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PICList Thread
'?Utilization of Calculator LCD displays?'
1998\02\10@150557 by Thomas M. Alldread

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Greetings:

       I wish to utilize the display in a low cost calculator for a
micro-controller project to provide output to the user. The cost of a
calculator can be much less than the cost of a new LCD display module with
similar size characters. If I can successfully adapt the display then I may
look into the possibility of also utilizing the calculator keyboard to
complete the user-machine interface.

       I purchased an inexpensive (about $5) solar/battery dual power desktop
calculator for exploring the possibilities of using the bold 0.5 inch 8
character display as the readout for my project. The characters are in the
solid bar 7 segment format as opposed to the dot matrix type. I am hoping
that I will be able to interface the micro controller to the display via a
standard multi-character 7 segment LCD driver chip.

       I wonder if someone in this list that has made past attempts at adapting
similar displays for micro controller projects or has general knowledge
about typical calculator LCD displays?

       My initial examination of the display with my scope revealed the followi
ng
information:

       1) The display is connected to the calculator PCB via a 28 lead ribbon
cable.

       2) Battery positive supply is connected via a 1 K resistor to lead 1.

       3) There is a 120 Hz symmetrical square wave clock signal applied to lea
d
# 3.

       4) There are 3 phases of sub harmonic 40 Hz 1/3 duty cycle clock signals
applied to leads 2, 27 and 28.

       5) A 40 Hz symmetrical square wave is applied to the remaining leads. Th
e
relative phase of these signals vary with respect to the clock depending on
the particular characters displayed.

       Does anyone on this list know of a source of information that describes
the time multiplexing characteristics of similar displays?

       Any other helpful information would also be appreciated.

1998\02\10@153234 by Thomas CHRISTIAN

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>Does anyone on this list know of a source of information that describes
>the time multiplexing characteristics of similar .. LCD ... displays?
>
>

Have a look at:

http://www.microchip.com/10/Appnote/AppNotes.htm

AN563
Description: Using PIC16C5x Microcontrollers as LCD Drivers


AN587
Description: Interfacing to an LCD Module


AN658
Description: LCD Fundamentals Using PIC16C92X Microcontrollers

T.C.

1998\02\10@154503 by John Payson

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>         I purchased an inexpensive (about $5) solar/battery dual power desktop
> calculator for exploring the possibilities of using the bold 0.5 inch 8
> character display as the readout for my project. The characters are in the
> solid bar 7 segment format as opposed to the dot matrix type. I am hoping
> that I will be able to interface the micro controller to the display via a
> standard multi-character 7 segment LCD driver chip.
>
>         1) The display is connected to the calculator PCB via a 28 lead ribbon
> cable.
>
>         2) Battery positive supply is connected via a 1 K resistor to lead 1.
>
>         3) There is a 120 Hz symmetrical square wave clock signal applied to
lead
> # 3.
>
>         4) There are 3 phases of sub harmonic 40 Hz 1/3 duty cycle clock
signals
> applied to leads 2, 27 and 28.
>
>         5) A 40 Hz symmetrical square wave is applied to the remaining leads.
The
> relative phase of these signals vary with respect to the clock depending on
> the particular characters displayed.

Interesting; from your description, the display is using a 1/3 multiplex
design though I'd have to look at things to see exactly what's going on.
A common way of doing 3:1 displays is to use nine elements per digit (seven
segments, a dot, and something else) and then have those in a 3x3 matris (one
side of the 3x3 is common throughout the entire display).

The thing I find odd, though, is that you say most of the other pins have
40Hz "square waves"; I'd expect them to look kinda wierd depending upon what
is being shown.

Perhaps if you could clarify what sort of annunciators, punctuation, etc.
the display has I could better guess what you've got.

1998\02\11@153308 by Russell McMahon

picon face
A non-purist but effective solution would be to use the
calculator electronics intact and access its keyboard
functions from the PIC.

