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ECT 206 Computer Architecture And Microcontrollers Lecture Notes
MODULE III
PROGRAMMING AND INTERFACING OF 8051
3.1 SIMPLE PROGRAMMING EXAMPLES IN ASSEMBLY LANGUAGE
ASSEMBLER DIRECTIVES.
Assembler directives tell the assembler to do something other than creating the machine code
for an instruction. In assembly language programming, the assembler directives instruct the
assembler to
1. Process subsequent assembly language instructions
2. Define program constants
3. Reserve space for variables
The following are the widely used 8051 assembler directives.
ORG (origin)
The ORG directive is used to indicate the starting address. It can be used only when the
program counter needs to be changed. The number that comes after ORG can be either in hex
or in decimal.
Eg: ORG 0000H ; Set PC to 0000.
EQU and SET

EQU and SET directives assign numerical value or register name to the specified symbol
name.
EQU is used to define a constant without storing information in the memory. The symbol
defined with EQU should not be redefined.
SET directive allows redefinition of symbols at a later stage.
DB (DEFINE BYTE)
The DB directive is used to define an 8 bit data. DB directive initializes memory with 8 bit
values. The numbers can be in decimal, binary, hex or in ASCII formats. For decimal, the 'D'
after the decimal number is optional, but for binary and hexadecimal, 'B' and ‘H’ are required.
For ASCII, the number is written in quotation marks (‘LIKE This).
DATA1: DB 40H ; hex
DATA2: DB 01011100B ;binary
DATA3: DB 48 ; decimal
DATA4: DB ' HELLO W’ ; ASCII
END
The END directive signals the end of the assembly module. It indicates the end of the program
to the assembler. Any text in the assembly file that appears after the END directive is ignored.
If the END statement is missing, the assembler will generate an error message
Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |1
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3.2 ASSEMBLY LANGUAGE PROGRAMS.
1. Write a program to add the values of locations 50H and 51H and store the result in
locationsin 52h and 53H.
ORG 0000H ; Set program counter 0000H

MOV A,50H ; Load the contents of Memory location 50H into
ADD A,51H ; Add the contents of memory 51H with CONTENTS A
MOV 52H,A ; Save the LS byte of the result in 52H
MOV A, #00 ; Load 00H into A
ADDC A, #00 ; Add the immediate data and carry to A
MOV 53H,A ; Save the MS byte of the result in location 53h
END
2. Write a program to store data FFH into RAM memory locations 50H to 58H
using directaddressing mode

MOV A, #0FFH ; Load FFH into A
MOV 50H, A ; Store contents of A in location 50H
MOV 51H, A ; Store contents of A in location 5IH
END
3. Write a program to subtract a 16 bit number stored at locations 51H-52H from 55H-
56H and store the result in locations 40H and 41H. Assume that the least significant
byte of data or theresult is stored in low address. If the result is positive, then store
00H, else store 01H in 42H.

MOV A, 55H ; Load the contents of memory location 55 into A
CLR C ; Clear the borrow flag
SUBB A,51H ; Sub the contents of memory 51H from contents of A
MOV 40H, A ; Save the LS Byte of the result in location 40H
MOV A, 56H ; Load the contents of memory location 56H into A
SUBB A, 52H ; Subtract the content of memory 52H from the content A
MOV A, 41H, ; Save the MS byte of the result in location 41H.
MOV A, #00 ; Load 00 into A
ADDC A, #00 ; Add the immediate data and the carry flag to A
MOV 42H, A ; If result is positive, store00H, else store 0lH in 42H
END

4. Write a program to add two 16 bit numbers stored at locations 51H-52H and 55H-56H
and store the result in locations 40H, 41H and 42H. Assume that the least significant
byte of data and the result is stored in low address and the most significant byte of
data or the resultis stored in high address.

MOV A,51H ; Load the contents of memory location 51H into A
ADD A,55H ; Add the contents of 55H with contents of A
MOV 40H,A ; Save the LS byte of the result in location 40H
MOV A,52H ; Load the contents of 52H into A
ADDC A,56H ; Add the contents of 56H and CY flag with A
MOV 41H,A ; Save the second byte of the result in 41H
MOV A,#00 ; Load 00H into A
ADDC A,#00 ; Add the immediate data 00H and CY to A
MOV 42H,A ; Save the MS byte of the result in location 42H
END
5. Write a program to store data FFH into RAM memory locations 50H to 58H using
indirect addressing mode.
MOV A, #0FFH ; Load FFH into A
MOV RO, #50H ; Load pointer, R0-50H
MOV R5, #08H ; Load counter, R5-08H
Start:MOV @RO, A ; Copy contents of A to RAM pointed by R0
INC RO ; Increment pointer
DJNZ R5, start ; Repeat until R5 is zero
END
6. Write a program to add two Binary Coded Decimal (BCD) numbers stored at
locations 60H and 61H and store the result in BCD at memory locations 52H and
53H. Assume that the least significant byte of the result is stored in low address.
ORG 0000H ; Set program counter 00004

MOV A,60H ; Load the contents of memory location 60H into A
ADD A,61H ; Add the contents of memory location 61H with contents of A
DA A ; Decimal adjustment of the sum in A
MOV 52H, A ; Save the least significant byte of the result in location 52H
MOV A,#00 ; Load 00H into .A
ADDC A,#00H ; Add the immediate data and the contents of carry flag to A
MOV 53H,A ; Save the most significant byte of the result in location 53H
END
7. Write a program to clear 10 RAM locations starting at RAM address 1000H.
ORG 0000H ;Set program counter 0000H

MOV DPTR, #1000H ;Copy address 1000H to DPTR
CLR A ;Clear A
MOV R6, #0AH ;Load 0AH to R6
again: MOVX @DPTR,A ;Clear RAM location pointed by DPTR
INC DPTR ;Increment DPTR
DJNZ R6, again ;Loop until counter R6=0
END

8. Write a program to compute 1 + 2 + 3 + N (say N=15) and save the sum at70H
ORG 0000 H ; Set program counter 0000H

N EQU 15
MOV R 0 ,#00 ; Clear R0
CLR A ; Clear A
again: INC R 0 ; Increment R0
ADD A, R0 ; Add the contents of R0 with A
CJNE R 0,# N, again ; Loop until counter, R0, N
MOV 70 H,A ; Save the result in location 70H
END
9. Write a program to multiply two 8 bit numbers stored at locations 70H and 71H and store the
result at memory locations 52H and 53H. Assume that the least significant byte of the result is
stored in low address.
ORG 0000H ; Set program counter 00 OH
MOV A, 70H ; Load the contents of memory location 70h into A
MOV B, 71H ; Load the contents of memory location 71H into B
MUL AB ; Perform multiplication
MOV 52H,A ; Save the least significant byte of the result in location 52H
MOV 53H,B ; Save the mostsignificant byte of the result in location 53
END
10. Ten 8 bit numbers are stored in internal data memory from location 5oH. Write
a program to increment the data.
Assume that ten 8 bit numbers are stored in internal data memory from location
50H, henceR0 or R1 must be used as a pointer.
The program is as follows.
OPT 0000H
MOV R0,#50H
MOV R3,#0AH
Loopl: INC @R0
INC RO
DJNZ R3, loop
END
11. Write a program to find the average of five 8 bit numbers. Store the result in H.
(Assume that after adding five 8 bit numbers, the result is 8 bit only).
ORG 0000H
MOV 40H,#05H
MOV 41H,#55H
MOV 42H,#06H
MOV 43H,#1AH
MOV 44H,#09H
MOV R0,#40H
MOV R5,#05H
MOV B,R5CLR A
Loop: ADD A,@RO
INC RO
DJNZ R5,Loop
DIV AB
MOV 55H,A
END

