This is alpha software that is currently under development, and interfaces may be subject to change (see stability for details). However, it is very much in a usable state, so if you’re fine living on the bleeding edge, give it a try.


Testers2 is a test harness for Chisel-based RTL designs, currently supporting directed testing (all test stimulus manually specified - no constrained random and coverage-driven flows). Testers2 emphasizes tests that are lightweight (minimizes boilerplate code), easy to read and write (understandability), and compose (for better test code reuse).

The core primitives are similar to nonsynthesizable Verilog: input pin assignment (poke), pin value assertion (expect), and time advance (step). Threading concurrency is also supported with the use of fork and join, and concurrent accesses to wires are checked to prevent race conditions.

Migrating from chisel-testers

The core abstractions (poke, expect, step) are similar to chisel-testers, but the syntax is inverted: instead of doing tester.poke(wire, value) with a Scala number value, in testers2 you would write write.poke(value) with a Chisel literal value. Furthermore, as no reference to the tester context is needed, test helper functions can be defined outside a test class and written as libraries.

Currently, this should support all the functionality that was in chisel-testers, and provides additional features. This project is meant to supersede chisel-testers, and eventually may become a default part of chisel3.

Test cases written in chisel-testers cannot be directly used in testers2, as the syntax is significantly different.

Getting Started


To use chisel-testers as a managed dependency, add this in your build.sbt:

libraryDependencies += "edu.berkeley.cs" %% "chisel-testers2" % "0.1-SNAPSHOT"

Note, chisel-testers2 snapshots generally track chisel3 snapshots, and requires the use of a chisel3 snapshot. We may introduce versioned snapshots and releases in the future, tied to a particular release of chisel3.

You can also build testers2 locally with publishLocal.

Writing a Test

Testers2 integrates with the ScalaTest framework, which provides a framework for detection and execution of unit tests.

Assuming a typical Chisel project, create a new file in src/test/scala/, for example, BasicTest.scala.

In this file:

  1. Add the necessary imports:
    import org.scalatest._
    import chisel3._
    import chisel3.tester._
  2. Create a test class:
    class BasicTest extends FlatSpec with ChiselScalatestTester with Matchers {
      behavior of "MyModule"
      // test class body here
    • FlatSpec is the default and recommended ScalaTest style for unit testing.
    • ChiselScalatestTester provides testdriver functionality and integration (like signal value assertions) within the context of a ScalaTest environment.
    • Matchers provides additional syntax options for writing ScalaTest tests. Potentially optional, since it’s mainly for Scala-land assertions and does not inter-operate with circuit operations.
  3. In the test class, define a test case:
    it should "do something" in {
      // test case body here

    There can be multiple test cases per test class, and we recommend one test class per Module being tested, and one test case per individual test.

  4. In the test case, define the module being tested:
    test(new MyModule) { c =>
      // test body here

    test automatically runs the default simulator (which is treadle), and runs the test stimulus in the block. The argument to the test stimulus block (c in this case) is a handle to the module under test.

  5. In the test body, use poke, step, and expect operations to write the test:
  6. With your test case complete, you can run all the test cases in your project by invoking ScalaTest. If you’re using sbt, you can either run sbt test from the command line, or test from the sbt console. testOnly can also be used to run specific tests.

Usage References

See the test cases for examples:

  • BasicTest shows basic peek, poke, and step functionality
  • QueueTest shows example uses of the DecoupledDriver library, providing functions like enqueueNow, expectDequeueNow, their sequence variants, expectPeek, and expectInvalid. Also, check out the DecoupledDriver implementation, and note that it is not a special case, but code that any user can write.
  • BundleLiteralsSpec shows examples of using bundle literals to poke and expect bundle wires.
    • Note: Bundle literals are still an experimental chisel3 feature and need to be explicitly imported:
      import chisel3.experimental.BundleLiterals._
  • ShiftRegisterTest shows an example of using fork/join to define a test helper function, where multiple invocations of it are pipelined using fork.
  • VerilatorBasicTests shows an example using Verilator as the simulator.
    • Note: the simulator is selected by passing an annotation into the test function, which requires experimental imports:
      import chisel3.tester.experimental.TestOptionBuilder._
      import chisel3.tester.internal.VerilatorBackendAnnotation
      test(new MyModule).withAnnotations(Seq(VerilatorBackendAnnotation)) { c =>
        // test body here

New Constructs

  • fork to spawn threads, and join to block (wait) on a thread. Pokes and peeks/expects to wires from threads are checked during runtime to ensure no collisions or unexpected behavior.
  • Regions can be associated with a thread, with fork.withRegion(...), which act as a synchronization barrier within simulator time steps. This can be used to create monitors that run after other main testdriver threads have been run, and can read wires those threads have poked.
  • timescope allows pokes to be scoped - that is, pokes inside the timescope block “disappear” and the wire reverts to its previous value at the end of the block. This fits well with the pattern of assigning a default pull-up/down to a wire, and temporarily overriding that value, for example a Decoupled valid signal defaulting low but driven high during an enqueue transaction. See TimescopeTest for examples.

Quick References

To dump VCDs (into the test_run_dir subfolder) using sbt:

testOnly chisel3.tests.BasicTest -- -DwriteVcd=1


These APIs may be considered stable and are unlikely to change significantly:

  • Test invocation test
  • Basic operations: poke, peek, expect, step, including on Bundles
  • Basic concurrency operations: fork, join
  • Decoupled library: enqueueNow, expectDequeueNow, their sequence variants, expectPeek, expectInvalid - though the names may be refactored
  • timescope - though the name may be refactored

These are subject to change:

  • Multiclock behavior, which is currently not well defined, including poke on clocks and step when there are multiple clocks.


These features are on our roadmap, but are not implemented yet. No timeframe is currently available, but feel free to let us know if some feature is critical to your use case in the relevant issue thread, and we may adjust our development priorities accordingly.

  • #14 support for multi-clock designs, and in particular, supporting clock poke to clock step inter-thread dependencies.
  • #28 faster Verilator / VCS testing using mechanisms that avoid interprocess communication, like JNI
  • #58 faster threading (note, unclear if there are good solutions here, especially ones that are fully API compatible - Scala generally lacks good coroutine support)
  • #60, #34, #2 support for testing non-Chisel modules, such as post-syn Verilog, or generally a separation of DUT interface and implementation