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LCD display basics

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 18 February, 2006 » LCD display basics « Previous Next »

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Dave_squibb
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Username: Dave_squibb

Post Number: 31
Registered: 04-2005

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Posted on Tuesday, 14 February, 2006 - 03:06 pm:   Edit Post Delete Post Print Post

Hi all,
I'm after some information on basic LCD modules and how to use them.
I've used "Intelligent LCD" modules and have interfaced them to Pics but what about the standard lcd modules without intelligence.
For example I have a cheap pedometer with an LCD display which increments the display every step.
This uses a single 1.5V battery and uses so little current that it doesn't have an off switch.
I'd like to experiment by driving one with a PIC using a 2 - 3V supply.
Can anyone point me in the right direction on how to drive these things please, (preferably directly from a PIC?)
And what sort of processor technology does the pedometer use to run from 1.5 volts? (No chip numbers, chip on board!)

Thanks,

Dave
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Epithumia
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Username: Epithumia

Post Number: 168
Registered: 07-2005

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Posted on Tuesday, 14 February, 2006 - 03:56 pm:   Edit Post Delete Post Print Post

If it's anything like the LCDs in digital multimeters then you'll have a common terminal on the LCD, and one pin per segment.

To turn on a segment you put a voltage between common and the segment.

BUT these displays are damaged by DC so you actually put a clock (say, 100Hz) on the common pin and toggle all the segment pins in phase or antiphase with the clock depending on whether you want the segments on or off.

It's no doubt possible in a PIC, if fiddly to get the timing right. You can use external CMOS XOR gates on each segment to do the phase/antiphase conversion rather than toggling all the PIC outputs.

I think there may be PICs with built-in LCD controllers.

You might be interested in the 74HC4543 chip which does binary-7 segment conversion - one chip per LCD digit.

I did an LCD project recently and used the Philips PCF8577CP chip - only £2.41 from Farnell. This is a 40 pin DIL that can drive 32 LCD segments and connects to your PIC via an I2C bus.

http://www.farnell.com/datasheets/59452.pdf
'Bother,' said the Borg. 'We've assimilated Pooh!'
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Dave_squibb
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Post Number: 32
Registered: 04-2005

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Posted on Tuesday, 14 February, 2006 - 04:15 pm:   Edit Post Delete Post Print Post

Thanks for that Epithumia, very useful.
The pedometer display must be muliplexed as well as it has 57 segments but only 19 connections.

Dave.
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Zeitghost
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Username: Zeitghost

Post Number: 59
Registered: 01-2006

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Posted on Tuesday, 14 February, 2006 - 04:38 pm:   Edit Post Delete Post Print Post

I designed a project for a client that used a Holtek 48R50 with LCD direct drive.

It's remarkably similar to a PIC, but the assembler mnemonics are rather more understandable...

All the AC drive jiggery pokery for the LCD is done by internal logic & all you have to do is write bit patterns to memory.

Yes, it's bit patterns according to the type of LCD that you're using...

The other option would be some type of LCD driver chip... which is where we came in...
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Obiwan
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Post Number: 108
Registered: 12-2005

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Posted on Tuesday, 14 February, 2006 - 05:07 pm:   Edit Post Delete Post Print Post

The best place I have found for information like that is the LCD manufacturers web sites. They normally have a lot of technical back ground information on how they do what they do.

Most of them generate an AC signal from two or more DC signals. At the summing node, you get what looks like a really crummy stepped AC wave.

I think it would be a lot of trouble, but it would also be a good learning experience. And this is the kind of learning that will stick with you. I'd be nice to be able to use old LCDs from other stuff. But some times, the connections can be hard to make. I saw a place that sold those "Zebra" strips a couple of weeks ago, I'll look up the address again if you need it (those rubber strips that connect to the actual glass)
May the Force be with (most of) you.....
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Dave_squibb
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Post Number: 33
Registered: 04-2005

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Posted on Tuesday, 14 February, 2006 - 05:40 pm:   Edit Post Delete Post Print Post

Thanks Guys,
The Holtek stuff looks good & some of it works down to 1.2V! Also seems to have the LCD interfacing sorted. I'll have a look at the cost of the development tools.
The pedometer LCD has the "zebra strip", just pressed against the pcb pads - very neat.

