claude-skill-verilog

Verilog/SystemVerilog Guidance

Apply when working with .v, .sv, .vh, .svh files or running Verilator.

Documentation

All modules, wires, and registers require comments:

// Module: counter
// Purpose: Simple up-counter with synchronous reset
module counter #(
    parameter WIDTH = 8  // Counter bit width
) (
    input  logic             clk,      // System clock
    input  logic             rst_n,    // Active-low reset
    output logic [WIDTH-1:0] count     // Current count value
);

Fixed-Point Notation

Document all fixed-point values using TI-style Q notation:

• Qm.n — signed: m integer bits (including sign bit), n fractional bits, total width = m + n bits.
• UQm.n — unsigned: m integer bits, n fractional bits, total width = m + n bits.

Use Q notation in signal comments, localparam descriptions, and module-level documentation.

logic signed [15:0] scale_factor;  // Scaling multiplier, Q4.12
logic        [15:0] timer_count;   // Free-running tick counter, UQ16.0
logic signed [15:0] delta;         // Sample-to-sample difference, Q4.12

Naming Conventions

• Active-low signals: use _n suffix (e.g., rst_n, chip_select_n)
• Clocks: clk or clk_<domain>
• Use descriptive names over abbreviations

always_ff: Simple Assignments Only

always_ff blocks must contain ONLY simple non-blocking assignments. No logic, no expressions — this ensures Verilator simulation matches synthesized behavior. (Exceptions: memory inference and async reset synchronizers require conditional logic — see those sections.)

// CORRECT - simple assignment
always_ff @(posedge clk) begin
    count <= count_next;
    state <= state_next;
end

// WRONG - logic in always_ff
always_ff @(posedge clk) begin
    count <= count + 1;                // Move to always_comb
    state <= enable ? RUNNING : IDLE;  // Move to always_comb
end

always_comb: All Logic Here

All combinational logic belongs in always_comb blocks:

always_comb begin
    count_next = count + 8'd1;
    state_next = enable ? RUNNING : IDLE;
end

Formatting

• One statement per line — never chain multiple statements or assignments on a single line
• One declaration per line
• Explicit bit widths on all literals
• Start files with `default_nettype none
• Always use begin/end blocks for if, else, case items (prevents bugs when adding code later)
• Prefer to keep modules under ~500 lines; if a module grows significantly larger, consider refactoring into smaller sub-modules

`default_nettype none

module example (
    input  logic        clk,
    input  logic        rst_n,
    input  logic [7:0]  data_in,
    output logic [7:0]  data_out
);

    logic [7:0] data_reg;    // Registered data
    logic [7:0] data_next;   // Next state value
    logic       valid;       // Data valid flag

    localparam logic [7:0] INIT_VAL = 8'd0;

endmodule

`default_nettype wire

Yosys Synthesis Compatibility

Synthesizable RTL should work with both Verilator (lint/simulation) and Yosys (synthesis for open-source FPGA flows). Yosys supports a subset of SystemVerilog via read_verilog -sv. Code that passes Verilator may still fail Yosys synthesis.

Constructs to avoid in synthesizable RTL:

Avoid	Use instead
return <expr>; in functions	function_name = <expr>; (Verilog-2005 style)
interface / modport	Explicit port lists
unique case / priority case	Plain case with default
Multi-dimensional packed arrays in ports	Flatten to single vectors

// CORRECT - Yosys-compatible function
function automatic logic [7:0] add_saturate(input logic [7:0] a, input logic [7:0] b);
    logic [8:0] sum;
    sum = {1'b0, a} + {1'b0, b};
    add_saturate = sum[8] ? 8'hFF : sum[7:0];
endfunction

// WRONG - return statement (Yosys rejects this)
function automatic logic [7:0] add_saturate(input logic [7:0] a, input logic [7:0] b);
    logic [8:0] sum;
    sum = {1'b0, a} + {1'b0, b};
    return sum[8] ? 8'hFF : sum[7:0];
endfunction

Always verify with the actual synthesis flow (e.g. yosys -p "synth_ecp5 ...", yosys -p "synth_ice40 ...", or the project's build target), not only verilator --lint-only, when using SystemVerilog features. Lint-clean does not imply synthesizable.

Testing with Verilator

Every module requires a testbench. Build and run with Verilator:

# Build testbench
verilator --binary -Wall module_tb.sv module.sv

# Run simulation
./obj_dir/Vmodule_tb

Testbench structure:

module counter_tb;
    logic       clk = 1'b0;  // System clock
    logic       rst_n;       // Active-low reset
    logic [7:0] count;       // DUT output

    counter dut (
        .clk(clk),
        .rst_n(rst_n),
        .count(count)
    );

    always begin
        #5 clk = ~clk;
    end

    initial begin
        rst_n = 1'b0;
        #20 rst_n = 1'b1;
        #100;
        $display("Test complete, count=%d", count);
        $finish;
    end
endmodule

Verilator Linting

Run linting on all files and fix all warnings:

verilator --lint-only -Wall module.sv

• Fix all warnings — do not suppress with pragmas
• Key warnings: WIDTH (bit-width mismatch), UNUSED, UNDRIVEN

