PC fan speed and tachometer on an Arduino Uno

// PC fan speed and tachometer: a 4-pin 12 V PWM fan, its speed set by a
// 25 kHz PWM on pin 9 and read back from its tachometer on pin 2.
//
//   9       the fan's PWM wire (blue), Timer1's OC1A
//   2       the fan's TACH wire (green), with the Uno's own pull-up
//   GND     the fan's GND (black), shared with the 12 V adapter's minus
//   12 V    the adapter's plus to the fan's +12V (yellow)
//
// The fan's PWM input is pulled up inside it: left open it runs flat out,
// which is how a fan with no controller keeps a machine cool. Intel's fan
// spec asks for 25 kHz. analogWrite() on an Uno is 490 Hz, which most fans
// follow but sing at, so the sketch sets Timer1 itself: fast PWM, mode 14,
// TOP in ICR1 = 639, no prescaler, so 16 MHz / 640 = 25 kHz, and OCR1A sets
// the duty in 640 steps.
//
// TACH is an open collector that pulls low twice a turn, so rpm is the
// falling edges in a second times 30. The fan here is a Noctua NF-A12x25 PWM:
// 2000 rpm at 100 %, 450 rpm at 20 % and under, and stopped at 0 %.
//
// Press Run and watch the serial monitor: the duty steps 100, 75, 50, 25, 0
// percent, six seconds each, and the rpm follows as the rotor spins down.

const int PWM_PIN = 9;   // OC1A
const int TACH_PIN = 2;  // INT0

const int STEPS[] = {100, 75, 50, 25, 0};
const int STEP_COUNT = 5;
const unsigned long STEP_MS = 6000;

volatile unsigned int pulses = 0;

void countPulse() {
  pulses++;
}

// Duty in percent. 0 % takes the pin off the timer and holds it low: fast PWM
// with OCR1A at 0 still gives one high tick each period.
void setDuty(int percent) {
  if (percent <= 0) {
    TCCR1A &= ~(1 << COM1A1);
    digitalWrite(PWM_PIN, LOW);
    return;
  }
  OCR1A = (unsigned int)((long)percent * 640 / 100 - 1);
  TCCR1A |= (1 << COM1A1);
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: PC fan speed and tachometer");
  pinMode(PWM_PIN, OUTPUT);
  TCCR1A = (1 << WGM11);
  TCCR1B = (1 << WGM13) | (1 << WGM12) | (1 << CS10);
  ICR1 = 639;
  pinMode(TACH_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(TACH_PIN), countPulse, FALLING);
  setDuty(STEPS[0]);
}

int step = 0;
unsigned long stepStart = 0;
unsigned long lastPrint = 0;

void loop() {
  unsigned long now = millis();
  if (now - stepStart >= STEP_MS) {
    stepStart = now;
    step = (step + 1) % STEP_COUNT;
    setDuty(STEPS[step]);
  }
  if (now - lastPrint >= 1000) {
    lastPrint = now;
    noInterrupts();
    unsigned int n = pulses;
    pulses = 0;
    interrupts();
    Serial.print("duty ");
    Serial.print(STEPS[step]);
    Serial.print("%  ");
    Serial.print((long)n * 30);
    Serial.println(" rpm");
  }
}