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how to produce milli seconds delay in pic timer0?

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suvaraj

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hi to all
i need to create milli second delay in pic timer0.i hav confuse with calculation.any one can help me to solve my problem.pls explain me the timer calculation clearly.
thank u
 

Hi, and first question - what is the frequency of XTAL?
 
Clock frequency divided by 4 gives instruction cycle time.
Multyply by timer prescale value gives timer increment time.

Example with 4 Meg clock and 1:8 prescale:

Instruction cycle time = 4/4 = 1M;

1/1000000 = 0.000001 seconds

Timer increment time = 0.000001 * 8 prescale = 0.000008 seconds;

Counts for 1mS = 0.001 / 0.000008 = 125;

Result 125 counts of timer take 1mS.

If you subtract the result from 256 and preload the timer with that value in an interrupt routine.
You will have a 1mS interrupt you can use as a clock;

Code: 
/*--- Global ---*/

volatile unsigned int sysclock;

/*--- Interrupt routine ---*/

void interrupt int_vector(void)
  { 

  if(T0IF)       /* Timer_0 overflow interrupt ~1mS system clock */
    {
    sysclock++;
  
    TMR0 = 131;  /* Preload Timer_0 for 1mS interrupt */
    T0IF = 0U;
    }
   }

/*--- Delay routine ---*/

void delay_mS(unsigned int ms)
  {
  unsigned int timer = sysclock;

  while((sysclock - timer) < ms){
    ;
    }
  }

You don't have to worry about overflow when subtracting one unsigned from another.

You could also use a macro and let the compiler work it out for itself.

Code: 
/*--- Macro to calculate 1mS Timer period (1:8 Prescale value) ---*/

#define FREQ_OSC  4.0
#define PRESCALE  8.0

#define TMR_1mS ((1000.0)/(((1.0)/((FREQ_OSC)/(4.0)))*(PRESCALE)))
 
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