Conceptually, what is the main behavioral difference between a level sensitive D latch and an edge triggered D flip flop when described at the register transfer level RTL in a digital design?

Introduction / Context:
This question compares the behavior of a level sensitive D latch and an edge triggered D flip flop at the register transfer level. These two storage elements are fundamental building blocks in digital design. Understanding how they respond to clock or enable signals and data input D is essential when writing RTL code in Verilog or VHDL, because incorrect modeling can lead to timing problems, glitches, and mismatches between simulation and hardware.

Given Data / Assumptions:

We are dealing with D type storage elements, specifically D latches and D flip flops.

The question concerns behavior at RTL level, not transistor level.

A D latch is level sensitive, not edge triggered.

A D flip flop is edge triggered, typically on the rising or falling edge of a clock.

We need to identify which statement correctly compares their timing behavior.

Concept / Approach:
A level sensitive D latch has an enable or clock input that makes the latch transparent when active. When the enable is asserted, the output Q follows the input D, so any change on D appears at Q. When the enable is deasserted, the latch closes and holds the last value of D. An edge triggered D flip flop behaves differently. It samples the D input only at a specific clock edge, such as the rising edge, and then holds that sampled value until the next active edge. Changes on D between edges do not affect Q. This difference in transparency versus discrete edge sampling is the key behavioral distinction between latches and flip flops at RTL.

Step-by-Step Solution:
Step 1: Recall that a D latch is level sensitive: when the enable or clock input is high for an active high latch, Q follows D; when the enable is low, Q holds its previous value.Step 2: Recall that a D flip flop is edge triggered: it captures the value of D only at a specific clock edge, then holds the value until the next active edge.Step 3: Identify the option that states that the latch is transparent while enabled and that the flip flop samples D only at an edge.Step 4: Compare option C with this expected description and see that it matches.Step 5: Select option C as the correct answer and reject options that reverse these behaviors or mix analog concepts.

Verification / Alternative check:
Standard digital design textbooks describe latches as level sensitive devices that can introduce unwanted transparency if the enable is active for too long, which can allow glitches and timing hazards. Flip flops are described as edge triggered elements that are preferred for synchronous design because they change state only at well defined clock edges. RTL coding patterns reflect this difference: latches are often modeled using always blocks sensitive to level changes on an enable signal, while flip flops are modeled using always blocks sensitive to posedge or negedge of a clock. These consistent descriptions confirm that option C is correct.

Why Other Options Are Wrong:
Option A reverses the behaviors by stating that latches update only on edges and that flip flops are transparent, which is incorrect. Option B claims there is no behavioral difference, which contradicts standard definitions. Option D introduces a false distinction about bit width; both latches and flip flops can store single bits or vectors at RTL. Option E incorrectly claims that flip flops are used only in analog circuits and that D latches are asynchronous only, which is misleading and factually wrong.

Common Pitfalls:
Designers new to RTL often accidentally infer latches when they intended flip flops, usually by writing incomplete if statements without else branches. Another pitfall is to forget that latches can cause timing issues in synchronous designs when not carefully controlled. To avoid these problems, remember the key rule: latches are level sensitive and transparent when enabled, while flip flops are edge triggered and update only on clock edges.

Final Answer:
The main behavioral difference is that a level sensitive D latch is transparent and follows D while its enable is active, whereas an edge triggered D flip flop samples D only at a clock edge and holds that value until the next active edge.