When working with digital logic circuits, especially sequential elements like flip-flops, it's crucial to understand the role of both synchronous and asynchronous inputs. J-K flip-flops, like many other types of flip-flops, are often equipped with asynchronous control inputs, typically labelled PRESET (or SET) and CLEAR (or RESET).

Flip-Flop Asynchronous Inputs

Asynchronous inputs are special control signals that directly affect the output of a flip-flop, bypassing the clock and the main data inputs (J and K in this case). They are called "asynchronous" because their effect is immediate and does not depend on the clock signal. They are usually active low, meaning they perform their function when the input is at a logic 0.

• PRESET (Active Low): When the PRESET input is driven to a logic 0 (active low), it forces the flip-flop's output Q to a logic 1, and the $\bar{Q}$ (Q-bar) output to a logic 0. This happens regardless of the states of the J, K, or clock inputs. It "sets" the flip-flop.
• CLEAR (Active Low): Similarly, when the CLEAR input is driven to a logic 0 (active low), it forces the flip-flop's output Q to a logic 0, and the $\bar{Q}$ (Q-bar) output to a logic 1. This also happens independently of the J, K, or clock inputs. It "resets" or "clears" the flip-flop.

Undefined Output with Simultaneous PRESET and CLEAR

The question describes a scenario where both asynchronous active-low inputs, PRESET and CLEAR, are applied simultaneously to a J-K flip-flop. This means both PRESET = 0 and CLEAR = 0 at the same time.

• When PRESET is active (logic 0), it tries to force the output Q to 1.
• When CLEAR is active (logic 0), it tries to force the output Q to 0.

Since the Q output of a flip-flop cannot simultaneously be both a logic 1 and a logic 0, applying both PRESET and CLEAR at the same time creates a conflicting condition. This situation leads to an undefined or indeterminate output state. The exact output might vary depending on the internal gate delays and manufacturing variations of the specific flip-flop. This is often referred to as a "race condition" because the two inputs are "racing" to control the output in opposite directions.

It's important to avoid this state in practical digital circuit design as it leads to unpredictable behavior and can cause system instability.

Therefore, when both active low PRESET and CLEAR are applied to a J-K flip-flop, the output will be undefined.