I am sharing LCD I/O pins with inputs from key switches on an SX=20
chip. The keys can only pull down an input through diodes when=20
it's common connection is set low by an output (key enable) and=20
the internal clamping resistors are used to pull the line high=20
when the switch is open.

One problem I had with this was that it sems that the LCD I/O=20
pins has a pretty high capacitance. If I read the keys right=20
after I had written a zero to the LCD input and the key switch=20
on the same line is open, the clamping resistors was sometimes=20
too weak to pull the (now input) line high quick enough. This=20
resulted in that the switch would sometimes be read as closed=20
even though it was open. The solution was simple though - write=20
a one to the I/O line for a short time, just before the keys are=20
read. It took a while to debug since LCD updates (writes) is not=20
synchronized with key switch reads.

Ruben

> I think about sharing    PORTB RB4:7 for hd44780 lcd, 4 DAC's
> data/clk, not enable and not load pin.
>=20
> I think it should fine. But if it is normally people doing and any
> disadvantages?
>=20
> BR,
>=20
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Elektronik
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

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