Re: LCD drivers

On Saturday, in article <11jtoklh2s3esf4@xxxxxxxxxxxxxxxxxx>
delman@xxxxxxxxxxxxxxx "Howard Delman" wrote:

>Paul Carpenter wrote:
>> On Friday, in article <11jqui3pu3lbl63@xxxxxxxxxxxxxxxxxx>
>> delman@xxxxxxxxxxxxxxx "Howard Delman" wrote:
>>>Paul Carpenter wrote:
>>>>On Friday, in article <11jppu5dhvrg53e@xxxxxxxxxxxxxxxxxx>
>>>> delman@xxxxxxxxxxxxxxx "Howard Delman" wrote:
>>>>>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?
>>>>
.....
>>
>>>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.
>>
.....
>> At the moment I would say it is at least 60:40 to not being a problem.
>>
>The micro is an Elan EM78869. The LCD has not yet been sourced. When

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.

>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

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.

>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.

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.

--
Paul Carpenter | paul@xxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

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