|
In an attempt to help me maintain a better
work/life balance, my wife suggested that I
pick up a hobby. The first thing that came
to mind was developing FPGA-based video
games. I have never been a good game player,
but I am fascinated by game hardware.
The reason I studied electrical engineering
in the first place was to get a job at Atari
Games, but they closed their doors before I
earned my degree.
Silicon, software, and solutions from
Xilinx® are all you need to develop game
platforms and write games. You can get
started for less than $100 with the Xilinx
Spartan-3 Starter Kit and ISE
WebPACK software. Implementing
video games in FPGAs is not only fun, it
creates an opportunity to learn more
about FPGA devices and development
tools through experimentation without
the stress of deadlines.
FPGA Gaming Platforms
Simple gaming platforms only require
three components: a human interface
device for input, another for output, and a
mechanism for implementing the game
logic or game program. Many commercial
development boards with Xilinx FPGAs
satisfy these requirements.
However, to create something that I
could truly call my own, I chose to design
a platform that was:
- Entirely Xilinx-based
- Inexpensive to build
- Suitable for cooperative development
- Capable of respectable performance
After a number of iterations, I created a
block diagram (Figure 1) for a simple and
yet highly flexible reconfigurable video
game platform that met my requirements.
The platform is designed to support the
largest Spartan-3 device available in the free
ISE WebPACK tools the XC3S1000; this
device is used to implement the game logic
or game program. The platform also uses
the Xilinx System ACE CF configuration
controller to great advantage.
At power on, the SystemACE CF controller
configures the XC3S1000 from one
of eight user-selectable configurations on the
Compact Flash card. After the initial poweron
configuration loads, the XC3S1000
can instruct the controller to reconfigure
using any of the other configurations.
Furthermore, the design in the XC3S1000
may access the Compact Flash card through
the System ACE CF controller as a disk. You
can use the extra space on the Compact
Flash card for program and data storage.
In terms of human interface devices,
there are two joystick ports, a stereo audio
output jack, and a VGA port capable of
4,096 colors. The platform also provides a
standard serial port to use for linking systems,
textual interaction through a terminal,
or debugging. Schematics and
photoplots for this design are available at
the FPGA Games website (see the resources
list at the end of the article).
A completed prototype of the platform
(Figure 2) is quite small at 4 x 6 inches and
could easily be smaller, possibly fashioned into a handheld unit with an integrated
LCD screen. The Compact Flash card
becomes, in effect, the game cartridge. The
fun doesnt start, though, until you have
some games to play!
Games Via Emulation
Emulation is the sincerest form of flattery.
With a suitable platform, you can emulate a
wide variety of games and game systems,
including those found exclusively in arcades.
There are two general methods: software
emulation and hardware emulation.
In either case, emulation may involve
the use of ROM images, which contain
copyrighted software and audiovisual content.
Make sure that you obtain legal ROM
images for emulation either by purchasing
original ROMs or by obtaining the legal
rights to use downloaded ROM images
from services such as StarROMS. Dont let
flattery turn into thievery.
The Atari 2600 Video Computer
System, originally released in 1977, makes
a wonderful case study of emulation.
Although the system is nearly 30 years old,
only recently have people been able to
emulate it well.
Software Emulation
Software emulation involves the creation of
a program that runs on one system (the
host) to simulate the behavior of
another (the target). For game systems,
the emulation of the target
must include human interface
devices as well as the game logic,
which is typically a microprocessor
running a game program.
In most cases, software emulation
requires a host with performance
at least an order of
magnitude greater than the target.
This is because it may take a fair
number of instructions (or cycles)
on the host to emulate what happens
in one instruction (or cycle)
of the target.
Software emulation of game
systems is entirely feasible if you
use Xilinx Platform FPGA devices
such as the Virtex-4 FX family
with integrated PowerPC
processors. For example, you can
emulate the Atari 2600 Video Computer
System on a PowerPC host created with the
Xilinx Embedded Development Kit. If you
are coding-challenged like me, porting an
existing emulator is quicker than writing
one from scratch.
For each emulated cycle of the target
system, the emulator must exactly match
the target behavior. However, most emulators
do not run with cycle-for-cycle
accuracy in real time. One technique is to
periodically emulate behaviors faster than
real time, and then halt until the next
period begins.
Hardware Emulation
Hardware emulation involves re-creating
the original hardware or something functionally
equivalent in programmable
logic. This approach may yield the most
authentic emulation, but requires detailed
knowledge of the emulated system, sometimes
down to the transistor level.
Again, consider the Atari 2600 Video
Computer System. It is based on a 6507
processor (a derivative of the venerable
6502), the 6532 peripheral (RAM, I/O, and
a timer), and a ROM (4 KB). The processor
is available from the Open Cores website,
and the rest are easy to create.
An additional custom peripheral called
the Television Interface Adapter (TIA)
implements audio and video output and
user inputs. The TIA was designed in
NMOS, and while its schematics are available,
the TIA is surprisingly complex for a
few thousand gates. The TIA design uses
dynamic logic, latches, all manner of asynchronous
resets and presets, gated clocks,
and circuits cascaded using ripple techniques.
To further complicate the situation,
game programmers learned how to
exploit undocumented features in TIA
hardware. As a result, any hardware implementation
must truly reproduce the original
behavior in real time, cycle for cycle.
Ed Henciak, an expert Xilinx user from
Pennsylvania, had the understanding,
skills, and patience to implement a hardware
emulation of the Atari 2600 Video
Computer System in an FPGA. I was so
amazed with his results that I sent him a
prototype unit of my game platform. He
sent me back some exciting pictures of his
design running on it (Figure 3). In the
near future, Ed plans to release his design
with a low-cost FPGA-based game platform
of his own.
Unleash Your Imagination
If you arent nostalgic for the video game systems
of yesteryear, unleash your imagination
and implement a game you enjoy
from the ground up. I find wonderful
ideas by scouring the websites
of hobbyists working with
discrete embedded processors.
Some of the best games are
simple. You can start small with an
addictive puzzle or action game
(for example, Tetris, Minesweeper,
Pong, or Breakout), implementing
it directly in logic or with a Xilinx
PicoBlaze processor.
Once you are comfortable
with games like this, you can
graduate to more complex ones
running on Xilinx MicroBlaze
or PowerPC processors. The
Xilinx Embedded Development
Kit is a great tool for this activity,
as it provides a wealth of
peripherals to choose from and
complete development environments
for both processors.
Conclusion
You can develop games on a wide variety
of Xilinx FPGA development boards. It is
a fun and rewarding activity. If you are
interested in reading more, the following
websites provide great information, motivation,
and example projects:
- www.fpgaarcade.com
- http://office-dsan.hp.infoseek.co.jp
(Japanese)
- http://members.iinet.net.au/
~msmcdoug/pace
- www.fpga-games.com
- www.retrogames.com
- www.starroms.com
- www.opencores.org.
Printable PDF version of this article with graphics.  (6/25/05) 300 KB
|
