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Science Forum Index » Electronics - Components Forum » Driving a little LCD
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| John Fields |
 Posted: Tue Feb 06, 2007 4:15 pm |
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On Tue, 06 Feb 2007 08:08:14 -0600, John Fields
<jfields@austininstruments.com> wrote:
Quote: On Tue, 06 Feb 2007 05:51:08 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:
I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
Any suggestions for a cheap and cheerful controller chip ? Or is it cheaper to
get a display with integrated driver straight off ?
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Figure out a slick way to drive that 32nd pin and you can use this:
http://www.national.com/pf/MM/MM5483.html
---
It just hit me that if you've got 32 pins and you weren't excluding
the backplane, then the MM5483 will do just what you need.
Which display are you planning to use?
--
JF |
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| Eeyore |
| Posted: Tue Feb 06, 2007 5:01 pm |
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John Fields wrote:
Quote: On Tue, 06 Feb 2007 08:08:14 -0600, John Fields
jfields@austininstruments.com> wrote:
On Tue, 06 Feb 2007 05:51:08 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:
I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
Any suggestions for a cheap and cheerful controller chip ? Or is it cheaper to
get a display with integrated driver straight off ?
---
Figure out a slick way to drive that 32nd pin and you can use this:
http://www.national.com/pf/MM/MM5483.html
---
It just hit me that if you've got 32 pins and you weren't excluding
the backplane, then the MM5483 will do just what you need.
Which display are you planning to use?
Just as a 'first off' that I can easily get from Farnell I'm looking at the VI-402
from Trident Microsytems. I'm sure it'll be a widely '2nd sourced' part. In fact I'm
sure Trident will just buy it in to distribute.
Graham |
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| John Fields |
| Posted: Tue Feb 06, 2007 6:27 pm |
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On Tue, 06 Feb 2007 21:01:32 +0000, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
Quote:
John Fields wrote:
---
It just hit me that if you've got 32 pins and you weren't excluding
the backplane, then the MM5483 will do just what you need.
Which display are you planning to use?
Just as a 'first off' that I can easily get from Farnell I'm looking at the VI-402
from Trident Microsytems. I'm sure it'll be a widely '2nd sourced' part. In fact I'm
sure Trident will just buy it in to distribute.
---
Hmm... Farnell rejects that part number. Are you sure it isn't a
VI-402 from Varitronix LTD?
http://www.varitronix.com/Product/LCD/VI-402-DP(R0).pdf
In any case, no matter who's display it is, if you want to use the
National part it'll work because you won't need the decimal points.
That is, unless you want to display temperature in degrees and
tenths of a degree, and then you'll only need DP3, so worst case
would be 28 segments for the digits and the colon, so you'd only
need to drive 29 pins.
Incidentally, segment 'f' of digit 1 is never used either (unless
you want to arrange your display to display temp on the two left
digits, then leave a space, and then display 'C' on the fourth digit
and the temp goes above 40C) so you could connect it to the
backplane with all the other unused stuff and get by with 28
segments like this Intersil part has available:
http://www.intersil.com/data/fn/fn3158.pdf
--
JF |
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| John Fields |
| Posted: Tue Feb 06, 2007 7:03 pm |
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On Tue, 06 Feb 2007 16:27:28 -0600, John Fields
<jfields@austininstruments.com> wrote:
Quote: On Tue, 06 Feb 2007 21:01:32 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:
John Fields wrote:
---
It just hit me that if you've got 32 pins and you weren't excluding
the backplane, then the MM5483 will do just what you need.
Which display are you planning to use?
Just as a 'first off' that I can easily get from Farnell I'm looking at the VI-402
from Trident Microsytems. I'm sure it'll be a widely '2nd sourced' part. In fact I'm
sure Trident will just buy it in to distribute.
