15.2 Boolean Algebra and Logic Circuits (3)
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1.
The Boolean function f(x, y, z) is defined as the sum of the minterms in the following truth table:
x | y | z | f(x, y, z) |
0 | 0 | 0 | 0 |
0 | 0 | 1 | 1 |
0 | 1 | 0 | 0 |
0 | 1 | 1 | 1 |
1 | 0 | 0 | 0 |
1 | 0 | 1 | 0 |
1 | 1 | 0 | 1 |
1 | 1 | 1 | 1 |
Construct a Karnaugh map to simplify the Boolean expression for f(x, y, z). State the simplified Boolean expression in terms of x, y, and z.
Answer:
The Karnaugh map for a 3-variable function is as follows:
We can group the '1's as follows:
- The two '1's in the first row (000 and 001) can be grouped.
- The two '1's in the fourth column (011 and 111) can be grouped.
- The '1' in the third row, second column (101) can be grouped with the '1' in the third row, third column (110).
This gives us the simplified expression: f(x, y, z) = (A'C' + A'BC) + (AB'C + ABC) = A'C' + A'BC + AB'C + ABC = A'C' + AB'C + BC
Or, using Boolean algebra: f(x, y, z) = x'z' + x'yz + xy'z + xyz
2.
Consider a system requiring a counter that increments on each clock pulse. Compare and contrast the use of JK flip-flops versus D flip-flops in implementing such a counter. Highlight the key differences in their operation and the resulting impact on the counter's functionality and design.
JK Flip-Flop for a Counter: A JK flip-flop can be used to create a counter by cascading multiple JK flip-flops. The J and K inputs are connected to generate the desired state transitions. For example, to create a 2-bit counter, the J and K inputs of the flip-flops are connected to generate the sequence 00, 01, 10, 11, and then back to 00. The clock signal triggers the state transitions.
- Operation: The JK flip-flop's ability to toggle its state when both inputs are '1' is crucial for counter functionality. The specific combination of J and K inputs determines the sequence of states.
- Design: Requires careful wiring to ensure the correct sequence of state transitions. The design can become complex for larger counters.
D Flip-Flop for a Counter: A D flip-flop is a simpler alternative for implementing a counter. The D input is connected to the desired next state of the counter. The clock signal triggers the transfer of the value on the D input to the Q output. To create a counter, the D input is set to the desired next state for each clock pulse.
- Operation: The D flip-flop directly captures the value on the D input at the active clock edge. This makes it easier to implement counters with predictable state transitions.
- Design: Generally simpler to design than a counter based on JK flip-flops, especially for larger counters. The D flip-flop's behavior is more straightforward to analyze.
Comparison:
Feature | JK Flip-Flop | D Flip-Flop |
Complexity | More Complex | Simpler |
Design | More wiring required | Less wiring required |
State Transitions | Toggle-based | Direct state capture |
3.
Describe the operation of both SR and JK flip-flops, including their truth tables and identifying their key advantages and disadvantages. Consider how these differences impact their suitability for different digital logic applications.
SR Flip-Flop: The SR flip-flop is a basic sequential logic circuit with two inputs: Set (S) and Reset (R).
- Operation: When S is asserted (typically high), the flip-flop transitions to a '1' state. When R is asserted (typically high), it transitions to a '0' state. If both S and R are asserted simultaneously, the output state is undefined (often considered a metastable state).
- Truth Table:
S | R | Q(t+1) |
0 | 0 | Q(t) |
0 | 1 | 0 |
1 | 0 | 1 |
1 | 1 | Undefined |
- Advantages: Simple design, easy to understand.
- Disadvantages: The 'S' and 'R' inputs can conflict, leading to an undefined state. This makes it unsuitable for applications requiring reliable state transitions.
- Suitability: Limited use in practical digital circuits due to the undefined state. May be used in specific applications where the undefined state is acceptable or can be managed.
JK Flip-Flop: The JK flip-flop addresses the problem of the SR flip-flop by providing a third input, J (Justify), which eliminates the undefined state.
- Operation:
- When J=0 and K=0, the flip-flop holds its current state.
- When J=0 and K=1, the flip-flop resets to '0'.
- When J=1 and K=0, the flip-flop sets to '1'.
- When J=1 and K=1, the flip-flop toggles its state (flips from '0' to '1' or '1' to '0').
- Truth Table:
J | K | Q(t+1) |
0 | 0 | Q(t) |
0 | 1 | 0 |
1 | 0 | 1 |
1 | 1 | !Q(t) |
- Advantages: Eliminates the undefined state of the SR flip-flop. Provides a clear and predictable state transition.
- Disadvantages: Slightly more complex than the SR flip-flop.
- Suitability: Widely used in digital circuits due to its reliability and predictable behavior. Suitable for a broad range of applications, including registers, counters, and memory elements.