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Lab 1: Hello World in Chisel

The lab session will show you a Hello World example (a blinking LED) in Chisel. We will explore the tools needed for the Chisel based design flow.

After the lab you will have a good overview of the tools used to edit and compile a hardware design coded in Chisel. You will be able to synthesize this design and configure the FPGA board.

We assume that you have downloaded the complete lab material from GitHub and it is placed in folder chisel-lab.

Background Reading

Exploring and Compiling the Hello World Component

With IntelliJ import the lab1 project as follows:

  • Start IntelliJ
  • When you start IntelliJ the first time, click Import Project, otherwise select File - New - Project from Existing Sources...
  • Select sbt
  • Navigate to .../chisel-lab/lab1 and select the file build.sbt, press Open
  • Select the JDK 1.8 (or later)
    • On Project JDK select New
    • Select JDK
    • Select the path to your OpenJDK 8 installation, something like ```C:\Program Files\AdoptOpenJDK\jdk-8.0.232.09-hotspot'''
  • Press OK on the next dialog box

This importing from the project may take some time at the first import, as Scala and Chisel files need to be downloaded. Wait until it is finished.

If you have an IntelliJ project already open, you can create a new project with: File - New - Project from Existing Sources...

Then navigate to the Chisel component Hello by following in the Project navigator along: lab1 - src - main - scala - Hello. Open Hello with a double click.

This is a complete Chisel component including generation of a slower logical time to drive a blinking LED at 1 Hz from the 100 MHz clock from the Basys3 board. Do not get intimidated by those about 20 lines of code, you will soon understand the details. Today's lab topic is to get the tool flow working down to a configured FPGA.

Open the terminal at the bottom of the IntelliJ window start the build process with:

sbt run

to compile and run your project.

In the Run window you should see something like:

[info] Running HelloMain 
Hello World, I will now generate the Verilog files!
[info] [0.001] Elaborating design...
[info] [2.205] Done elaborating.
Total FIRRTL Compile Time: 916.6 ms
[success] Total time: 22 s, completed Jul 22, 2019 11:09:25 AM

This is the output of the Chisel compiler und runtime. To see that the program runs we provide a friendly greeting message, starting with the famous "Hallo World". If your design contains errors, you will see error messages in this window.

Running the Chisel program generates a Verilog file (Hello.v) that we use for synthesizing our design for an FPGA. The content of this file is not important, but for curiosity you might open it right from within IntelliJ.

CLI Alternative:

If you do not like to use an IDE, you can work at the command line (shell, terminal). Use a program editor of you choice and open Hello.scala found at .../lab1/src/main/scala/Hello.scala. You compile and run the Hello World in Chisel from the command line with a simple:

sbt run

End CLI Alternative

Synthesizing and Configuring the FPGA with Vivado.

We use the Xilinx tool Vivado to synthesize our design and configure the Basys3 board.

The process is described in detail in the document A digital circuit design flow guide using VHDL and Xilinx Vivado targeting a Digilent BASYS 3 FPGA board from last semester. Please use that document (from the first semester). The following list is only a brief summary.

Vivado Project Creation

  • Open Vivado
  • Click Create Project
  • Click Next
  • Pick a name and a location and click Next
    • You might place your project under chisel-lab/lab1
  • Click Next to accept an RTL Project
  • In the next dialog box click Add Files and navigate to Hello.v and add it
  • At the Add Constraints dialog box clock Add Files and select Basis3Hello.xdc and press OK.
    • For the following labs you will need to edit the constraints file derived from the Basys3 Master constraints file.
  • Click Next
  • In the Default Part dialog box select Boards and the Basys3, press Next
  • Press Finish to create the project

Synthesize and Configure the FPGA Board

We are just a few clicks away from running our great Hello World design in the Basys3 board.

  • Connect your Basis3 board with the USB cable to your laptop
  • Click on Generate Bitstream at the bottom of the Project Manager to start synthesis, implementation, and bitstream generation
    • This process may take some time (minutes)
  • Configure your FPGA with Program Device under Open Hardware Manager
    • Open hardware manager after bitfile generation
    • Open target, Auto Connect
    • Program device

You should now see an LED blinking at 1 Hz.

Congratulation! You have build your first digital design in Chisel

After this lengthy setup, the next run of the design flow should be smooth. Try to change the CNT_MAX constant to a slightly smaller value (e.g., 50000000 instead of 100000000) to change the blinking frequency. Run the Chisel code in IntelliJ again and synthesize and configure again with Vivado. The LED should now blink at a different frequency. Faster or slower? At what frequency?

Simulation Without an FPGA Board

If it happens that you do not have access to an FPGA board, you can run the blinking LED in simulation. To avoid to simulate for 100000000 clock cycles change the factor in Hello.scala on following line from:

  val CNT_MAX = (100000000 / 2 - 1).U;

to

  val CNT_MAX = (50000 / 2 - 1).U;

and run the simulation with

sbt test

You should see in the terminal a simulation of the blinking LED.