RTL Problems / Level 2 · Sequential Logic
What You Will Learn
By solving D Flip-Flop with Enable — Conditional Register Update, you will acquire core industry skills:
- Synthesizable hardware modeling in standard Verilog-2001 / SystemVerilog.
- Correct handling of interface port contracts, edge-triggered clocks, and active-low/high resets.
- Simulation waveform interpretation and debugging using timing diagrams (VCD traces).
- Architectural avoidance of timing violations, glitching hazards, and synthesis warnings.
Problem Statement & Requirements
Design a D flip-flop with active-high synchronous enable (en) and active-low asynchronous reset (areset_n).
Hardware design teams at leading semiconductor firms evaluate these exact port contracts, race-condition assertions, and corner cases during technical design reviews and ASIC engineer interviews.
Prerequisites: DFF with async reset, conditional branching.
Port List & Interface Timing
The hidden testbench instantiates top_module. Module port names and bit-widths must match the interface contract exactly:
| Name | Dir | Width | Description |
|---|---|---|---|
clk | input | 1 | Master positive-edge clock |
areset_n | input | 1 | Active-low asynchronous reset |
en | input | 1 | Clock enable (active high) |
d | input | 1 | Data input |
q | output | 1 | Registered output |
Timing Constraints & Synthesizable RTL Rules
Timing Contract: Reset has highest priority. Enable only samples `d` when `areset_n=1` and `en=1` at `posedge clk`.
Synthesis Constraints: Non-blocking assignments. Reset overrides enable.
Architectural Deep Dive & Implementation Walkthrough
Common RTL Designer Pitfalls
['Attempting to gate the clock in RTL: `assign gated_clk = clk & en; always @(posedge gated_clk)`. NEVER do this in synthesizable RTL due to clock skew, race conditions, and glitches.', 'Placing `en` inside the sensitivity list.']
Silicon & Hardware Engineering Notes
Always prefer synchronous clock enables over manual clock gating logic. For ASIC low-power design, use dedicated Integrated Clock Gating (ICG) cells instantiated from the vendor library.
Starter RTL Code
module top_module(
input clk,
input areset_n,
input en,
input d,
output reg q
);
// DFF with async reset and clock enable
endmodule
Verified Reference Solution
🔒 Reveal Verified Reference RTL Solution
module top_module(
input clk,
input areset_n,
input en,
input d,
output reg q
);
always @(posedge clk or negedge areset_n) begin
if (!areset_n)
q <= 1'b0;
else if (en)
q <= d;
end
endmodule
Related RTL Problems in Level 2
- D Flip-Flop — Positive-Edge-Triggered Storage — Implement a basic 1-bit positive-edge-triggered D flip-flop without reset.
- D Flip-Flop with Asynchronous Reset — Reset Behavior — Design a D flip-flop with active-low asynchronous reset (areset_n). Reset forces q to 0 immediately, independent of clk.
- D Flip-Flop with Synchronous Reset — Clocked Reset — Implement a D flip-flop with active-high synchronous reset (sync_rst). The reset takes effect only on the rising clock edge.
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Frequently Asked Questions
What does D Flip-Flop with Enable — Conditional Register Update require?
Design a D flip-flop with active-high synchronous enable (en) and active-low asynchronous reset (areset_n). Implement it as Verilog module top_module matching the listed ports.
Is this design combinational or sequential?
Tags: sequential, fsm, counters, medium, registers. If a clock signal is present in the port table, implement it as sequential logic; otherwise, use continuous assignments or combinational blocks.
How is my solution graded?
A hidden SystemVerilog testbench simulates your module in the cloud judge. You pass when all directed corner cases match the golden model and the VCD timing waveform matches the contract.
How does this challenge help with semiconductor interviews?
Technical interviewers at NVIDIA, Qualcomm, Apple, and Intel routinely ask candidates to code, debug, and explain the timing waveforms of D Flip-Flop with Enable — Conditional Register Update to assess real-world synthesizability and race-condition awareness.