Question

Setup time is the time:

• A. After clock
• B. Before the clock edge
• C. During clock
• D. Between outputs

Hint:

Remember:
Setup Time \( \implies \) Before the clock edge.
Hold Time \( \implies \) After the clock edge.
Both are critical to avoid metastability in synchronous digital designs.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the definition of "Setup time" in digital sequential circuits, specifically in relation to flip-flops and clock signals.
Setup time is a fundamental timing parameter that must be met to ensure reliable state transition.

Step 2: Key Formula or Approach:
For any edge-triggered flip-flop, the data input cannot change instantaneously at the exact moment the clock edge arrives.
The logic gates inside the device need a finite duration to establish steady voltages before they are latched by the clock.
This lead-time is defined as the setup time (\( t_s \)).

Step 3: Detailed Explanation:
Let us define and detail the timing requirements around a clock edge:

• Setup Time (\( t_s \)): It is the minimum time interval for which the data signal must remain stable and unchanged before the active transition edge of the clock signal.
If the data changes within this setup time window, the internal gates may latch an intermediate state, leading to a condition called metastability.

• Hold Time (\( t_h \)): It is the minimum time interval for which the data must remain stable after the active clock edge.

• Metastability: Failing to meet either the setup time or the hold time can cause the flip-flop output to oscillate between 0 and 1 for an extended period, leading to system failure.

Step 4: Final Answer:
Setup time is the time for which data must be stable before the clock edge.