Consider the 4-variable Boolean function
where is the most significant bit and the least significant bit, and there are no don't-care terms. Which one of the following is the minimal sum-of-products form of ?
(Here denotes complement.)
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Solution MCQ · 2 marks · −⅔ if wrong
Answer: (A)
Method: place the minterms on the K-map and take the largest groups.
With rows and columns in Gray order ():
| 00 | 01 | 11 | 10 | |
|---|---|---|---|---|
| 00 | 1 (m0) | 1 (m2) | ||
| 01 | 1 (m5) | 1 (m7) | ||
| 11 | 1 (m13) | 1 (m15) | ||
| 10 | 1 (m8) | 1 (m10) |
- m0, m2, m8, m10 are the four corners. The map wraps left-right and top-bottom, so they form one group of four where and : the term .
- m5, m7, m13, m15 form the central block where and : the term .
Both groups are essential prime implicants and together cover all eight minterms, so (that is, exactly when ).
Trap. Not seeing that the corners are adjacent leads to two separate pairs (), which is correct as a function but has more literals.
What was tested: wrap-around adjacency on a 4-variable K-map.
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