LCD drivers

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I am considering using an MCU that has built in LCD drivers, but it only
implements 1:8 multiplexing.  In other words, it has eight "common"
lines that are sequentially asserted.  I need to connect it to an eight
character seven segment display.  None of the displays that I have found
are designed for 1:8 multiplexing.  Is it possible, and reasonable, to
use this MCU with a display that only has four commons (1:4
multiplexing)?  If so, would I use four consecutive commons, or every
other one?  Would the appearance suffer in any way?  And what if the
display is 1:3?

Thanks for any help.
Howard Delman
--
Delman Design
Digital design from conception to production
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Re: LCD drivers
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To drive 8 characters with four commons suggests more segment drives required
in other words the common drives TWO characters at the same time, so requires
*14* segment drives (16 including DP).

Does the micro support sufficient segment drives to match the LCD.

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The micro probably has the ability to drive UPTO 8 common drives and
upto SOME number of segment drives, you must match this to your LCD
and vice versa.

The information should be in the micrprocessor and LCD datasheets and/or
application notes.

As you have not said WHICH micro and WHICH display you are using, no more
detailed help can be given.

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Re: LCD drivers


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The MCU is not configurable for anything other than 1:8 or 1:9 drive.
It has more than enough segment drivers.  I know I can leave the
unneeded segment drivers unconnected.  It is the commons that concern me.

Howard
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Delman Design
Digital design from conception to production
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Re: LCD drivers

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Answer this question and someone might be able to help you.
 
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If you answer the question above about which processor someone might
be able to help you. At the moment everything is just handwaving in
a hurricane.

Things may or may not work depending on WHICH micro and WHICH LCD.
When you can at least give this information, those with experience
of that will be able to see if it is a problem.

At the moment I would say it is at least 60:40 to not being a problem.

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Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: LCD drivers
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The micro is an Elan EM78869.  The LCD has not yet been sourced.  When
it is, it will be a generic display from some company in China that no
one would have heard of.  The specifics of the components are not the
issue.  I am very well versed in the use of LCDs, and their methods of
driving.  I have just never implemented a design in which the driver
multiplexing wasn't matched to the LCDs requirements.  My guess is that
it will display correctly, but the contrast will be poor.  I was hoping
someone might have done this, and could tell me for sure.

Howard
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Delman Design
Digital design from conception to production
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Re: LCD drivers

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As I can see this is one of those processors that not many will have
experience of in a group like this. However if you have a development
version of the micro you can use try it out on some LCDs.

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As the LCD has not been sourced yet is it not possible to get a 8 character
seven segment with 8 commons from your sources? These are available from
other sources I have seen as standard parts, but I do not know enough
of the application to determine if they fit your application.

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I would expect contrast would diminish (dependent on the LCD hold/response
times). I think only a test setup would tell you for sure.

From the micro choice I guess this is a very low cost, almost throwaway
device. I personally have no direct experience with this micro, so answering
as bet as I can, hopefully others may

However the options as I see it are:-

 1/  Try a test setup with a standard part with 4 commons on a
     development version and compare against 8 common variant and 4 common
     variant on a four common driving device.

 2/  If tests above show appreciable difference, see if you can devise an
     external LCD voltage 'OR'ing so common 1 and 5 outputs drive first
     segment through common 4 and 8 outputs driving fourth common.
     Being careful not to mess up any key scan columns this may affect
     in your application. This is not an easy option or a cheap option.

 3/  If not fixed on that processor, think about using a different one
     from that manufacturer for better match to common drive.
     Other manufacturers do have micros for doing such things that
     can be bought in masked versions (for example Renesas H8/38024)
     which in my view have better documentation and flash versions
     for development and are easier to programme, with advantages of serial
     port for changing messages etc.

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Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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