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'[PIC]: Weak pullups'
2000\12\14@115822 by David Dunn

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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.


Thanks,

David Dunn



On Thu, 14 Dec 2000 11:06:45 -0500, Dan Michaels wrote:

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2000\12\14@144418 by Dan Michaels

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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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2000\12\15@100510 by J Nagy

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Andrew McDermott wrote:
>
>> Being a research technician I understand and use pull up resistors.
>> I am however confused by the 16F84 data sheet using the term "weak
>> pullups" on the I/O port pins.  Can somebody explain to me please what
>> qualifies a pullup circuit as "weak?"
>>

       I believe that the term 'weak pullup' comes from the fact that the
'resistors' are actually transistors connected between Vdd and the input.
The trouble is that if standard output transistors were used, they would be
too 'strong' and would require high current drivers to overcome. By
purposely varying the geometry/doping, the transistor can be made to be
'weak', and the input logic level varied easily.
       The term 'weak' is of course relative. Some of Microchips for
example effectively provide a 300K resistance, while others are in the
ballpark of 22k.




       Jim Nagy
       Elm Electronics
 ICs for Experimenters
http://www.elmelectronics.com/

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