12. Write a program to find the cube of an 8 bit number program is as follows
ORG 0000H
MOV R1,#N
MOV A,R1
MOV B,R1
MUL AB //SQUARE IS COMPUTED MOV R2, B
MOV B, R1
MUL AB
MOV 50,A
MOV 51,B
MOV A,R2
MOV B, R1
MUL AB
ADD A, 51H
MOV 51H, A
MOV 52H, B
MOV A, # 00H
ADDC A, 52H
MOV 52H, A //CUBE IS STORED IN 52H,51H,50H
END
13. Write a program to exchange the lower nibble of data present in external memory
6000H and6001H
ORG 0000H ; S e t p r o g r a m c o u n t e r 0 0h
MOV DPTR, # 6000 H ; Copy address 6000 H to DPTR
MOVX A, @DPTR ; Copy contents of 6000H to A
MOV R0, #45H ; Load pointer, R0=45H
MOV @RO, A ; C o p y c o n t o f A t o R A M p o i n t e d b y R0
INC DPL ; Increment pointer
MOVX A, @DPTR ; Copy contents of 6001H to A
XCHD A, @R0 ; Exchange lower nibble of A with RAM pointe
d by RO
MOVX @DPTR, A ; Copy contents of A to 6001H
DEC DPL ; Decrement pointer
MOV A, @R0 ; C o p y c o n t of R A M p o i n t e d b y R 0 t o A
MOVX @DPTR, A ; Copy cont of A to RAM pointed by DPTR
END
14. Write a program to count the number of and o's of 8 bit data stored in location 6000H.
ORG 00008 ; Set program counter 00008
MOV DPTR, #6000h ; Copy address 6000H to DPTR
MOVX A, @DPTR ; Copy num be r t o A
MOV R0,#08 ; Copy 08 in RO
MOV R2,#00 ; C o py 00 in R 2
MOV R3,#00 ; C o py 00 in R 3
CLR C ; Clear carry flag
BACK: RLC A ; Rotate A through carry flag
JC NEXT ; If CF = 1, branch to next
INC R2 ; If CF = 0, increment R2
AJMP NEXT2
NEXT: INC R3 ; If CF = 1, increment R3
NEXT2: DJNZ RO,BACK ; Repeat until RO is zero
END

15. Write a program to shift a 24 bit number stored at 57H-55H to the left logically four
places.Assume that the least significant byte of data is stored in lower address.
ORG 0000H ; Set program counter 0000h
MOV R1,#04 ; Set up loop count to 4
again: MOV A,55H ; Place the least significant byte of data in A
CLR C ; Clear tne carry flag
RLC A ; Rotate contents of A (55h) left through carry
MOV 55H,A
MOV A,56H
RLC A ; Rotate contents of A (56H) left through carry
MOV 56H,A
MOV A,57H
RLC A ; Rotate contents of A (57H) left through carry
MOV 57H,A
DJNZ R1,again ; Repeat until R1 is zero
END
3.3 INTERFACING WITH 8051 USING ASSEMBLY LANGUAGE PROGRAMMING:
LED INTERFACING TO 8051
Blinking 1 LED using 8051
This is the first project regarding 8051 and of course one of the simplest, blinking LED using
8051. The microcontroller used here is AT89S51 In the circuit, push button switch S1,
capacitor C3 and resistor R3 forms the reset circuitry. When S1 is pressed, voltage at the
reset pin (pin9) goes high and this resets the chip. C1, C2 and X1 are related to the on chip
oscillator which produces the required clock frequency. P1.0 (pin1) is selected as the output
pin. When P1.o goes high the transistor Q1 is forward biased and LED goes ON. When P1.0
goes low the transistor goes to cut off and the LED extinguishes. The transistor driver circuit
for the LED can be avoided and the LED can be connected directly to the P1.0 pin with a
series current limiting resistor(~1K). The time for which P1.o goes high and low (time
period of the LED) is determined by the program. The circuit diagram for blinking 1 LED is
shown below.

Program
START: CPL P1.0

ACALL WAIT
SJMP START
WAIT: MOV R4,#05H
WAIT1: MOV R3,#00H
WAIT2: MOV R2,#00H
WAIT3: DJNZ R2,WAIT3
DJNZ R3,WAIT2
DJNZ R4,WAIT1
RET
Blinking 2 LED alternatively using 8051.
This circuit can blink two LEDs alternatively. P1.0 and P1.1 are assigned as the outputs.
When P1.0 goes high P1.0 goes low and vice versa and the LEDs follow the state of the
corresponding port to which it is connected. Here there is no driver stage for the LEDs and
they are connected directly to the corresponding ports through series current limiting
resistors (R1 & R2). Circuit diagram is shown below.

Program
START: CPL P1.0

ACALL WAIT
CPL P1.0
CPL P1.1
ACALL WAIT
CPL P1.1
SJMP START
WAIT: MOV R4,#05H
WAIT1: MOV R3,#FFH
WAIT2: MOV R2,#FFH
DJNZ R3,WAIT2
DJNZ R4,WAIT1
RET

INTERFACING 7 SEGMENT DISPLAY TO 8051.
A NOTE ABOUT 7 SEGMENT LED DISPLAY.
This article is about how to interface a seven segment LED display to an 8051
microcontroller. 7 segment LED display is very popular and it can display digits from 0 to 9
and quite a few characters like A, b, C, ., H, E, e, F, n, o,t,u,y, etc. Knowledge about how to
interface a seven segment display to a micro controller is very essential in designing
embedded systems. A seven segment display consists of seven LEDs arranged in the form of
a squarish ‘8’ slightly inclined to the right and a single LED as the dot character. Different
characters can be displayed by selectively glowing the required LED segments. Seven
segment displays are of two types, common cathode and common anode. In common
cathode type , the cathode of all LEDs are tied together to a single terminal which is usually
labeled as ‘com‘ and the anode of all LEDs are left alone as individual pins labeled as a, b, c,
d, e, f, g & h (or dot) . In common anode type, the anode of all LEDs are tied together as a
single terminal and cathodes are left alone as individual pins. The pin out scheme and
picture of a typical 7 segment LED display is shown in the image below.

Digit drive pattern.
Digit drive pattern of a seven segment LED display is simply the different logic combinations
of its terminals ‘a’ to ‘h‘ in order to display different digits and characters. The common
digit drive patterns (0 to 9) of a seven segment display are shown in the table below.
* P1.7 P1.6 P1.5 P1.4 P1.3 P1.2 P1.1 P1.0 *

Character h g f e d c b a HEX
0 0 0 1 1 1 1 1 1 0x3F
1 0 0 0 0 0 1 1 0 0x06
2 0 1 0 1 1 0 1 1 0x5B
3 0 1 0 0 1 1 1 1 0x4F
4 0 1 1 0 0 1 1 0 0x66
5 0 1 1 0 1 1 0 1 0x6D
6 0 1 1 1 1 1 0 1 0x7D
7 0 0 0 0 0 1 1 1 0x07
8 0 1 1 1 1 1 1 1 0x7F
9 0 1 1 0 1 1 1 1 0x6F
The circuit diagram shown is of an AT89S51 microcontroller based 0 to 9 counter which has
a 7 segment LED display interfaced to it in order to display the count. This simple circuit
illustrates two things. How to setup simple 0 to 9 up counter using 8051 and more
importantly how to interface a seven segment LED display to 8051 in order to display a
particular result. The common cathode seven segment display D1 is connected to the Port 1
of the microcontroller (AT89S51) as shown in the circuit diagram. R3 to R10 are current
limiting resistors. S3 is the reset switch and R2,C3 forms a debouncing circuitry. C1, C2 and
X1 are related to the clock circuit. The software part of the project has to do the following
tasks.
 Form a 0 to 9 counter with a predetermined delay (around 1/2 second here).
 Convert the current count into digit drive pattern.
 Put the current digit drive pattern into a port for displaying.
All the above said tasks are accomplished by the program given below.
Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad | 10

