Electronics > Microcontrollers

Need Advise on Driving an LED Display

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fcb:
+1 for uwezi.

To explain the smearing more. If you had a sequence such as COLUMN 1/2/3/4/1/2/3/4/etc... and you wrote 1811 on your display, then you would notice that the COLUMN 3 unlit segments would be glowing a little. How much would depend on on how fast you multiplex the display.

If this bothers you (it may not) - you can counter act this by inserting a dead-time (allowing the COLUMN drivers to switch off properly) between column drive events, but this will lead top a reduction in brightness (again, how much is dependent on your scheme).

This problem is very annoying with moving characters (such as timecode clocks), so much so that I've abandoned multiplexing on timecode in-favour M5450 drivers running each segment.

dannyf:
-The smearing would show up between digits, i.e. the contents of the first digit would smear out to the next digit, because both digits would be powered at the same time during the transition period. -

are you sure that it is not a coding issue? Those chips may be slow but likely much faster than human vision. What you described sounds like switching the digits too fast for the eyes.

mariush:
No, uwezi is right, I simply forgot about that issue.

So yes, it's possible to get some bleeding into the other digits because it takes more time for the darlington elements to turn off compared to the time it would take to turn off one digit and turn on next digit.

Like fcb says, it would be possible to just put a pause between turning off a digit and turning on next digit, which would allow the darlington arrays to fully turn off. The brightness would be lower, but if you're using darlington array, you could afford higher currents into each element of the seven segment display in the first play.

A microcontroller is capable usually of 20-25 mA per pin (for pics), maybe more for avr , and if i remember correctly about 300-400mA in total through all pins chip and this is already a lot of current available to light up on digit at a time. If you have seven segments that need more power or if you want to run them from higher voltage than what the pic can handle, then it makes sense to use those uln2803/4/5a arrays.   

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