3-input/7-output decoder for a 7 segment display

Discussion in 'Homework Help' started by Raphaello, Aug 23, 2015.

  1. Raphaello

    Thread Starter New Member

    Aug 23, 2015
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    Does anyone know how to build a 3-input/7-output decoder? all segments are also in active high. I have been having some serious trouble with this concept, any help would be greatly appreciated.
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    Assuming you are looking for a display decoder:

    How about a SN7447/8? Study the gate diagram in the datasheet to resolve your questions about the concepts involved.
    MC4513B does it in CMOS with active high outputs.

    For a 1 of 8 decoder, look at the 74138 and its variants.
     
    Last edited: Aug 23, 2015
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Are you talking about 7-segment display (digits 0-9)? If so, then you will need more than 3 inputs to get all 10 possible digits.
     
  4. Raphaello

    Thread Starter New Member

    Aug 23, 2015
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    I have to design a 3 input (g0,g1,g2) decoder with a 7 line output (a to g) to link into a 7 segment display. all the segments of the display are activated on active high. and I have to assume that when 6 and 9 are displayed, the system only displays 5 segments not six. (so segment a in 6 is not active and segment d in 9 is not active)

    g2 g1 g0 | a b c d e f g | display
    0 0 1 | ? ? ? ? ? ? ? | 1
    0 0 0 | ? ? ? ? ? ? ? | 7
    0 1 0 | ? ? ? ? ? ? ? | 2
    0 1 1 | ? ? ? ? ? ? ? | 4
    1 0 0 | ? ? ? ? ? ? ? | 5
    1 0 1 | ? ? ? ? ? ? ? | 0
    1 1 0 | ? ? ? ? ? ? ? | 3
    1 1 1 | ? ? ? ? ? ? ? | 6

    I just cant figure out how to construct the logic or manipulate the logic of a 4 to 8 decoder. Ive tried to manipulate a 3 to 8 decoder's logic gates but Im trumped
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    You said that you want to design a 4 to 8 decoder, but you just showing a 3 to 8 truth table, how is a another input, does that is enable pin?
     
  6. Raphaello

    Thread Starter New Member

    Aug 23, 2015
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    I want to build a THREE input to SEVEN output decoder... the truth table above has 7 possible outputs a, b, c, d, e, f, g. The truth table has 3 inputs G0 G1 and G2 (1, 2 an 4 respectively) Is that even possible?
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Your input G0,G1,G2 and the display number didn't like the normal number as :
    000 - 0,
    001 - 1,
    010 - 2
    011 - 3
    100 - 4
    010 - 5
    110 - 6
    111 - 7

    The number what you post is specific of your home work or you setup?
     
  8. absf

    Senior Member

    Dec 29, 2010
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    Here is the truth table for your customized 7-segment decoder.

    Now, are you able to design your decoder circuit using discrete logic TTL/CMOS gates?

    TRUTH TABLE.PNG

    Allen
     
    Last edited: Aug 24, 2015
  9. WBahn

    Moderator

    Mar 31, 2012
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    The first step is to go through row by row and determine whether each question mark needs to be a 0 or a 1. If the segment is supposed to be lit, then it is a 1 since the display segment is active high.

    You talk about when 6 and 9 are displayed, but 9 is not one of the values you can display.
     
    absf likes this.
  10. WBahn

    Moderator

    Mar 31, 2012
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    Isn't it nice when you find someone willing to do a good part of your homework for you, even if they don't quite read the problem and do it wrong.
     
  11. absf

    Senior Member

    Dec 29, 2010
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    I though that may be if doing the truth table would get him start thinking why are the "1" and "0" are there. If he didn't understand he would start asking questions and me or someone else would slowly guide him. But after seeing post #9, that should give him the "ah ha" moment.

    Allen
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    When you did that, it seems as you already did for him half of his work, since this is a home work, so we can't do too much things for the student, only guiding him.
     
    absf likes this.
  13. Raphaello

    Thread Starter New Member

    Aug 23, 2015
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    Hi gentlemen,

    Firstly, thank you for your advice.

    Is this an accurate portrayal of the desired decoder ?

    image.jpg
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    Think about your circuit. Each of your 7-seg inputs will be HI for exactly one combination of your G inputs and LO for all other combinations.

    Is that what you want?
     
  15. Raphaello

    Thread Starter New Member

    Aug 23, 2015
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    I assume so, as I am told "the segments of the display a, b, c, d, e, f and g are activated on active -high of the corresponding a, b, c, d, e, f and g input connections" and i can only display one numereric output as i only have one display to work with
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    But the whole idea is that as you change the values of the G inputs that the digit that is displayed changes.

    You can apply eight different patterns to the G inputs, meaning that you can display eight different digits depending on which pattern is currently being applied.

    Once you have the (correct) truth table, focus on each segment as a separate problem.
     
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