Issue
142 May2002
Design
Your Own Microprocessor
by
Jim Turley
PROCESSOR
FACTORY
Although
building your own foundry is out of reach for most engineers
(it costs more than $1 billion just to upgrade an existing
chip plant), it’s easy to make a few of your own processors
using low-cost FPGAs. Altera and Xilinx both encourage
this behavior for the obvious reasons. In fact, both
companies have entered the business of supplying soft
processor core designs to encourage engineers to use
more FPGA chips. Altera’s Nios (see Figure 1) and Xilinx’s
MicroBlaze (see Figure 2) are both free but only work,
of course, in the parent company’s chips.
|
|
Figure
1—NIOS
is a 16-bit processor Altera designed specifically
to work in its own FPGA devices. |
Why
put a processor in an FPGA? Why not buy a real microprocessor
chip and solder it down? If you’re going to use an FPGA
for random logic anyway, you can eliminate the extra
chip by putting the processor in the FPGA, as well.
You also could experiment with different processors
in your FPGA, evaluating various ones until you find
the right balance of performance, size, cost, and power
consumption.
|
|
Figure
2—Xilinx’s MicroBlaze, like NIOS, is an FPGA-only
processor that resides in the "soft logic"
of an FPGA. |
The
most interesting prospect, of course, is modifying the
processor to suit your own whims. Or, dare I say, even
designing your own processor from scratch (the first
CPYou)? Well, you can, and there are some good reasons
why you would (or would not) want to try this at home.
A
ONE-MAN SHOW
For
most engineers, designing your own processor architecture
is a cool idea but not a practical one. A custom CPU
has zero software support—no tools, no compiler, no
debugger, and no one to turn to when bugs crop up. You’re
on your own.
Using
an existing CPU, on the other hand, makes a lot of sense
even if you’re going to embed it into an FPGA or high-volume
ASIC. In fact, according to Dataquest, the business
of embedding CPUs into other chips is growing at about
25% per year and stood at about $20 billion in 2000.
That’s no small potatoes. The average is 2.3 processors
per intelligent ASIC (those with any programmability)
and rising.
There’s no shortage of processors to choose from, either.
From little-known 8-bit processors created as university
research projects to popular 16- and 32-bit processors
like the ’286, SPARC, ARM, or MIPS, the market is ready
and willing to supply you with microprocessor intellectual
property (IP). Some are free (and worth every penny),
while others might cost you as much as a new Mercedes.
One
high-end free core is LEON-1, a SPARC clone distributed
by the European Space Agency. This is a real SPARC processor,
created with Sun’s blessing, but without floating point
capability or anywhere near the performance of Sun’s
current processors. Still, SPARC has good software tools
(available for free) and a certain amount of appeal
and prestige. LEON-1 is provided with VHDL source code
(you supply the hardware compiler) and runs at 20 to
40 MHz in Xilinx or Altera FPGAs. At that speed, it
would probably take you a week to boot Solaris.
At
the low end (and also in the free category), are 8-bit
processor designs from CMOSexod and Free-IP cores. Both
designs have little 8-bit processors you’ve never heard
of but come with free tools to get you started. The
Free-IP Project also has a 6502 core, in case you’ve
been hankering to resurrect your old Apple II. VAutomation
also sells its 8-bit V8 processor with free tools. For
your money, VAutomation gives you real technical support
and the warm fuzzy feeling that their processor really
works. They even use the V8 in some of their own products.
The company also has a 6502 as well as Z80, 8086, and
’186 cores. Now that Intel and AMD are both out of the
embedded ’x86 business, soft cores like VAutomation’s
may be your only way to get low-end ’x86 processors.
