CDC Day 11 — CDC Testbenches

Testing Multi-Clock Designs

CDC verification needs independent clock generators for each domain:

module cdc_tb();
  reg clk_a = 0, clk_b = 0, reset;

  // Independent clocks (non-integer ratios stress CDC)
  always #10 clk_a <= ~clk_a;     // 50 MHz
  always #15 clk_b <= ~clk_b;     // 33.33 MHz

  // Reset synchronized in each domain
  initial begin
    reset = 1;
    #100 reset = 0;
  end

  // DUT
  my_cdc_fifo dut (
    .clk_a(clk_a), .clk_b(clk_b),
    .reset(reset),
    .wr_data(wr_data), .wr_en(wr_en),
    .rd_data(rd_data), .rd_en(rd_en)
  );

  // Random stimulus across domains
  initial begin
    repeat(10000) begin
      @(posedge clk_a) wr_en <= $random;
      wr_data <= $random;
      @(posedge clk_b) rd_en <= $random;
      // Check rd_data consistency
    end
  end
endmodule

Key Testbench Techniques

✅ Use non-integer clock ratios (stress corner cases)
✅ Run 1M+ cycles to catch rare metastability events
✅ Random clock enables (skew, jitter)
✅ Monitor for data corruption and deadlock

Day 12: Formal CDC verification.