LowFrequencyOnPin.pde

// Low frequency output on an arbitrary pin
//
// Brett Hagman (bhagman@loftypremises.com)
// (or bhagman@wiring.org.co)

const int lfPin = 6;

// Number of milliseconds between flipping the pin state
// 30 Hz = twice every 1/30 second
// Frequency = f
// time between flips = (1/f)/2 in seconds
//                    = 1/(2*f)
//                    = 1/(2*f) * 1000000 in microseconds
//                    = 1000000/(2*f)
//                    = 500000/x microseconds
#define FlipTime(x) (500000L / x)

#define myFrequency 30

void setup()
{
  pinMode(lfPin, OUTPUT);
}

void loop()
{
  static uint32_t lastFlip = 0;
  static bool state = false;

  if ((micros() - lastFlip) > FlipTime(myFrequency))
  {
    state = !state;
    digitalWrite(lfPin, state);
    lastFlip = micros();
  }
  
  // You can do whatever you want here, but remember that it can't
  // take up too much time (i.e. less than FlipTime(x))
}