Illustrates an investigation between the vcd output of verilator vs interpreter when reset is called

AccumulatorTester.scala

// See LICENSE for license details.

package examples

/**
  * verilator and interpreter have inconsistent behavior around reset.
  */

import java.io.File

import chisel3._
import chisel3.iotesters.{Driver, PeekPokeTester, TesterOptionsManager}
import org.scalatest.{FreeSpec, Matchers}


class Accumulator extends Module {
  val io = IO(new Bundle {
    val enable = Input(Bool())
    val in1 = Input(Bool())
    val in2 = Input(UInt(4.W))
    val in3 = Input(UInt(8.W))
    val freeCount = Output(UInt(16.W))
    val constrainedCount = Output(UInt(16.W))
  })

  // The registers are here to see when reset happens and when accumulation starts
  val freeAccumulator = RegInit(0.U(16.W))
  freeAccumulator := freeAccumulator + 1.U()

  val constrainedAccumulator = RegInit(0.U(16.W))

  when(io.enable) {
    constrainedAccumulator := constrainedAccumulator + 1.U()
  }

  io.freeCount := freeAccumulator
  io.constrainedCount := constrainedAccumulator
}

class AccumulatorTests(c: Accumulator) extends PeekPokeTester(c) {
  poke(c.io.enable, 0)
  step(2)

  reset(2)
  step(2)

  poke(c.io.enable, 1)
  step(2)

  // The following poke will be lost by verilator.
  poke(c.io.enable, 0)
  reset(2)

  poke(c.io.enable, 1)

  poke(c.io.in1, 1)
  poke(c.io.in2, 3)
  poke(c.io.in3, 17)

  step(2)


  for(base <- Seq(1, 4)) {

    poke(c.io.enable, 0)

    step(1)

    reset(base)

    step(1)

    poke(c.io.enable, 1)

    step(base)

    poke(c.io.enable, 0)

    step(base)

    poke(c.io.enable, 1)

    step(base)
  }
}

class AccumulatorTester extends FreeSpec with Matchers {

  "working on differences between verilator and interpreter reset behavior" in {
    val optionsManager = new TesterOptionsManager {
      testerOptions = testerOptions.copy(backendName = "verilator",
        waveform = Option(new File("A.vcd"))
      )
    }

    Driver.execute(() => new Accumulator(), optionsManager)(c => new AccumulatorTests(c)) should be(true)

    Driver.execute(Array("--fint-write-vcd"), () => new Accumulator())(c => new AccumulatorTests(c)) should be(true)
  }
}

Idea: add an io.enableOut := io.enable and add an expect(io.enableOut, ...) throughout....