#asm
.equ __lcd_port=0x15
#endasm

#include <lcd.h>
#include <stdio.h>
#include <delay.h>
#include <mega16.h>

// quartz crystal frequency [Hz]
#define F_XTAL 3686400L
// PIND0..3 will be row inputs
#define KEYIN PIND
// PORTD4..7 will be column outputs
#define KEYOUT PORTD
// used for TIMER0 count initialization
#define INIT_TIMER0 TCNT0=0x100L-F_XTAL/64L/500L
#define FIRST_COLUMN 0x80
#define LAST_COLUMN 0x10

typedef unsigned char byte;
// store here every key state as a bit,
// bit 0 will be KEY0, bit 1 KEY1,...
unsigned keys;
// LCD display buffer
char buf[33];

// TIMER 0 interrupt at every 2 ms
interrupt [TIM0_OVF] void timer0_int(void)
{
static byte key_pressed_counter=20;
static byte key_released_counter,column=FIRST_COLUMN;
static unsigned row_data,crt_key;
// reinitialize TIMER0
INIT_TIMER0;
row_data<<=4;
// get a group of 4 keys in in row_data
row_data|=~KEYIN&0xf;
column>>=1;
if (column==(LAST_COLUMN>>1))
{
column=FIRST_COLUMN;
if (row_data==0) goto new_key;
if (key_released_counter) --key_released_counter;
else
{
if (--key_pressed_counter==9) crt_key=row_data;
else
{
if (row_data!=crt_key)
{
new_key:
key_pressed_counter=10;
key_released_counter=0;
goto end_key;
};
if (!key_pressed_counter)
{
keys=row_data;
key_released_counter=20;
};
};
};
end_key:;
row_data=0;
};
// select next column, inputs will be with pull-up
KEYOUT=~column;
}

// test if a key was pressed
unsigned inkey(void)
{
unsigned k;
if (k=keys) keys=0;
return k;
}

void init_keypad(void)
{
DDRD=0xf0;
INIT_TIMER0;
TCCR0=3;
TIMSK=2;
#asm("sei")
}

main() {
unsigned k;
init_keypad();
lcd_init(20);
lcd_putsf("CVAVR Keypad");
// read keys and display key code
while (1)
{
lcd_gotoxy(0,1);
if (k=inkey())
{
sprintf(buf,"Key code=%Xh",k);
lcd_puts(buf);
}
else lcd_putsf("NO KEY ");
delay_ms(500);
}
}