online.co.uk on 9/24/2009

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2009\09\23@160343 by olin piclist

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cdb wrote:
> I don't have it to hand at the moment, but I do recall uChip produced
> a low power consumption application note that required all unused pins
> to be tied to a supply node, and IIRC correctly, there was a power
> consumption difference between tying high and tying low, don't think
> resistors were mentioned though.

Inputs definitely need to be held full high or full low for minimum power.
A CMOS input stage draws supply current when its input is not solidly high
or low.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\09\23@164010 by Barry Gershenfeld

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> Guess what? Microchip recommends resistors to GND on unused outputs in the
DSPIC Datasheets

I had this vision of a thousand voices asking "What do we do with our unused
pins?".  And one day Microchip realized, "You know, if we just pick one
answer and publish it, everyone will leave us alone".  Because, really, any
answer is sufficient for 95% of designs.  And because of the conflicting
criteria, there's never going to be One Right Answer.  If someone is worried
about ground bounce, or high RF fields, or power consumption, maybe there's
a special answer for them.  The rest of us?  Just pick one and get back to
work!

I wasn't going to say anything but then I saw others had doubts, too.

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