CIRCT

RTL Simulation

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RTL simulation verifies your design at the register-transfer level — the same abstraction you write in. It is the fastest feedback loop during development.

Running RTL Simulation

rhdl simulate lib/counter.rb --level rtl

Or programmatically in Ruby:

dut = Counter.new
dut.reset!
dut.enable = 1
 
10.times do |cycle|
  dut.tick!
  puts "Cycle #{cycle}: count = #{dut.count}"
end

Simulation Backends

RTL simulation can use different backends:

  • Ruby interpreter — slowest, but easiest to debug with binding.pry
  • Rust native — compiled to native code for fast execution
  • WASM — compiled to WebAssembly for browser-based simulation

Waveform Output

Capture signal traces in VCD format for viewing in GTKWave or other waveform viewers:

dut.trace_to("output.vcd") do
  1000.times { dut.tick! }
end

Comparison with Gate-Level

RTL simulation is faster because it operates on higher-level abstractions. Use it for functional verification. Switch to gate-level simulation when you need to verify timing and synthesis correctness.

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