void timer0() interrupt 1 // interrupt address is 0x000b 
{ 
    P0_1 = !P0_1;      // 5uS hi / 5uS low makes 10uS period.
}
void main(void)
{
    TH1 = 0xFD;
    SCON = 0x50;           // for serial communication
    TMOD = 0x20 | 0x02;    // uart Timer1 auto-reload as well.
    TH0 = -5;              // every 5 clocks assuming 12 clocker.
    TR0 = 1;               // start timer
    EA = 1;                // start IRQ
    while (1) ;
}