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#include "range.h"

extern "C"

{

#include <avr/io.h>

#include <avr/interrupt.h>

}

/* Ultrasonic Sensor:

 * -if RX pin is left open, it will continuously take readings

 * -PW output is 147us/in.

 * -PW will be high for a maximum of 37.5ms if no target is detected

 * 

 * 37.5ms * 8 MHz / 8 prescaler = 37500 max wait

 */

// so that we can test on a 328

#if defined(__AVR_ATmega128RFA1__)

#  define read_INT0 (PIND & _BV(PD0))

#  define read_INT1 (PIND & _BV(PD1))

#elif defined(__AVR_ATmega328P__) || defined(__AVR_ATmega328__)

#  define read_INT0 (PIND & _BV(PD2))

#  define read_INT1 (PIND & _BV(PD3))

#else

#  error "Please define read_INTx for this device"

#endif

struct range_t {

  unsigned int start; // timer value on rising edge

  unsigned int value; // last measured range

} range[2];

static void on_edge(int which)

{

  unsigned int time = TCNT1;

  if (read_INT0)

  {

    range[which].start = time;

  }

  else

  {

    // if timer overflowed since start, this arithmetic should still work out

    range[which].value = time - range[which].start;

  }

}

ISR(INT0_vect)

{

  on_edge(0);

}

ISR(INT1_vect)

{

  on_edge(1);

}

void range_init()

{

  // ISCx = 1, edge triggered

  EICRA |= _BV(ISC10) | _BV(ISC00);

  // enable INT0 and INT1

  EIMSK |= _BV(INT1) | _BV(INT0);

  // CS1 = 2, 1/8 prescaler

  TCCR1B = _BV(CS11);

  range[0].value = RANGE_ERR;

  range[1].value = RANGE_ERR;

}

unsigned int range_get(int which)

{

  unsigned int ret;

  if (0 <= which && which <= 1)

  {

    cli();

    ret = range[which].value;

    sei();

    return ret;

  }

  else return RANGE_ERR;

}