CIRCT

Testing with RSpec

1 min read

One of RHDL’s biggest advantages is using Ruby’s mature test ecosystem for hardware verification. Write testbenches with RSpec — the same framework Ruby developers already know.

Basic Test Structure

RSpec.describe Counter do
  let(:dut) { Counter.new }
 
  before { dut.reset! }
 
  it "starts at zero after reset" do
    expect(dut.count).to eq(0)
  end
 
  it "increments on each clock edge" do
    dut.enable = 1
    3.times { dut.tick! }
    expect(dut.count).to eq(3)
  end
 
  it "wraps around at max value" do
    dut.enable = 1
    16.times { dut.tick! }
    expect(dut.count).to eq(0)  # 4-bit counter wraps
  end
end

Clock and Reset

  • dut.tick! — advance one clock cycle
  • dut.reset! — assert reset for one cycle
  • dut.tick!(n) — advance n clock cycles

Waveform Capture

Record signals over time for debugging:

it "generates correct waveform" do
  trace = dut.capture(:clk, :count, :enable, cycles: 20)
  trace.save("counter_trace.vcd")
end

Property-Based Testing

Combine RSpec with property-based testing gems for exhaustive verification:

it "addition is commutative" do
  100.times do
    a, b = rand(256), rand(256)
    dut.a = a; dut.b = b; dut.tick!
    result_ab = dut.result
 
    dut.a = b; dut.b = a; dut.tick!
    expect(dut.result).to eq(result_ab)
  end
end

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