[求助]关于延时程序

#include<pic.h>
             int counter;
             void main()
             {   
             ECCPAS=10;         
             counter=0;
             OPTION=0xc3;
             TMR0=5;
             T0IE=1;
             GIE=1;
             while(1);
             }
             void interrupt timer0_isr(void)
             {
             T0IF=0;
             TMR0=5;
             counter++;
             if(counter==250)
             {
             counter=0;
             ECCPAS=ECCPAS+1;
             }
             }

按F9，执行到TMR0=05就停了，

按F7执行到TMR0=FF，进入到中断函数，但ECCPAS的值不改变

这段程序是抄自书上，只是将原来的lcd控制改成了变换寄存器的值，确定每抄错请高手看看

这是原程序

             #include<pic.h>
             int counter;
             void main()
             {                       
             counter=0;
             TRISA=0;
             OPTION=0xc3;
             TMR0=5;
             T0IE=1;
             GIE=1;
             while(1);
             }
             void interrupt timer0_isr(void)
             {
             T0IF=0;
             TMR0=5;
             counter++;
             if(counter==250)
             {
             counter=0;
             RA0=!RA0;
             }
             }

我用的是MPLAB IDE v8.00     pic16f716