apa102_led_strip.js

// Raspberry Pi with a led strip connected to hardware SPI pins
// There seem to be 3 ways to use SPI:
// - /dev/gpiomem - software bit banging (slow), enough to be in `gpio` group
// - /dev/mem - fast hardware SPI, requires root
// - /dev/spidevX.X - fast hardware SPI, enough to be in `spi` group (raspbian)
//
// You need spi-bcm2708 kernel module.
// On raspbian `sudo raspi-config` -> Interfacing -> enable SPI.

// `LEDS_APA102=` to disable, `LEDS_APA102=/dev/...` to override.
const DEV = process.env.LEDS_APA102 ?? "/dev/spidev0.0"

let piSPI = null
let spi = null
let status = ''

if (DEV) {
  try {
    piSPI = require('pi-spi')
    spi = piSPI.initialize(DEV)
    // People say 4MHz and more work fine, Pi supports powers of two up to 32MHz.
    spi.clockSpeed(1e6)
    status = `LEDS_APA102: opened ${DEV}`
  } catch(e) {
    status = `LEDS_APA102 error: ${e}`
  }
} else {
  status = 'LEDS_APA102 disabled'
}
console.log(status)

//const NLEDS = 150  // strip length, TODO make configurable
// In Maayan's room, I cut off a few leds.
const NLEDS=136

// Inspired by https://github.com/livejs/pixels-apa102 but don't want
// to mess with ndpixels and `cwise` which does heavy voodoo.
//
// I'm keeping a buffer in a format good for the led strip, and
// changing a pixel mutates this buffer directly.

// https://cpldcpu.wordpress.com/2014/08/27/apa102/
// https://cpldcpu.wordpress.com/2014/11/30/understanding-the-apa102-superled/

// TODO: in theory, end frame of ceil((NLEDS-1) / 16) should be enough.
// but I saw some artifacts, let's try more?  (Or was this just brigtness flicker?)
let buf = Buffer.alloc(/*start*/1 + /*pixels*/4 * NLEDS + /*end*/Math.ceil(NLEDS))

let reverse = process.env.LEDS_REVERSE === 'true'

// r, g, b, brightness ∈ [0..255]
//
// The r,g,b values are PWM'd at high frequncy ~19.2kHz.  Brightness modulation
// is overlaid on top of that, at ~32 times slower PWM ~582Hz.
// So it's best to stick to max brightness, except for extending dynamic
// range towards blacks...
const setRGBb = (pixel, r, g, b, brightness) => { 
  if (pixel < 0 || pixel > NLEDS) {
    //throw
    console.log(`setRGBb: pixel ${pixel} out of bounds [0..${NLEDS})`)
  }
  const i = 1 + 4 * (reverse ? NLEDS - 1 - pixel : pixel)
  // TODO auto brightness?
  if(b < 16 && g < 16 && r < 16) {
  	brightness
  }
  buf[i] = 0xE0 | (brightness >> 3)  // 111bbbbb
  buf[i + 1] = b
  buf[i + 2] = g
  buf[i + 3] = r
}

const clear = () => {
  for(let i = 0; i < NLEDS; i++) {
    setRGBb(i, 0, 0, 0, 0)
  }
}

// after start frame, all pixels must have MSB turned on
clear()

const send = () =>
  new Promise((resolve) => {
    //console.log(buf)
    if (spi) {
      spi.write(buf, resolve)
    } else {
      resolve()
    }
  })

module.exports = {status, NLEDS, clear, setRGBb, send, buf, spi} // buf, spi only for debug