---
Hmm... Farnell rejects that part number. Are you sure it isn't a
VI-402 from Varitronix LTD?
http://www.varitronix.com/Product/LCD/VI-402-DP(R0).pdf
In any case, no matter who's display it is, if you want to use the
National part it'll work because you won't need the decimal points.
That is, unless you want to display temperature in degrees and
tenths of a degree, and then you'll only need DP3, so worst case
would be 28 segments for the digits and the colon, so you'd only
need to drive 29 pins.
Incidentally, segment 'f' of digit 1 is never used either (unless
you want to arrange your display to display temp on the two left
digits, then leave a space, and then display 'C' on the fourth digit
and the temp goes above 40C) so you could connect it to the
backplane with all the other unused stuff and get by with 28
segments like this Intersil part has available:
http://www.intersil.com/data/fn/fn3158.pdf
---
Oops...
That's a BCD to seven-segment driver, and you can't get 'C' out of
it, if that was your plan...
--
JF |
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| Eeyore |
| Posted: Tue Feb 06, 2007 7:38 pm |
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John Fields wrote:
Quote: Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
John Fields wrote:
---
It just hit me that if you've got 32 pins and you weren't excluding
the backplane, then the MM5483 will do just what you need.
Which display are you planning to use?
Just as a 'first off' that I can easily get from Farnell I'm looking at the VI-402
from Trident Microsytems. I'm sure it'll be a widely '2nd sourced' part. In fact I'm
sure Trident will just buy it in to distribute.
---
Hmm... Farnell rejects that part number. Are you sure it isn't a
VI-402 from Varitronix LTD?
http://www.varitronix.com/Product/LCD/VI-402-DP(R0).pdf
Same beast.
Quote: In any case, no matter who's display it is, if you want to use the
National part it'll work because you won't need the decimal points.
That is, unless you want to display temperature in degrees and
tenths of a degree, and then you'll only need DP3, so worst case
would be 28 segments for the digits and the colon, so you'd only
need to drive 29 pins.
Very true. I'd like to keep the colon and 1 decimal point so that saves 2 connections.
Quote: Incidentally, segment 'f' of digit 1 is never used either (unless
you want to arrange your display to display temp on the two left
digits, then leave a space, and then display 'C' on the fourth digit
and the temp goes above 40C) so you could connect it to the
backplane with all the other unused stuff and get by with 28
segments like this Intersil part has available:
http://www.intersil.com/data/fn/fn3158.pdf
Neat thinking !
Graham |
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| Guest |
| Posted: Tue Feb 06, 2007 7:53 pm |
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On Feb 6, 12:51 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
Quote: I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
Use an Atmel AVR - ATmega169P. Get yourself a $20 Butterfly board
(which has an LCD on it) and experiment.
The chip itself is very cheap in large volume, and has almost anything
you could have wanted in an 8051. |
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| Ben Jackson |
| Posted: Wed Feb 07, 2007 3:15 am |
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On 2007-02-06, Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
Quote: I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
It's worse than that, isn't it? You have to drive "on" segments with
an AC square wave. DC is 'off' for LCD.
--
Ben Jackson AD7GD
<ben@ben.com>
http://www.ben.com/ |
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| Mark Harriss |
| Posted: Wed Feb 07, 2007 3:33 am |
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Eeyore wrote:
Quote: I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
Any suggestions for a cheap and cheerful controller chip ? Or is it cheaper to
get a display with integrated driver straight off ?
Graham
Been using the Maxim MAXQ2000 for a LCD project, it's good for 4x 132
segments but it's biggest package is a 100pin LQFP. It's got a free
4K limited C compiler from IAR. |
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| Don McKenzie |
| Posted: Wed Feb 07, 2007 5:24 am |
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| Al |
| Posted: Wed Feb 07, 2007 10:11 am |
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In article <45c99ac4$0$9774$afc38c87@news.optusnet.com.au>,
Don McKenzie <look@mysig.com> wrote:
Don't forget to add the postage. I tried to buy a "cheaper" unit from
Canada. Then I saw the delivery charge and the handling fee. It doubled
the price.