PROGRAM.
ORG 000H //initial starting address

START: MOV A,#00001001H // initial value of accumulator
MOV B,A
MOV R0,#0AH //Register R0 initialized as counter which counts from 10 to
0
LABEL: MOV A,B
INC A
MOV B,A
MOVC A,@A+PC // adds the byte in A to the program counters address
MOV P1,A
ACALL DELAY // calls the delay of the timer
DEC R0 //Counter R0 decremented by 1
MOV A,R0 // R0 moved to accumulator to check if it is zero in next
instruction.
JZ START //Checks accumulator for zero and jumps to START.
Done to check if counting has been finished.
SJMP LABEL
DB 3FH // digit drive pattern for 0

DB 06H // digit drive pattern for 1

DB 5BH // digit drive pattern for 2
DB 6DH // digit drive pattern for 5
DB 7DH // digit drive pattern for 6
DELAY: MOV R4,#05H // subroutine for delay
WAIT1: MOV R3,#FFH
WAIT2: MOV R2,#FFH
DJNZ R3,WAIT2
DJNZ R4,WAIT1
RET
END
ABOUT THE PROGRAM.
Instruction MOVC A,@A+PC is the instruction that produces the required digit drive pattern
for the display. Execution of this instruction will add the value in the accumulator A with the
content of the program counter(address of the next instruction) and will move the data
present in the resultant address to A. After this the program resumes from the line after
MOVC A,@A+PC.
In the program, initial value in A is 00001001B. Execution of MOVC A,@A+PC will add
oooo1001B to the content in PC ( address of next instruction). The result will be the
address of label DB 3FH (line15) and the data present in this address ie 3FH (digit drive
pattern for 0) gets moved into the accumulator. Moving this pattern in the accumulator to
Port 1 will display 0 which is the first count.
At the next count, value in A will advance to 00001010 and after the execution of MOVC
A,@+PC ,the value in A will be 06H which is the digit drive pattern for 1 and this will display
1 which is the next count and this cycle gets repeated for subsequent counts.
The reason why accumulator is loaded with 00001001B (9 in decimal) initially is that the
instructions from line 9 to line 15 consumes 9 bytes in total.

3.4 PROGRAMMING IN C
Embedded C
For programming the embedded hardware devices, we need to use Embedded C language
instead of our conventional C language.
The key differences between conventional C and Embedded C are
 Embedded C has certain predefined variables for registers, ports etc. which are in
8051 e.g. ACC, P1, P2, TMOD etc.
 We can run super loop (infinite loop) in embedded C language.
We know that the programming in C language is solely done by dealing with different
variables.
In case of Embedded C, these variables are nothing else but the memory locations of
different memories of the microcontroller like code memory (ROM), data memory (RAM),
external memory etc. To use these memory locations as variables, we need to use data types.
Data types
There are 7 different data types in embedded C for 8051…
1) unsigned char
This data type is used to define an unsigned 8-bit variable. All 8-bits of this variable
are used to specify data. Hence the range of this data type is (0)10 𝑡𝑜 (255)10 .
e.g. unsigned char count;
2) signed char
This data type is used to define a signed 8-bit variable. Here MSB of variable is used
to show sign (+/-) while rest 7 bits are used to specify the magnitude of the variable.
Hence the range of this data type is (−128)10 𝑡𝑜 (127) 10.
e.g. signed char temp;
3) unsigned int
This data type is used to define a 16-bit variable. Hence from this we can comment
that this data types combines any 2 memory locations of the data memory as one
variable. Here all 16 bits are used to specify data. So the range of this data type is
(0)10 𝑡𝑜 (65535)10.
4) signed int
This data type is used to define a signed variable like signed char but of 16-bit size.
Hence its range is (−32768)10 𝑡𝑜 (32767)10
5) sfr
This is an 8-bit data type used for defining names of Special Function Registers
(SFR’s) that are located in RAM memory locations 80 H to FF H only.
e.g. sfr P0 = 0x80;

6) bit
This data type is used to access single bits from the bit-addressable area of RAM.
e.g. bit MYBIT = 0x32;
7) sbit
The sbit data type is the one which is used to define or rather access single bits of the
bit addressable SFR’s of 8051 microcontroller.
e.g. sbit En = P2^0;
With these data types in mind, let's take a look at the structure of a program in Embedded C.
DOCUMENTATION/COMMENTARY
Effective coding requires the use of documentation or commentary to indicate any

important details of what the code is doing. An Embedded C program typically begins with
some documentation information like the name of the file, the author, the date that the code
was created, and any specific details about the functioning of the code. Embedded C
supports single-line comments that begin with the characters "//" or multi-line comments
that begin with "/*" and end with "*/" on a subsequent line.
Pre-processor Directives
Pre-processor directives are not normal code statements. They are lines of code that begin
with the character "#" and appear in Embedded C programming before the main function. At
runtime, the compiler looks for pre-processor directives within the code and resolves them
completely before resolving any of the functions within the code itself. Some pre-processor
directives can skip part of the main code based on certain conditions, while others may ask
the pre-processor to replace the directive with information from a separate file before
executing the main code or to behave differently based on the hardware resources available.
Many types of pre-processor directives are available in the Embedded C language.
Global Variable Declaration
Global declarations happen before the main function of the source code. Engineers can
declare global variables that may be called by the main program or any additional functions
or sub-programs within the code. Engineers may also define functions here that will be
accessible anywhere in the code.

Main Program
The main part of the program begins with main( ). If the main function is expected to return
an integer value, we would write int main( ). If no return is expected, convention dictates
that we should write void main(void).
 Declaration of local variables - Unlike global variables, these ones can only be
called by the function in which they are declared.
 Initializing variables/devices - A portion of code that includes instructions for
initializing variables, I/O ports, devices, function registers, and anything else needed
for the program to execute
 Program body - Includes the functions, structures, and operations needed to do
something useful with our embedded system
Subprograms
An embedded C program file is not limited to a single function. Beyond the main( ) function,
programmers can define additional functions that will execute following the main function
when the code is compiled.
Decision control structures
The decision control structures are used to decide whether to execute a particular block of
code depending on the condition specified. Following are some decision control structures:
 if statement
 if…else statement
if...else statement if(condition)
{
if statement
statement-1; statement-2;
if(condition)
………
{
}
statement-1; statement-2;
else
……..
{
}
statement n;

Loop statements
The loop statements are the one which are used when we want to execute a certain block of
code for more than one times either depending on situation or by a predefined number of
times.
Embedded C is basically having two loop statements:
 for loop
 while loop
1) for loop
for loops are used to repeat any particular piece of code a predefined number of
times.
for(initializations ; conditions ; updates)
{
statement-1;
statement-2;
………
}
2) while loop
while loop also has the provision to repeat a certain block of code but here the block is
repeated depending on the condition specified. The loop keeps on repeating until the
condition becomes false.
Format of while loop is:
while(condition)
statement-1;
statement- 2;
………
Break & Continue Statements

1) break

The break statement, whenever is encountered in the loop, it forces the control to terminate the
loop in which it is written.
2) continue
Whenever this statement is encountered in any loop, the statements in the loop after it won’t be
executed i.e. will be skipped and again control will be transferred to check the condition of the loop.
Format of any C Program
#include <reg51.h> Header File
sbit <name>=<bit address>; sfr bit definitions
sfr <name>=<sfr address>; sfr definition
Data-type udf1(data-type var_name); User defined function
Data-type udf2(data-type var_name);
void main(void) main function
statement-1;
statement-2;
………………….;
Functions
Sometimes, there comes a situation in which in a program a group of statements is used
frequently. Writing these statements again & again makes our program clumsy to write as
well as it consumes more memory space. To overcome this problem there is a facility in C
language to define a function. In function we can write the particular group of statements
which is getting repeated continuously. Now anytime when we want to use that code group,
we just have to call the function and it’s done.
Types of functions
No arguments, no return values
With no arguments and a return value
With arguments but no return value
With arguments and return value
There are 3 ways to deal with a function:

 Define first, then use

 Do prototyping (i.e. Define first, use after main( ) )
 Do prototyping in header file
a) Define first, then use
In this case, before writing the main function, we define the user-defined
function and then use it in main( ) function whenever required.
b) Do prototyping and define after main function
In this case the function name, data type and argument data type are specified
before writing main function to declare that we’ll later implement this
function.
c) Do prototyping in header file
In this case, define the function in a (user defined) header file and then just
include that header file in your program.
Data Types in Embedded C
Delay generation in 8051
The delay length in 8051 microcontroller depends on three factors:
 The crystal frequency

 the number of clock per machine
 the C compiler.
The original 8051 used 1/12 of the crystal oscillator frequency as one machine cycle. In
other words, each machine cycle is equal to 12 clocks period of the crystal frequency
connected to X1-X2 pins of 8051. To speed up the 8051, many recent versions of the 8051
have reduced the number of clocks per machine cycle from 12 to four, or even one. The
frequency for the timer is always 1/12th the frequency of the crystal attached to the 8051,
regardless of the 8051 version. In other words, AT89C51, DS5000, and DS89C4x0 the
duration of the time to execute an instruction varies, but they all use 1/12th of the crystal's
oscillator frequency for the clock source.
8051 has two different ways to generate time delay using C programming, regardless of
8051 version.

