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. This delay of ONE Second can be changed by changing the values loaded into the R7, R6 and R5 in the delay subroutine. The higher the value loaded the higher the delay. Since the controller is 8 bit microcontroller values greater than 255 cannot be loaded.

led1                       bit                           P2.0

ORG 0000h

loop:

                clr led1                                  //TURN ON LED

                call delay

                setb led1                              //TURN OFF LED

                call delay

                ajmp loop                                           

delay:

                mov r7,#200

l1_delay:

                mov r6,#217

l2_delay:

                mov r5,#10

                djnz r5,$                             // "$" over here indicates jump at the same location

                djnz r6,l2_delay

                djnz r7,l1_delay

                ret

 end

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