Hacking the Xbox

Appendix D - Getting Started with FPGAs 243
verilog syntax and verilog tutorial are both good sets of keywords to
start out with when searching for syntax references or tutorials. Xilinx’s
website also has a good Verilog reference for FPGA designers, and
Sutherland HDL, Inc. has a free Verilog quick reference guide at http://
www.sutherland-hdl.com/on-line_ref_guide/vlog_ref_body.html.
Another advantage of the HDL design entry approach is the availability
of free and paid “softcores.” Websites such as www.opencores.org offer
general-public licensed HDL cores for functions such as USB interfaces, DES
and AES crypto-engines, and various microprocessors. In addition, almost
every standard function is offered by third-party vendors who will sell you
cores for a fee.
After design entry, I highly recommended that you simulate your design
before compiling it into hardware. Trying to track down bugs by
twiddling code, pushing it to hardware and probing for changes is very
inefficient. Simulation allows you to probe any node of the circuit with
the push of a button. In addition, the effort required to simulate a code
change is very small, especially when compared to the effort of pushing a
change all the way through to hardware.
Once the design has been entered and simulated, it needs to be compiled or
translated into a common netlist format. This netlist format is fed into a
program that maps the netlist primitives into the target FPGA hardware
primitives, after which the mapped primitives are placed and routed. The
resulting design is analyzed for compliance with a set of constraints
specified by the designer. If the design does not meet the designer’s
specifications, it is iteratively refined through successive place and route
passes. Once the design passes its design constraints, it goes to a configuration
bitstream generator where the internal representation of the
FPGA is translated into a binary file that the FPGA can use to configure
itself. (All of these steps happen fairly seamlessly at the touch of a button
in the later versions of the FPGA design tools.)
Project Ideas
Now that you know a little bit about what an FPGA is and how you can
program them, what sorts of things can you do with them?
As it turns out, FPGAs have enough logic capacity and performance
these days to accomplish a very impressive range of tasks. The obvious
industrial application of FPGAs is in the emulation of designs intended
for hard-wired silicon. The cost of building a custom chip has been
skyrocketing, and it will soon be the case where a single critical mistake
can cost hundreds of thousands of dollars, if not millions, to fix.
On the other hand, fixing a mistake made in an FPGA HDL description
pretty much only costs time and design effort; you don’t throw away any
parts, and you don’t have to buy any new parts. Thus, many companies
have adopted the strategy of fully simulating a mock-up of the design in