To my knowledge, this is not possible with a Raspberry Pi. You should see glitches on the output when your loop generating the 1s and 0s is interrupted.
That may be ok it if you just when to send one static light pattern but the glitches should be more evident if you want to create beautiful, flowing light patterns.
That's why it's better to drive those WS2812b with an Arduino that you code bare metal.
When you still want bare metal, pick a modern uC like ESP8266 (cheap, 32 bit, integrated WiFi for trivial connectivity) instead of "classic" overpriced, outdated 8/16 bit AVR (Attiny/Atmega) based Arduino hardware.
Arduino the software platform still works great of course, but so does NodeMCU which is the go to stack on ESP8266.
That said the ESP8266 still has a place because of how cheap you can usually find them. Lowest price arm board that i know of from a manufacturer is the Teensy LC at ~$12USD, but there are cheaper ones out there on ebay and aliexpress. I just can't say anything about how well they work or are supported by anyone.
That having been said, if this works reliably, it's fantastic. The ESP32 is pretty great, but controlling WS2812b LEDs from a Pi Zero would be extremely convenient as well.
You can then manually assign a thread to an isolated core, and it will run there without any interruptions from thread switching.
You can also map interrupt handlers away from those isolated CPUs.
There may be architectural things that cause interruptions you can’t mitigate though (like SMIs on Intel).
And the userspace sends entire buffer in one syscall, so those underruns will have to be caused by kernel stall, which usually only happens when buggy device drivers are present.
> Now taking a look at the duties of the PWM signal we need to send, we can approximate the highs and lows to some multiple of 1/3. This means that if we run a MOSI SPI signal at triple the frequency the lights expect, then we can send a single WS2812b "panel bit" using a combination of 3 SPI bits.
Let me know what you guys think.
One question about the code, why does the LedPanel struct use a String as a buffer rather than using a Vec<u8>?
edit - I think I see the reason now. Using a string made it easy to print the u8 as a string of bits, which you could then iterate over using chars. This was used to help perform the conversion from bits directly representing colors to the on the wire representation.
Note: God bless the Oscilloscope, essential for these projects.
We built a similar WS2812b array at university for an embedded systems design project: https://github.com/joe-wright/wiisel
We looked at using a Raspberry Pi but instead settled on an mbed FRDM-KL25Z so we had more control over the signal timing. The trade-off was that the KL25Z only had 16KB memory instead of the Pi's 512MB, so getting everything to fit took a bit of creativity and a few late nights...
Source: wrote a software UART for an embedded project
