I've done it and it works fine. If you were driving an HD44780-type LCD
with internal pullups on some pins I'd suggest swapping and using
pullups instead. But since you're driving a shift register then
pulldowns are a better choice for this configuration, you chose the
better way already.

You could probably go higher on the resistor values if you wanted more
immunity, but no need to.

Cheerful regards,

Bob

On Mon, May 13, 2013, at 09:58 AM, Justin Richards wrote:
> Just wanted to run this past the wisdom of the list.
>=20
> Port Pins RA4 - RA7 on a 16F887 are successfully driving a standard 2 x
> 16
> LCD (via a shift
> register).
>=20
> Want to add 4 push buttons (ENTER, EXIT, UP,DOWN).
>=20
> Possible solution
>=20
> use weak pull downs approx 4.7K from port pin to ground.
>=20
> use 680ohm from port pin to N/O push button then  to +V.
>=20
> PIC should still be able to drive pins high or low as needed to control
> LCD
> even when switch is pressed.  Quick test seems to indicate this is ok.
>=20
> Current when PIN driven low and switch is pressed, I =3D V/R =3D 5/680 ~ =
7 mA
> (+ LCD and shift reg loading)
>=20
> Is this plausible.
>=20
> Cheers Justin
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