• 8051-8052 , Assembly Language , Tutorials ,
• Tags: 8051 assembly, 8051 assembly code for led blinking, 8051 assembly tutorial, 8051 led interfacing, 8051 microcontroller, 8051 operating frequency, led blink 8051 assembly, led blinking program in assembly for 8051, proteus assembly code
• 09 Sep

Blink and LED is one of the simplest task which we are going to solve in this tutorial. This could be difficult if we try to do this in assembly language. We are going to write LED Blink 8051 Assembly language code in this tutorial. Before we get started let’s review what kind of previous understanding we need to start writing this code.

Prerequisite

Before jumping into the code we must understand the basic registers and internal architecture of the 8051 microcontroller. Also basic understanding of simulation is required because we are going to test this code in proteus. So if you have not watch my basic assembly language and lecture before you must check out my previous YouTube video about assembly language Introduction.

Sudo code for LED Blinking

Now that you may know basic assembly introduction, let’s start understanding the development logic for LED blinking code. So the main steps to make one LED blink we need to follow these orders

1. Label LED to some Pin of 8051
2. Make LED HIGH
3. wait for few milliseconds
4. Turn LED ON by writing LOW to the LED Pin
5. Take some delay again
6. Loop back to Turn the LED Off on step # 2

So these are the basic steps which you need to follow.

What is a Delay in Assembly Language?

Well, you may already aware of the word delay which means to lag some event by some time. But the question arises what is the Delay in Assembly language? Specially in Embedded systems?

To answer the above question we must understand that the Microcontroller runs very very fast. Just like, in case of 8051, connected with 12MHz crystal, the one instruction is executed in one micro seconds.

So if in normal case of blinking, where we only need to turn on led and then turn off LED and repeat. Microcontroller will do this so rapidly that we won’t be able to see the actual blinking. We (humans) cannot visualize the on and off of LED at 1us. So for making this visualize able we need to slow down microcontroller.

But the microcontroller always following some instructions. That’s why to slow down the microcontroller we also have to write some code to let microcontroller do some task all the time. We create a Delay task for this purpose and call this a Delay subroutine in assembly language.

Delay function in 8051 Assembly

To write the Delay function in 8051 assembly language we fill some 8bit register with a fix number and then ask 8051 microcontroller to decrement that register with DJNZ Instruction. Once it’s done we repeat the task until we wasted enough microseconds of the 8051 microcontroller. Here is the one register delay function which is called the Delay subroutine in Assembly Language

DELAY1: MOV  R7,#255
        DJNZ R7,$
        RETCode language: PHP (php)

Complete LED Blink Code

Even though we took 255 decrements for the R7 register but still this is not enough for human’s to visualize the blinking. So we need at-least 3 registers delay to accomplish LED blinking task. So here is the final complete code for the LED blink in 8051 assembly language.

LED 	BIT 	P1.0
;--------------------------------------------
	ORG 	00H
;--------------------------------------------
MAIN:
	CLR 	LED
	ACALL 	DELAY3
	SETB	LED
	ACALL	DELAY3
	SJMP	MAIN

;-------------------------------------------

DELAY3:
	    MOV 	R7,#5
D3L1:	MOV 	R6,#255
D3L2:	MOV 	R5,#255
	
	    DJNZ	R5,$
     	DJNZ	R6,D3L2
	    DJNZ	R7,D3L1
	    RET
;-----------------------------------------------
END
;-------------------------------------------------Code language: PHP (php)

Here is the YouTube video I created for this tutorial if you like to see things in action