5.9. Sky130 + OpenROAD Tutorial

The vlsi folder of this repository contains an example Hammer flow with the TinyRocketConfig from Chipyard. This example tutorial uses the built-in Sky130 technology plugin and OpenROAD tool plugin.

5.9.1. Project Structure

This example gives a suggested file structure and build system. The vlsi/ folder will eventually contain the following files and folders:

  • Makefile, sim.mk, power.mk

    • Integration of Hammer’s build system into Chipyard and abstracts away some Hammer commands.

  • build

    • Hammer output directory. Can be changed with the OBJ_DIR variable.

    • Will contain subdirectories such as syn-rundir and par-rundir and the inputs.yml denoting the top module and input Verilog files.

  • example-vlsi-sky130

    • Entry point to Hammer. Contains example placeholders for hooks.

  • example-sky130.yml, example-openroad.yml, example-designs/sky130-openroad.yml

    • Hammer IR for this tutorial.

  • example-design.yml, example-asap7.yml, example-tech.yml

    • Hammer IR not used for this tutorial but provided as templates.

  • generated-src

    • All of the elaborated Chisel and FIRRTL.

  • hammer, hammer/src/hammer-vlsi/<syn-par-drc-lvs>/<tool>, hammer/src/hammer-vlsi/technology/<tech>

    • Core repository, and open-source tool and technology plugins.

5.9.2. Prerequisites

5.9.3. Initial Setup

In the Chipyard root, run:

./scripts/init-vlsi.sh sky130 openroad

to pull the Hammer submodule. Note that for technologies other than sky130 or asap7, the tech plugin submodule is cloned into the vlsi folder, and for the commercial tool flow (set up by omitting the openroad argument), the tool plugin submodules are cloned into the vlsi folder.

Pull the Hammer environment into the shell:

cd vlsi
export HAMMER_HOME=$PWD/hammer
source $HAMMER_HOME/sourceme.sh

5.9.4. Building the Design

To elaborate the TinyRocketConfig and set up all prerequisites for the build system to push the design and SRAM macros through the flow:

make buildfile tutorial=sky130-openroad

The command make buildfile generates a set of Make targets in build/hammer.d. It needs to be re-run if environment variables are changed. It is recommended that you edit these variables directly in the Makefile rather than exporting them to your shell environment.

For the purpose of brevity, in this tutorial we will set the Make variable tutorial=sky130-openroad, which will cause additional variables to be set in tutorial.mk, a few of which are summarized as follows:

  • CONFIG=TinyRocketConfig selects the target generator config in the same manner as the rest of the Chipyard framework. This elaborates a stripped-down Rocket Chip in the interest of minimizing tool runtime.

  • tech_name=sky130 sets a few more necessary paths in the Makefile, such as the appropriate Hammer plugin

  • TOOLS_CONF and TECH_CONF select the approproate YAML configuration files, example-openroad.yml and example-sky130.yml, which are described below

  • DESIGN_CONF and EXTRA_CONFS allow for additonal design-specific overrides of the Hammer IR in example-sky130.yml

  • VLSI_OBJ_DIR=build-sky130-openroad gives the build directory a unique name to allow running multiple flows in the same repo. Note that for the rest of the tutorial we will still refer to this directory in file paths as build, again for brevity.

  • VLSI_TOP is by default ChipTop, which is the name of the top-level Verilog module generated in the Chipyard SoC configs. By instead setting VLSI_TOP=Rocket, we can use the Rocket core as the top-level module for the VLSI flow, which consists only of a single RISC-V core (and no caches, peripherals, buses, etc). This is useful to run through this tutorial quickly, and does not rely on any SRAMs.

5.9.5. Running the VLSI Flow example-vlsi-sky130

This is the entry script with placeholders for hooks. In the ExampleDriver class, a list of hooks is passed in the get_extra_par_hooks. Hooks are additional snippets of python and TCL (via x.append()) to extend the Hammer APIs. Hooks can be inserted using the make_pre/post/replacement_hook methods as shown in this example. Refer to the Hammer documentation on hooks for a detailed description of how these are injected into the VLSI flow. example-sky130.yml

This contains the Hammer configuration for this example project. Example clock constraints, power straps definitions, placement constraints, and pin constraints are given. Additional configuration for the extra libraries and tools are at the bottom.

First, set technology.sky130.<sky130A, openram_lib> to the absolute path of the respective directories containing the Sky130 PDK and SRAM files. See the Sky130 Hammer plugin README for details about the PDK setup. example-openroad.yml

This contains the Hammer configuration for the OpenROAD tool flow. It selects tools for synthesis (Yosys), place and route (OpenROAD), DRC (Magic), and LVS (NetGen). Synthesis

make syn tutorial=sky130-openroad

Post-synthesis logs and collateral are in build/syn-rundir. Place-and-Route

make par tutorial=sky130-openroad

After completion, the final database can be opened in an interactive OpenROAD session.

cd ./build/par-rundir

Below is the post-PnR layout for the TinyRocketConfig in Sky130 generated by OpenROAD.


Intermediate databases are written in build/par-rundir between each step of the par action. These databases can be restored in an interactive OpenROAD session as desired for debugging purposes.

openroad  # launch OpenROAD tool
openroad> read_db pre_global_route DRC & LVS

To run DRC & LVS:

make drc tutorial=sky130-openroad
make lvs tutorial=sky130-openroad

Some DRC errors are expected from this PDK, especially with regards to the SRAMs, as explained in the Sky130 Hammer plugin README. VLSI Flow Control

Firt, refer to the VLSI Flow Control documentation. The below examples use the redo-par Make target to re-run only place-and-route. redo- may be prepended to any of the VLSI flow actions to re-run only that action.

# the following two statements are equivalent because the
#   extraction step immediately precedes the write_design step
make redo-par HAMMER_EXTRA_ARGS="--start_after_step extraction"
make redo-par HAMMER_EXTRA_ARGS="--start_before_step write_design"

# example of re-running only floorplanning to test out a new floorplan configuration
make redo-par HAMMER_EXTRA_ARGS="--only_step floorplan_design -p example-sky130.yml"

See the OpenROAD tool plugin for the full list of OpenROAD tool steps and their implementations.

5.9.6. Documentation

For more information about Hammer’s underlying implementation, visit the Hammer documentation website.

For details about the plugins used in this tutorial, check out the OpenROAD tool plugin repo + README and Sky130 tech plugin repo + README.