8.1. Tops, Test-Harnesses, and the Test-Driver
The three highest levels of hierarchy in a Chipyard
SoC are the
TestHarness, and the
TestHarness are both emitted by Chisel generators.
TestDriver serves as our testbench, and is a Verilog
file in Rocket Chip.
ChipTop is the top-level module that instantiates the
System submodule, usually an instance of the concrete class
The vast majority of the design resides in the
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
IOBinders are responsible for instantiating the IO cells for
ChipTop IO that correspond to IO of the
HarnessBinders are responsible for instantiating test harness collateral that connects to the
Most types of devices and testing collateral can be instantiated using custom
184.108.40.206. Custom ChipTops
The default standard
ChipTop provides a mimimal, barebones template for
IOBinders to generate IOCells around
For tapeouts, integrating Analog IP, or other non-standard use cases, Chipyard supports specifying a custom
ChipTop using the
An example of a custom ChipTop which uses non-standard IOCells is provided in generators/chipyard/src/main/scala/example/CustomChipTop.scala
The system module of a Rocket Chip SoC is composed via cake-pattern.
DigitalTop extends a
System, which extends a
Subsystem, which extends a
BaseSubsystem is defined in
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).
Looking in generators/chipyard/src/main/scala/Subsystem.scala, we can see how Chipyard’s
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.
generators/chipyard/src/main/scala/System.scala completes the definition of the
HasHierarchicalBusTopologyis defined in Rocket Chip, and specifies connections between the top-level buses
HasExtInterruptsModuleImpadds IOs for external interrupts and wires them appropriately to tiles
CanHave...AXI4Portadds various Master and Slave AXI4 ports, adds TL-to-AXI4 converters, and connects them to the appropriate buses
HasPeripheryBootROMadds a BootROM device
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.
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
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.
TestDriver is defined in
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.