{Original Message removed}

1998\02\13@033229 by Thomas M. Alldread

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face
Greetings

       Thanks to all that responded to my posting!



>>         I purchased an inexpensive (about $5) solar/battery dual power
desktop
> >calculator for exploring the possibilities of using the bold 0.5 inch 8
>> character display as the readout for my project. The characters are in
the
>> solid bar 7 segment format as opposed to the dot matrix type. I am
hoping
>> that I will be able to interface the micro controller to the display via
a
>> standard multi-character 7 segment LCD driver chip.
>>
>>         1) The display is connected to the calculator PCB via a 28 lead
ribbon
>> cable.
>>
>>         2) Battery positive supply is connected via a 1 K resistor to
lead 1.
>>
>>         3) There is a 120 Hz symmetrical square wave clock signal
applied to
>> lead
>> # 3.
>>
>>         4) There are 3 phases of sub harmonic 40 Hz 1/3 duty cycle clock
>> signals
>> applied to leads 2, 27 and 28.
>>
>>         5) A 40 Hz symmetrical square wave is applied to the remaining
leads.
>> The
>> relative phase of these signals vary with respect to the clock depending
on
>> the particular characters displayed.

>Interesting; from your description, the display is using a 1/3 multiplex
>design though I'd have to look at things to see exactly what's going on.
>A common way of doing 3:1 displays is to use nine elements per digit
(seven
>segments, a dot, and something else) and then have those in a 3x3 matris
(one
>side of the 3x3 is common throughout the entire display).

>The thing I find odd, though, is that you say most of the other pins have
>40Hz "square waves"; I'd expect them to look kinda wierd depending upon
what
>is being shown.

>Perhaps if you could clarify what sort of annunciators, punctuation, etc.
>the display has I could better guess what you've got.

       I wish to add the following to my original description:

       1) Each digit of the display has a right hand decimal point.

       2) The voltage applied to pin 1 is more or less equal to the battery
voltage of approximately + 1.5 Volts. This may be a reference baseline for
bipolar logic levels as the clock and signal pulses extend from 0 to + 3
Volts.

       3) All of the waveforms have a peak-to-peak amplitude of approximately 3
Volts and the step for the square waves is from 0 to +3 Volts with respect
to battery negative. As the 3 Volt level is twice that of the battery I
presume there must be a charge pump voltage doubler in the circuitry to
provide the higher voltage.

       4) The baselines for the 1/3 duty cycle, 1/3 clock frequency signals on
pins 2, 27 and 28 are at +1.5 Volts with negative pulses extending down to
0 Volts and positive pulses up to +3 Volts.

       5) The shape of the remaining 24 squarewave logic waveforms does not var
y
from a blank display (with only the right hand 0 showing) to a display
filled with 8 digit eight characters showing all segments. The relative
phases with respect to the clock signals do vary thus the algebraic sum of
the signal lines and clock signals would also vary and I presume this is
what is used to selectively illuminate the digit segments.

       Although I have had success utilizing LCD alpha/numeric display
modules in some of my projects which simply interface with ASCII code I
have not gotten around to studying the theory of the LCD element operation.
Thanks to some of those that responded to my first posting on this subject
I now understand an LCD display must be continuously clocked and requires
bipolar drive. I am in the process of checking out some of the references
given to me to try and obtain a better understanding. Any additional
references on the subject would be appreciated.

       BTW I should have mentioned in my first post that in this case the displ
ay
would be used for a frequency counter readout which is updated about 10
times per second. The direct interface to the keyboard for input suggested
by some is an interesting idea that I might try in the future for a steady
state display application. However, for this application I think there
would be too much flicker caused by the clearing of the display and
sequential entry of digits for each count update. If only the cursor were
addressable!



       Thanks again to all that responded to my posting!


Very Best Regards,
Thomas M. Alldread
e-mail: spam_OUTtmaldredTakeThisOuTspammail.island.net




Very Best Regards,
Thomas M. Alldread
e-mail: .....tmaldredKILLspamspam@spam@mail.island.net

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