From Simulation Success to Interview Errors: A Student’s Journey into RTL-Correct Verilog Coding

Introduction

Most students feel confident when their Verilog code runs perfectly in simulation. Waveforms look correct, outputs match expectations, and the testbench prints green tick marks. But this confidence collapses during RTL interviews or real hardware design tasks.

Why?

Because writing Verilog that behaves correctly in simulation is not the same as writing synthesizable, hardware-accurate RTL.

This article explains the journey from simulation comfort to real RTL understanding, and highlights the common interview errors students face—and how to avoid them.

STAGE 1: The Simulation Comfort Zone

In academic labs, Verilog is tested only for functional correctness. Students rely heavily on:
– initial blocks
– delays (#5)
– print statements ($monitor, $display)
– forced stimulus
– procedural clocks

This builds the misconception:
“If my simulation works, my design is correct.”

STAGE 2: The Interview Reality Check

Interviewers ask for synthesizable RTL, not simulation tricks.

Interview question:
“Write synthesizable Verilog for a mod‑10 counter.”

Example student response:

These constructs are simulation-only and have no meaning to synthesis tools. This is where students realize simulation success is not equal to hardware correctness.

STAGE 3: Understanding RTL Thinking

Simulation Thinking:
– Executes sequentially like software
– Uses delays for timing
– Registers start initialized

RTL Thinking:
– Hardware runs in parallel
– Timing controlled only by clock edges
– Registers power up unknown
– Must avoid latches
– Must use proper reset logic

Real RTL requires:
– always @(posedge clk) for sequential logic
– always @(*) for combinational logic

STAGE 4: Rising to RTL Excellence

Once students adopt proper RTL discipline, they:

Write correct sequential logic 
Avoid unintended latches 
Use clean reset structures 
Understand synthesis warnings 
Think in terms of hardware timing

Simulation proves functionality. 
RTL correctness proves hardware feasibility.

EXAMPLE 1 — Blocking vs Non-Blocking Assignments

Blocking ‘=’ is WRONG inside sequential logic but correct inside combinational logic. Below is the wrong sequential example:

Wrong Sequential Block

Explanation (Why Wrong?)

Blocking ‘=’ updates immediately in simulation, causing r to receive the new q instead of the old one. Hardware flip-flops update simultaneously, so simulation and hardware behavior diverge.

Correct Sequential Block

Explanation (Why Correct?)

Non-blocking ‘<=’ schedules updates for the next clock edge, matching real flip-flop behavior. ‘r’ receives the old value of ‘q’, exactly like hardware should behave.

Correct Combinational Block (Blocking OK)

Explanation

Blocking ‘=’ is correct in combinational logic because it models gate-level immediate propagation, which reflects real combinational hardware behavior.

EXAMPLE 2 — Missing Else Causes Latch

Wrong vs correct coding for latch avoidance.

Wrong: Missing Else

Explanation (Why Wrong?)

Without an ELSE, the output ‘y’ holds its previous value when enable=0, causing synthesis to infer a latch. Latches introduce unintended memory and timing problems.

Correct: Full Assignment

Explanation (Why Correct?)

All conditions assign ‘y’, so no latch is inferred and the logic remains purely combinational.

EXAMPLE 3 — Delays in RTL (Not Synthesizable)

Delays (#5) are simulation-only and ignored by synthesis.

Wrong Delay Example

Explanation (Why Wrong?)

The ‘#5’ delay works only in simulation. Synthesis tools ignore delays completely, causing mismatch and unintended behavior in hardware.

Correct RTL Example

Explanation (Why Correct?)

Pure combinational logic without delays synthesizes correctly, and real hardware timing depends on actual gate propagation delay—not ‘#5’ constructs.

EXAMPLE 4 — initial Register Initialization

Registers must be initialized through resets in synthesizable RTL.

Wrong Initialization

Explanation (Why Wrong?)

Most FPGA/ASIC registers power up in unknown (X) states. Simulation initialization does not translate to hardware, leading to unpredictable startup behaviour.

Correct Reset Logic

Explanation (Why Correct?)

Reset logic ensures deterministic initialization of registers in hardware, making the design reliable on every power-up.

Final Message

Simulation is practice. RTL is the real match.
Write Verilog the way hardware behaves — not the way software executes.