On some 8051, you have special feathures that givs you the possibility to toggle a pin automatically when a timer interrupt occures. Normaly, if you have the 3rd timer (timer2), you will have this feature (eg.: Philips 8051, any 8052, etc....)
Or you have to toggle you pin by soft at each timer interrupt.

Good day form the Milk-cows

Juan-Carlos / DIGMESA AG