Searching \ for 'PIC16C924 LCD question' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/io/lcd/pic.htm?key=lcd
Search entire site for: 'PIC16C924 LCD question'.

Truncated match.
PICList Thread
'PIC16C924 LCD question'
1997\08\20@183757 by Guy Farebrother

flavicon
face
Hello everyone.

Does anybody know if it possible to drive two separate LCD's from one '924.
I would like to use separate 4 digit LCD's and tie there segments together
but use the COM0 and COM1 backplane drivers to drive them separately.

Is this possible? What should I be aware of and watch out for?

Thanks for any assistance.

Guy

1997\08\20@220747 by John Payson

flavicon
face
> Does anybody know if it possible to drive two separate LCD's from one '924.
> I would like to use separate 4 digit LCD's and tie there segments together
> but use the COM0 and COM1 backplane drivers to drive them separately.
>
> Is this possible? What should I be aware of and watch out for?

I see no problem with doing this.  The only caveats I can really see are
that:

[1] Your contrast won't be as good as it would be with static drive.  Even
a 2:1 mux severely limits your contrast (often not objectionably so, but
with static drive the clear parts get no juice and the dark parts get max
juice; by comparison, with 2:1 mux your RMS_on:RMS_off ratio is only
sqrt(5):1(*) This may necessitate the addition of a contrast knob since
you will probably no longer be able to get totally dark blacks or totally
clear whites.
(*) You'll get a sqrt(5):1 RMS ratio whether you use a bias voltage of 1/2
or 1/3.  If you use a bias voltage of 1/(1+sqr(2)), however, you can get a
contrast ratio of up to sqrt(3+2*sqrt(2)):1 [about sqrt(5.8:1)]  The PIC
16C924, however cannot bias the display with such a voltage.

[2] Your displays will have to be 31 segments or less, each.

[3] It may be easier to wire things if you use two displays, each of which
is internally 2:1 or 3:1 multiplexed.  If you go 2:1, your net output will
be the same as if you use your existing displays, but you'll only have to
connect about half as many pins.  If you go 3:1, your contrast won't be
quite as good, but you'll have even fewer pins.

1997\08\21@235015 by Mike Ghormley

flavicon
face
Guy Farebrother wrote:

> Does anybody know if it possible to drive two separate LCD's from one '924.
> I would like to use separate 4 digit LCD's and tie there segments together
> but use the COM0 and COM1 backplane drivers to drive them separately.

How about using a descrete 7-segment LCD controller chips such as Teledyne's
TC7211?  It handles 4 digits, so two of them would give you what you need.
Don't know about price and availability these days, though.

Michael

1997\08\22@024320 by blunn

flavicon
face
Bob Lunn
08/22/97 04:43 PM


> [1] Your contrast won't be as good as it would be
>     with static drive.  Even a 2:1 mux severely
>     limits your contrast

    Up till a month ago I would have agreed with
    this whole heartedly.

    I have, however, been blown away by the custom
    duplex displays we are now using.

    I find no perceptible difference in the contrast
    between these new displays and our older static
    drive displays (and they were _very_ good static
    displays).

    Yes, you have to pay attention to display fluid,
    temperature range, and drive voltage.  But there's
    no longer, in my mind, any contrast-driven reason
    to discriminate against duplex driven displays.

> If you use a bias voltage of 1/(1+sqr(2)), however,
> you can get a contrast ratio of up to sqrt(3+2*sqrt(2)):1
> [about sqrt(5.8:1)].
>
> The PIC 16C924, however cannot bias the display with
> such a voltage.

    I don't have the datasheet here, but as I recall
    the '924 uses an external resistor ladder to generate
    the drive voltages.

    Won't the correct selection of resistor values give
    the drive bias you want?

> [3] It may be easier to wire things if you use two
>     displays, each of which is internally 2:1 multiplexed.

    Big yes to this.  The simplification in board
    layouts that came with the move to duplex displays
    alone made the exercise worthwhile.

___Bob

1997\08\22@032834 by John Payson

flavicon
face
> > [1] Your contrast won't be as good as it would be
> >     with static drive.  Even a 2:1 mux severely
> >     limits your contrast
>
>      Up till a month ago I would have agreed with
>      this whole heartedly. I have, however, been blown
>      away by the custom duplex displays we are now using.
>
>      I find no perceptible difference in the contrast
>      between these new displays and our older static
>      drive displays (and they were _very_ good static
>      displays).

It's possible to adjust things so that you can get good contrast with
multiplexed displays--don't get me wrong.  On the other hand, static
displays have the definite advantage that you can just hook them up for
maximum drive and they'll pretty consistently look great.

>      Yes, you have to pay attention to display fluid,
>      temperature range, and drive voltage.  But there's
>      no longer, in my mind, any contrast-driven reason
>      to discriminate against duplex driven displays.

If you can manage to consistently drive the display optimally, this is
true.  But the display very definitely goes from being something you can
just drive hard independent of circumstances to being something that you
must drive 'just right'.