The first method is simply using Loop program function in which Delay( ) function is
made or by providing for(); delay loop in Embedded C programming. You can define your
own value of delay and how long you want to display. For example- for(i=0;i<"any decimal
value";i++); this is the delay for loop used in embedded C.
Code to generate 250 ms delay on Port P1 of 8051:
#include "REG52.h"
void MSDelay(unsigned int);
void main( )
{
while (1) //repeat forever
{
P1=0x55;
MSDelay(250);
P1=0xAA;
MSDelay(250);
}
}
void MSDelay(unsigned int itime)
{
unsigned int i,j;
for (i=0;i<itime;i++) // this is For( ); loop delay used to define delay value in
Embedded C
{
for (j=0;j<1275;j++);
}
}
The second method is using Timer registers TH, TL and TMOD that are accessible in
embedded C by defining header file reg52.h Both timers 0 and 1 use the same register,
called TMOD (timer mode), to set the various timer operation modes in 8051 C
programming. There are four operating modes of timer 0 and 1.
To generate Time delay using timer registers:
Load the TMOD value register indicating which timer (timer 0 or timer 1) is to be
used and which timer mode (0 or 1 is selected)
Load registers TL and TH with initial count value
Start the timer
Keep monitoring the timer flag (TF) until it rolls over from FFFFH to 0000.
After the timer reaches its limit and rolls over, in order to repeat the process - TH and
TL must be reloaded with the original value, and TR is turned off by setting value to 0
and TF must be reloaded to 0.

Code generating delay using timer register:

#include <REG52.h>
void T0Delay(void);
void main(void){
while (1)
{
P1=0x55;
T0Delay( );
P1=0xAA;
T0Delay ( );
}
}
void T0Delay( )
{
TMOD=0x01; // timer 0, mode 1
TL0=0x66; // load TL0
TH0=0xFC; // load TH0
TR0=1; // turn on Timer0
while (TF0==0); // wait for TF0 to roll over
TR0=0; // turn off timer
TF0=0; // clear TF0
}
Steps for generating precise Delay using 8051 Timers
In order to produce time delay accurately,

1. Divide the time delay with timer clock period.
NNNN=time delay/1.085μs
2. Subtract the resultant value from 65536.
MMMM=65536-NNNN
3. Convert the difference value to the hexa decimal form.
MMMMd = XXYYh
4. Load this value to the timer register.

TH=XXh
TL=YYh
Delay Function to Generate 1 ms Delay

In order to generate a delay of 1ms, the calculations using above steps are as follows.
1. NNNN = 1ms/1.085μs ≈ 922.

2. MMMM = 65536-922 = 64614
3. 64614 in Hexadecimal = FC66h
4. Load TH with 0xFC and TL with 0x66
The following function will generate a delay of 1 ms using 8051 Timer 0.
Void delay ( )
{
TMOD = 0x01; // Timer 0 Mode 1
TH0= 0xFC; //initial value for 1ms
TL0 = 0x66;
TR0 = 1; // timer start
while (TF0 == 0); // check overflow condition
TR0 = 0; // Stop Timer
TF0 = 0; // Clear flag
}
Port programming
1. Write an 8051 C program to send values 00 – FF to port P1.
#include <reg51.h>
void main(void)
{
unsigned char i; for (i=0;i<=255;i++)
P1=i;
}
2. Write an 8051 C program to send the ASCII characters of 0, 1, 2, 3, 4, 5, A, B, C, and D to
port P1
#include <reg51.h>
void main(void)
{

unsigned char mynum( )=“012345ABCD”;

unsigned char i;
for (i=0;i<=10;i++)
P1=mynum(i);
}
3. Write an 8051 C program to toggle all the bits of P1 continuously.
#include <reg51.h>
void main(void)
{
While (1)
{
p1=0x55;
p1=0xAA;
}
}
4. Write an 8051 C program to send values of –4 to +4 to port P1.
//Singed numbers
#include <reg51.h>
void main(void)
{
char mynum[ ]={+1,-1,+2,-2,+3,-3,+4,-4};
unsigned char i; for (i=0;i<=8;i++)
P1=mynum[i];
}
5. Write an 8051 C program to send values of –4 to +4 to port P1
//Singed numbers
#include <reg51.h>
void main(void)
{
char mynum[ ];
signed char i;
for (i=-4;i<=4;i++)
P1=mynum[i];
}
6. Write an 8051 C program to toggle bit D0 of the port P1 (P1.0) 50,000 times.
#include <reg51.h>
sbit MYBIT=P1^0;
void main(void)

{
unsigned int z;
for (z=0;z<=50000;z++)
{
MYBIT=0;
MYBIT=1;
}
}
Note: sbit keyword allows access to the single bits of the SFR registers
7. LEDs are connected to bits P1 and P2. Write an 8051 C program that shows the count
from 0 to FFH (0000 0000 to 1111 1111 in binary) on the LEDs.
#include <reg51.h>
#define LED P2;
void main(void)
{
P1=00; //clear P1
LED=0; //clear P2
while(1)
{
P1++; //increment P1
LED++; //increment P2
}
}
Note: Ports P0 – P3 are byte-accessable and we can use the P0 – P3 labels as defined in the
8051 header file <reg51.h>
8. Write an 8051 C program to get a byte of data form P1, wait 1/2 second, and then send it to P2.
#include <reg51.h>
void MSDelay(unsigned int); void MSDelay(unsigned int itime)
void main(void) {
{ unsigned int i,j;
unsigned char mybyte; for (i=0;i<itime;i++)
P1=0xFF; //make P1 input port for (j=0;j<1275;j++);
while (1) }
{
mybyte=P1; //get a byte from P1
MSDelay(500);
P2=mybyte; //send it to P2
}
}
9. Write an 8051 C program to get a byte of data form P0. If it is less than 100, send it to P1;
otherwise, send it to P2.

#include <reg51.h>
void main(void) void MSDelay(unsigned int itime)
{ {
unsigned char mybyte; unsigned int i,j;
P0=0xFF; //make P0 input port for (i=0;i<itime;i++) for
while (1) (j=0;j<1275;j++);
{ }
mybyte=P0; //get a byte from P0
if (mybyte<100)
else
}
}
10. Write an 8051 C program to toggle only bit P2.4 continuously without disturbing the rest of the
bits of P2
//Toggling an individual bit

#include <reg51.h>
sbit mybit=P2^4;
void main(void)
{
while (1)
{
mybit=1; //turn on P2.4
mybit=0; //turn off P2.4
}
}
Note:
 Ports P0 – P3 are bit-addressable and we use sbit data type to access a single
bit of P0 - P3
 Use the Px^y format, where x is the port 0, 1, 2, or 3 and y is the bit 0 – 7 of
that port
11. Write an 8051 C program to monitor bit P1.5. If it is high, send 55H to P0; otherwise, send AAH
to P2
#include <reg51.h>
sbit mybit=P1^5;
void main(void)
{
mybit=1; //make mybit an input
while (1)
{
if (mybit==1)

