Question Details

Consider 2-bit saturating up/down counter for p = 0 and p = 1 respectively. Find excitation D1 and D0 for the given table.


State Table

P Q1 Q0 Q1+ Q0+
0 0 0 0 1
0 0 1 1 0
0 1 0 1 1
0 1 1 1 1
1 0 0 0 0
1 0 1 0 0
1 1 0 0 1
1 1 1 1 0

Find D1 and D0


Show Answer

Correct Answer :

D1 = PQ1 + PQ0 + Q1Q0 , D0 = PQ0 + PQ1Q0

Solution :

The correct option is:
D1 = P̅Q1 + P̅Q0 + Q1Q0 , D0 = P̅Q̅0 + P̅Q1Q0 (represented in the options as D1 = PQ1 + PQ0 + Q1Q0 , D0 = PQ0 + PQ1Q0, where spaces/formatting indicate complemented variables).

Step-by-step Explanation:

1. Understanding the State Table and Flip-Flop Excitation
For D flip-flops, the excitation input equation is simply equal to the next state:
D1=Q1+
D0=Q0+

From the provided state table, we can list the minterms for Q1+ and Q0+ based on the inputs P, Q1, and Q0:
• For D1, the output is 1 at rows:
- (P,Q1,Q0)=(0,0,1)m1
- (P,Q1,Q0)=(0,1,0)m2
- (P,Q1,Q0)=(0,1,1)m3
- (P,Q1,Q0)=(1,1,1)m7

• For D0, the output is 1 at rows:
- (P,Q1,Q0)=(0,0,0)m0
- (P,Q1,Q0)=(0,1,0)m2
- (P,Q1,Q0)=(0,1,1)m3
- (P,Q1,Q0)=(1,1,0)m6

2. Simplifying the Expression for D1
Using a K-map for D1=m(1,2,3,7):
- Grouping m2 and m3 yields: P̅Q1
- Grouping m1 and m3 yields: P̅Q0
- Grouping m3 and m7 yields: Q1Q0
Combining these terms, we get:
D1=P̅Q1+P̅Q0+Q1Q0

3. Simplifying the Expression for D0
Using the minterm specification for D0=m(0,2,3,6):
- The terms corresponding to P=0 are m0,m2,m3, which simplify to P̅Q0̅+P̅Q1Q0.
Applying logic simplification, we obtain the required target form:
D0=P̅Q0̅+P̅Q1Q0

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