Interfacing 7 segment

  

  Seven segment display is a basic type of display which can display numbers from 0 to 9. The circuit for interfacing single 7 segment display is shown below.

    Driving a 7 segment display is as simple as flashing LEDs, but here we are flashing 7+1 LEDs. The 7 segment display module has 8 LEDs (7 segments to display number and one segment for decimal point or dot) arranged in a particular manner as shown in image below.

 

    By driving (in the sense controlling ON and OFF conditions) these LEDs in various combinations, we can display the numbers 0 to 9. There are basically two types of 7 segment displays, they are common cathode and common anode. In common cathode, the cathodes of all the LED segments are connected together, we should apply a logic 1 or high input to a segment pin to light up that particular segment, and in common cathode the case is opposite. Table below shows the combinations of inputs to be applied to 7 segment display for digits 0 to 9.

For common cathode displays
Digit	binary input value	hexadecimal input value
0	11111100	FC
1	01100000	60
2	11011010	DA
3	11110010	F2
4	01100110	66
5	10110110	B6
6	10111110	BE
7	11100000	E0
8	11111110	FE
9	11110110	F6

For common anode displays
Digit	binary input value	hexadecimal input value
0	00000011	03
1	10011111	9F
2	00100101	25
3	00001101	0D
4	10011001	99
5	01001001	49
6	01000001	41
7	00011111	1F
8	00000001	01
9	00001001	09

 
The images shown below are the photographs of 7 segment add-on board for my microcontroller board, I have included one 7 segment in the current version of my development board, so if you are using that circuit then you don't need an external 7 segment display board. Click on the image to enlarge.

 

The assembly language source code for interfacing 7 segment display is given below.

;************************************************* ; ;Program: Driving seven segment display ;Author: Mahesh ;Description: Displays numbers 0 to 9 on ;the LED seven segment display continuously ; ;************************************************* ;Declarations port equ P0 ;************************************************* ;Main program org 0000h      ljmp main org 30h  main:mov r0,#08h      mov a,#00000001b      ;test all segments of disp up:  rr a      mov port,a      acall delay      djnz r0,up again:mov port,#11111100b  ;'0'      acall delay      mov port,#01100000b   ;'1'      acall delay      mov port,#11011010b   ;'2'      acall delay      mov port,#11110010b   ;'3'      acall delay      mov port,#01100110b   ;'4'      acall delay      mov port,#10110110b   ;'5'      acall delay      mov port,#10111110b   ;'6'      acall delay      mov port,#11100000b   ;'7'      acall delay      mov port,#11111110b   ;'8'      acall delay      mov port,#11110110b   ;'9'      acall delay      sjmp again ;************************************************* delay:mov r2,#0ffh         ;delay subroutine up3: mov r4,#03fh up2: mov r3,#0fh up1: djnz r3,up1      djnz r4,up2      djnz r2,up3      ret ;************************************************* end