P0=0x55;
else
P2=0xAA;
}
}
12. A door sensor is connected to the P1.1 pin, and a buzzer is connected to P1.7. Write an
8051 C program to monitor the door sensor, and when it opens, sound the buzzer. You can
sound the buzzer by sending a square wave of a few hundred Hz.
#include <reg51.h>
void MSDelay(unsigned int); void MSDelay(unsigned int itime)
sbit Dsensor=P1^1;
sbit Buzzer=P1^7; {
void main(void)
{ unsigned int i,j;
Dsensor=1; //make P1.1 an input
for (i=0;i<itime;i++) for
while (1)
(j=0;j<1275;j++);
{
while (Dsensor==1)//while it opens }
{
Buzzer=0; MSDelay(200);
Buzzer=1; MSDelay(200);
}
}
}
13. Write an 8051 C program to toggle all the bits of P0, P1, and P2 continuously with a 250 ms
delay. Use the sfr keyword to declare the port addresses
sfr P0=0x80;
void MSDelay(unsigned int itime)
sfr P1=0x90;
{
sfr P2=0xA0;
unsigned int i,j;
void MSDelay(unsigned int);
for (i=0;i<itime;i++) for
void main(void)
(j=0;j<1275;j++);
{
}
while (1)
{
P0=0x55;
P1=0x55;
P2=0x55;
MSDelay(250);
P0=0xAA;

P1=0xAA;
P2=0xAA;
MSDelay(250);
}
}
14. The data pins of an LCD are connected to P1. The information is latched into the LCD
whenever its Enable pin goes from high to low. Write an 8051 C program to send “ECED-
JCET” to this LCD
#include <reg51.h>
#define LCDData P1 //LCDData declaration
sbit En=P2^0; //the enable pin void main(void)
{
unsigned char message[ ] =“ECED-JCET”;
unsigned char z;
for (z=0;z<9;z++) //send 9 characters
{
LCDData=message[z];
En=1; //a high-
En=0; //-to-low pulse to latch data
}
}
15. Write an 8051 C program to turn bit P1.5 on and off 50,000 times.
#include <reg51.h>
sbit MYBIT=0x95;
void main(void)
{
unsigned int z;
for (z=0;z<50000;z++)
{ MYBIT=1; MYBIT=0;
}
}
Note
 We can access a single bit of any SFR if we specify the bit address
16. Generate a square wave with ON time 3ms and OFF time 5ms at port 0.Assume crystal
frequency 11.059MHz .

#include <reg51.h>
sbit wave =P0^0; void MSDelay(unsigned int itime)
void MSdelay (unsigned int ); {
void main(void) unsigned int i,j;
{ for (i=0;i<itime;i++) for
wave =1; (j=0;j<1275;j++);
MSdelay(3); }
wave =0;
MSdelay (5);
}
17. Generate a square wave with ON time 3ms and OFF time 5ms at port 0.Assume crystal
frequency 22MHz , Timer 0 in mode 1.
#include <reg51.h> Void delay3 ( )

sbit wave =P0^0; {
void delay3 ( ); TMOD = 0x01; // Timer 0 Mode 1
TH0= 0xEA; //initial value for 1ms
void delay5 ( ); TL0 = 0x8A;
void main(void) TR0 = 1; // timer start
{ while (TF0 == 0); // check overflow condition
wave =1; TR0 = 0; // Stop Timer
delay(3); TF0 = 0; // Clear flag
}
wave =0; Void delay5 ( )
delay (5); {
} TMOD = 0x01; // Timer 0 Mode 1
TH0= 0xDC; //initial value for 1ms
TL0 = 0x3B;
TR0 = 1; // timer start
while (TF0 == 0); // check overflow condition
TR0 = 0; // Stop Timer
TF0 = 0; // Clear flag
}
Code Conversion Programs

1. Write an 8051 C program to convert packed BCD 0x29 to ASCII and display the bytes
on P1 and P2.
#include <reg51.h>
void main(void)
{
unsigned char x,y,z;
unsigned char mybyte=0x29;
x=mybyte&0x0F;
P1=x|0x30;

y=mybyte&0xF0;
y=y>>4;
P2=y|0x30;
}
2. Write an 8051 C program to convert ASCII digits of ‘4’ and ‘7’ to packed BCD and display
them on P1.
#include <reg51.h>
void main(void)
{
unsigned char bcdbyte;
unsigned char w=‘4’;
unsigned char z=‘7’;
w=w&0x0F;
w=w<<4;
z=z&0x0F;
bcdbyte=w|z;
P1=bcdbyte;
}
3. Write an 8051 C program to calculate the checksum byte for the data 25H, 62H, 3FH, and
52H.
#include <reg51.h>
void main(void)
{
unsigned char mydata[ ]={0x25,0x62,0x3F,0x52};
unsigned char sum=0;
unsigned char x;
unsigned char chksumbyte; for (x=0;x<4;x++)
{
P2=mydata[x];
sum=sum+mydata[x];
}
chksumbyte=~sum+1;
P2=chksumbyte;
}
4. Write an 8051 C program to perform the checksum operation to ensure data integrity. If data is
good, send ASCII character ‘G’ to P0. Otherwise send ‘B’ to P0.
#include <reg51.h>
void main(void)
{
unsigned char mydata[ ]={0x25,0x62,0x3F,0x52,0xE8};

unsigned char chksum=0;

unsigned char x;
for (x=0;x<5;x++) chksum=chksum + mydata[x];
if (chksum==0)
P0=‘G’;
else
P0=‘B’;
}
5. Write an 8051 C program to convert 11111101 (FD hex) to decimal and display the digits
on P0, P1 and P2.
#include <reg51.h>
void main(void)
{
unsigned char x,binbyte,d1,d2,d3;
binbyte=0xFD;
x=binbyte/10;
d1=binbyte%10;
d2=x%10;
d3=x/10;
P0=d1;
P1=d2;
P2=d3;
}
INTERFACING THE KEYBOARD TO 8051 MICROCONTROLLER
The key board here we are interfacing is a matrix keyboard. This key board is designed with
a particular rows and columns. These rows and columns are connected to the
microcontroller through its ports of the micro controller 8051. We normally use 8*8 matrix
key board. So only two ports of 8051 can be easily connected to the rows and columns of the
key board.
When ever a key is pressed, a row and a column gets shorted through that pressed key
and all the other keys are left open. When a key is pressed only a bit in the port goes
high. Which indicates microcontroller that the key is pressed. By this high on the bit key in
the corresponding column is identified.

Once we are sure that one of key in the key board is pressed next our aim is to identify
that key. To do this we firstly check for particular row and then we check the corresponding
column the key board.
To check the row of the pressed key in the keyboard, one of the row is made high by
making one of bit in the output port of 8051 high . This is done until the row is found
out. Once we get the row next out job is to find out the column of the pressed key. The
column is detected by contents in the input ports with the help of a counter. The content of
the input port is rotated with carry until the carry bit is set.
The contents of the counter is then compared and displayed in the display. This display
is designed using a seven segment display and a BCD to seven segment decoder IC 7447.
The BCD equivalent number of counter is sent through output part of 8051 displays the
number of pressed key.
MICRO
KEY PAD DISPLAY
CONTROLLER
Circuit diagram of INTERFACING KEY BOARD TO 8051.
The programming algorithm, program and the circuit diagram is as follows. Here program is
explained with comments.

