Multiplexing 7 segment displays

In some cases we need more than one 7 segment display, But we will be having limited pins to interface, so we cannot use for example 3X8 port pins for 3 seven segment displays. Therefore the effective approach is multiplexing the displays. This method takes advantage of POV i.e. Persistence Of Vision of a human eye, that means a human eye cannot observe changes that occur rapidly within milliseconds or microseconds. So here is the secret, we will access LED displays one by one so fast that a human eye cannot recognize and it appears like we are driving all the displays simultaneously.

Figure below shows the circuit diagram for driving multiple 7 segment displays. Here all the display inputs are joined together and connected to a single port of the microcontroller, the display access is controlled by the transistors connected to the common anode terminals of the displays.

I have built a quickie circuit with only two displays on a breadboard (same as above circuit, just ignored dis3 and dis4) I have written the program for only two displays, you can easily modify the program for more than two displays.

The assembly language source code for this circuit is given below. The programs is self explanatory. The first program uses polling method therefore it cannot be used in actual projects since all the microcontrollers time is utilized for driving displays, whereas second one uses timer and interrupts, therefore microcontroller can perform the main required program and simultaneously drive the displays.

Using timer and interrupts:

;*************************************************
;
;Program: Driving multiple 7 segment
;Author: Srikanth
;Website: http://shree-electronics.com/
;Description: Displays 0-9 in one display
;and 9-0 in another
;
;*************************************************
;declarations
dis1 equ P1.0;
dis2 equ P1.1;
;*************************************************
;Main program

org 00h
ljmp main

org 0bh                 ; Timer ISR
ljmp disp

org 30h
main:mov P2,#00h
     mov tmod,#01h      ;Configure timer
     mov TH1,#05h
     mov TH0,#0ffh
     mov IE,#82h        ;Enable timer interrupts
     setb TR0           ;start timer

up: mov r2,#11111100b ;'0'
     mov r3,#11111100b ;'0'
     acall delay
     mov r2,#01100000b ;'1'
     mov r3,#0F6h       ;'9'
     acall delay
     mov r2,#0DAh       ;'2'
     mov r3,#0FEh       ;'8'
     acall delay
     mov r2,#0F2h       ;'3'
     mov r3,#0E0h       ;'7'
     acall delay
     mov r2,#066h       ;'4'
     mov r3,#0BEh       ;'6'
     acall delay
     mov r2,#0B6h       ;'5'
     mov r3,#0B6h       ;'5'
     acall delay
     mov r2,#0BEh       ;'6'
     mov r3,#066h       ;'4'
     acall delay
     mov r2,#0E0h       ;'7'
     mov r3,#0F2h       ;'3'
     acall delay
     mov r2,#0FEh       ;'8'
     mov r3,#0DAh       ;'2'
     acall delay
     mov r2,#0F6h       ;'9'
     mov r3,#060h       ;'1'
     acall delay
     sjmp up

;*************************************************
;display subroutine
disp:clr dis1
     clr dis2
     mov a,r2
     cpl a
     mov P2,a
     setb dis1
     mov r4,#0fh
     djnz r4,$
     clr dis1
     mov a,r3
     cpl a
     mov P2,a
     setb dis2
     mov r4,#0fh
     djnz r4,$
     clr dis2
     mov TH1,#05h
     mov TH0,#0ffh

reti

;*************************************************
;delay subroutine
delay:mov r7,#0ffh
rep:mov r6,#0ffh
rep1:mov r5,#05h
     djnz r5,$
     djnz r6,rep1
     djnz r7,rep
ret
;*************************************************

end