verilog_rra

Verilog RRA Testbench Skill

Trigger

Use this skill when the user asks to build, verify, or test a Round Robin Arbiter (RRA) in Verilog, or when a Foundry requirement mentions RRA/verilog.

Capabilities

• Write a parametric Round Robin Arbiter module (Verilog-2001)
• Generate a self-checking testbench with pass/fail assertions
• Compile using iverilog and simulate using vvp
• Parse simulation output to determine pass/fail

Implementation Guide

RRA Module (rra.v)

Use a mask-based approach for fair round-robin scheduling:

• Maintain a 4-bit mask register
• After granting bit[i], set mask to clear bits <= i
• Use lowest-set-bit extraction: x & (~x + 1)
• When masked_req is empty, wrap around to unmasked req

// Round Robin Arbiter - 4 bit (Verilog-2001)
module rra (
    input  wire        clk,
    input  wire        rst_n,
    input  wire [3:0]  req,
    output reg  [3:0]  grant
);
    reg [3:0] mask;
    reg [3:0] masked_req;
    reg [3:0] sel;

    function [3:0] lsb;
        input [3:0] x;
        begin
            lsb = x & (~x + 4'b0001);
        end
    endfunction

    always @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            mask  <= 4'b1111;
            grant <= 4'b0000;
        end else if (|req) begin
            masked_req = req & mask;
            if (|masked_req)
                sel = lsb(masked_req);
            else
                sel = lsb(req);
            grant <= sel;
            case (sel)
                4'b0001: mask <= 4'b1110;
                4'b0010: mask <= 4'b1100;
                4'b0100: mask <= 4'b1000;
                default: mask <= 4'b1111;
            endcase
        end else begin
            grant <= 4'b0000;
        end
    end
endmodule

Testbench (rra_tb.v)

• Use $dumpfile / $dumpvars for waveform capture
• Include reset sequence (rst_n=0 for 20ns, then rst_n=1)
• Test single requests, round-robin all-request, and no-request cases
• Print PASS/FAIL for each test case
• Print summary with ALL_TESTS_PASSED or SOME_TESTS_FAILED