/************************************************************************/
/*                                                                      */
/*                      Debouncing 8 Keys				*/
/*			Sampling 4 Times				*/
/*			With Repeat Function				*/
/*                                                                      */
/*              Author: Peter Dannegger                                 */
/*                      danni@specs.de                                  */
/*                                                                      */
/************************************************************************/
#pragma cd pl(9999)
#include <reg51.h>


#define KEY_INPUT	P2
#define LED_OUTPUT	P0


#define REPEAT_MASK	0x0F		// repeat only key 0 ... key 3
#define REPEAT_START	125		// after 500ms
#define REPEAT_NEXT	25		// every 100ms


char key_state;				// debounced and inverted key state:
					// bit = 1: key pressed
char key_press;				// key press detect or repeat


void t0_int( void ) interrupt INT_T0
{
  static char ct0, ct1, rpt;
  char i;

  TH0 |= -16;				// interrupt every 4.096ms at 12MHz

  i = key_state ^ ~KEY_INPUT;		// key changed ?
  ct0 = ~( ct0 & i );			// reset or count ct0
  ct1 = ct0 ^ ct1 & i;			// reset or count ct1
  i &= ct0 & ct1;			// count until roll over ?
  key_state ^= i;			// then toggle debounced state
  key_press |= key_state & i;		// 0->1: key press detect

  if( (key_state & REPEAT_MASK) == 0 )	// check repeat function
     rpt = REPEAT_START;		// start delay
  if( --rpt == 0 ){
    rpt = REPEAT_NEXT;			// repeat delay
    key_press |= key_state & REPEAT_MASK;
  }
}


#pragma disable				// disable interrupts
char get_key_press( void )
{
  char i;

  i = key_press;			// read keys
  key_press = 0;			// clear keys
  return i;
}


void main( void )
{
  key_state = 0;
  key_press = 0;

  TMOD = T0_M0_;			// T0: 16 bit timer
  IE = EA_ + ET0_;			// global + T0 interrupt enable  

  for(;;)				// main loop
    LED_OUTPUT ^= get_key_press();	// toggle LEDs on key press
}