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'DTMF sample code to LCD'
1995\11\09@103803 by David G. Schmidt

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Check out the Sept 95 issue of Popular Electronics.  Terry Weeder of
Weeder technologies has done an article and sells such a unit cheaply,
kit form.  He has
interfaced a PIC to a serial EEprom and an lcd display with scrollback
buffers.  This issue is within the last year.  He used a 4 bit binary
output DTMF chip, but his code can be modified for your 2 of 8 output
chip.  BTW, does the chip have a selector for 4 bit binary out or 2 of 8?

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( Dave Schmidt       DSchmidt Technologies )
( spam_OUTdschmidtTakeThisOuTspamrain.org     Ventura, CA        )
( Own an FME? ->finger .....dschmidtKILLspamspam@spam@rain.org   )
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On Tue, 7 Nov 1995 John.P.HollingsheadspamKILLspamHAPPY.FIRSTNETHOU.COM wrote:

> I'm looking for code, pinouts etc for LCD matrix displays.

1995\11\09@130508 by David Brenegan

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I was wondering if you could modify his design to use a 16c84
and have the dtmf chip and the lcd use some of the same
data lines and seperate enable lines? Does this seem possible
or would you potentially lose some dtmf tones comming in?
(I'm new at this stuff....).

Thanks.
_David

1995\11\10@023556 by adrian

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> I was wondering if you could modify his design to use a 16c84
> and have the dtmf chip and the lcd use some of the same
> data lines and seperate enable lines? Does this seem possible
> or would you potentially lose some dtmf tones comming in?
> (I'm new at this stuff....).


Hmmm.

Typical LCD - 4 data lines, R/W, E, RS       total 7 lines
Typical DTMF - 4 lines only, maybe five      lets say 4 lines
Serial EEPROM, clock and data                total 2 lines
                                             13 lines = 16C84

You could probably make it work without too much hassle.
I guess control control input keys, but DTMF could be used for this also.
You can use the EEPROM clock without a start condition as one of
the LCD lines (say RS) to free anothr line. This could be used with a diode
on the 4 LCD data lines to give a 4 key keypad.

piece of cake <-:

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1995\11\10@055356 by Mike Keitz

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>I was wondering if you could modify his design to use a 16c84
>and have the dtmf chip and the lcd use some of the same
>data lines and seperate enable lines? Does this seem possible
>or would you potentially lose some dtmf tones comming in?
>(I'm new at this stuff....).
>
If the DTMF chip has an enable pin, then it can be used to share a bus with
the LCD, serial EEPROM, and other chips assuming they have enables as well.
Each chip needs a dedicated line back to the PIC to select it.  In a larger
system a decoder like a 74HCT138 could be used (unfortunately, the LCD,
93CXX, and most DTMF chips have active high enables, so an active-low
decoder wouldn't work well).  Data will not be lost since it only takes a
few microseconds to access each chip and go to the next one.  Also, many
DTMF chips latch the last tone code received until another one is detected.
I usually use the LCDs in the 4-bit mode to save PIC pins as well.

Using a bus like this, with none of the peripheral chips selected the PIC
pins can be used as inputs by driving them through resistors.  This could be
used for the 'tone detected' function from the DTMF chip as well as any user
controls that the project has.

Another note about this project, the HD44780-based LCD units have 80 bytes
of display RAM and 64 bytes of custom-character RAM, regardless of the size
of the screen.  Especially if the display is smaller than 80 characters, the
extra 128 bytes or so of RAM could be used for (volatile) scroll-back
storage instead of a serial EEPROM.  The specifications I have don't say how
much power the display units use with the display disabled though, it may
not be practical to 'sleep' them and keep the RAM alive if the
decoder/display is expected to retain the data for a long time.

-Mike

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