Multiplexed Seven Segment Displays.

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We have discussed the basics of seven segment displays on our tutorial “Using Seven Segment Displays with AVR MCUs”. So you should be familiar with them. In this tutorial we will discuss about multiplexing of seven segment displays.Multiplexing is required when we want to interface 3 or 4 or even more such displays with MCUs since it we go for normal way it will require lots of IO port. So the smart way is multiplexing. Multiplexing achieved by tricking our eyes. Only one display is active at a time but we see all of them active. For multiplexing all the displays are connected in parallel such that if you activate any segment, say ‘a’ the ‘a’ segment of all displays glows up. But the trick is that we can switch on and off the “common” line of the displays under MCU control. So if we wish to light up the ‘a’ segment of display 2 we simply switch on display 2 first by applying proper level at the base of its driving transistor as shown in figure.

Fig – Multiplexed Seven Segment Displays.

If we like to display the digit say “123” on three displays first we select disp-3 by applying a “low” level at the base of transistor Q1 and output the code of required digit at the data input terminals. Since display 3 is selected “1” is displayed in disp-3 then we wait for some time and select disp-2 and output code of digit “2”. This “2” will be shown in disp-2. Then we select display-1 and output code for digit “3”. And now you can guess it will be displayed in display-1. This process is shown in animation below.

Fig – Multiplexed Seven Segment Displays.

If we repeat this step fast enough (not a problem with MCUs !) what we get due to persistence of vision is that we see the number “123” displayed “still” in the display as if all the displays are active simultaneously.

Programming

The actual programming will be covered in next tutorial. In this part we will simply see the steps involved. We will use the TIMER0 to refresh our displays. To learn about timer please see the previous tutorial. The timer will interrupt the CPU at predefined time interval an the CPU will switch to next display and display a digit there. An array variable uint8_t digits[3]; will hold the digits for the three displays what ever we need to display on the displays we store them there. So to display “911” on the display we store. digits[0]=1; digits[1]=1; digits[2]=9; The ISR of TIMER0 Overflow is as follows

We set the TIMER0 prescaler as 256 so the sytem clock i.e. 16MHz is divided by 256 and the timer increment its value at 16000000/256 = 62500Hz i.e. 62.5KHz And timer overflows when its value changes from 255 to 0 so the frequency of Overflow is 62.5 KHz/ 256 = 244.140625 Hz As we switch display in each ISR the display of switching is 244.14 Hz since 3 switching are needed to completely draw a three digit number so the frequency of display update is 244.14/3= 81.38 Hz So we draw the complete digits approximately 81 times per second !!! This is too fast for our eyes to catch up so we see all three digits lit simultaneously. So friends this was the theory behind one of the most used techniques in MCU design world. You must have seen these Seven Segments used in weighing machines,PCOs and even currency counters in banks they all use this technique of Multiplexing the displays. Many microcontroller projects published in net of magazines uses multiplexed seven segment displays and this article will help you understand them. I said it all, now its your turn so all of you get ready for some typing because I need your comment on this post, simply say anything you think good or bad. While I write the next tutorial I will be heartily waiting for your comments.

WhatsNext ?

We will write the code for this in AVR-GCC. Proceed to next part.

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Avinash

Avinash Gupta is solely focused on free and high quality tutorial to make learning embedded system fun !

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About Avinash

Avinash Gupta is solely focused on free and high quality tutorial to make learning embedded system fun !

31 thoughts on “Multiplexed Seven Segment Displays.

  • By vishal - Reply

    nothing to say.

    great job!

    thank you very much.

  • By Tiago - Reply

    In the display animation, the display names are the same (DISP-1), but I think that the names would be DISP-1, DISP-2 and DISP-3.

    By the way, it´s a great job.

    Thank you.

  • By chandrasekaran - Reply

    Thanks a lot for your work.

  • By Jerry B. Halibas - Reply

    Hmm, few topics where not included particulary the effect of multiplexing to its power requirement and frequency response specially when Large 7 segment (5″) are used.

  • By Jerry B. Halibas - Reply

    Hmm, few topics where not included particulary the effect of multiplexing to its power requirement and frequency response specially when Large 7 segment (5?) are used as the number of 7 segment increases to 4 and up.

  • By Amit Rana - Reply

    How to multiplexed the common anode 4-inch big seven segment display with microcontrollers? cos they need 12V and if i apply 12V to emitter, it will burn my micro

  • By Anonymous - Reply

    Use NPN transistor for big seven segment such as TIP31/33/35 or BC337/338 but using PNP transistor to supply 12V to seven segment and that too controlled by MCU may have many problem to appear.

  • By andri - Reply

    i need to handle with 64 seven segment, is this work? and how about wire connection with atmel 89S52?, thank u.

  • By Guptah - Reply

    Well done with your explanations. I have enjoyed your graphic animations in many of your posts. I plan recommend your tutorials to several people I know just starting out in MCUs. Keep up the good work.

  • By Ali - Reply

    Love your tutorial. very easy to understand. its help me a lot. thank you.

  • By andyy - Reply

    Great explaination..:) thank you…But I would ask u How to write program to display data onto 4-digit multiplexed 7-segmentdisplays? in asemmbly not C….?so controlling for digits by PORTD(D0…D3) and controlling segment’s leds by PORTB(B0…B7)..thank U

  • By sagar - Reply

    Thanx i needed this info.
    nd, u helped in d nik of time :)

    all D Best, buddy.

  • By carlos - Reply

    mate you born to teach
    what is your occupation?
    can i have your mail address?
    regards
    carlos

    • By Avinash - Reply

      @Carlos,

      Thanks! But I don’t like teaching.

  • By Jorge González - Reply

    Great! Tu muy bien!

  • Pingback: Interfacing Seven Segment Displays | eXtreme Electronics

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  • By elecengr - Reply

    Great work! However I have a query. Is system clock 16mhz of 16/4, i.e, 4 mhz.

    • By Avinash - Reply

      @elecengr

      Why you think the clock is 16/4 ? Any reason ? where did you get that ’4′?

      • By elecengr -

        I thought 16MHz is external crystal frequency. So the frequency of executing each instruction is 16/4 as execution of each instruction takes 4 clock cycles. Isn’t it?

        I am a beginner and trying to learn. Please help.

    • By Avinash - Reply

      The value ’4′ probably came from your guess, but it should really come from the ‘datasheet’

      Yes you are a beginner but you should avoid picking up those value from your dream (I would suggest datasheet indeed)

      In AVR CPU we have 1 instruction per clock (for most instuctions) and the clock input is not divided.

      In PIC MCUs the input clock is divided by 4.

      So a PIC @ 20MHz CPU clock is 5MHz
      while AVR @ 20MHz CPU clock is 20MHz!!!

      • By elecengr -

        thanks!

  • By Jerry Halibas - Reply
    • By Avinash - Reply

      @Jerry,

      Good!

  • Pingback: watch using 4 digit 7 segment and avr

  • By Vikas arora - Reply

    too good yaar

  • By vijay - Reply

    better to implement a code.carry on

  • By DIVKA - Reply

    I like your tutorial..nice and cute to read it..easy to understand..

  • By Hariom Dubey - Reply

    Really,you are genious !
    what you put on your website is very creative and Amazing,i can’t express how much your website helpfull
    Here i should say Electronics means Avinash .
    Thankyou so much.

  • By Shereef - Reply

    You just saved me from the 36 pin usage headache !
    Thank you sir Avinash

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