 The 8051 has 4 I/O ports P0 to P3 each with 8 I/O pins, P0.0 to P0.7,P1.0 to P1.7, P2.0
to P2.7, P3.0 to P3.7. The one of the port P1 (it understood that P1 means P1.0 to
P1.7) as an I/P port for microcontroller 8051, port P0 as an O/P port of
microcontroller 8051 and port P2 is used for displaying the number of pressed key.
 Make all rows of port P0 high so that it gives high signal when key is pressed.
 See if any key is pressed by scanning the port P1 by checking all columns for non
zero condition.
 If any key is pressed, to identify which key is pressed make one row high at a time.
 Initiate a counter to hold the count so that each key is counted.
 Check port P1 for nonzero condition. If any nonzero number is there in
[accumulator], start column scanning by following step 9.
 Otherwise make next row high in port P1.
 Add a count of 08h to the counter to move to the next row by repeating steps from
step 6.
 If any key pressed is found, the [accumulator] content is rotated right through the
carry until carry bit sets, while doing this increment the count in the counter till carry
is found.
 Move the content in the counter to display in data field or to memory location
 To repeat the procedures go to step 2.
Start of main program:
to check that whether any key is pressed
start: mov a,#00h

mov p1,a ;making all rows of port p1 zero
mov a,#0fh
mov p1,a ;making all rows of port p1 high
press: mov a,p2
jz press ;check until any key is pressed
after making sure that any key is pressed

mov a,#01h ;make one row high at a time

mov r4,a
mov r3,#00h ;initiating counter
next: mov a,r4
mov p1,a ;making one row high at a time
mov a,p2 ;taking input from port A
jnz colscan ;after getting the row jump to check
column
mov a,r4
rl a ;rotate left to check next row
mov r4,a
mov a,r3
add a,#08h ;increment counter by 08 count
mov r3,a
sjmp next ;jump to check next row
after identifying the row to check the column following steps are followed
colscan: mov r5,#00h

in: rrc a ;rotate right with carry until get the carry
jc out ;jump on getting carry
inc r3 ;increment one count
jmp in
out: mov a,r3
da a ;decimal adjust the contents of counter
before display
mov p2,a
jmp start ;repeat for check next key.
INTERFACING DAC TO 8051

The Digital to Analog converter (DAC) is a device, that is widely used for converting digital
pulses to analog signals. There are two methods of converting digital signals to analog
signals. These two methods are binary weighted method and R/2R ladder method. In this
article we will use the MC1408 (DAC0808) Digital to Analog Converter. This chip uses R/2R
ladder method. This method can achieve a much higher degree of precision. DACs are judged
by its resolution. The resolution is a function of the number of binary inputs. The most
common input counts are 8, 10, 12 etc. Number of data inputs decides the resolution of DAC.
So if there are n digital input pin, there are 2n analog levels. So 8 input DAC has 256 discrete
voltage levels.
The MC1408 DAC (or DAC0808)
In this chip the digital inputs are converted to current. The output current is known as Iout by
connecting a resistor to the output to convert into voltage. The total current provided by
the Iout pin is basically a function of the binary numbers at the input pins D0 - D7 (D0 is the
LSB and D7 is the MSB) of DAC0808 and the reference current Iref. The following formula is
showing the function of Iout
𝐷7 𝐷6 𝐷5 𝐷4 𝐷3 𝐷2 𝐷1 𝐷0
𝐼𝑜𝑢𝑡 = 𝐼𝑟𝑒𝑓 ( + + + + + + + )
2 4 8 16 32 64 128 256

The Iref is the input current. This must be provided into the pin 14. Generally 2.0mA is used
as Iref
We connect the Iout pin to the resistor to convert the current to voltage. But in real life it may
cause inaccuracy since the input resistance of the load will also affect the output voltage. So
practically Iref current input is isolated by connecting it to an Op-Amp with Rf = 5KΩ as
feedback resistor. The feedback resistor value can be changed as per requirement.
Generating Sinewave using DAC and 8051 Microcontroller
For generating sinewave, at first we need a look-up table to represent the magnitude of the
sine value of angles between 0° to 360°. The sine function varies from -1 to +1. In the table
only integer values are applicable for DAC input. In this example we will consider 30°
increments and calculate the values from degree to DAC input. We are assuming full-scale
voltage of 10V for DAC output. We can follow this formula to get the voltage ranges.
Vout = 5V + (5 ×sinθ)
Let us see the lookup table according to the angle and other parameters for DAC.
Angle(in θ ) sinθ Vout (Voltage Values sent to DAC
Magnitude) (Vout* 25.6)
0 0 5 128
30 0.5 7.5 192
60 0.866 9.33 238
90 1.0 10 255
120 0.866 9.33 238
150 0.5 7.5 192
180 0 5 128
210 -0.5 2.5 64
240 -0.866 0.669 17
270 -1.0 0 0
300 -0.866 0.669 17
330 -0.5 2.5 64
360 0 5 128

Circuit Diagram −
Program
#include<reg51.h>
sfr DAC = 0x80; //Port P0 address
void main(){
int sin_value[12] = {128,192,238,255,238,192,128,64,17,0,17,64};
int i;
while(1){
//infinite loop for LED blinking
for(i = 0; i<12; i++){
DAC = sin_value[i];
}
}
}

INTERFACING ADC TO 8051

An analog to digital converter or ADC, as the name suggests, converts an analog signal to a
digital signal. An analog signal has a continuously changing amplitude with respect to time.
A digital signal, on the contrary, is a stream of 0s and 1s. An ADC maps analog signals to their
binary equivalents. To do this, ADCs use various methods like Flash conversion, slope
integration, or successive approximation.
To understand the ADC in a better way, let us look at an example. Let us say we have an
input signal which varies from 0 to 8 volt, and we use a 3-bit ADC to convert this signal to
binary data. A 3-bit ADC can represent 2^3 or 8 different voltage levels using 3 bits of data.
How convenient! In this case, the ADC maps the data in the following manner.
Input voltage Binary equivalent

0-1 volt 000B
1-2 volt 001B
2-3volt 010B
3-4 volt 011B
4-5 volt 100B
5-6 volt 101B
6-7 volt 110B
7-8 volt 111B
If you look at the table above, you will understand how the ADC maps analog data to digital values. In
the case mentioned above, we can see that the tiniest change we can detect is that of 1 volt. If the
change is smaller than 1 volt, the ADC can’t detect it. This minimum change that an ADC can detect is
known as the step size of the ADC. To calculate it, we can use the formula:
Step size=(Vmax-Vmin)/2n (where n is the number of bits(resolution) of an ADC)

The step size of an ADC is inversely proportional to the number of bits of an ADC. So using
an ADC with higher bits can detect smaller changes, but this increases the cost of production.
Due to this reason, most on-chip ADCs’ have an 8-bit/10-bit resolution. Given below is the
resolution vs. step size for various configurations with a range of 0-5v input signal.
Number of bits Number of steps step size(mV)

8 256 5/256=19.53
10 1024 5/1024=4.88
12 4096 5/4096=1.2
16 65536 0.076 (precise conversion)

ADC 0808
The ADC 0808 is a popular 8-bit ADC with a step size of 19.53 millivolts. It does not have an
internal clock. Therefore, it requires a clock signal from an external source. It has eight input
pins, but only one of them can be selected at a time because it has eight digital output pins. It
uses the principle of successive approximation for calculating digital values, which is very
accurate for performing 8-bit analog to digital conversions. Let us look at the pin description
to get more insights into ADC 0808.
Input pins (INT0-INT7)

The ADC 0808 has eight input analog pins. These pins are multiplexed together, and only
one of them can be selected using three select lines.
Select lines and ALE
It has three select lines, namely A, B, and C, that are used to select the desired input lines.
The ALE pin also needs to be activated by a low to high pulse to select a particular input. The
input lines are selected as follows:
A B C Selected analog channel ALE pin
0 0 0 INT0 Low to High pulse
Output pins (D0-D7)

The ADC has eight output pins that give the binary equivalent of a given analog value.
VCC and Ground
These two pins are used to provide the required voltage to power the microcontroller. In
most cases, the ADC uses 5V DC to power up.

