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'[PIC] How to control a dumb LCD'
2000\06\05@120522 by Lea

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Hi People of the list

I'm trying to control an LCD 3 1/2 digit display, this one has no controller,
the main reason for use this, is the cost, about 3$ each.
the data sheets says that it works with 30 - 150 Hz AC 5V ps, so I build
an small oscilator and test the display, all Ok, It works, but how can I
control it with the pic without a dedicated chip 4 it.
If I connect the oscilator to the common lcd pin and all the
segments to a PIC, all segments will lite on, doesn't matter if the ouputs
are high or low because it is AC.
So, the question is: It's possible to turn on/off the segments setting the
tris registers???, so if the pin is input the segment should turn off.
I don't know if the small leakeage of the pic's input is enough to turn on
the LCD segment because they need almost no current to lite on.

Also I was reading, (don't remember where) that an intermediate voltage
on a pic's digital input will cause an increment of the pic current. is it
true?, if almost all ports are connected to the LCD, it will be dangerous to
turn all as inputs?.

I know that what I am trying to do is quite weird but if it works will be
nice :-).

Thanks.
  Leandro J. Laporta (LU2AOQ)     mail: spam_OUTlu2aoqTakeThisOuTspamyahoo.com
wrk: Arg. Assoc. for Space Tech.   ham: TCP/IP high speed group HSG


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2000\06\05@134636 by Jason Wolfson

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face
Direct LCD drive with a PIC is real easy.
Attached one output pin to the LCD common. Drive the LCD segment inputs
with more PIC outputs. Setup an interrupt to run at about 50 hz. At each
interrupt
switch (invert) the common state and the segment states.
Make the segments you want on, the opposite of the common state.
The segments you don't want on, make them the same as the common state.
This will give you AC across all the LCD segments, lighting the ones you
want.

For 2:1 or 3:1 multiplexing put a resistor divider across the commons. For a
high,
drive it high, for a low, drive low, for a mid voltage (2.5 typ), make the
pin
an input.

With a 16C62 and 22 I/O's, you can direct drive a 3:1 mux LCD with up to 57
segments!

Jason Wolfson

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> {Original Message removed}

2000\06\05@170505 by Harold M Hallikainen

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       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
parts!
       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
permanently black).
       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.

Harold



On Mon, 5 Jun 2000 01:02:46 -0300 Lea <lu2aoqspamKILLspamYAHOO.COM> writes:
{Quote hidden}

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2000\06\05@184335 by Bob Ammerman

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LCD's require an AC waveform. There is no reason that the PIC cannot
directly generate this waveform. Just tie one PIC output pin the common, and
other pins to the segments. You toggle the common pin periodically, and each
time you do so you also toggle all the segment pins (try COMF PORTx,F for
example to do 8 at a time).

To turn a segment on you just make sure that it is toggled 180 degrees out
of phase with the common. To turn it off you toggle it in phase with the
common.

Alternately, you can run each segment line thru an external XOR gate, the
other input of which is connected to an oscillator. This is much simpler
from a code standpoint, but requires more hardware.

Bob Ammerman
RAm Systems
(high function, high performance, low-level software)


{Original Message removed}

2000\06\05@210218 by Brian Kraut
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There is a Microchip app note on driving raw LCD displays.  Don't have the number
offhand.  I can look it up for you if you can't find it.

Lea wrote:

{Quote hidden}

2000\06\10@032749 by picxpert

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As I recall, you need to invert the wave that you're putting in through the
common terminal, and use THAT to drive the segments.

-Randy Glenn
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