#define F_CPU 1000000
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#define TIMER_SNAPSHOT_MS() ((ticks << 8) | TCNT0)
#define DIAL_DELAY   4500    // How much time to allow for dialing (ms)
#define DEB_DELAY    2       // Debounce delay


// Variable declarations
volatile int call_recipient; // The recipient of the phone call
volatile int caller; // The caller
volatile int dial_var; // The number of pulses received by the caller phone
volatile uint32_t ticks;
volatile uint32_t DialTimeSnapshot;
volatile uint32_t DebounceSnapshot;

/* Interrupt service routine to keep track of time
 */
ISR(TIMER0_OVF_vect)
{
  ticks++;
  return;
}

/* Interrupt service routine for pulse dialing sensing
 * Increments dial_var for each pulse received
 */
ISR(PCINT_vect)
{
  if ((TIMER_SNAPSHOT_MS() - DebounceSnapshot) >= DEB_DELAY) {
    dial_var++;
    DebounceSnapshot = TIMER_SNAPSHOT_MS();
  }
  return;
}

/* Initialise I/O ports */
void
ioinit(void)
{
  // Setup outputs
  DDRB |= (1 << PB4); // Ringer output for telephone 1
  DDRB |= (1 << PB5); // Ringer output for telephone 2
  DDRB |= (1 << PB6); // Ringer output for telephone 3
  DDRB |= (1 << PB7); // Ringer output for telephone 4

  DDRD |= (1 << PD0); // Voice path activation output for telephone 1
  DDRD |= (1 << PD1); // Voice path activation output for telephone 2
  DDRD |= (1 << PD2); // Voice path activation output for telephone 3
  DDRD |= (1 << PD3); // Voice path activation output for telephone 4

  DDRD |= (1 << PD4); // Special function 6
  DDRD |= (1 << PD5); // Special function 5
  DDRD |= (1 << PD6); // Special function 7
  DDRA |= (1 << PA0); // Special function 8
  DDRA |= (1 << PA1); // Special function 9

  
  // Make sure no telephone is ringing, nor connected to the voice path
  // and that all special function outputs are off
  PORTB &= ~ _BV(4); // Ringer output for telephone 1
  PORTB &= ~ _BV(5); // Ringer output for telephone 2
  PORTB &= ~ _BV(6); // Ringer output for telephone 3
  PORTB &= ~ _BV(7); // Ringer output for telephone 4
  PORTD &= ~ _BV(0); // Voice path activation output for telephone 1
  PORTD &= ~ _BV(1); // VOice path activation output for telephone 2
  PORTD &= ~ _BV(2); // Voice path activation output for telephone 3
  PORTD &= ~ _BV(3); // Voice path activation output for telephone 4
  PORTD &= ~ _BV(4); // Special function 6
  PORTD &= ~ _BV(5); // Special function 5
  PORTD &= ~ _BV(6); // Special function 7
  PORTA &= ~ _BV(0); // Special function 8
  PORTA &= ~ _BV(1); // Special function 9

  // Enable internal pull-ups
  PORTB |= _BV(0); // Off-hook detection, and pulse dialing for telephone 1
  PORTB |= _BV(1); // Off-hook detection, and pulse dialing for telephone 2
  PORTB |= _BV(2); // Off-hook detection, and pulse dialing for telephone 3
  PORTB |= _BV(3); // Off-hook detection, and pulse dialing for telephone 4

  // Enable interrupts for PB0-PB3 (offhook, and pulse dialing detection)
  GIMSK |= _BV(PCIE);
  sei();

  // Reset variables
  caller = 0;
  call_recipient = 0;
  dial_var = 0;
  DialTimeSnapshot = 0;
  DebounceSnapshot = 0;

  return;
}

/* Initiate the timer */
void
timer_init(void)
{
  TCCR0B = _BV(CS02) | _BV(CS00); /* set prescaler to use clock/1024  */
  TIMSK  = _BV(TOIE0);   /* enable the overflow interrupt */

  ticks = 0;
}

// Ring the telephone
void ring(int times, int phone) {
  int i;
  for(i=0;i<times;i++){
    PORTB |= _BV(3 + phone);
    _delay_ms(500);
    PORTB &= ~ _BV(3 + phone);
    _delay_ms(500);
    /* If the phone we are ringing has been
     * picked up, exit the function immediately */
    if ((PINB & _BV(-1 + phone)) == 0) {
      i=times;
    }
  }
}

int main(void)
{

  // Initialise I/O and timer
  ioinit();
  timer_init();
  
