At 11:07 AM 6/3/2006 +0530, you wrote:
>Hi all,
>
>What is weak pullup ? The strength 'strong' or 'weak' related
>to voltage level or current level ?
>
>I am using PIC18f2525 and datasheet gives only 'weak' pullup current
>and not resistor value,  Why it is ?.

They are not resistors. They are small p-channel MOSFETs typically
operating in the pinch-off region when conducting, so they are
(relatively) constant currrent sources for small values of output
voltage relative to ground. As the voltage on the output approaches
Vdd-Vth, the current will drop in a nonlinear fashion. The gate of
the MOSFET is switched to ground to turn the weak pullup on, and to
Vdd to turn it off (see port pin schematics in your datasheet).

They are very loosely specified, since process parameters can vary
over a wide range and Microchip doesn't want to throw any chips out
because of pullup current out of spec, so the specs are made loose.

The current is relatively large, partly because of Microchip's loose
port pin leakage specification (as much as 1uA leakage is possible),
and for EMC considerations, but perhaps smaller at the low end than
you'd like in an electrically hostile environment. You can always
add parts outside if you don't like it, it's basically free if you
can use it.

>My another doubt is , Weak pullups are done by resistors or active
>components  ? ( Because, In academics, i have read somewhere , it is
>difficult to fabricate passive components on IC than active components).

Yes, the typical current is determined by the geometry of the MOSFET
for a given process.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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