Question

Define cross-coupled pair in digital logic circuits.

Answer 1

In digital logic circuits, a cross-coupled pair refers to a specific configuration of two logic gates (typically two NOR gates or two NAND gates) that are interconnected in a feedback loop. This configuration is commonly used to implement bistable elements, such as flip-flops or latches, which are essential building blocks in sequential logic circuits.

The cross-coupled pair consists of two gates connected in such a way that the output of each gate is fed back to the input of the other gate. This feedback loop creates a stable state, which means that the circuit can hold its output value even after the input signal is removed. The two possible stable states correspond to binary values (0 and 1), allowing the circuit to store one bit of information.

For example, in a basic implementation using NOR gates, the cross-coupled pair looks like this:

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       +---+
       |   |
-------|   |------- Q
       |   |
       +---+

       +---+
       |   |
-------|   |------- ~Q (Q-bar)
       |   |
       +---+


Here, the output of each NOR gate is connected to the input of the other gate, forming a loop. The state of the circuit is determined by the input conditions and the initial state of the cross-coupled pair. Depending on the logic levels at the inputs, the circuit will remain in one of the stable states (either Q=1, ~Q=0 or Q=0, ~Q=1) until the input conditions change.

The cross-coupled pair is a fundamental building block used to create more complex sequential logic elements like SR flip-flops, D flip-flops, and JK flip-flops, which are essential for storing and processing data in digital systems.