Re: Positional/physical addressing

On 6/25/2025 10:30 AM, Ian wrote:

On 2025-06-24, Jeroen Belleman <jeroen@nospam.please> wrote:

WS2812 LED chips do something like that. The first chip in the chain
absorbs the first three bytes and passes on all that come after, and
so on down the chain. Wait 50 us and the first chip in the chain is
ready again to intercept its three bytes.

 That's not quite how they work. The whole chain is one big shift register,
(24 bits per device), with 1's and zero's being shifted along as broad
or narrow pulses. When the pulses stop for a while*, the 24 bits sitting
in the "shift register" are latched into the "DAC register" and the
colour changes.

And, you (the controller) have to send the entire bit pattern; you can't
"address" the 47th device without also having sent the data for the
preceding 46.
Nor can those devices talk to each other (even if they had something to
say).
The UART "trick" makes the fabric more capable with the biggest
(operating) downside being non-constant latency.

I think I saw that there was some variant of the WS2812 that went into
a bypass mode on failure, to avoid breaking all that follow. Exactly what
counted as failure wasn't clear.

Yes.  It's possible to create a bypass much like removing a device
from a fiber optic "tap" and letting the optical switch act to bypass
its removal.

* An intersting / irritating bug had the firmware not waiting long enough
(= at all) between sending updates, so although the string was buisily
clocking data through it, nothing was happening on the lights. Getting the
bit count wrong can produce some pretty patterns, if you like migraines.

It is actually amusing that this even works, at all (given there are no
semi-precise timing references in the technology!)