/************************************************************************************/
/*                                 port_d_check                                     */
/*Reads all port D switches and places them in a 16 location circular queue and     */
/*logically ANDs all columns.  If all 16 samples of the bit is true, a one set      */
/*in that bit position in the global variable "debounced_port_d".  Also returned    */
/* is a "1" if something did change, else a "0" is retuened. Since port D is an     */
/*active low asserted port, if a button is pushed and qualifies for the debounce,   */
/*it returns a "1" and not a zero.  In othe words, it inverts the state of Port D.  */
/************************************************************************************/
uint8_t port_d_check() {
static uint8_t index = 0;   //queue pointer 
static uint8_t state[16] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};  //circular queue for state
uint8_t i,new_state;

  state[index] = (PIND ^ 0xFF); //put inverted PIND inputs into queue
  ++index;                      //advance pointer
  if(index>=16) index=0;        //wrap the queue pointer if necessary 
  
  new_state = 0xFF;    //initalize summing variable
  for(i=0; i<15; i++)  //logically AND all column bits of port D
    new_state = new_state & state[i];

  if(new_state == debounced_port_d){return 0;} //everything still the same
  else{                //something changed
    debounced_port_d = new_state; //pass back new value through global variable
    return 1;          //note that something happened
  }
} //port_d_check