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PICList Thread
'7-segment LCD display'
1998\07\29@173025 by MattA

Good day to the PIC List:

I'm exploring various ways to drive a 2 digit, 7-segment LCD display (LCD
not LED!) and I'm looking for suggestions/comments.

I have looked at the PIC16C92X but consider the part too expensive.

I have considered driving the display with a PIC directly, but am concerned
about adding complexity to the software.  The PIC also has to take input
from a keypad, drive some indicator LEDs (separate from the LCD display),
and address and control an ISD sound chip.  Even with these chores, I think
the PIC could be programmed to also directly drive the LCD display, but I'm
interested in making my life easier if possible.

I have found some driver chips (National Semiconductor MM74HC4543).  The
extra parts cost and PCB real estate shouldn't be a problem and using these
parts should relieve some software complexities.

I also saw on Microchip's latest Technical Library CD-ROM (1998), a "Simple
Circuit" using a PIC12C508 and a 74LS demux chip to drive a LCD display.
No other details are given.

Another one of my objectives is to solve this problem in a way that can be
easily duplicated for the next similar project.  (Maybe the next project
will require more or less digits.)

Has anyone driven a 7-segment LCD display before, and if so, how did you do
it?  (Besides very carefully!)


1998\07\30@112110 by Peter L. Peres

picon face
imho, for only 2 digits, which are not multiplexed, you could go with
4543. The other solution (which I keep advocating <g>) is using CD4094s.
The problem with a LCD is the backplane of course. The 4094s will reduce
the complexity of the wiring on the board, as they are serial, but they
will add processing overhead required to push out inverted data every 20
msec. The backplane pin of the LCD I connected to a free PIC pin that is
toggled together with the STB pulse rising edge to the 4094s. The PIC pin
supplies enough current to have a nice pulse on the backplane even with
all segments 'on'. No problem adding digits, but larger LCDs are usually
multiplexed and it makes sense to use a PIC solely for that.

For larger things, you might want to investigate dumb segment drivers,
such as OKI MSM 6298/99. These will drive up to 80 segments using only 3
(resp. 5) pins and are fast enough for the fastest PICs. They also include
backplane drive arrangements and voltage ladder inputs for muxed LCDs.

hope this helps,


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