fan RPM computer

#include <Arduino.h>

#define PWM_PIN 23
#define TACH_PIN 22

volatile int ticks;
const int ticks_per_rev = 2;
volatile uint32_t old_tick_ts;
volatile uint32_t tick_ts;
static uint32_t ts = 0;

static void pwm_set(uint8_t value, void *state)
{
  // input is 8-bit
  // write a 4-bit duty
  ledcWrite(0, value >> 4);
}

static void update_ticks() {
  if((ticks++)==(ticks_per_rev-1)) {
    old_tick_ts = tick_ts;
    tick_ts = millis();
    ticks = 0;
  }
}
void setup()
{
  ticks = 0;
  tick_ts = 0;
  old_tick_ts = 0;
  Serial.begin(115200);
  ledcAttachPin(PWM_PIN, 0);
  ledcSetup(0, 25 * 1000, 4);
  attachInterrupt(TACH_PIN,update_ticks,RISING);
  Serial.println("Booted");
  pwm_set(255,nullptr);
}

void loop()
{
  double rpm = NAN;
  if(tick_ts>old_tick_ts && old_tick_ts!=0) {
    rpm = (60*1000.0)/(tick_ts-old_tick_ts);
  }
  if(millis()>ts+250) {
    ts=millis();
    Serial.print(rpm);
    Serial.println(" RPM");
  }
}