Donald L Burdette wrote:
>I just got done building my second prototype with a 44780 driver, and I
>swear the first one tri-states only when E and R/W are BOTH low (I tested
>this by measuring the pins as I switched R/W) and the second one doesn't
>tri-state even when they are both low.  The boards and the chips on them
>are exactly the same, except one has a Phillips 22V10 and the other an
>AMD.
>
>Anyone have an explanation?  I'll be working on these boards for the next
>few days, so I'll keep you posted on my efforts.  I'm hoping to find I've
>gone brain dead somewhere, but right now it seems unlikely.
>
>                        Don


Don, you do know the HD44780 has a MOS weak pull-up on almost all the I/O
lines? This means if the input floats (tristated) it will be pulled high.
MOSFET pull-up current is specified at 50 to 250uA with a typical value of
125uA. Try adding a 10K resistor to ground as a test and you should see the
pin "float" about in the middle of the voltage range.

Other thoughts: the data sheet wants R/W to be stable 25 nsec before E goes
high and to stay stable for 25 nsec after E goes low again. The data lines
should go "tristate" (with MOS weak pull-ups always ON) if E OR R/W is low.
E is the only microprocessor side pin which does not have a weak pull-up on
it.

Good Luck, Chip