Barry Cooper wrote:
>  Myke, I have a copy of your book 'Programming and Customizing the PIC
>Microcontroller'. Is there any specific reason why you chose 4.7k resistors
>for pullups? I tend towards using 10k and even up to 1 meg depending on
>what I am doing and how big the battery is.
>
>  Thanks
>  Barry

Hi Barry,

It's pretty arbitrary; I use 4.7K resistors because it simplifies worst case
current calculations.  5 Volts across 4.7K will pass roughly 1 mA; after
adding up the currents of all the digital components, count the number of
resistors and multiply by 1 mA.

When using Pull-Ups with the PIC, the only consideration I would make is to
choose a value that will limit the current sufficiently to prevent the I/O
pins from being in an overcurrent situation.  With this criteria, anything
over 50 Ohms (the PIC Pins can source/sink roughly 20 mA) would be okay with
the PIC.

Personally, I would keep in the 1 K to 10 K range because anything greater
than that could have problems with the input capacitance of the pins (the RC
time constant would grow so large as to affect the responsiveness of the
circuit).  Anything less than this will start having some serious current
(power) requirements.


I'm passing these comments out to the PICList.  Does anybody else have
anything else to add?

myke

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