Be careful.
Al |
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| John Fields |
| Posted: Wed Feb 07, 2007 1:36 pm |
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On Wed, 07 Feb 2007 01:15:47 -0600, Ben Jackson <ben@ben.com> wrote:
Quote: On 2007-02-06, Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
It's worse than that, isn't it? You have to drive "on" segments with
an AC square wave. DC is 'off' for LCD.
---
The way it's usually done for a static display is that the 'on'
segments are driven out of phase with the backplane and the 'off'
segments are driven in phase with the backplane.
The driving waveform should be square so as not to leave a net DC
between the segments and the backplane, like this:
_ _ _ _
BP _| |_| |_| |_| |_
_ _ _ _
OFF _| |_| |_| |_| |_
_ _ _ _ _
ON |_| |_| |_| |_|
This is easily done by continuously driving the backplane with a
square wave at about 30 Hz and the segment's on/off signal to the
segment through an exclusive-or, like this: (View in Courier)
SEGa>---------A
EXOR Y---[seg]---+
+----B |
| |
SEGb>----|----A |
| EXOR Y---[seg]---+
+----B |
| |
SEGc>----|----A |
| EXOR Y---[seg]---+
+----B |
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SEGd>----|----A |
| EXOR Y---[seg]---+
+----B |
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SEGe>----|----A |
| EXOR Y---[seg]---+
+----B |
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SEGf>----|----A |
| EXOR Y---[seg]---+
+----B |
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SEGg>----|----A |
| EXOR Y---[seg]---+
+----B |
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BP>------+-------------------+
So, in order "light up" a segment, the appropriate SEG signal would
be held high and to kill it would be pulled low.
Since the liquid crystal fluid is non-conductive it becomes the
dielectric between the capacitances formed by the segments and the
backplane, so when the SEG lines go low the voltage across the
corresponding segments and the backplane will be the same, resulting
in a voltage of zero between them, which will turn the segment off.
However, when the SEG signal is pulled high, the voltage between the
segment and the backplane will be twice the peak-to-peak output
voltage of the EXOR, (if the EXOR's output can swing rail-to-rail
and is the same amplitude as the square wave on the backplane)
causing the segment to turn on.
--
JF |
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| Spehro Pefhany |
| Posted: Wed Feb 07, 2007 9:35 pm |
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On Wed, 07 Feb 2007 01:15:47 -0600, the renowned Ben Jackson
<ben@ben.com> wrote:
Quote: On 2007-02-06, Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
I've got a low cost application where I want to drive a small LCD ( probably 4
digit ) from a microcontroller.
I looked at the data on the 'bare glass' and there are 32 pins needing driving
which doesn't leave enough for my other I/O.
It's worse than that, isn't it? You have to drive "on" segments with
an AC square wave. DC is 'off' for LCD.
Not much worse- static drive requires a backplane plus one line per
segment, so 33 pins for 4 digits plus decimal points, or 32 if you can
lose one of the decimal points.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com |
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| Eric Smith |
| Posted: Fri Feb 09, 2007 3:03 pm |
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Dave Platt wrote:
Quote: Microchip sells a bunch of PIC microcontrollers which are
designed specifically to drive LCDs. Ferexample, their PIC16F913
is a 28-pin part which can drive certain LCD configurations having up
to 60 addressable segments.
Eeyore wrote:
Quote: Do you know how they do that ? Presumably there's an app note ?
Dave wrote:
Quote: I would guess that the LCDs in question are generally multiplexed in a
matrixed arrangement, with some fraction being anodes and the rest
cathodes.
They are multiplexed, but since LCDs require AC drive, thy don't
really have anodes and cathodes. A small multiplexed segment
LCD typically has 2, 3, or 4 "backplane" lines, and 16 to 64 "segment"
lines, depending on the total segment count. |
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