0
\$\begingroup\$

I have the following state transition table:

enter image description here

where,
A, B - current states of two D flip flops

A+, B+ - next states of the two D flip flops

X1, X2, X3 - inputs to the current states

There are don't care conditions in inputs X1, X2 and X3.

I am trying to make a K-map for A+ using four variables A, X1, X2 and X3. I have dealt with don't cares in the output. But, I don't know how to deal with don't cares when they are on the input side.

How to deal with input don't care conditions ?

(I somehow tried to solve and got the answer: A+ = A'*X2'. Answer key has something else the answer. My state transition table is same as the one given in the answer key. So, I am sure that my state transition table is correct. I am sure that I am making mistakes in building K-maps from table as I don't know to incorporate input don't cares.)

\$\endgroup\$
  • \$\begingroup\$ The A+ K-Map needs to have 5 variables, A, B, X1, X2 and X3 \$\endgroup\$ – Michael Karas Nov 22 '15 at 10:06
  • \$\begingroup\$ The B+ K-Map also needs 5 variables. The next state of both flops depends upon the current state of both plus the inputs. \$\endgroup\$ – Michael Karas Nov 22 '15 at 10:57
1
\$\begingroup\$

In this case the input don't cares are quite simple. When you code each row into the K-Map you should code your desired output in cells that meet the input variable requirements. If a certain Xi is a don't care you have more places to fill in your outputs.

What I am trying to say is that you need to find all cells in the KMap that specifically satisfy the output ignoring inputs where there is a don't care.

Here are the direct fill outs in the K-Maps for A+ and B+. I'll leave it to you to perform the minimization steps.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ @Micheal Karas Thank you very much! Although, some of the terms in my simplified expression are not matching with given answer, I got to know how to incorporate input don't cares. \$\endgroup\$ – maverick Nov 27 '15 at 5:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.