On the perils of trying to pack your bits too tightly

While superficially, it might seem like a good idea to try to pack your bits in as tightly as possible. You want to update your string as rapidly as possible to reduce transmission time. Unfortunately, this can actually cause some hairy problems that get worse as your strings get longer.

Remember how signal reshaping will take a short bit and stretch it out to make it into an “ideal” bit? If you try to be cute and pack your bits in as tightly as you can, eventually these stretched bits are going to use up the gap before the next bit and violate the VDL minimum. When this happens, you see a glitch on the string.

You might think that as long as you keep your bits at least as long as the “ideal” size, then they will not get stretched and you will avoid glitches.  Unfortunately, the “ideal” bit size varies from pixel and even changes over time for a single pixel depending on temperature and voltage

Heck, you can even have cases where a chip will turn tightly packed 1-bits into 0-bits thanks to signal shaping. Take at look at this capture…

Here we were trying to to squeeze our 1-bits to be as small as possible while still meeting the letter of the law. The bottom blue trace shows a perfectly valid 560ns wide 1-bit (spec says 700ns ±150ns = 550ns min) coming in, but the purple trace shows it going out the other side now reshaped to  a 376ns wide 0-bit. Why did it do that? Maybe it was hot. Maybe it was just having a bad day. It doesn’t really matter because if this happened to you, what you would see is that all of the LEDs past this one went black – a symptom that would surely have you wasting lots of time looking for a power or signal break.  And the longer your strings, the wider the range of behavior like this you are likely to see.

Give your 1-bits and your gaps a little room the stretch their legs and it will save you all kinds of intermittent and hard to debug problems

The moral of the story: Give your 1-bits and your gaps a little room to stretch their legs and it will save you all kinds of intermittent and hard to debug problems. You don’t want to stretch your 0-bits, however, because then they might become 1-bits (see T0H maximum), instead you can stretch the gap after the 0-bit to give it some room to expand into. For some simple bit-pampering code, check out…

NeoPixels Revealed: How to (not need to) generate precisely timed signals