Clock
As mentioned earlier, the 0808 does not have an internal clock and needs an external clock
signal to operate. It uses a clock frequency of 20Mhz, and using this clock frequency it can
perform one conversion in 100 microseconds.
VREF (+) and VREF (-)
These two pins are used to provide the upper and the lower limit of voltages which
determine the step size for the conversion. Here Vref(+) has a higher voltage, and Vref(-) has
the lower voltage. If Vref(+) has an input voltage 5v and Vref(-) has a voltage of 0v then the
step size will be 5v-0v/28= 15.53 mv.
Start conversion
This pin is used to tell the ADC to start the conversion. When the ADC receives a low to high
pulse on this pin, it starts converting the analog voltage on the selected pin to its 8-bit digital
equivalent.
End of conversion
Once the conversion is complete, the ADC sends low to high signal to tell a microcontroller
that the conversion is complete and that it can extract the data from the 8 data pins.
Output enable
This pin is used to extract the data from the ADC. A microcontroller sends a low to high
pulse to the ADC to extract the data from its data buffers
Interfacing 8051 with 0808
Most modern microcontrollers with 8051 IP cores have an inbuilt ADC. Older versions of
8051 like the MCS-51 and A789C51 do not have an on-chip ADC. Therefore to connect these
microcontrollers to analog sensors like temperature sensors, the microcontroller needs to
be hooked to an ADC. It converts the analog values to digital values, which the
microcontroller can process and understand. Here is how we can interface the 8051 with
0808.
To interface the ADC to 8051, follow these steps.

Connect the oscillator circuit to pins 19 and 20. This includes a crystal oscillator and
two capacitors of 22uF each. Connect them to the pins, as shown in the diagram.
Connect one end of the capacitor to the EA’ pin and the other to the resister. Connect
this resistor to the RST pin, as shown in the diagram.
We are using port 1 as the input port, so we have connected the output ports of the
ADC to port 1.
As mentioned earlier, the 0808 does not have an internal clock; therefore, we have to
connect an external clock. Connect the external clock to pin 10.
Connect Vref (+) to a voltage source according to the step size you need.
Ground Vref (-) and connect the analog sensor to any one of the analog input pins on
the ADC. We have connected a variable resistor to INT2 for getting a variable voltage
at the pin.
Connect ADD A, ADD B, ADD C, and ALE pins to the microcontroller for selecting the
input analog port. We have connected ADD A- P2.0; ADD B- P2.1; ADD C- P2.2 and the
ALE pin to port 2.4.
Connect the control pins Start, OE, and Start to the microcontroller. These pins are
connected as follows in our case Start-Port-2.6; OE-Port-2.5 and EOC-Port-2.7.
Logic to communicate between 8051 and ADC 0808

Several control signals need to be sent to the ADC to extract the required data from it.
Step 1: Set the port you connected to the output lines of the ADC as an input port.
You can learn more about the Ports in 8051 here.
Step 2: Make the Port connected to EOC pin high. The reason for doing this is that the
ADC sends a high to low signal when the conversion of data is complete. So this line
needs to be high so that the microcontroller can detect the change.
Step 3: Clear the data lines which are connected to pins ALE, START, and OE as all
these pins require a Low to High pulse to get activated.
Step 4: Select the data lines according to the input port you want to select. To do this,
select the data lines and send a High to Low pulse at the ALE pin to select the address.
Step 5: Now that we have selected the analog input pin, we can tell the ADC to start
the conversion by sending a pulse to the START pin.
Step 6: Wait for the High to low signal by polling the EOC pin.
Step 7: Wait for the signal to get high again.
Step 8: Extract the converted data by sending a High to low signal to the OE pin.
Program
#include <reg51.h>
sbit ALE = P2^4;
sbit OE = P2^5;
sbit SC = P2^6;
sbit EOC = P2^7;
sbit ADDR_A = P2^0;
sbit ADDR_B = P2^1;
sbit ADDR_C = P2^2;
sfr MYDATA =P1;
sfr SENDDATA =P3;

void MSDelay(unsighned int) // Function to generate time delay

{
unsighned int i,j;
for(i=0;i<delay;i++)
for(j=0;j<1275;j++);
}
void main()
{
unsigned char value;
MYDATA = 0xFF;
EOC = 1;
ALE = 0;
OE = 0;
SC = 0;
while(1)
{
ADDR_C = 0;
ADDR_B = 0;
ADDR_A = 0;
MSDelay(1);
ALE = 1;
MSDelay(1);
SC = 1;
MSDelay(1);
ALE = 0;
SC = 0;
while(EOC==1);
while(EOC==0);
OE=1;
MSDelay(1);
value = MYDATA;
SENDDATA = value;
OE = 0 ;
}
}
STEPPER MOTOR INTERFACING WITH 8051
Stepper motors are used to translate electrical pulses into mechanical movements. In some
disk drives, dot matrix printers, and some other different places the stepper motors are
used. The main advantage of using the stepper motor is the position control. Stepper motors
generally have a permanent magnet shaft (rotor), and it is surrounded by a stator.

Normal motor shafts can move freely but the stepper motor shafts move in fixed repeatable
increments.
Some parameters of stepper motors −
Step Angle − The step angle is the angle in which the rotor moves when one pulse is
applied as an input of the stator. This parameter is used to determine the positioning
of a stepper motor.
Steps per Revolution − This is the number of step angles required for a complete
revolution. So the formula is 360° /Step Angle.
Steps per Second − This parameter is used to measure a number of steps covered in
each second.
RPM − The RPM is the Revolution Per Minute. It measures the frequency of rotation.
By this parameter, we can measure the number of rotations in one minute.
Interfacing Stepper Motor with 8051 Microcontroller
Weare using Port P0 of 8051 for connecting the stepper motor. HereULN2003 is used. This
is basically a high voltage, high current Darlington transistor array. Each ULN2003 has seven
NPN Darlington pairs. It can provide high voltage output with common cathode clamp
diodes for switching inductive loads.
The Unipolar stepper motor works in three modes.
 Wave Drive Mode − In this mode, one coil is energized at a time. So all four coils are
energized one after another. This mode produces less torque than full step drive
mode.
The following table is showing the sequence of input states in different windings.
Steps Winding A Winding B Winding C Winding D
1 1 0 0 0

2 0 1 0 0
3 0 0 1 0
4 0 0 0 1
 Full Drive Mode − In this mode, two coils are energized at the same time. This mode
produces more torque. Here the power consumption is also high
1 1 1 0 0
2 0 1 1 0
3 0 0 1 1
4 1 0 0 1
 Half Drive Mode − In this mode, one and two coils are energized alternately. At first,
one coil is energized then two coils are energized. This is basically a combination of
wave and full drive mode. It increases the angular rotation of the motor
1 1 0 0 0
2 1 1 0 0
3 0 1 0 0
4 0 1 1 0
5 0 0 1 0
6 0 0 1 1
7 0 0 0 1
8 1 0 0 1

Program
// Wave drive Mode
#include<reg51.h>
void ms_delay(unsigned int t) //To create a delay of 200 ms = 200 x 1ms
{
unsigned i,j ;
for(i=0;i<t;i++) //200 times 1 ms delay
for(j=0;j<1275;j++); //1ms delay
}
void main()
{
while(1) // To repeat infinitely
{
P2=0x08; //P2 = 0000 1000 First Step
ms_delay(200);
P2=0x04; //P2 = 0000 0100 Second Step
ms_delay(200);
P2=0x02; //P2 = 0000 0010 Third Step
ms_delay(200);
P2=0x01; //P2 = 0000 0001 Fourth Step
ms_delay(200);
}
}
// Full drive Mode
#include<reg51.h>
{
unsigned i,j ;
for(i=0;i<t;i++) //200 times 1 ms delay
for(j=0;j<1275;j++); //1ms delay
}
void main()
{
while(1) // To repeat infinitely
{
P2=0x0C; //P2 = 0000 1000 First Step
ms_delay(200);
P2=0x06; //P2 = 0000 0100 Second Step
ms_delay(200);
P2=0x03; //P2 = 0000 0010 Third Step
ms_delay(200);
P2=0x09; //P2 = 0000 0001 Fourth Step
ms_delay(200);
}
}
// Half Drive Mode

#include<reg51.h>
{
unsigned i,j ;
for(i=0;i<t;i++)
for(j=0;j<1275;j++);

}
void main()
{
while (1)
{
P2 = 0x08; //P2 = 0000 1000 First Step
ms_delay(200)
P2 = 0x0C; //P2 = 0000 1100 Second Step
ms_delay(200)
P2 = 0x04; //P2 = 0000 0100 Third Step
ms_delay(200)
P2 = 0x06; //P2 = 0000 0110 Fourth Step
ms_delay(200)
P2 = 0x02; //P2 = 0000 0010 Fifth Step
ms_delay(200);
P2 = 0x03; //P2 = 0000 0011 Sixth Step
ms_delay(200);
P2 = 0x01; //P2 = 0000 0001 Seventh Step
ms_delay(200);
P2 = 0x09; //P2 = 0000 1001 Eight Step
ms_delay(200);
}
}
The circuit diagram is shown below: It uses the full drive mode.