Verilator Simulation Flags

Recommended flags for simulation builds:

verilator --binary \
    -Wall \
    -Wno-fatal \
    -j 0 \
    --assert \
    --timing \
    --trace-fst \
    --trace-structs \
    --main-top-name "-" \
    --x-assign unique \
    --x-initial unique \
    module_tb.sv module.sv

Flag	Purpose
-Wall	Enable all warnings
-Wno-fatal	Don't exit on warnings (allows full report)
-j 0	Fully parallelized compilation
--assert	Enable SystemVerilog assertions
--timing	Enable timing constructs
--trace-fst	Dump waveforms as FST (compressed)
--trace-structs	Human-readable struct dumps
--main-top-name "-"	Remove extra TOP module wrapper
--x-assign unique	Replace X with random constant per-build
--x-initial unique	Randomly initialize uninitialized variables

Module Instantiation

• One module per file, filename matches module name
• Always use named port connections (never positional)

// CORRECT - named connections
counter #(
    .WIDTH(16)
) u_counter (
    .clk    (clk),
    .rst_n  (rst_n),
    .count  (count_value)
);

// WRONG - positional connections
counter u_counter (clk, rst_n, count_value);

Avoiding Latches

Latches are inferred when signals aren't assigned in all paths. Prevent with:

• Default assignments at start of always_comb
• Cover all cases including default

always_comb begin
    // Default assignments first
    data_next = data_reg;
    valid_next = 1'b0;

    case (state)
        IDLE: begin
            data_next = 8'd0;
        end
        LOAD: begin
            data_next = data_in;
        end
        default: begin
            data_next = data_reg;
        end
    endcase
end

Reset Handling

Use synchronous resets when possible. For external async resets, synchronize first.

Note: Async reset synchronizers require conditional logic in always_ff for the reset condition — this is a necessary exception similar to memory inference.

// Synchronous reset (preferred)
logic [7:0] count;       // Counter register
logic [7:0] count_next;  // Next counter value

always_comb begin
    count_next = rst_n ? (count + 8'd1) : 8'd0;
end

always_ff @(posedge clk) begin
    count <= count_next;
end

// Reset synchronizer for external async reset
logic [1:0] rst_sync;       // Synchronizer flip-flops
logic [1:0] rst_sync_next;  // Next synchronizer value

always_comb begin
    rst_sync_next = {rst_sync[0], 1'b1};
end

always_ff @(posedge clk or negedge rst_async_n) begin
    if (!rst_async_n) begin
        rst_sync <= 2'b00;
    end else begin
        rst_sync <= rst_sync_next;
    end
end
assign rst_n = rst_sync[1];

FSM Patterns

Separate state register from next-state logic. Use enums for state encoding.

typedef enum logic [1:0] {
    IDLE,
    RUN,
    DONE
} state_t;

state_t state;       // Current state register
state_t state_next;  // Next state value

// Next-state logic (combinational)
always_comb begin
    state_next = state;
    case (state)
        IDLE: begin
            if (start) begin
                state_next = RUN;
            end
        end
        RUN: begin
            if (finish) begin
                state_next = DONE;
            end
        end
        DONE: begin
            state_next = IDLE;
        end
        default: begin
            state_next = IDLE;
        end
    endcase
end

// State register (sequential)
always_ff @(posedge clk) begin
    state <= state_next;
end

Clock Domain Crossing (CDC)

Single-bit signals: use 2-FF synchronizer. Multi-bit: use gray coding or handshake.

// 2-FF synchronizer for single-bit CDC
logic [1:0] sync_reg;       // Synchronizer flip-flops
logic [1:0] sync_reg_next;  // Next synchronizer value
logic       signal_sync;    // Synchronized output

always_comb begin
    sync_reg_next = {sync_reg[0], signal_src};
end

always_ff @(posedge clk_dst) begin
    sync_reg <= sync_reg_next;
end
assign signal_sync = sync_reg[1];

// Gray code for multi-bit counters crossing domains
function automatic logic [WIDTH-1:0] bin2gray(input logic [WIDTH-1:0] bin);
    bin2gray = bin ^ (bin >> 1);
endfunction

Memory Inference

Use standard patterns for RAM/ROM inference by synthesis tools.

Note: Memory patterns are an exception to the "simple assignments only" rule for always_ff. Synthesis tools require these specific patterns to correctly infer RAM/ROM primitives.

// Single-port RAM
logic [DATA_WIDTH-1:0] mem [0:DEPTH-1];  // Memory array

always_ff @(posedge clk) begin
    if (we) begin
        mem[addr] <= wdata;
    end
    rdata <= mem[addr];
end

// ROM (initialized memory)
logic [7:0] rom [0:255];  // ROM array
initial $readmemh("rom_data.hex", rom);

always_ff @(posedge clk) begin
    rdata <= rom[addr];
end

Assertions (SVA)

Use assertions for verification. They're enabled with --assert in Verilator.

// Immediate assertion
always_comb begin
    assert (count < MAX_COUNT) else $error("Count overflow");
end

// Concurrent assertions
property p_valid_handshake;
    @(posedge clk) disable iff (!rst_n)
    valid |-> ##[1:3] ready;
endproperty

assert property (p_valid_handshake)
    else $error("Handshake timeout");

// Cover property (for functional coverage)
cover property (@(posedge clk) state == DONE);