ESP32-C3 PWM fade

// PWM fade: a breathing LED on hardware PWM.
//
// The `fade` example next door bit-bangs its waveform: it holds the pin high
// for a while and low for a while, and the core is awake for every microsecond
// of it. This one hands the pin to LEDC, the chip's LED PWM controller, and
// then does nothing but change a number thirty-three times a second. The
// waveform comes out of the hardware, so the core spends its life asleep and
// the edges still land where they should.
//
// analogWrite is the Arduino spelling and it is all this needs: 0 to 255 at
// 1 kHz on any GPIO, because LEDC reaches the pins through the GPIO matrix and
// not through a fixed set of `~` pins.
//
// Wire an LED and a resistor from GPIO 4 to ground to watch it; GPIO 8, the
// DevKitM-1's own LED, fades along with it.

const uint8_t LED = 4;
const int STEPS = 64;
const unsigned long STEP_MS = 30;  // a whole breath takes about two seconds

// One cycle of (1 - cos)/2 scaled to 0..255. A linear ramp looks wrong: the
// eye is roughly logarithmic, so an even climb in duty is an uneven climb in
// brightness, and this is the shape that reads as breathing.
const uint8_t breath[STEPS] = {
    0,   1,   2,   6,  10,  16,  22,  29,
   38,  47,  57,  68,  79,  91, 103, 116,
  128, 140, 153, 165, 177, 188, 199, 209,
  218, 226, 234, 240, 246, 250, 253, 255,
  255, 255, 253, 250, 246, 240, 234, 226,
  218, 209, 199, 188, 177, 165, 153, 140,
  128, 116, 103,  91,  79,  68,  57,  47,
   38,  29,  22,  16,  10,   6,   2,   1,
};

void setup() {
  // Nothing to do: the first analogWrite attaches the pin to a free channel.
}

void loop() {
  for (int step = 0; step < STEPS; step++) {
    analogWrite(LED, breath[step]);
    analogWrite(LED_BUILTIN, breath[step]);
    delay(STEP_MS);
  }
}