On Fri, 6 Nov 1998, Steven Kosmerchock wrote:

> Friends, I am using an LCD (2X20) to display some results of a test
> performed by a PIC16C65A. The PIC performs all of it's tests and
> display's the appropriate data for the appropriate tests. Unfortunately,
> the first two spaces on the top line seems to have an incorrect
> character (It's supposed to be blank). The funny thing is that when I
> unplug the LCD from it's socket (leave the PIC circuit powered up) and
> then plug it right back in, the "trash" characters aren't there anymore.

The LCD needs to be initialized according to the data sheet sequence, or
reset by a fast power on flank. When you remove it from the socket and put
it back you do just that. The power on flank for the whole circuit is much
slower.

If you don't want to use the whole funky initialization sequence, use a
transistor switch to turn on the LCD power abruptly from a PIC pin or
from the reset chip that drives the PIC (if any).

> But if I turn off the whole circuit and then restart it, the "trash"
> characters are still there. It's a mystery to me!? I would appreciate
> any/all help I can get!! Thank you all!

The HD47xxx chips are genuine CMOS parts and as such they will keep the
bit patterns for several seconds or minutes without power applied (using
the very low voltage remaining in the decoupling caps). It is known that
some CMOS timekeeping chips need to be shorted without power applied for
up to 5 minutes to convince them to forget some of what they contain.

If you use the initialization sequence with the LCD it will be erased
properly by the time it lights up.

imho the missong reset pin from the industry standard LCDs is a pain in
the rear.

Peter