RPM sensor and reading

Projects General Guidance
2

Well one technique that you can use is to connect the relevant pin to an interrupt (pins 2 or 3 on regular Arduinos) and have the interrupt handler record the time difference between successive pulses.

#define MINUTE (60 * 1000000L)
#define FIVE_MINUTES  (5 * MINUTE)

volatile unsigned long last_ts, current_ts, last_period, rotation_count ;

setup ()
{
  current_ts = micros () ;
  last_period = FIVE_MINUTES ;
  last_ts = current_ts - last_period ; // force initial period to be large enough that RPM reads 0
  rotation_count = 0L ;
  attachInterrupt (0, handle_ints, RISING) ; // pin 2 is channel 0.  Can trigger on RISING or FALLING, shouldn't matter
}

void handle_ints ()
{
  current_ts = micros () ;     // get time of transition
  rotation_count ++ ;
  last_period = current_ts - last_ts ;  // determine period of last rotation
  last_ts = current_ts ;
}

unsigned long latest_period ()
{
  cli () ;    // inhibit interrupts while reading the long value to prevent garbling
  unsigned long result = last_period ;
  sei () ;
  return result ;
}

unsigned long get_rotations ()
{
  cli () ;    // inhibit interrupts while reading the long value to prevent garbling
  unsigned long result = rotation_count ;
  sei () ;
  return result ;
}

Or similar - calling latest_period() gives the most recent rotation period, which can be converted to RPM thus:

  rpm = MINUTE / latest_period() ;

You can also keep track of total rotations against the clock using get_rotations ()