Allen Mulvey wrote 2013-04-03 19:46:
> Sorry, this is my first post. I forgot to tag the subject
> line.
>
>
>
> Is there a way to compute the optimum value of pull up (or
> pull down) resistors on PIC digital inputs?
>

No, there is not, for the general case. For specific cases
(such as pullups for I2C lines) there is. But we don't know
anything about your specific case. It could be a unusual
noicy environment. :-)

> Most example circuits I have seen seem to use 10k. My
> EasyPIC7 board uses 4k7.

Both are popular "junk-box" and "lab-set" values. :-)

> I have been experimenting and 20k
> resistors produce unreliable results.

It should work perfectly well, in the *general* case.
We don't know what was special in your specific case.

> I don't see anything
> in the data sheet to help with this.

No, there can't be since "it depends" on things that
Microchip has no way to know in advance.

>
>
> I am using a pic16f877a. The leakage current is one
> micro-amp.

Note that for a modern PIC (16F1938) it's 5 nA (typ)
and 125 nA max @ 85 degC. It's time, not only for
this reason, to take a 15 year leap forward. :-)

Anyway, 100 kohm at 1 uA only give an voltage drop
of 0.1 V, so that isn't an issue.

> Sampling the port must draw considerably more.

What is "sampling the port" ? Reading from the port?
Reading the PORTx register doesn't "draw" anything.

> My current project is not battery operated but, if it were,
> I would like to use the largest resistors possible.
>

Without knowing anything else about the curcuit, I'd
say that 100 kohm would not be any problem. Probably
several 100's kohm also. On a modern PIC one could
probably use a few Mohm's. Not in a noicy environment,
but in the general case.

Then you have to add any specifics about your application
that we can't know anything about into the equation.

Best Regards,
Jan-Erik.


>
>
> Thanks,
>
>
>
> Allen
>
>
>
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