//	ATMEL AVR based s88 feedback unit
//
//	This software was developed using a MEGA162, can be compiled for other AVR's as well
//
//	S88 clock should be connected to INT0 pin, other S88 signals can be routed to any available port
//
//	PINA and PINB ports are used as inputs for the shift register.
//
//	This software emulates a 64 bits s88 unit in the s88_val[] array
//
//	S88_val[0] will hold the inputs from PINB
//	S88_val[1] will hold the inputs from PINA
//
//	Other S88_val[] values can be calculated, or can be derived using a keyboard matrix scan 

#include  <avr/io.h>
#include  <avr/interrupt.h>

#define S88_CLOCK 2 // S88 Clock,       PD2 (with INT0)
#define S88_LOAD  3 // S88 Latch bus,   PD3 
#define S88_OUT   4 // S88 Data Output, PD4
#define S88_RESET 5 // S88 Reset,  		PD5	(not further used in this code, only used to enable port pullup)
#define S88_IN    6 // S88 Data Input, 	PD6 (not further used in this code, only used to enable port pullup)

unsigned char col2val[8] = 
{
    0b10000000,         // 0
    0b01000000,         // 1
    0b00100000,         // 2
    0b00010000,         // 3
    0b00001000,         // 4
    0b00000100,         // 5
    0b00000010,         // 6
    0b00000001,         // 7
};

volatile unsigned char s88_val[10]; 	//	this array holds the values of the shift register.

volatile unsigned char  cnt,			//	s88 clock counter
						col,			//	3 lowest bits, derived from counter, value 0 .. 7
						row,			//	5 (or 3 to start with highest bits, derived from counter
						val;

ISR (INT0_vect)							//	INT0 = S88 clock 
{
	if (PIND & _BV(S88_LOAD))			//	restart shift register if load signal is active
	{
		cnt = 0;						//	restart is done by starting over at the first element again
	}

	col = cnt & 0b00000111;				//	calculate row and column using counter value, column = last 3 bits

	row = cnt & 0b00111000;				//	mask 3 bits for row

	row >>= 3;							//	row = now last 3 bits

	val = s88_val[row] & col2val[col];	//	get value based upon row and column

	if(val)								//	set dataout bit based upon value
	{
		PORTD |= _BV(S88_OUT);
	}
	else
	{
		PORTD &= ~_BV(S88_OUT);
	}

	cnt++;								//	point to next value in shift register
}

void setup_hw(void)
{
	MCUCR |= _BV(ISC00) 				//	INT0 enable at rising edge (S88 clock)
		  |  _BV(ISC01);  

	GICR = _BV(INT0);					// 	Enable INT0

	DDRA  = 0x00;						//	PORTA = input
	PORTA = 0xff;						//	enable pull up's for inputs

	DDRB  = 0x00;						//	PORTB = input
	PORTB = 0xff;						//	enable pull up's for inputs

	DDRD  |= _BV(S88_OUT);   			//	S88 Data = output

	PORTD |= _BV(S88_CLOCK)				//	enable pull up's for inputs
		  |  _BV(S88_LOAD) 
		  |  _BV(S88_RESET)
		  |  _BV(S88_IN);
}

int main (void)
{		

    setup_hw();					//	Initialise

	cnt = 0;					//	initialize shift register, start with first element

    sei();						//	enable interrupts

    while(1)					//	Loop forever, read inputs and fill arrays in memory
	{		
		s88_val[0] = ~PINB;		//	inputs are active low, so xor port value

		s88_val[1] = ~PINA;
	} 

    return (0);
}