source: avrstuff/libs/amiga_keyboard/amiga_keyboard.c@ b82c7b3

/* Amiga Keyboard reading on Atmel AVR
 * Copyright 2010, Adrien Destugues <pulkomandy@pulkomandy.ath.cx>
 * Distributed under the terms of the MIT Licence */

#define F_CPU 16000000UL

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <util/delay.h>

#include "keymap.h"


//PIN configuration
#define AK_CLK PD3 /* Also INT1 */
#define AK_PORT PIND
#define AK_DATA PD4

volatile uint8_t kbd_data;
volatile uint8_t char_waiting;
uint8_t started;
uint8_t bit_count;
uint8_t shift;


// Notify the keyboard we got the last char right
inline void acknowledge_char()
{
        // We have to pull down the "DATA" line
        
        // Set it as an input
        DDRD |= (1<<AK_DATA);

        // Down for 85 us
        PORTD &= ~(1<<AK_DATA);
        _delay_us(85);

        // Up again to let the keybord talk
        PORTD |= (1<<AK_DATA);

        // And it's now an input again
        DDRD &= ~(1<<AK_DATA);
}


// Interrupt vector - Triggered when there is activity on the clock line
ISR(INT1_vect)
{
        PORTD ^= (1<<PD6);

        //make sure clock line is low, if not ignore this transition
        if(AK_PORT & (1<<AK_CLK)){
                return;
        }

        // First bit, reset everything then continue below
        if(!started){
                started = 1;
                bit_count = 0;
                kbd_data = 0;
        } 

        if(bit_count < 8) { //we started, read in the new bit
                //put a 1 in the right place of kdb_data if PC2 is high, leave
                //a 0 otherwise
                if(!(AK_PORT & (1<<AK_DATA))) kbd_data |= (128>>bit_count);
                bit_count++;
        } 
        
        if(bit_count >= 8)
        {  // enough bits
                started = 0;
                bit_count = 0;

                /*
                if(kbd_data == 0xF0){ //release code
                        release = 1;
                        kbd_data = 0;
                        return;
                } else if (kbd_data == 0x12) { //hanlde shift key
                        if(release == 0){
                                shift = 1;
                        } else {
                                shift = 0;
                                release = 0;
                        }
                        return;
                } else { //not a special character
                        if(release){ //we were in release mode - exit release mode
                                release = 0;
                                //ignore that character
                        } else {
                                char_waiting = 1;
                        }
                }*/
                char_waiting=1;
        }
}


char ak_scancode_to_ascii(uint8_t data){
        char to_ret = pgm_read_byte(&(keymap[data])); //grab character from array
        if(shift) to_ret -= 0x20;
        return to_ret;
}


uint8_t ak_read_scancode(){
        while(!char_waiting);

        while (AK_PORT & (1<<AK_CLK) == 0);
        _delay_us(30);
        acknowledge_char();

        char_waiting = 0;
        return kbd_data;
}


void ak_init_keyboard(){
        started = 0;
        kbd_data = 0;
        bit_count = 0;

        //make AK_CLK input pin
        DDRD &= ~(1<<AK_CLK);
        //turn on pullup resistor
        AK_PORT |= (1<<AK_CLK);

        // Interrupt configuration
        // We use INT1 falling edge
        MCUCR |= (1<<ISC11);
        MCUCR &= ~(1<<ISC10);

        // Enable INT1
        GIMSK |= (1<<INT1);

        // Enable interrupts
        sei();

        acknowledge_char();
}