General Purpose Timer 1

Hi

I have been going through example and I am struck on one thing....

Need to generate 250mS

So I look at the table, the fastest rate which allws timer T3 to have a period of 250mS is 156.25 kHz hence T31= 100b

250mS/420 = 59.52 where 420mS is the maximum period.

The intial value of the timer register is 39009 (65535*0.595)...

I am still baffled with value of T3_RELOAD=100 so can someone kindly tell where 100 come from?

Thank you

#include <reg167.h>

#define T3_RELOAD 39009      /* T3 reload value : 250 ms */
//#define T3_RELOAD 100      /* T3 reload value : 250 ms */

/* Timer T3 ISR */
void T3_elapsed(void) interrupt 0x23 {

   T3 = T3_RELOAD;           /* reset T3 */
   P2 ^= 0x0001;             /* toggle P2.1 */

}

/* main program */
void main(void) {

  DP2 |= 0x0001;             /* P2.1 : output */
  P2  |= 0x0001;             /* initially: LED off */

  T3 = T3_RELOAD;            /* load initial timer value T3 */

  /* T3 in timer mode, counting down, pre-scale factor 128, period: 420 ms  */
  /* alternate output function disabled */
  /* T3CON = 0000.0000.1000.0100 = 0x0084 */
  T3CON = 0x0084;
  T3IC  = 0x0044;            /* enable T3 interrupt, ILVL = 1, GLVL = 0 */

  IEN = 1;                   /* allow all interrupts to happen */

  T3R = 1;                   /* start timer (T3CON |= 0x0040) */

  while(1);                  /* forever... */

} /* main */