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Thread: Weak pullups
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face BY : Dan Michaels email (remove spam text)



DAvid Dunn wrote:
>By this defintion, I'd think it would be fine to run as a pullup on logic
stuff.
>
>Somebody else in the list didn't think it would work for that .. any way we
can get a "for sure" on this ? It's working in my circuit,
>and i'm low on space so i don't want to have to use an external pullup if i
don't have to, but need to know if the internal pullups are
>ok to bring up the output of a COMPARATOR.
>


David, I'll wager a guess here and say I think you are ok.

Looking at the schematic for PortB in the d/s, you see the
weak pullup is implemented as a PMOSFET tied to Vdd, and in
that respect is just like any other hi-side PMOSFET in a CMOS
gate - EXCEPT that it has very small "channel" dimensions,
and can only push 250uA versus 25mA for the normal PMOSFET.
And which means it acts much like a 20K resistor in the "short
circuit" situation.

CMOS gate outputs are normally looked at as being a resistance
voltage divider with the hi-side and lo-side resistors [ie,
MOSFET channels] being variable, and "alternately" going from
hi-R to lo-R as the output swings hi and lo.

Personally, however, I never use the internal pullups as I
generally like to use smaller values for better noise
rejection [ie, lower R --> less noise pickup].

hope this helps,
- dan michaels

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