I wrote this in Keil ,but it compiles and runs okay under Raisonance.

/* * * * * * * * * * * * * * * * * * * * * *
*
* Name: LED Test Program
* Purpose: To demonstrate the use of a timer
* ISR to toggle a LED.
* Assumes use of 11.059 xtal
*/



#include <reg51.h>
#include <stdio.h>

#define LED_freq 5 // in seconds

// Port assignation - edit to suit you board layout.
sbit LED1 = P0^0; // toggled by ISR

// Memory
data unsigned char tick_count;
data unsigned int second_count;

//*****************************************************
// timer interrupt(11.059/12) /65535 =14 per second(ish)
void Timer0(void) interrupt 1 using 2 {
tick_count++;
//if one second has passed ,incement second_count.
if(tick_count>14){
tick_count=0;
second_count++;
if(second_count>(LED_freq-1)){
second_count=0;
LED1^=LED1;

}/* end of second_count>3 */

}/* end of tick_count>14 */


}



/* * * * * * * * * * * * * * * * * * * * * * *
*
* Name: main()
* Purpose: main procedure for this program
*/
void main(void) {

TMOD = 0x01; // timer 0, mode 1, 16bit timer
TR0 = 1; // Enable timer 0
ET0 = 1; // Enable Timer 0 interrupts
EA = 1; // Enable interrupts

while (1) {} /* end of while */
} /* end of main() routine */