8.1. Tops, Test-Harnesses, and the Test-Driver

The three highest levels of hierarchy in a Chipyard SoC are the ChipTop (DUT), TestHarness, and the TestDriver. The ChipTop and TestHarness are both emitted by Chisel generators. The TestDriver serves as our testbench, and is a Verilog file in Rocket Chip.

8.1.1. ChipTop/DUT

ChipTop is the top-level module that instantiates the System submodule, usually an instance of the concrete class DigitalTop. The vast majority of the design resides in the System. Other components that exist inside the ChipTop layer are generally IO cells, clock receivers and multiplexers, reset synchronizers, and other analog IP that needs to exist outside of the System. The IOBinders are responsible for instantiating the IO cells for ChipTop IO that correspond to IO of the System. The HarnessBinders are responsible for instantiating test harness collateral that connects to the ChipTop ports. Most types of devices and testing collateral can be instantiated using custom IOBinders and HarnessBinders. Custom ChipTops

The default standard ChipTop provides a mimimal, barebones template for IOBinders to generate IOCells around DigitalTop traits. For tapeouts, integrating Analog IP, or other non-standard use cases, Chipyard supports specifying a custom ChipTop using the BuildTop key. An example of a custom ChipTop which uses non-standard IOCells is provided in generators/chipyard/src/main/scala/example/CustomChipTop.scala

8.1.2. System/DigitalTop

The system module of a Rocket Chip SoC is composed via cake-pattern. Specifically, DigitalTop extends a System, which extends a Subsystem, which extends a BaseSubsystem. BaseSubsystem

The BaseSubsystem is defined in generators/rocketchip/src/main/scala/subsystem/BaseSubsystem.scala. Looking at the BaseSubsystem abstract class, we see that this class instantiates the top-level buses (frontbus, systembus, peripherybus, etc.), but does not specify a topology. We also see this class define several ElaborationArtefacts, files emitted after Chisel elaboration (e.g. the device tree string, and the diplomacy graph visualization GraphML file). Subsystem

Looking in generators/chipyard/src/main/scala/Subsystem.scala, we can see how Chipyard’s Subsystem extends the BaseSubsystem abstract class. Subsystem mixes in the HasBoomAndRocketTiles trait that defines and instantiates BOOM or Rocket tiles, depending on the parameters specified. We also connect some basic IOs for each tile here, specifically the hartids and the reset vector. System

generators/chipyard/src/main/scala/System.scala completes the definition of the System.

  • HasHierarchicalBusTopology is defined in Rocket Chip, and specifies connections between the top-level buses

  • HasAsyncExtInterrupts and HasExtInterruptsModuleImp adds IOs for external interrupts and wires them appropriately to tiles

  • CanHave...AXI4Port adds various Master and Slave AXI4 ports, adds TL-to-AXI4 converters, and connects them to the appropriate buses

  • HasPeripheryBootROM adds a BootROM device Tops

A SoC Top then extends the System class with traits for custom components. In Chipyard, this includes things like adding a NIC, UART, and GPIO as well as setting up the hardware for the bringup method. Please refer to Communicating with the DUT for more information on these bringup methods.

8.1.3. TestHarness

The wiring between the TestHarness and the Top are performed in methods defined in traits added to the Top. When these methods are called from the TestHarness, they may instantiate modules within the scope of the harness, and then connect them to the DUT. For example, the connectSimAXIMem method defined in the CanHaveMasterAXI4MemPortModuleImp trait, when called from the TestHarness, will instantiate SimAXIMems and connect them to the correct IOs of the top.

While this roundabout way of attaching to the IOs of the top may seem to be unnecessarily complex, it allows the designer to compose custom traits together without having to worry about the details of the implementation of any particular trait.

8.1.4. TestDriver

The TestDriver is defined in generators/rocketchip/src/main/resources/vsrc/TestDriver.v. This Verilog file executes a simulation by instantiating the TestHarness, driving the clock and reset signals, and interpreting the success output. This file is compiled with the generated Verilog for the TestHarness and the Top to produce a simulator.