CIRCT

Your First Circuit

1 min read

Build a simple LED blinker circuit from scratch — your first complete RHDL design from source to simulation.

The Design

We will build a clock divider that blinks an LED at a visible rate. The design takes a fast clock input and produces a slow toggle output.

class Blinker < RHDL::Component
  input  :clk, :reset
  output :led
 
  DIVIDE = 25_000_000  # 1 Hz at 50 MHz clock
 
  sequential(clk: :posedge, reset: :sync) do
    counter = signal(width: 25)
 
    if reset
      counter <= 0
      led <= 0
    elsif counter == DIVIDE - 1
      counter <= 0
      led <= ~led
    else
      counter <= counter + 1
    end
  end
end

Simulate It

# spec/blinker_spec.rb
RSpec.describe Blinker do
  it "toggles led after DIVIDE cycles" do
    dut = Blinker.new
    dut.reset!
 
    Blinker::DIVIDE.times { dut.tick! }
    expect(dut.led).to eq(1)
  end
end

Run the test:

rspec spec/blinker_spec.rb

Export to Verilog

rhdl export verilog lib/blinker.rb -o output/blinker.v

Next Steps

Now that you have a working design, explore the Ruby DSL Fundamentals to learn the full language.

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