November 10, 2020

New Clash FPGA Starter

For this tutorial, you will need at least Clash version 1.2.3. We’ll be programming the Terasic DE0-Nano FPGA development board using the functional hardware description language Clash. The end result of this tutorial is demonstrated in the video below:

Watch this video on YouTube.

This tutorial is not a general introduction to Clash, nor to programming FPGAs. It is meant to demonstrate how to use Synthesize annotations and SynthesisAttributes to configure your Clash designs for an FPGA, without writing a single line of VHDL or (System)Verilog.

Blinker circuit

We start with some general information about the DE0-Nano board:

  • It has a 50 MHz crystal connected to a clock pin of FPGA, which we will connect to one of the PLLs that is on the FPGA to create a stable 20 MHz clock signal.
  • It has 8 green LEDs that that are active-high (turn on at logic level 1).
  • It has two buttons that are already properly debounced by a Schmitt trigger. The buttons are active-low, meaning that their logic level is 0 when they are pressed, and logic level 1 when they are not pressed.

The circuit that we are making will repeatedly do one of two things:

  • Invert the state of the LEDs
  • Rotate the state of the LEDs

We switch between these two modes when the KEY1 button on the board is pressed and subsequently released. We reset the circuit by pressing the KEY0 button.

Clash circuit specification

The complete description of the circuit is given below, where the description of the core behavior of the circuit only starts at line 95, and everything that comes before it is there to correctly set up the FPGA.

The two things that we wanted to highlight from the “setup” part of the descriptions are:

The Synthesize annotation we see on line 15:

lets you control the port names of the generated entity (VHDL) or module (Verilog), and the name of the entity/module itself. It also tells the Clash compiler from which function it should start synthesis. So in the above case, the compiler should start from the topEnity function, and should call the generated entity corresponding to that function: blinker. Then the ports corresponding to the arguments clk50MHz, rstBtn and modeBtn should be called: CLOCK_50, KEY0 and KEY1. The output port, corresponding to the result of the function should be named: LED.

Next up are the Annotate attributes, such as the one on line 30:

where we annotate the type Clock Input with two string attributes:

  • "chip_pin" "R8", which informs Quartus to connect the port corresponding to this argument to the FPGA pin called R8.
  • "altera_attribute" "-name IO_STANDARD \"3.3-V LVTTL\"", which informs Quartus about the voltage characteristics of the pin associated with the port associated with this argument.

All of this allows us to simply import the generated VHDL later on in our Quartus project without having to do any further FPGA configuration.

VHDL generation

Now it’s time to generate some VHDL. Copy the above description into a file called Blinker.hs. Then from the command line, run the Clash compiler to generate VHDL:

This will create a ./vhdl/Blinker/blinker directory with all the generated files. Note we passed in the additional flag -fclash-hdlsyn Quartus, that is because Quartus is somewhat quirky as to where it expects attributes for entity ports: in the architecture. By passing -fclash-hdlsyn Quartus, we ensure that Clash generates VHDL that is “compliant” with these quirks.

You can inspect ./vhdl/Blinker/blinker/blinker.hs and the entity and its ports are named as specified in the Synthesize ANN pragma.

Further down, we can see that the synthesis attributes have propagated to the generated VHDL:

Quartus Project

Next up, we create a Quartus project. We could go through the GUI to set things up, but in this case, we will simply create the project manually. Create the following files, with their given content, and put them in the directory where Clash generated the VHDL: ./vhdl/Blinker/blinker.




You should now have a directory with the following files:

  • altpllE588ED61A4853C9C.qsys
  • blinker.qpf
  • blinker.qsf
  • blinker.sdc
  • blinker.vhdl
  • blinker_types.vhdl

FPGA Bitstream creation

Now start Quartus Prime; once started, in the menu bar, click File -> Open Project and open blinker.qpf. In the menu bar, click: Processing -> Start Compilation. This can take up to a minute depending on your machine. If everything worked as it was supposed to the last messages in the logs should be in the spirit of:

Programming the FPGA

After synthesis has finished, it is time to program our FPGA board. Connect the FPGA board to a USB port, and start the programmer from the menu bar: Tools -> Programmer. Press the Start button on the left to program your FPGA and wait until the progress bar says 100% (Successful).

Your FPGA should now be fully configured with the Clash generated design for you to play with. So yeah, go press those buttons!

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