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 mode to generate One second delay. The maximum delay that can be generated using TIMER 0 in any mode is 65535 clock cycles & since we are using 12MHz crystal 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.

led1                                       BIT                          P2.0

multiplier                             EQU                       30h                         //RAM Location               

MULTIPLIER_DEFAULT   DATA                     20                           //20 x 0.05 = 1 sec

ORG 0000h

                mov multiplier,#MULTIPLIER_DEFAULT

                mov TMOD,#01h                                              // Timer 0 Mode 1(16 bit mode)

                mov TH0,#3ch                                                  //0.05 sec           

                mov TL0,#0B8H

                setb TR0                                                          //Start Timer 0

loop:

                clr led1                                                                  //TURN ON LED

                call check_timer                               //wait for 1 second

                setb led1                                                             //TURN OFF LED

                call check_timer                               //wait for 1 second

                ajmp loop                                           

check_timer:

                jnb TF0,$                                              //wait for timer to over flow

                clr TF0                                                   //clear TIMER 0 flag

                clr TR0                                                   //Stop Timer 0

                mov TH0,#3ch                                   //0.05 sec, Reload timer 0

                mov TL0,#0B8H

                setb TR0                                               //Start Timer 0

                djnz multiplier,check_timer           //Decrement multiplier by 1 & check if its 0

                mov multiplier,#MULTIPLIER_DEFAULT                

                ret

end

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