BTW, it would seem a simple improvement to an LCD display would be to
aligh the two polarizers such that the clear parts of the display were
just a tiny bit dark, but such that a small amount of drive would make the
display get more clear (and a bit more would make it get darker).  It
would seem this should allow the unlit segments to match the background
perfectly, improving contrast.  Anyone know if this has been done?

{Quote hidden}

Unfortunately not the way the PIC is designed.  Generally, when driving a
multiplexed display you want to ensure that all segments on an inactive
row are driven the same.  The normal driving pattern, assuming bias Q,
is... [for each parameter, there are two values; the pins should go
between the two values 'in phase']

Active   row: 0 and 1
Passive  row: Q and 1-Q
Dark  column: 2Q and 1-2Q
Blank column: 0 and 1

The segments thus receive the following voltages:
Dark  column - Active  row: 2Q and -2Q
Blank column - Active  row:  0 and   0
Dark  column - Passive row:  Q and  -Q
Blank column - Passive row: -Q and   Q

Note that the magnitude of voltage on a segment on an inactive row is
indepent of whether its column is on or off; it's Q regardless.

The difficulty with the PIC is that the magnitude of "blank column,
passive row" is equal to the difference between the two center voltages,
while the magnitude of "dark column, passive row" is the difference
between an outer voltage and its nearest center voltage (note that if the
two outside gaps aren't equal, this will create a DC bias that may destroy
the LCD).  For the PIC to use the optimum drive on a duplex LCD, it would
need to have seperate inputs for the passive row and column voltages.

> > [3] It may be easier to wire things if you use two
> >     displays, each of which is internally 2:1 multiplexed.
>
>      Big yes to this.  The simplification in board
>      layouts that came with the move to duplex displays
>      alone made the exercise worthwhile.

Personally, I have something of a bent for 3:1 multiplexing, in part
because I came up with a technique that gets decent contrast without any
biasing nonsense (ideal RMS on:off, using 1/3 bias, would be sqrt(11/3);
my method produces an RMS on:off of sqrt(3) using clever timings alone.
Unfortunately, while the method would also 'work' with a 2:1 display and
the programming would be simpler than with 3:1, the contrast ratio would
actually be no better.

FYI, the technique would also be possible for displays muxed higher:

2:1 or 3:1 --- sqrt(3/1)
4:1 or 5:1 --- sqrt(11/5)
6:1 or 7:1 --- sqrt(21/11)

Note, however, that the 7:1 mux require a 128-step display frame; I
suspect the approach would be practical for--at most--a 5:1 display mux.

1997\08\22@102139 by lilel

flavicon
face
Here's one for you LCD guys

Whose display are you using?  What subspecies of display?
Alphanumeric, dot matrix, custom?  What suppliers have you had good
luck with?  What kind of cost are you seeing in quantity?

I am needing to get my hands on a relatively simple and relatively
low cost display, from someone who can do custom displays once I get
into volumes.


-- Lawrence Lile

Download AutoCad blocks for electrical drafting at:
http://members.sockets.net/~llile/index.htm

1997\08\22@103206 by Martin R. Green

picon face
I, for one would be interested in knowing what duplex displays you are now
using.

TIA - Martin R. Green
spam_OUTelimarTakeThisOuTspambigfoot.com

----------
From:   .....blunnKILLspamspam@spam@KEYCORP.COM.AU[SMTP:blunnspamKILLspamKEYCORP.COM.AU]
Sent:   Friday, August 22, 1997 12:43 PM
To:     .....PICLISTKILLspamspam.....mitvma.mit.edu
Subject:        Re: PIC16C924 LCD question

Bob Lunn
08/22/97 04:43 PM


> [1] Your contrast won't be as good as it would be
>     with static drive.  Even a 2:1 mux severely
>     limits your contrast

    Up till a month ago I would have agreed with
    this whole heartedly.

    I have, however, been blown away by the custom
    duplex displays we are now using.

    I find no perceptible difference in the contrast
    between these new displays and our older static
    drive displays (and they were _very_ good static
    displays).

    Yes, you have to pay attention to display fluid,
    temperature range, and drive voltage.  But there's
    no longer, in my mind, any contrast-driven reason
    to discriminate against duplex driven displays.

> If you use a bias voltage of 1/(1+sqr(2)), however,
> you can get a contrast ratio of up to sqrt(3+2*sqrt(2)):1
> [about sqrt(5.8:1)].
>
> The PIC 16C924, however cannot bias the display with
> such a voltage.

    I don't have the datasheet here, but as I recall
    the '924 uses an external resistor ladder to generate
    the drive voltages.

    Won't the correct selection of resistor values give
    the drive bias you want?

> [3] It may be easier to wire things if you use two
>     displays, each of which is internally 2:1 multiplexed.

    Big yes to this.  The simplification in board
    layouts that came with the move to duplex displays
    alone made the exercise worthwhile.

___Bob

More... (looser matching)
- Last day of these posts
- In 1997 , 1998 only
- Today
- New search...