Update src/fan_driver.h

src/fan_driver.h

@@ -108,6 +108,7 @@ static void fan_driver_compute_rpm(void) {
         delta += _fan_driver_delta[i];
     delta /= rpm_buf_size;
     fan_driver_current_rpm = delta ? 30000000 / 2 / delta : 0;
+    // delta is the number of ticks elapsed between tach pin pulses
     // each tick is 2us. one minute is 30000000 ticks, each rotation is 2 pulses
 }