Spreadsheet

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Software

• This spreadsheet is intended for code of the following structure...
  
  
  
  

  // DECLARE GLOBAL VARIABLE
  static char cOneSecondFlag;
  
  
  
  
  
  // MAIN ROUTINE
  void main ( void )
      {
      .
      .
      // Timer0 (RTCC) roll is...
      //          256 * 1 / ( ClockFreq / PrescalerSetting / 4 )
      setup_counters ( RTCC_INTERNAL, RTCC_DIV_64 );
      .
      .
      // Enabling the RTCC (Timer0) interrupt will cause an RTCC
      // interrupt to occur every time the eight-bit timer rolls
      // from 255 to 0.  For a 4MHz clock and prescaler of 64,
      // and when the RTCC counter is reset to 8 after each interrupt
      // (so that it starts out at eight and counts to 255), the
      // interrupt will occur every 15.872mS.  The spreadsheet shows
      // that this will have an accuracy of 0.01%.
      //
      // And, a hint from Mike Rigby-Jones, 10/03/05, if you need to
      // pre-load the timer0 register to get a particular period, it's
      // a much better idea to add an offset to the current value rather
      // than just set a static value.  For instance, TMR0 += 5 rather
      // than TMR0 = 5.  The latter method will cause the period to vary
      // depending on how many cycles elapsed before the timer was loaded.
      // By adding an offset, the interrupt period becomes insensitive
      // to interrupt latencies or even the position of the statement
      // within the interrupt code.
      
      enable_interrupts ( INT_RTCC );
      enable_interrupts ( GLOBAL );
      .
      .
      while ( 1 )   // main loop is here
          {
          while ( cOneSecondFlag == 0 ) // interrupt turns this flag on
              {
              // drops thru to here when one second interval completed
              .
              .
              cOneSecondFlag = 0;  // reset flag to wait for next second
              }
          }
      .
      .
      }
  
  
  
  
  
  // INTERRUPT ROUTINE
  #int_rtcc
  void Timer0Interrupt ( void )
      {
      // Gets here every 15.872mS.
      // CtimerCount is incremented every time.
      // When it gets to 63 it has been 0.999936 seconds.
      if ( cTimer0Count < 63 )
          {
          cTimer0Count++;
          }
      else
          {
          // turn on one-second flag, turned off later by main routine
          cOneSecondFlag = ON;
          // start count over again
          cTimer0Count = 0;
          }
      // reset timer for 248 counts, rather wrap at 255
      set_rtcc ( 8 );
      }