Vfill

Verilog Writer

Overview

This guide defines the workflow for implementing a Verilog module from an approved design proposal.

Before starting, read:

1. The target Verilog source file — for //@ annotations and port declarations
2. The proposal file ./ddoc/<module_name>_proposal.md (same folder as the requirement file) — this is the source of truth for the implementation

Prerequisites

Tool	Purpose	Check
Verilator	Verilog lint	verilator --version
xvlog	Xilinx Verilog compiler / lint	xvlog --version
xelab	Xilinx elaborator	xelab --version
xsim	Xilinx simulator	xsim --version

Before running any lint or simulation step, verify the tool it needs is installed using the check command above. If a required tool is missing, stop and report which tool is unavailable — never fabricate lint, simulation, or elaboration results. The Verilog can still be filled in (Step 1) without the tools, but Steps 2 and 3 must not be reported as passed unless they actually ran. (When invoked under vflow, this surfaces as ✗ BLOCKED (tool unavailable: <tool>) for the dependent phase.)

Before You Start — Re-run Safety

Before editing, check rtl/<module>.v for //@ markers (the signal vflow uses for completion, CodingStyle §18):

State	Detect	Action
Backbone	//@ markers present	First run — implement, then convert every //@ away.
Already filled	no //@ left / real logic present	Filled before: do not re-fill from scratch. Make the requested change as an incremental edit, update the proposal's ## Implementation Notes (vfill) section in place rather than appending a duplicate, then re-lint and re-simulate.

Step 1 - Fill in the Verilog Code

Complete the target Verilog source file based on:

• The approved proposal file
• The coding style rules defined in .claude/skills/shared/CodingStyle.md
• The design patterns in .claude/skills/shared/DesignPatterns.md — when the proposal names a pattern (or the backbone is clearly shaped like one of those indexed there), build that pattern's canonical shape and honor its Ruling rather than picking a different valid shape. Adapt the names, widths, and reset scope to this module.

The filled file is synthesizable RTL, so it must be plain Verilog-2005 (IEEE 1364-2005) only — no SystemVerilog constructs (CodingStyle §0), and must carry `default_nettype none at the top restored with `default_nettype wire at the end (CodingStyle §14).

When the target module uses submodules, follow this lookup order:

1. Read the submodule documentation first — only its port/parameter section, not the whole file. It could be in ./doc, ./lib and sub-folder, or ./rtl
2. Read the submodule RTL code only if the documentation is not sufficient

Do not read RTL code unless additional information is required to understand the interface or behavior.

After completing the Verilog code:

1. Record the design updates and important notes in the proposal's ## Implementation Notes (vfill) section (seeded empty by vdesign; add the section if an older proposal lacks it). Write inside this section only, updating in place on re-runs — vdesign's re-run safety check detects a filled module by this section having content. This information will help understand the module's functionality and behavior, and can be used during the debugging phase.
2. True up the module doc. doc/<module>.md was written by vdesign with intended values; if the implementation deviated — actual latency or pipeline depth, a reset value, a protocol detail, a register field or access type — update the affected sections (ModuleDocContract, esp. Timing §6 and Register Map §9) to the as-built behavior. The doc's whole purpose is to be used instead of reading the RTL; a doc that still describes the intent after the implementation drifted silently breaks every downstream consumer (vassert, vtestgen, vveri, vdoc, vpackage). No deviation → no edit.
3. Convert all //@ annotations to regular // comments.

Step 2 - Clean the Code

Run lint in two passes. Both must pass before proceeding to simulation.

Run these commands from a scratch directory (e.g. ./tb/<module_name>, the same folder used for simulation in Step 3) so xvlog/xelab write xsim.dir and their logs there instead of polluting the repository root. Express RTL/library paths relative to that directory.

Redirect each tool to a log file and read back only the finding lines, never the full transcript — Verilator prefixes findings with %Error/%Warning:

verilator … 2>&1 | tee lint.log    # then inspect: grep -nE '%(Error|Warning)' lint.log

Pass 1 — Verilator

verilator --lint-only --top-module <top_module> -y <path_to_rtl_folder> -y <path_to_library_folder> -y <path_to_library_submodule_folder> <path_to_top_module_file>

Where:

• --top-module <top_module> specifies the top-level module name
• -y <path_to_rtl_folder> specifies the path to the project RTL folder, relative to the scratch dir, for example -y ../../rtl
• -y <path_to_library_folder> specifies the path to the library root folder, for example -y ../../lib
• -y <path_to_library_submodule_folder> specifies the path to a submodule folder inside the library, for example -y ../../lib/module_1
• <path_to_top_module_file> specifies the path to the file containing the top-level module, for example ../../rtl/top_module.v

Add one -y option for each required library subfolder. The -y paths are search directories: Verilator discovers each instantiated submodule by name itself, so this pass needs no explicit per-file list — just point it at every folder that holds a required module.

