Seven-Segment Display

The segments in a 7-segment display are arranged to form a single digit from 0 to F as shown in the animation:

We can display a multi-digit number by connecting additional displays. Even though LCD displays are more comfortable to work with, 7-segment displays are still standard in the industry. This is due to their temperature robustness, visibility and wide viewing angle. Segments are marked with non-capital letters: a, b, c, d, e, f, g and dp, where dp is the decimal point. The 8 LEDs inside each display can be arranged with a common cathode or common anode. With a common cathode display, the common cathode must be connected to the 0V rail  and the LEDs are turned on with a logic one. Common anode displays must have the common anode connected to the +5V rail. The segments are turned on with a logic zero. The size of a display is measured in millimeters, the height of the digit itself (not the housing, but the digit!). Displays are available with a digit height of 7,10, 13.5, 20, or 25 millimeters. They come in different colors, including: red, orange, and green.

The simplest way to drive a display is via a display driver. These are available for up to 4 displays. Alternatively displays can be driven by a microcontroller and if more than one display is required, the method of driving them is called "multiplexing."

The main difference between the two methods is the number of "drive lines." A special driver may need only a single "clock" line and the driver chip will access all the segments and increment the display. If a single display is to be driven from a microcontroller, 7 lines will be needed plus one for the decimal point. For each additional display, only one extra line is needed. To produce a 4, 5 or 6 digit display, all the 7-segment displays are connected in parallel. The common line (the common-cathode line) is taken out separately and this line is taken low for a short period of time to turn on the display. Each display is turned on  at a rate above 100 times per second, and it will appear that all the displays are turned on at the same time. As each display is turned on, the appropriate information must be delivered to it so that it will give the correct reading. Up to 6 displays can be accessed like this without the brightness of each display being affected. Each display is turned on very hard for one-sixth the time and the POV (persistence of vision) of our eye thinks the display is turned on the whole time. Therefore, the program has to ensure the proper timing, else the unpleasant blinking of display will occur.

Connecting a microcontroller to 7-segment displays in multiplex mode

Program "7seg.asm" displays decimal value of a number stored in variable D.


movlw .21  
movlw D ; number 21 will be printed on 7seg display

Displaying digits is carried out in multiplex mode which means that the microcontroller alternately prints ones digit and tens digit. TMR0 interrupt serves for generating a time period, so that the program enters the interrupt routine every 5ms and performs multiplexing. In the interrupt routine, first step is deciding which segment should be turned on. In case that the tens digit was previously on, it should be turned off, set the mask for printing the ones digit on 7seg display which lasts 5ms, i.e. until the next interrupt.

For extracting the ones digit and the tens digit, macro digbyte is used. It stores the hundreds digit, the tens digit, and the ones digit into variables Dig1, Dig2, and Dig3. In our case, upon macro execution, Dig1 will equal 0, Dig2 will equal 2, and Dig3 will equal 1.

Realization of the macro is given in the following listing:

            LOCAL    PONO        
            LOCAL    EXITL        
            LOCAL    EXIT2        
            LOCAL    POZITIV    
            CLRF    DIGL        
            CLRF    DIG2        
            CLRF    DIG3        
            MOVF    PARO,W        
            MOVWF    DIGTEMP        
            MOVLW    .100        
            INCF    DIGL,F        
            SUBWF    DIGTEMP ,F        
            BTFSC    STATUS, C        
            GOTO    PONO        
            DECF    DIGL,F        
            ADDWF    DIGTEMP, F        
            MOVLW    .10        
            INCF    DIG2,F        
            SUBWF    DIGTEMP, F        
            BTFSC    STATUS,C        
            GOTO    EXITL        
            DECF    DIG2,F        
            ADDWF    DIGTEMP, F        
            MOVF    DIGTEMP,W        
            MOVWF     DIG3        

The following example shows the use of the macro in a program. Program prints a specified 2-digit number on a 7seg display in multiplex mode.

