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

#define KEY_INPUT	P2

char key_state, key_press;

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

  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 pressing detect
}