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 and defining the test harness collateral that connects to the top-level ports.
Most of these types of devices can be instantiated using custom IOBinders
, so the provided ChipTop
and ChipTopCaughtReset
classes are sufficient.
However, if needed, the BaseChipTop
abstract class can be extended for building more custom ChipTop
designs.
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
.
8.1.2.1. 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).
8.1.2.2. 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.
8.1.2.3. 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 busesHasAsyncExtInterrupts
andHasExtInterruptsModuleImp
adds IOs for external interrupts and wires them appropriately to tilesCanHave...AXI4Port
adds various Master and Slave AXI4 ports, adds TL-to-AXI4 converters, and connects them to the appropriate busesHasPeripheryBootROM
adds a BootROM device
8.1.2.4. 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.