Timer Based Interrupts syntax (adafruit flow meter)

Firmware
2

There is an ingenious library that wraps the timer programming up in an easy to use way
https://build.particle.io/libs/SparkIntervalTimer/1.3.7

And I’d count the sensor revolutions (and measure the speed) this way

#include "SparkIntervalTimer.h"

IntervalTimer msTimer;	

volatile uint32_t usLastTime  = 0;
volatile uint32_t usDeltaTime = 0;
volatile uint32_t msCount     = 0;
volatile double   revPerSec   = 0;
volatile double   revPerMS    = 0;

void msISR() {
  revPerMS = msCount;
  msCount = 0;
}

void senseISR() {
  uint32_t us = micros();
  msCount++;
  usDeltaTime = us - usLastTime;
  usLastTime = us;
  revPerSec =  1000000.0 / usDeltaTime;
}

void setup() {
  ...
  msTimer.begin(msISR, 1000, uSec);  // trigger every 1000µs

  pinMode(FLOWSENSORPIN, INPUT_PULLUP);
  attachInterrupt(FLOWSENSORPIN, senseISR, FALLING);
  ...
}