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  • Author Author: tariq.ahmad
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Charlieplexing Buttons and LEDs at the Same Time - How Hard Can It Be? -- Episode 604

So many buttons so little IO. When you want to drive a large number of LEDs or read buttons the obvious choice is multiplexing them. But there is a method called charlieplexing that lets you use even more LEDs and buttons with even less IO pins. Clem wants to interface a large amount of buttons and LEDs with a microcontroller but has a limited number of pins.   He's also trying to save money on parts. Could you just replace expensive signal diodes with cheap LEDs? Could you use the same pins to read buttons and drive LEDs in the same matrix? Could you also achieve true n-key rollover without having a diode on every single button? How hard can it be?

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LED, Super Red, Through Hole, T-1 3/4 (5mm), 12 mA, 5 V, 640 nm Multicomp 9 Buy Now
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  • From what little I've seen, Charlieplexing can work well. Plus, it's ultra-low cost which can be a huge advantage.

    As another approach, sometimes dot-matrix boards use a single serial-to-parallel logic IC, like (say) 74HC164 for (say) a row, and then does a round-robin scan across the column with a johnson counter type IC like 74HC4017.

    Another classic way is the MAX7219, which (contrary to the typical use for it) doesn't need to be used with 7-segment displays, it can control arbitrary LEDs in any matrix/array up to 8x8 (and they can be chained up for multiples of 64). Works brilliantly, I used a couple of them once for driving 120 LEDs for a clock dial display. The benefit is that the code doesn't need to do any scanning, but the disadvantage is the cost (MAX7219 is really expensive nowadays, but there are ultra-cheap clones of it). There are lots of modern LED driver replacements (some with bult-in keyboard scanning), but the MAX7219 probably has a ton of example code since it is so mature.

    For buttons, a cheap approach that eliminates scanning code effort, is to use an I2C or SPI I/O expander IC, because most of them have an interrupt output. The code just needs to read the I/O expander whenever the interrupt occurs. But as mentioned, all of these approaches will cost more in hardware (but simplify the software).

  • in reply to shabaz

    loading off code complecity to dedicated ICs is clearly adavantageous for sophisticated designs. the thing i inparticular like a botu a combined chalieplexed matrix of buttons and leds is the ability t ochange its purpose in software.

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