On 30 Mar 2017 at 8:48, alan.b.pearce@stfc.ac.uk wrote:

> If you are really after low power then you don't want any pull-ups.
>=20
> Default state is set port to output with all pins at 0 (assuming
> switches are to ground). When you need to read it set all pins to 1,
> do read and return pins to 0. Yes the output FETS will see short
> circuit current for about 5us while you do the read, but the average
> of that with 0 current over a significant period of time will be
> much less than even the internal pull-ups.=20

I like this idea. Asbolute minimal parts, high noise immunity, power consum=
ption=20
great so long as not reading too often. PIC's are generally OK with brief s=
horts on=20
outputs, though for complete confidence you'd want to establish that doing =
so=20
doesn't take anything outside of datasheet specs.

Variations on the theme include combining with internal or external pull-up=
s and=20
setting idle output states hi/lo to match switches after reading them. Smal=
l delay=20
before reading to give pull-up time to overcome capacitance.


--=20
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