  // Main loop
  while(1)
    {
      // Check if a telephone is off hook
      // and, if yes, set 'caller' to
      // its number, and temporarily enable
      // interrupts PCINT_vect on the input pin associated
      // to that phone (and only that one).
      if ((PINB & _BV(0)) == 0) { // Telephone 1 
	_delay_ms(100);
	caller = 1;
	PCMSK |= _BV(PCINT0);
      } else if ((PINB & _BV(1)) == 0) { // Telephone 2 
	_delay_ms(100);
	caller = 2;
	PCMSK |= _BV(PCINT1);
      } else if ((PINB & _BV(2)) == 0) { // Telephone 3 
	_delay_ms(100);
	caller = 3;
	PCMSK |= _BV(PCINT2);
      } else if ((PINB & _BV(3)) == 0) { // Telephone 4
	_delay_ms(100);
	caller = 4;
	PCMSK |= _BV(PCINT3);
      }      

      // If a telephone has been lifted and is
      // presumably about to dial, allow it
      // DIAL_DELAY time to complete dialing,
      // store the dialled number in 'call_recipient',
      // and disable interrupts again on input pins
      // for all phones.
      if (caller > 0) {
	_delay_ms(DIAL_DELAY);
	PCMSK &= ~ _BV(PCINT0);
	PCMSK &= ~ _BV(PCINT1);
	PCMSK &= ~ _BV(PCINT2);
	PCMSK &= ~ _BV(PCINT3);
	call_recipient = dial_var / 2;
	dial_var = 0;
      }

      // If a telephone has dialed a number associated with another phone
      if ((call_recipient > 0) && (call_recipient < 5)) {
	// As long the recipient doesn't answer, and the caller doesn't give up
	while ((PINB & _BV(-1 + call_recipient)) &&
	       !(PINB & _BV(-1 + caller))) {
	  // Ring the phone
	  ring(2, call_recipient);
	  _delay_ms(500);
	}
	// Check if both parties are still on the line
	if (!(PINB & _BV(-1 + call_recipient)) && !(PINB & _BV(-1 + caller))) {
	  // Both parties are on the line
	  // Provide a short delay
	  _delay_ms(300);
	  // Then connect the call
	  PORTD |= _BV(-1 + caller);
	  _delay_ms(100);
	  PORTD |= _BV(-1 + call_recipient);
	}

	// Wait for the call to be over
	while (!(PINB & _BV(-1 + call_recipient)) && !(PINB & _BV(-1 + caller))) {
	  _delay_ms(1);
	}

	// Then disconnect the call
	_delay_ms(500);
	PORTD &= ~ _BV(-1 + caller);
	_delay_ms(100);
	PORTD &= ~ _BV(-1 + call_recipient);
	
      } else if (call_recipient == 6) {        /* If the dialled number is any of the special functions,
						* activate/deactivate the corresponding GPIO output.
						* When dealing with numbers greater than 9, remember,
						* call_recipient contains a count of pulses. Thus,
						* if 51 was dialled, call_recipient would contain
						* the value of 5 + 1, thus 6, and NOT 51. The digit 0
						* is equal to 10 pulses */
	PORTD |= _BV(5);
      } else if (call_recipient == 15) {
	PORTD &= ~ _BV(5);
      } else if (call_recipient == 7) {
	PORTD |= _BV(4);
      } else if (call_recipient == 16) {
	PORTD &= ~ _BV(4);
      } else if (call_recipient == 8) {
	PORTD |= _BV(6);
      } else if (call_recipient == 17) {
	PORTD &= ~ _BV(6);
      } else if (call_recipient == 9) {
	PORTA |= _BV(0);
      } else if (call_recipient == 18) {
	PORTA &= ~ _BV(0);
      } else if (call_recipient == 10) {
	PORTA |= _BV(1);
      } else if (call_recipient == 19) {
	PORTA &= ~ _BV(1);
      } 

      // Reset the variables
      caller = 0;
      call_recipient = 0;
    }  

  // We will never get here
  return 0;
}