# 3 bit inputs decoder for a 7 segment display

#### Tohien

Joined Mar 12, 2020
8
Hello everyone, i've just learn encoder/decoder and my homework is "Design a decoder with 3bit input to 7 segment display with Cathode Common then display the character: 9,0,5,b,A"
I confused that with decoder BCD to 7 Segment display we have 4 input and we can display "9" . So with 3 input (3 bit) how can we display "9" and another ?

Thanks you !

#### RBR1317

Joined Nov 13, 2010
563
Have you considered the possibility that "3bit" is a typographical error? Is there even such a thing as 3-bit BCD?

#### Tohien

Joined Mar 12, 2020
8
Have you considered the possibility that "3bit" is a typographical error? Is there even such a thing as 3-bit BCD?
Oh my fault, i'm sorry.

#### Tohien

Joined Mar 12, 2020
8
Have you considered the possibility that "3bit" is a typographical error? Is there even such a thing as 3-bit BCD?
i mean, what is the diffrent from "decoder BCD to 7 segment display" and "decoder Binary 3-bit to 7 segment display" ? BCD we have 0-9 and Binary 3-bit we have 0-7 so how the 7 segment can display "9". Thanks you.

#### WBahn

Joined Mar 31, 2012
26,145
If the assignment is to display eight or fewer specific patterns, then all you need is, at most, three bits. You are being asked to display five different patterns, so you can't do that with just two bits, so you need at least three.

Imagine that I asked you to design a 1-bit to 7-segment decoder and display 7 and F. Could you do that?

#### MrChips

Joined Oct 2, 2009
21,853
Your brain is fixated on BCD which stands for Binary Coded Decimal.

#### Tohien

Joined Mar 12, 2020
8
If the assignment is to display eight or fewer specific patterns, then all you need is, at most, three bits. You are being asked to display five different patterns, so you can't do that with just two bits, so you need at least three.

Imagine that I asked you to design a 1-bit to 7-segment decoder and display 7 and F. Could you do that?
Yah, Thanks for your help!, i don't know how to solve it . Can u show me the hint ?
"binary 3 bits to led 7 segment common cathode decoder to display 9, 5, b, A, 0"

#### Tohien

Joined Mar 12, 2020
8
Your brain is fixated on BCD which stands for Binary Coded Decimal.
thanks you i think with 3-bits the 7 segment can display from 0 to 8 right?

#### Kjeldgaard

Joined Apr 7, 2016
410
How many codes can you get with 3 bits binary?
How many different numbers are there between 0 and 8?

#### MrChips

Joined Oct 2, 2009
21,853

Draw a table of the desired display: 9, 0, 5, b, A.
Show which LEDs need to be lit.

#### djsfantasi

Joined Apr 11, 2010
6,802
thanks you i think with 3-bits the 7 segment can display from 0 to 8 right?
Actually, that’s not the problem you’ve been asked to solve. This is the question. With 3 bits input, can you display 5 different patterns on a 7-segment display?

A 7-segment display isn’t restricted to numbers. It is one component with 7 LEDs than can display numbers. Or 128 different patterns.

Note that BCD hasn’t even entered my discussion

#### WBahn

Joined Mar 31, 2012
26,145
Yah, Thanks for your help!, i don't know how to solve it . Can u show me the hint ?
"binary 3 bits to led 7 segment common cathode decoder to display 9, 5, b, A, 0"
I gave you a hint.

Let's take it a step back even further.

Design a 0-bit to 7-seg decoder that displays the character '7' on the display. Can you do that? Don't overthink it -- wire up a seven segment display so that it displays the character '7'.

Now modify the circuit so that you have a single input signal and if it is a 0 the display shows '7' and if it is a 1 the display shows 'F'. Can you do that? Don't overthink it.

#### Tohien

Joined Mar 12, 2020
8
I do in this way:
3 inputs X,Y,Z
9 : leds a-b-c-d-f-g on
b : Leds c-d-e-g-f on
A : leds a-b-c-e-g-f on
0 : leds a-b-c-d-e-f on
then i replace "a" acording to X,Y,Z , right?

#### MrChips

Joined Oct 2, 2009
21,853
No, no, no!

Your values in a-f table are all wrong. Your assignment has nothing to do with BCD!
Erase what you have in the XYX column for now.

#### WBahn

Joined Mar 31, 2012
26,145
What do you mean "replace "a" according to X,Y,Z"?

What about the pattern that looks like '5'?

You are still stuck on the notion that the inputs represent a number between 0 and 7 and that the display shows that number.

Forget about any link between the number associated with the input pattern and the character that is displayed given that input.

When the input pattern is "011" that does NOT represent a '3' -- it is a particular input pattern consisting of 3 bits. YOU decide which of the seven segments are on and which are off when that particular pattern is applied. It can be ANY of the 128 possible patterns of 7 LEDs. YOU pick what you want it to look like and assign the values on that row of the truth table accordingly.

#### djsfantasi

Joined Apr 11, 2010
6,802
What do you mean "replace "a" according to X,Y,Z"?

What about the pattern that looks like '5'?

You are still stuck on the notion that the inputs represent a number between 0 and 7 and that the display shows that number.

Forget about any link between the number associated with the input pattern and the character that is displayed given that input.

When the input pattern is "011" that does NOT represent a '3' -- it is a particular input pattern consisting of 3 bits. YOU decide which of the seven segments are on and which are off when that particular pattern is applied. It can be ANY of the 128 possible patterns of 7 LEDs. YOU pick what you want it to look like and assign the values on that row of the truth table accordingly.
This is exactly what I was implying in my answer. 11 represents the third SYMBOL; not the number 3.

And note WBahn also referred to 128 patterns. Again, they don’t refer to any specific number, just the fact that seven LEDs can be lit in 128 unique patterns.

Open your mind. Stop thinking about what you think is the solution. It’s not a 7-segment display; its 7 LEDs. it’s not BCD. It’s not a number; it’s an ordinal.

Concentrate on the problem. Not the solution first

#### MrChips

Joined Oct 2, 2009
21,853
Fill in the columns a-f.
Do not fill in the columns X, Y, Z.

#### Tohien

Joined Mar 12, 2020
8

is it right ?

#### MrChips

Joined Oct 2, 2009
21,853
Ok. Now we are making progress.

Do you know how to draw a Karnaugh map?