So I am looking to drive 6 digits worth of 7 segment LED digit display... I want to use a 3v PIC, the interface IC's that I am finding (ICM7218AIPI+ for instance) list voltages like 2-6V, but upon investigation, the 2V is some low power shutdown mode, and require +4v input for serial communication. So am i barking up the wrong tree, or do I need some sort of charge pumped serial IC (or UART) in between?
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asked Jan 25, 2013 at 19:43
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\$\begingroup\$ Let me know if I linked to the wrong datasheet. That chip has a parallel interface, not a serial one. \$\endgroup\$– Dave TweedCommented Jan 25, 2013 at 20:23
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\$\begingroup\$ sorry, i just grabbed one out of the matrix that I thought fit. \$\endgroup\$– Grady PlayerCommented Jan 25, 2013 at 20:34
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2 Answers
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You can avoid the use of external display driver by choosing a PIC with higher pin-count. Driving six 7-segment digits will need 14 extra pins on your PIC. You can multiplex the display in software. Depending on your application you can use a 28 pin PIC (minimum 18 pin PIC that gives you 16 IO) and go higher if your application demands it.
Removing extra variables (like the driver chip) will ease your debugging efforts.
Here (below image) is something that you would need. You just have to add two more displays. You may not need transistors depending on the PIC and display current source and requirements. Make sure you check the current requirements of your display and the current that your PIC can sink.
answered Jan 25, 2013 at 21:07
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\$\begingroup\$ I have considered it... I may very well end up going that way, I wanted an dedicated IC so that I could only have one PIC and drive several of these groups... but 28pin PICs are cheaper at the 1-25 quantity than these IC's so I still may end up going that route. \$\endgroup\$ Commented Jan 25, 2013 at 21:34
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\$\begingroup\$ I guess i need 6 tri state output pins and 7 ground/+5 pins, is that right? \$\endgroup\$ Commented Jan 25, 2013 at 21:37
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\$\begingroup\$ @GradyPlayer You don't need tristate pins. I have updated my answer with an example. \$\endgroup\$ Commented Jan 25, 2013 at 21:42
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\$\begingroup\$ I am accepting because I already have common anode displays and other pics, thank you for the diagram \$\endgroup\$ Commented Jan 26, 2013 at 7:18
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\$\begingroup\$ Common anode displays are better as the sink capabilities of PICs are greater than source. \$\endgroup\$ Commented Jan 26, 2013 at 7:29
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answered Jan 25, 2013 at 20:13
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\$\begingroup\$ I looked at some from maxim... I can use the IC you specified, i just have to re-order common cathode displays, but every other spec would jive. \$\endgroup\$ Commented Jan 25, 2013 at 20:25