ESP32-C6 rainbow

// Rainbow: the DevKitC-1's own RGB LED, and a strip beside it.
//
// GPIO 8 on this board is not a plain LED. It is an addressable one (a
// WS2812 in the corner by the USB socket), and it wants twenty-four bits, not
// a level. That is the pin `LED_BUILTIN` names, so this is what the board's
// LED is actually for.
//
// Wiring, strip to board:
//   DIN -> GPIO 8      VDD -> 5 V      VSS -> GND
//
// One wire carries the whole chain: the board's own LED first, then anything
// wired after it. There is no clock: a bit is a high pulse whose length is
// the bit, 0.4 us for a 0 and 0.8 us for a 1, twenty-four bits an LED, green
// first. Fifty microseconds of quiet at the end is the latch.
//
// The driver in mokxi_ws2812.h makes those pulses with a counted loop rather
// than a timer, because SYSTIMER ticks at 16 MHz, 62.5 ns a tick, and
// reading it takes longer than the pulse being measured. The RMT peripheral
// that would normally do this on a C6 is not modelled (docs/esp32c6.md section
// 7), so bit-banging is not a shortcut here: it is the only way.
//
// There is no floating point in this runtime, so the colour wheel is integers:
// 1536 steps round the six edges of the RGB cube, one channel at full, one at
// nothing and one sliding between them.

#include "mokxi_ws2812.h"

const uint8_t DIN_PIN = 8;
const uint16_t LEDS = 8;

// Three bytes an LED, green first: the order the wire wants them in.
uint8_t pixels[LEDS * 3];
Ws2812 strip(DIN_PIN, pixels, LEDS);

// Where the wheel has got to, and how far apart two neighbours are on it.
uint16_t phase = 0;
const uint16_t SPREAD = 1536 / LEDS;
const uint16_t SPEED = 16;

// Full scale on all eight would be a floodlight. This is a sixth of it.
const uint8_t LEVEL = 40;

const int FRAME_MS = 20;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi ESP32-C6: a rainbow on the RGB LED and seven more");
  strip.begin();
  strip.clear();
  strip.show();
}

void loop() {
  for (uint16_t i = 0; i < LEDS; i++) {
    uint8_t r;
    uint8_t g;
    uint8_t b;
    ws2812_wheel((uint16_t)(phase + i * SPREAD), &r, &g, &b);
    strip.setPixel(i, ws2812_scale(r, LEVEL), ws2812_scale(g, LEVEL),
                   ws2812_scale(b, LEVEL));
  }
  strip.show();

  phase = (uint16_t)((phase + SPEED) % 1536u);
  delay(FRAME_MS);
}