Pass 2 — Xilinx xvlog + xelab

xvlog -nolog <rtl_files>
xelab -nolog --top <top_module>

Where <rtl_files> is the explicit list of all Verilog source files required by the design (unlike Verilator's -y search paths, xvlog needs the files named). Build the list by the walk in .claude/skills/shared/HierarchyFilelist.md: recurse from the filled module, searching ./rtl/ first then ./lib/, de-duplicate a file shared by multiple parents, and treat a submodule whose source cannot be found as a hard error — report the missing module and its parent rather than silently dropping it. The same <rtl_files> list feeds the simulation step (Step 3). For a single small module this walk is short, so do not generate a persisted filelist — that artifact belongs to the IP-level skills.

Step 2A - Resolve Lint Issues

Review all warnings and errors from both tools.

For each warning or error:

1. Determine whether it is caused by incorrect RTL code, missing files, missing include paths, or intentional design behavior.
2. Propose a fix.
3. If the user chooses to ignore a warning, suppress it explicitly using a lint directive.
4. Apply the fix only if it does not conflict with the approved design.

For Verilator, use the following format to suppress a specific warning:

/* verilator lint_off <WARNING_CODE> */
    a <= b;     // line of code that causes the warning
/* verilator lint_on <WARNING_CODE> */

After all proposed fixes have been applied, re-run the lint pass that failed and re-grep its log — do not re-echo whole logs each iteration.

Repeat this process until:

• No remaining lint errors exist in either tool
• Remaining warnings, if any, are explicitly justified and cannot be safely removed without changing the intended design

Do not ignore warnings silently.

Step 3 - Functional Simulation

Generate a self-checking testbench file <module_name>_tb.sv in the folder ./tb/<module_name>. All compile and simulation commands must run from inside this folder. The testbench is verification code, so it is SystemVerilog (.sv) — permitted under tb/ per CodingStyle §0; only the synthesizable RTL is restricted to Verilog-2005.

The testbench must:

• Cover every operating scenario from the approved design
• Assert outputs against expected values using $error on mismatch
• End by printing exactly one verdict token (CodingStyle §19): [FINISH] PASS when every scenario passed, [FINISH] FAIL on any mismatch — then $finish. This is the single machine-readable result; print diagnostic context on [ERROR] time= ... lines, not in the verdict.

Run simulation using xsim from inside ./tb/<module_name>. All paths must be relative to that folder — use ../../rtl for the RTL folder and ../../lib for the library folder.

xvlog -nolog <rtl_files>
xvlog -nolog -sv <testbench_file>
xelab -nolog --top <testbench_module> --snapshot sim_snap --timescale 1ns/1ps --override_timeunit --override_timeprecision
xsim sim_snap --runall --nolog | tee sim.log

The bench prints many $display lines, but only the result matters: read back grep -nE '\[ERROR\]|\[FINISH\]' sim.log, not the full transcript. This is what the [FINISH]/[ERROR] convention is for. On a re-run after a fix, re-grep — do not re-ingest the whole log.

To allow users to rerun the testbench later, write these commands into a script at ./tb/<module_name>/run.sh and run that file, rather than issuing the commands ad-hoc.

Step 3A - Resolve Simulation Failures

For each failing scenario:

1. Determine whether the fault is in the RTL or the testbench.
2. Propose a fix.
3. Apply the fix only if it does not conflict with the approved design.

Repeat until all scenarios pass.

Step 4 - Self-Check

Confirm before declaring the module complete — this is the handoff vflow relies on:

• No //@ markers remain in rtl/<module>.v (a stray one keeps vflow at IN PROGRESS — CodingStyle §18).
• Verilog-2005 only, with `default_nettype none/wire bookends (§0, §14).
• Both lint passes clean; any surviving warning has a paired narrow waiver.
• Simulation passes every scenario; ./tb/<module_name>/run.sh reproduces the run.
• Proposal notes updated once (not duplicated — see Re-run Safety).
• doc/<module>.md matches the as-built behavior — latency, reset values, register map — updated wherever the implementation deviated from the proposal.

If a tool was unavailable, report which check could not run — don't mark it passed.

Known Issues and Fixes

This section records issues encountered when running this skill and how they were resolved. Use it to avoid repeating the same mistakes.

<!-- Add entries below as issues are discovered. Format: ### [Tool] - <short issue title> **Symptom:** what was observed **Cause:** root cause **Fix:** what was done to resolve it -->