CHDL on FPGA

by chad

FPGAs are fun. They provide an execution environment for RTL models quick enough to rival custom hardware, at a tiny fraction of the initial investment. This makes them highly appealing to builders of prototypes, those needing to use high-frequency interfaces, and hobbyists, who build copies of historically significant computers from the mid 1980’s that run slower than emulators on modern hardware because why not (http://www.bigmessowires.com/plus-too/).

So of course CHDL code can be run on FPGAs. Just not directly. The configuration formats for FPGAs are proprietary and technology mapping to FPGA lookup tables has not yet been implemented. But why bother to generate an FPGA configuration bitstream for one product by one vendor when you can generate synthesizable Verilog and let the vendor’s proprietary tools do the translation and optimization?

This is what I’ve done. The result is satisfying:

shot0001

So, of course, if you do FPGA development at all, this is something you should do as well. Here’s how:

  • Get CHDL. (https://github.com/cdkersey/chdl)
  • Create a design…
  • Compile the design with an ordinary C++ compiler.
  • Write a program that:
    • Instantiates the design.
    • Calls optimize().
    • Calls print_verilog().
  • Place this Verilog code in a file called “chdl_design.v” and import it into the FPGA toolchain of your choice.

The rest is your standard FPGA workflow, for which there are plenty of tutorials on the Internet. Among the advantages of the CHDL workflow is that it pushes the proprietary tools out to the margins of the design flow. They are still responsible for much of the optimization, technology mapping, pin assignment, and the like, but as long as they speak a simple synthesizable subset of Verilog, they become completely interchangeable.

A Bit About the Demo

The demo code is a simple VGA terminal meant to be clocked at 50 MHz, with a simple parallel interface. The character ROM is stored in human-readable format in the file FONT, from which it is converted to hex by the simple program in font2hex.cpp. Attached to this VGA controller is a text generator, which outputs characters at a human-readable rate from a ROM, whose contents are initialized based on the file TEXT, which is converted into hex in the makefile using a simple hexdump command.

The entire design uses 1691 LUTs and 29k bits of block RAM on an Altera Cyclone II. Somewhat surprising is that this is more than the total number of nodes in the design after optimization by the CHDL toolchain, but this is easily explained away by duplication for the purpose of performance, since area is plentiful (on the demo board, this is ~10% of available resources).