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
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Delman Design
Digital design from conception to production
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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.
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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Paul Carpenter | paul@pcserviceselectronics.co.uk
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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
http://www.blueneptune.com/~delman
Answer this question and someone might be able to help you.
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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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
--
Delman Design
Digital design from conception to production
http://www.blueneptune.com/~delman
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.
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.
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 | paul@pcserviceselectronics.co.uk
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