Palaniappan C wrote:

> What is weak pullup ? The strength 'strong' or 'weak' related 
> to voltage level or current level ? 

You answer your own question below.
Strong or weak is relative to what is needed, based on transition
frequency. What is adequate at low frequencies is not sufficient
at high transition rates.

> I am using PIC18f2525 and datasheet gives only 'weak' pullup current
> and not resistor value,  Why it is ?. 

Because it's a CURRENT.

> 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).

Correct.
So they fabricate a current source that gets turned on to enable
'weak pullup'.

R

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