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#include  <msp430x20x2.h>

#define LED0 BIT0
#define LED1 BIT6
#define PHASE1 BIT1
#define PHASE2 BIT2
#define PHASE_MASK 6
#define PHASE_SHIFT 1

int state;     // used to hold current state of encoder bits (00, 01, 10 or 11) 
int lastState; // used to hold previous state of encoder bits (00, 01, 10 or 11) 
int pos;  // used to count steps. A 1/150" encoder (the most common kind) yields 600 steps per inck. 

// -- prototype of common bit of code used to compute the present state value 
void updateState(void);

int main(void)
{
	WDTCTL = WDTPW + WDTHOLD; // Stop watchdog timer 
	// --- set up some initial values 	
        pos = 0;
	P1DIR &= ~(PHASE1 + PHASE2); // Set P1.1 and P1.2 as inputs 	
        P1DIR |= (LED0 + LED1); // Set P1.0 and P1.6 to outputs. These are the LEDs on a LaunchPad. 

	// --- get initial conditions so that the first interrupt 	
 // has the right "edge" values and so that position calculations are based on feasible transitions 	
        updateState();
	lastState = state;

	P1IFG &= ~(PHASE1 | PHASE2); // clear interrupt flags for P1.1 and P1.2 	
        P1IE |= (PHASE1 | PHASE2); // P1.1 and P1.2 interrupt enabled 
	__enable_interrupt(); // enable all interrupts 	
 for(;;)
	{ /* just loop forever and let the interrupts do all the work */ }
}

// updateState() calculates updates for the edge detectors used in the interrupts 
// and also updates the "lastState" variable 
void updateState(void){
	state = P1IN & PHASE_MASK;

	if (state & PHASE1)
		P1IES |= PHASE1; // PHASE1 high, then interrupt on falling edge 	
        else
		P1IES &= ~PHASE1; // PHASE1 low, then interrupt on rising edge 
	if (state & PHASE2)
		P1IES |= PHASE2; // PHASE2 high, then interrupt on falling edge 	
        else
		P1IES &= ~PHASE2; // PHASE2 low, then interrupt on rising edge 

	// ---"right-justify" state so that it is in the range 0..3	
       state =  (state >> PHASE_SHIFT);
}

// Port 1 interrupt service routine #pragma vector=PORT1_VECTOR
__interrupt void Port_1(void)
{
    updateState();

    // --- test for all clockwise incremental cases      
 // if we stepped clockwise, then increment the position     
    if ( (lastState == 0 & state == 2)
       ||(lastState == 2 & state == 3)
       ||(lastState == 3 & state == 1)
       ||(lastState == 1 & state == 0)){
         pos++;
    }
    // --- If we didn't move clockwise, then we must have moved counter-clockwise     
 // so decrement the position     
    else{
        pos--;
    }

    // --- record last state     
    lastState = state;

    // --- update LEDs just to help debug     
    // use "state" as intermediate variable so that we can see the output value in the debugger easily     
    // This is just so that we can see the encoder signal. No functional purpose.     
    state = state + ((state << 5) & 0x40);
    P1OUT = state;

    // --- clear the interrupt flags     
    P1IFG &= ~(PHASE1 | PHASE2); // clear interrupt flags for P1.1 and P1.2  
}

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