/************************************************************************/
/*                                                                      */
/*                      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
}