Dave
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Mark
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Username: Mark

Post Number: 25
Registered: 05-2005

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Posted on Tuesday, 14 February, 2006 - 10:11 pm:   Edit Post Delete Post Print Post

I can echo the appreciation of Holtek chips. I've used the 46R62 (OTP) in a commercial design. it has direct LCD drive on board. Only problem is the 56 pin SSOP package - so stripboard is out of the question.

The code is a doddle for anyone that has 'done' PIC assembler. The LCD section is as described - set and clear bits in memory -one bit for each segment.

I looked at PICs but they have only a couple of cranky old parts that do direct LCD drive.

Mark
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Zeitghost
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Username: Zeitghost

Post Number: 61
Registered: 01-2006

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Posted on Tuesday, 14 February, 2006 - 10:22 pm:   Edit Post Delete Post Print Post

The Holtek processor emulator came from Steatite.

It was about £200 +vat. The programmer was about £150.

Farnell used to stock the kit too.

Speaking professionally, that sort of money corresponds to about a day's rate, so it's really very cheap.

Especially compared with some of the processor emulators I've used previously (£5k - £10k).

Don't know if Holtek have progressed to flash programmable yet, would be nice if they had, that was the only criticism I had, though the board stuffers I did the design for were less than enchanted with the security of supply & had to have two versions of the pcb to take either package variant.
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Mark
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Post Number: 26
Registered: 05-2005

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Posted on Tuesday, 14 February, 2006 - 10:34 pm:   Edit Post Delete Post Print Post

I think Holtek are about to introduce some 'Flash' parts and also have a few devices with built in data EEPROM (just another small EEPROM die bonded in the package I think - not to be confused with program memory)

One criticism - the technical support is a bit thin on the ground - one user forum that gets occasional answers from their technical staff.
So you're on your own when things get difficult.

But the Chips are seriously Cheap. like the man says!


Mark

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Dave_squibb
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Post Number: 34
Registered: 04-2005

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Posted on Wednesday, 15 February, 2006 - 09:05 am:   Edit Post Delete Post Print Post

Thanks for that. Development tools do sound cheap.
Flash would be nice but not the end of the world if you have the Emulator.

Dave.
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Zeitghost
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Post Number: 64
Registered: 01-2006

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Posted on Wednesday, 15 February, 2006 - 10:16 am:   Edit Post Delete Post Print Post

Correction: it was a Holtek 49C50 / 49R50 processor that has the LCD drive capability.

There's no sign of the emulator/programmer on the Farnell site any more.
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Dave_squibb
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Username: Dave_squibb

Post Number: 35
Registered: 04-2005

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Posted on Wednesday, 15 February, 2006 - 11:40 am:   Edit Post Delete Post Print Post

Thanks Zeitghost

Dave
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Mike_b
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Post Number: 154
Registered: 05-2005

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Posted on Wednesday, 15 February, 2006 - 12:51 pm:   Edit Post Delete Post Print Post

On rainy days I have played around with a couple of displays taken from old CASIO watches.

The lcd glass panel has a row of 12 faint pads which are connected to the pcb using a sort of small rubber conductive block.

I have driven these with a 3V SOIC "L" 16LF628 PIC directly and they display random segments - no problem. I have not got around to playing with the lines to determine the segment allocations yet - (awaiting another rainy day).

rgds

mb
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Obiwan
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Post Number: 118
Registered: 12-2005

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Posted on Wednesday, 15 February, 2006 - 10:40 pm:   Edit Post Delete Post Print Post

That small rubber block is the "zebra strip".

I think they used to come in alternating white and black colors, hence the "zebra" notation.
May the Force be with (most of) you.....

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