Calculating PWM timing using interrupts.

pwm_interrupt.ino

#include <EnableInterrupt.h>

const int CH_1_PIN = 9;

volatile int pwmValue;
volatile unsigned long prevTime = 0;
 
void setup() {
  Serial.begin(9600);
  enableInterrupt(CH_1_PIN, rising, RISING);
}
 
void loop() {
  delay(33); // 30 FPS
  Serial.println(pwmValue);
}
 
void rising() {
  prevTime = micros();
  enableInterrupt(CH_1_PIN, falling, FALLING);
}
 
void falling() {
  pwmValue = micros()-prevTime;
  disableInterrupt(CH_1_PIN);
  enableInterrupt(CH_1_PIN, rising, RISING);
}