LCD INTERFACING WITH 8051 MICROCONTROLLER
Display units are the most important output devices in embedded projects and electronics
products. 16x2 LCD is one of the most used display unit. 16x2 LCD means that there are two
rows in which 16 characters can be displayed per line, and each character takes 5X7 matrix
space on LCD. In this tutorial we are going to connect 16X2 LCD module to the 8051
microcontroller (AT89S52). Interfacing LCD with 8051 microcontroller might look quite
complex to newbies, but after understanding the concept it would look very simple and easy.
Although it may be time taking because you need to understand and connect 16 pins of LCD
to the microcontroller. So first let's understand the 16 pins of LCD module.
We can divide it in five categories, Power Pins, contrast pin, Control Pins, Data pins and
Backlight pins.
Pin Pin
Category Function
NO. Name
1 VSS Ground Pin, connected to Ground
Power Pins
VDD or
2 Voltage Pin +5V
Vcc
Contrast V0 or Contrast Setting, connected to Vcc thorough a

3
Pin VEE variable resistor.
Register Select Pin, RS=0 Command

4 RS
mode, RS=1 Data mode
Control Read/ Write pin, RW=0 Write mode, RW=1

Pins 5 RW
Read mode
Enable, a high to low pulse need to enable the

6 E
LCD
Data Pins, Stores the Data to be displayed on

Data Pins 7-14 D0-D7
LCD or the command instructions

LED+ or
15 To power the Backlight +5V
A
Backlight
Pins
LED- or
16 Backlight Ground
K
All the pins are clearly understandable by their name and functions, except the control pins,
so they are explained below:
RS: RS is the register select pin. We need to set it to 1, if we are sending some data to be
displayed on LCD. And we will set it to 0 if we are sending some command instruction like
clear the screen (hex code 01).
RW: This is Read/write pin, we will set it to 0, if we are going to write some data on LCD.
And set it to 1, if we are reading from LCD module. Generally this is set to 0, because we do
not have need to read data from LCD. Only one instruction “Get LCD status”, need to be read
some times.
E: This pin is used to enable the module when a high to low pulse is given to it. A pulse of
450 ns should be given. That transition from HIGH to LOW makes the module ENABLE.
There are some preset command instructions in LCD, we have used them in our program
below to prepare the LCD (in lcd_init() function). Some important command instructions are
given below:
Hex Code Command to LCD Instruction Register
0F LCD ON, cursor ON
01 Clear display screen
02 Return home
04 Decrement cursor (shift cursor to left)
06 Increment cursor (shift cursor to right)
05 Shift display right
07 Shift display left
0E Display ON, cursor blinking

80 Force cursor to beginning of first line
C0 Force cursor to beginning of second line
38 2 lines and 5×7 matrix
83 Cursor line 1 position 3
3C Activate second line
08 Display OFF, cursor OFF
C1 Jump to second line, position 1
OC Display ON, cursor OFF
Circuit diagram for LCD interfacing with 8051 microcontroller is shown in the above
figure. If you have basic understanding of 8051 then you must know about EA(PIN 31),

XTAL1 & XTAL2, RST pin(PIN 9), Vcc and Ground Pin of 8051 microcontroller. I have used
these Pins in above circuit.
So besides these above pins we have connected the data pins (D0-D7) of LCD to the Port 2
(P2_0 – P2_7) microcontroller. And control pins RS, RW and E to the pin 12,13,14 (pin 2,3,4
of port 3) of microcontroller respectively.
PIN 2(VDD) and PIN 15(Backlight supply) of LCD are connected to voltage (5v), and PIN 1
(VSS) and PIN 16(Backlight ground) are connected to ground.
Pin 3(V0) is connected to voltage (Vcc) through a variable resistor of 10k to adjust the
contrast of LCD. Middle leg of the variable resistor is connected to PIN 3 and other two legs
are connected to voltage supply and Ground.
Program
// Program for LCD Interfacing with 8051 Microcontroller (AT89S52)
#include<reg51.h>
#define display_port P2 //Data pins connected to port 2 on microcontroller
sbit rs = P3^2; //RS pin connected to pin 2 of port 3
sbit rw = P3^3; // RW pin connected to pin 3 of port 3
sbit e = P3^4; //E pin connected to pin 4 of port 3
void msdelay(unsigned int time) // Function for creating delay in milliseconds.

{
unsigned i,j ;
for(i=0;i<time;i++)
for(j=0;j<1275;j++);
}
void lcd_cmd(unsigned char command) //Function to send command instruction to LCD
{
display_port = command;
rs= 0;
rw=0;
e=1;
msdelay(1);
e=0;
}
void lcd_data(unsigned char disp_data) //Function to send display data to LCD

{
display_port = disp_data;
rs= 1;

rw=0;
e=1;
msdelay(1);
e=0;
}
void lcd_init() //Function to prepare the LCD and get it ready

{
lcd_cmd(0x38); // for using 2 lines and 5X7 matrix of LCD
msdelay(10);
lcd_cmd(0x0F); // turn display ON, cursor blinking
msdelay(10);
lcd_cmd(0x01); //clear screen
msdelay(10);
lcd_cmd(0x81); // bring cursor to position 1 of line 1
msdelay(10);
}
void main()
{
unsigned char a[15]="CIRCUIT DIGEST"; //string of 14 characters with a null terminator.
int l=0;
lcd_init();
while(a[l] != '\0') // searching the null terminator in the sentence
{
lcd_data(a[l]);
l++;
msdelay(50);
}
}

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ECT 206 Computer Architecture And Microcontrollers Lecture Notes

The following are the widely used 8051 assembler directives.

EQU and SET

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |1

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3.2 ASSEMBLY LANGUAGE PROGRAMS.

ORG 0000H ; Set program counter 0000H

ORG 0000H ; Set program counter 0000H

ORG 0000H ; Set program counter 0000H

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |2

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ORG 0000H ; Set program counter 0000H

ORG 0000H ; Set program counter 00004

7. Write a program to clear 10 RAM locations starting at RAM address 1000H.

ORG 0000H ;Set program counter 0000H

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |3

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ORG 0000 H ; Set program counter 0000H

The program is as follows.

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |4

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Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |5

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3.3 INTERFACING WITH 8051 USING ASSEMBLY LANGUAGE PROGRAMMING:

LED INTERFACING TO 8051

Blinking 1 LED using 8051

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |6

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START: CPL P1.0

Blinking 2 LED alternatively using 8051.

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |7

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START: CPL P1.0

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |8

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INTERFACING 7 SEGMENT DISPLAY TO 8051.

A NOTE ABOUT 7 SEGMENT LED DISPLAY.

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad |9

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Digit drive pattern.

* P1.7 P1.6 P1.5 P1.4 P1.3 P1.2 P1.1 P1.0 *

 Form a 0 to 9 counter with a predetermined delay (around 1/2 second here).

 Convert the current count into digit drive pattern.

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad | 10

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ORG 000H //initial starting address

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad | 11

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DB 06H // digit drive pattern for 1

ABOUT THE PROGRAM.

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad | 12

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The key differences between conventional C and Embedded C are

There are 7 different data types in embedded C for 8051…

Sanish V S ,Assistant Professor,ECE,JCET,Lakkidi,Palakkad | 13

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Effective coding requires the use of documentation or commentary to indicate any

Global Variable Declaration