Limit for GPIO pixels

[lrhorer]

What is the hard node limit for pixels directly controlled by a GPIO pin on a Raspberry Pi?  A client is looking to control some WS-2111 pixels directly from a Pi Zero-W, and we need to know what the maximum number of pixels is so we can scale the strings accordingly.

[MikeKrebs]

FPP puts a limit of 1600 pixels per port (2 max) for a pi hat (which is gpio based output). WS2811 is also limited by frames per second. Forty FPS is about 640 pixels per port. Twenty FPS would double that. 

If you are not using FPP, you might get longer strings but FPS would have to go way down.

[lrhorer]

Understood, but this application may be able to live with less than 10 fps.  Id these a hard limit?  This will not be a Pi HAT.  I will drive the pixel strings directly (through a level shifter, of course).

[Poporacer]

There are a few things that determine the limit, but the largest one is the WS2811 protocol. The data signal for each pixel has some predefined pauses in it. If you send the next signal packet before the first one has been processed, it will cause tearing where the lights are not changing at the same time.
The other issue will be voltage and data loss, but that is a different topic.
The theoretical limits can be calculated using this:
Max Pixels per port= 33,333/FPS
So, if you are going to use 10 FPS, then you can theoretically control 3,333 pixels per port. The Pi will use up some of those pixels in the processing and the Pi-Zero has very little processing power so you will likely get less than this, but probably close.