How to control a dumb LCD
Harold M Hallikainen email (remove spam text)
These LCDs require you drive the segments either in-phase (segment off)
or out of phase (segment-on) with the back plane signal (5 Vpp square
wave, 30 Hz or so). Years ago I did this with an ICL7211. It's a 4 digit
BCD to 7 segment decoder/LCD driver in a 40 pin package. The BCD is
multiplexed (4 bits plus digit select lines). The other standard approach
is to use exclusive or gates on each segment, though this takes a lot of
As I recall, Microchip has a chip with an LCD driver in it, though I
have not used it.
Using your TRIS trick would work for a while, but when you enable the
output driver on the pin and then hold the line high or low, you are
putting 2.5Vdc across the LCD, and it doesn't like DC (they turn
If you don't want to use an LCD driver, I think you'll have to go to an
interrupt routine that generates the backplane and segment drive.
Hoepfully the interrupt response time is regular enough that you will not
have a DC component in the drive so the LCD will last.
On Mon, 5 Jun 2000 01:02:46 -0300 Lea <YAHOO.COM> writes: lu2aoq
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