> Is there a way to compute the optimum value of pull up (or
> pull down) resistors on PIC digital inputs?

Hi Allen,

yes and no, selection is going to depend a bit on the circuit. In theory
the value will have to be between a value which overcomes the pin
leakage current (1uA) by some margin and a value which does not
let the pin exceed Imax (20 or 25mA)

In practice though, a pull-up of several hundred k could leave the
pin susceptible to noise and a pull-up of a few hundred is going to
be wasteful of power

However, there is a guide in the datasheets

PortB pull-ups are often defined as 400uA when Vpin =3D Vss.
Assuming Vcc =3D 5V, this equates to a resistance of

V/I =3D R or 5,000,000uV/400uA =3D 12500 ohms

So that's what Microchip has decided is a generally useable pull-up

Joe
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