stick

if (global_count == 100)
{
TR0=0;
P1.1=~P1.1; 
global_count=0; /*reset global value global_count increment in timer0_ISR */
TR0=1;
}

into the timer ISR just before exit.

I applaud you for trying to keep the ISR as short as possible, but this need be there if precision is required.

Also, for precision, if more interrupts are active, you may want to set the timer IST at a highre priority.

Erik