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pimoroni / keybow-firmware

Keybow Firmware for the Raspberry Pi Zero
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Lack of documentation: Why 0b11100011? #23

Open mrusme opened 5 years ago

mrusme commented 5 years ago

As I simply could not find any documentation on this, I'm misusing GItHub issues to ask this question and also suggest documenting this:

https://github.com/pimoroni/keybow-firmware/blob/master/keybow/lights.c#L108

What's that magic number? Why not 255 instead of 227? 255 seems to do the job as well. And what's that field in first place? I first thought it's the brightness, however, while it does seem to make the LEDs brighter when running at 255 instead of 227, it does not seem to support all numbers.

Could you please give some insights on this?

Gadgetoid commented 5 years ago

I'd suggest reading https://cpldcpu.wordpress.com/2014/08/27/apa102/ if you're interested in driving APA102 LEDs.

This magic number is the LED frame marker, which is indicated by 3 high bits set to 1: 0b11100000 plus 5 bits of global (applies to R, G and B LEds in a package) brightness: 0b00011111 (giving an 0-31 brightness range). Since we don't want to drive at maximum brightness, but still want colours to have the full 255255255 range, we use global brightness to handle the dimming across the LEDs.

From the above article: image

mrusme commented 5 years ago

@Gadgetoid thank you for clarification! This information is a great piece of documentation.