            PROCESSOR  P16F84
            #INCLUDE "P16F84.INC"
            ORG 0X00 
            GOTO MAIN                ;Beginning  of  the program
            ORG 0X04 
            GOTO  ISR                ;Beginning  of  the  interrupt routine

Cblock 0x0c
Digl            ;Variable  for  storing the hundreds  digit of number D
Dig2            ;Variable  for  storing the  tens  digit of number D
Dig3            ;Variable  for  storing the  ones  digit of number D
D                ;D  stores  the number  to  be  displayed
One             ;Auxiliary variable  for multiplex  disp.
W_temp            ;Auxiliary variable
            #include   ""

            BANKSEL  TRISA
            MOVLW B'11111100'    ;RA0  and RAl  are  output pins used for    
            MOVWF TRISA            ;multip lexing
            CLRF TRISB            ;Port B  is   output
            MOVLW   B'10000100' ;Prescaler  is  32 meaning that TMR 0  is
            MOVWF   OPTI0N_REG    ;incremented  every 32ms supposition  that 
                                ;4HHz  oscilator  is used
            BANKSEL   PORTA
            MOVLW   .96            ;Starting value  of THRO  is  96,
            MOVWF   TMRO        ;interrupt occurs  every   (255-97)*32us=5.088ms
            MOVLW   B'10100000'    ;TMR0  interrupt enabled
            MOVWF   INTC0N

            MOVLW   .21            ;Print decimal value   21
            MOVWF    D
            CLRF     ONE
            CLRF     PORTA        ;Turn off both  displays  at the  start
            GOTO     LOOP        ;Main loop        

            MOVWF W_TEMP        ;Store   the  contents   of  W  register
            MOVLW    .96            ;
            MOVWF    TMRO        ;initialize  THRO  to  have  the next  interrupt in
                                ;approximately 5ms
            BCF    INTC0N,T0IF        ;Clear   flag overflow TMRO  to be  able
                                ;to  react to  the next interrupt
            BCF    PORTA,0            ;Turn  off both  displays
            BCF    PORTA,1            
            MOVF ONE,F            
            BTFSC STATUS,Z        ;Which  display should be  on?
            GOTO MSDON            ;
LSDON                           ;If HSD was previously on,
            BCF  ONE,0            ;0ne  stores  that LSD  is  on

            MOVLW HIGH BCDTO7SEG;Prior  to  jump  to  lookup  table,   PCLATH
            MOVWF  PCLATH        ;register  should be   initialized with the higher
                                ;byte  of  the  address  lookup   (HIGH  lookup)
                                ;to prevent errors  in addressing
            DIGBYTE D             ;Macro   for  separating digits  of number  D
            MOVF DIG3,RA        ;Dig3 holds  the  loner  digit
            CALL BCDTO7SEG        ;Call   the  lookup  table  from  which   the 
                                ;appropriate mask  is  read,   for  the  digit
            MOVWF PORTB            ;to be printed  on 7-seg display raith common
            BSF PORTA, 1        ;Turn  on LSD  display (remain on for  5ms,    
                                ;until   the next TMRO  interrupt)
            MOVF W_TEMP ,RA        ;W  registrer has  the  same value  it had before
                                ;entering the  interrupt subroutine

            BSF ONE,0            ;0ne  stores  that HSD  is  on
            MOVLW HIGH BCDTO7SEG        ;Initializing PCLATH  register before using 
            movwf    PCLATH               ;addwf   PCL,f  instruction   (when returning from
                                ;routine,   'call'   does  it automatically)
            DIGBYTE D             ;Macro   for  separating digits  of number  D
            MOVF DIG2,RA         ;HI holds  the higher  digit
            CALL BCDTO7SEG        ;Set the  appropriate mask  for number HI  to
            MOVWF P0RTB         
            BSF  PORTA,0        ;Turn on display for printing the more
                                ;significant digit
            MOVF W_TEMP,W        

            ORG       0X300     ;Lookup  table  is  located at the beginning
                                ;of  the  third page,   other  locations  are 
                                ;possible  as  long as  the  entire  table  is  on 
                                ;single page 
            ADDWF   PCL,F
            DT     0X3F,   0X06,   0X5B,   0X4F,   0X66,   0X6D,   0X7D,   0X07,   0X7F,   0X6F


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