Help Kmap don't care conditions

[trigon91]

I need help on this problem i was stuck the whole day..

My assignment is to design a octal to 7 segment common cathode decoder.

You are to assume that only octal digits 2 to 7 are applied to the inputs of the decoder, i.e. combination for digits 0 and 1 are never applied.

I have fill up my truth table..

When i tried to set input for A it lights up when 1 is switch on?

I just wanted it to show 2,3,4,5,6,7 digits only

What's the purpose don't care conditions?

[Chen_T]

Hi there,

Because you only need to show 2 to 7, therefore "don't care" means the decoder output abcdefg does not matter when 0 or 1 is switch on.

When you use the Kmap to simply the logic, just treat the don't care "x" as 1 to simplify the map.

Regards,

[trigon91]

what i actually meant was that when switch 1 and 0 is turn on, there will be no display since i wanted 2 to 7 digits output to be shown only..

[Chen_T]

If you want no display when switch 1 and 0 is turned on, then treat "x" as 0 to simply the map.

[trigon91]

Then what is the point of don't care condition for this case?

Is it because i have the choice to pick either 1 or 0 but i taught don't care condition is to simplify more further?

[Chen_T]

Yes, "don't care" is to simplify more further. But you said previously that you don't wanna 1 or 0 displayed, so you  need to treat "x" as 0 to simply the map which means you do care the "x" to be 0 now.

[trigon91]

Can you tell if the picture below is correct? I tried to implement the least gates possible.. But it doesn't looks right to me..

Set to 0.

[Chen_T · Accepted solution]

Hi there,

Right now your input is "0 0 0" which means "don't care", just ignore the pattern you see right now. Make sure the rest from 2 to 7 works.

Regards,