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0 pointskragen2y ago0 comments

oh, yeah, for sure, once you're trying to generate a separate pwm waveform for each pixel, you need something more sophisticated than a 74hc595 driving the columns. aside from the ws2812 approach and the cpld/fpga approach, these days you can get custom led driver chips that do this, a separate pwm waveform (or sometimes programmable current sink) for each output pin, driven by daisy-chainable shift registers with multiple bits per pin

one quibble. if two 32-pixel-wide panels are daisy-chained, doesn't that mean it's sending one 64-pixel-wide row one pixel at a time, which gets shifted through the first panel into the second one?

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jacquesm2y ago· 1 in thread

It works like that for every LED. So the data for LED 1536 in a set of 6 ganged 8x32 arrays goes through all 1535 preceding LEDs! It's a miracle it works at all given the amount of stages, you'd expect that at some point there would be some noise but there really isn't, it's rock solid. The 8x32 is arranged such that it's 32 LEDs in one row (or actually, 96+a tiny processor) and then it moves to the next row so they serpentine across the board. Daisychaining multiple board just extends the serpentine further, it's all still one string. That's also why you need to treat odd and even rows a bit different during pixel addressing.

kragenOP2y ago

sure, in the led 1536, but the older panels we're talking about were multiplexed, so the data would only have to go through the (shift registers for the) previous leds on the same row

you obviously know a lot more than i do, so maybe i'm missing something, but in either case i wouldn't expect noise or reflections, the signal levels and timing get restored at every shift stage, even low voltage cmos usually has more than a volt of noise margin, the traces are pretty short, and we're talking about longwave to shortwave frequencies, not microwave frequencies

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