/* Routine to decode quadrature signals on the AT89C2051 */

/* Basic Algorithm:
 * 
 * Program monitors 2 external input lines, for phases A and B of the quadrature signal.
 * Use an interrupt service routine to trigger a read, or just poll.
 * Compare the 'current' state of the inputs with the 'last' state of the inputs and 
 *   use a truth table to figure out which way we are going.
 * Use this info to increment or decrement a counter.
 * 
 *    | Last State | Current State | Direction |
 *    |  A    B    |    A     B    |           |
 *    |----------------------------------------|
 *    |  0    0    |    0     1    |   dec     |
 *    |  0    0    |    1     0    |   inc     |
 *    |            |               |           |
 *    |  0    1    |    1     1    |   dec     |
 *    |  0    1    |    0     0    |   inc     |
 *    |            |               |           |
 *    |  1    0    |    0     0    |   dec     |
 *    |  1    0    |    1     1    |   inc     |
 *    |            |               |           |
 *    |  1    1    |    1     0    |   dec     |
 *    |  1    1    |    0     1    |   inc     |
 *    |            |               |           |
 * 
 */

#include <at89x2051.h>

/* global variables */
int counter;
bit a, b, am1, bm1, dir;

void quad_dec(void);

void main(void)
{
	/* set port 1 for output mode */
	P1 = 0x00;

	/* set these 2 bits for input mode */
	P1_0 = 1;
	P1_1 = 1;

	a=0;
	b=0;
	am1=0;
	bm1=0;
	dir=0;

	while(1) {
		
		a = P1_0;
		b = P1_1;
		
		quad_dec();
		
		am1 = a;
		bm1 = b;
		
		P1_2 = counter&0x00000001;
		P1_3 = (counter&0x00000002)>>1;
		P1_4 = (counter&0x00000004)>>2;
		P1_5 = (counter&0x00000008)>>3;
		P1_7 = dir;
	}

}


void quad_dec(void)
{
	if( !am1 && !bm1) {	/* <00> - state #3 */

		if( a && !b ) {
			++counter;
			dir = 1;

		} else if( !a && b ) {
			--counter;
			dir = 0;
		}

	} else if( !am1 && bm1 ) { /* <01> - state #2 */

		if( a && b ) {
			--counter;
			dir = 0;

		} else if( !a && !b ) {
			++counter;
			dir = 1;
		}

	} else if( am1 && !bm1 ) { /* <10> - state #0 */

		if( a && b ) {
			++counter;
			dir = 1;

		} else if( !a && !b ) {
			--counter;
			dir = 0;
		}

	} else { /* <11> - state #1 */

		if( a && !b ) {
			--counter;
			dir = 0;

		} else if( !a && b ) {
			++counter;
			dir = 1;
		}

	}
}