#include "reg51.h"

#define KEY_INPUT       P1

#define uchar unsigned char


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


void int_t0( 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 press detect
}