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TMR0用作定时器时，定时器时钟＝系统时钟/4； 
写TMR0时，会产生2个周期的的延时，如果不使用前置分频器，可通过设置 
初值进行补偿。 
例如 需要定时200个时钟周期时，TMR0 = 256 -200 + 2= 58

OPTION_REG:

bit5 T0CS:TMR0 Clock Source Select bit

       1 = Transition on T0CKI pin

       0 = Internal instruction cycle clock(CLKO)

bit 4 T0SE:TMR0 Source Edge Select bit

       1 = Increment on high-to-low transition on T0CKI pin

       0 = Increment on low-to-high transition on T0CKI pin

bit 3 PSA:Prescaler Assignment bit

      1 = Prescaler is assigned to the WDT

      0 = Prescaler is assigned to the Timer0 module

bit2-0 PS2:PS0:Prescaler Rate Select bits

给大家一个意见：如果想用TMR0实现精确的定时，一旦用了预分频器后就不要对TMR0做任何写操作。

注意：在中断服务程序中查询TMR0中断时，既要查询T0IE位，还要查询T0IF位。 
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#include  
#include "../head/config.h"

__CONFIG(HS&WDTDIS&LVPDIS&PWRTEN); 
//HS振荡,禁止看门狗,低压编程关闭,启动延时定时器

uint16 count = 0; 
void main(void) 
{ 
    PSA = 1;//prescaler is assigned to the WDT 
    T0CS  = 0;//Internal instruction cycle clock

    TMR0 = 58;//timer 200 cycle clock

    GIE = 1;//global interrupt enable 
    T0IE = 1;//TMR0 interrupt enable

    PORTD = 0x00; 
    TRISD = 0x00;

    while(1) 
    {} 
}

void interrupt ISR(void) 
{ 
    if(T0IE && T0IF) 
    { 
        T0IF = 0; 
        count++; 
        if(count == 1000) 
        { 
        count = 0; 
        PORTD = ~PORTD; 
        } 
        TMR0 = TMR0 + 58; 
    } 
}