Here is a simple code for Blinking a LED. The LED is connected to P2.0 & 12MHz crystal is used. The Anode of the LED is given to Vcc through a resistor & the cathode is connected to the microcontroller pin. Please check this link to see how to interface LED to Microcontroller. The LED is ON for One Second & OFF for ONE Second.

Here we will be using Timer 0 in 16 bit INTERRUPT mode to generate One second delay. We load TH0 & TL0 with a specific value & on every clock cycle TH0 & TL0 increments by 1 and when the TIMER overflows from FFFF to 0000 a Timer0 overflow flag is set & interrupt is generated.  The maximum delay that can be generated using TIMER 0 in any mode is 65535 clock cycles & since we are using 12MHz crystal the maximum delay is 65.535ms. But we require 1second delay. So we generate 50ms delay using TIMER 0 & when 20 such delays have been generated we come to know that one second has elapsed. So basically 0.05ms X 20 =1 second. So we use a RAM location “multiplier” load it with 20 and whenever the timer overflows the ram location multiplier is decremented by one. Thus when multiplier becomes zero we come to know that one second has elapsed & we toggle the LED.