Seven segment display is a basic type of output device, 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 driving LEDs, but here the difference is we are lighting up 7+1 LEDs in aspecific pattern. The 7 segment display module has 8 LEDs (7 segments to display number and one segment for decimal point or dot) arranged in a 10 pin module as shown in the 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.
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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.
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The assembly language source code for interfacing 7 segment display is given below.
;*************************************************
;
;Program: Driving seven segment display
;Author: Srikanth
;Website: http://shree-electronics.com/
;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
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
