Soft CPU vs Hard CPU's

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Hai all,
          I heard that the SOFT CPU's have wide range of benefits than
the HARD CPU's. if so, then  why still HARD CPU's are preferred.

plz let me know the answer.


Re: Soft CPU vs Hard CPU's
Hi (hai means "shark" in my native language)

1 your question doesnt have an answer.
2 and your assumptions are not correct.
3 and if you keep using an identy like "teen" what hardly is your name and
wording in style 'Hai', 'plz' then you hardly will get answers to anything
you ask. If you dont understand what I mean then it would be of benefit for
you to go back to school and learn there something.
4 if you do not get upset to what I said above and do some homework then you
will be able to correct your wrong assumptions and answer your question


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Re: Soft CPU vs Hard CPU's
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Hey Antti,

Don't beat about the bush, man - tell him what you think, straight up!
This softly-softly approach never works [grin]

[Not to say that I don't agree with your points on the language though :-]


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Re: Soft CPU vs Hard CPU's
Hello Antti,

              Thank you for your reply. I had disclosed my identity in
the mail(regards, Kishore) . Please read the mail again.

Also I competely disagree with your assumption that I haven't done any
homework. After I had gone through a couple of pages collected through
I had some ambiguity in the differences between the two CPU's , So I
had asked the above question only to refine my web searching.

Pease be straight forward while giving any suggestions.


Re: Soft CPU vs Hard CPU's

Quite frankly, you have not even scratched the surface of the issue you
are asking about.  There is probably ten years of research, results,
products, etc. that you need to demonstrate a minimum of understanding of.

There is another twenty years of hardware uP knowledge that you seem to
be totally unaware of.

The question itself speaks volumes.

Open your mouth, and you remove all doubt of your competence.

So, now that we know you know practically nothing, we can reply with
something that will help you.

FPGAs have long been known to take thirty transistors to do the job of a
single transistor in an ASIC.  The same is true for a microprocessor.
It takes many more tranistors to the the job in an FPGA, than it does in
a uP.

Why would anyone in their right mind then use a FPGA?

Because it can be anything, for anyone (it is programmable).

An ASIC requires a great up front investment, and if anything is wrong,
you pretty much throw most of it away, and start over.  An ASIC also
requires a task that is not going to change -- ever.  Fine for a uP that
has a language that no one will change (too many lines of code already
written for it), but a system is always changing.

NTT saved billions of dollars by using Xilinx FPGAs in their cellular
base stations:  they didn't have to remove all the equipoment from the
field every time there was a change, they merely reprogrammed all the
base stations.

No genius required to see this advantage.

So why use a soft uP at all?

A uP is standard, so you don't need to change it, it uses software to
program it (hence the flexibility is maintained), and it is going to be
much faster, much smaller, and much lower power if the uP is hardened
(made into a non-changing mask).

Answer is, that unless you want to make a uP with a custom instruction
set (that now limits us to probably less that .001% of all applications
- and for them we offer the APU interface), there is no good reason.

Hence why we offer the IBM 405 PPC in Virtex II Pro, and Virtex 4.

But, what if I can replace a ton of gates and flip flops by a simple
program (less than 1K words of instruction)?  Hey, maybe there is a
benefit to using a soft uP:  I can actually use less
area/power/gates/transistors in the FPGA with a soft uP, than I would
use if I used FPGA logic to do the same job.

Hence the fabulous sucess of the simple microcontroller core IP that we
offer for both the 405 PPC, and MicroBlaze.


teen wrote:

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Re: Soft CPU vs Hard CPU's
Another reason, of course, to use a soft processor.. is one like mine... I
needed a processor.. nothing complex... But I have an FPGA only 1.2 full..
So I've got a picoblaze .. with a dual port RAM to handle inter processor
communication, a simple interpreter to handle initialization, and an
interrupt controller to handle unscheduled, but necessary events.. and a
real cool SPI interface which automatically reconfigures itself depending on
what external device is chip selected.

And it was free.. because the FPGA has to be there, so I saved myself a few
bucks and impressed my boss :-).


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Re: Soft CPU vs Hard CPU's
Not really related to the original question but intresting nonetheless:

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That is not entirely true.
Similar to FPGAs uP are extremly inefficient because they are universal.

If I want to add two numbers in a uP, something that basically takes a
few dozen gates, thousands of of gates start working. Lot's of stuff
that is there to preform the ISA abstraction like instruction
reordering, scoreboards and reservation stations. Security measures like
MMU, and stuff to hide memory access time like caches and translation
lookaside buffers. At the same time many, many more gates are sitting
around idle like unused cache parts and unused execution units.
In a high and processor it takes half a billion transistors to execute
something like six operations per clock cycle.
But it is universal an can run any algorithm any time.

On the other extreme you have an asic implementation of the algorithm
that only does work that is required for the task but can do nothing
else. This of course is a lot more efficient. (By a factor of 100.000 or
so for many algorithms)

The FPGA is a compromise between the two. It can use asic like
algorithms  getting away without all the CPU overhead stuff but you have
to pay the factor of thirty for the implementation because you can
change the algorithm universaly.

As a result it takes a lot - and I mean a lot - less transistors per
operation e.g. to perform a smith-waterman computation in an FPGA than
in a uP.

Kolja Sulimma

Re: Soft CPU vs Hard CPU's

I respectfully disagree.

If I want a uP, I can optimize it until I have only those transistors I

If the job is to run any program whatsoever, I can not choose to
implement just the logic I need, as that would be an infinite amount fo
logic (for an unbounded set of programs).

Now that I have an instruction set defined, it takes far more
transistors to do the same in a FPGA, than in a ASIC uP (or ASSP uP).


Kolja Sulimma wrote:
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Re: Soft CPU vs Hard CPU's

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You heard wrong. A hard CPU is faster and cheaper then a soft CPU. By
cheaper I mean they take much less die area then an equivalent soft CPU.
The downside of the hard CPUs is that they are only available in high end
FPGAs like the Virtex2P and Virtex4. The cheaper FPGAs like the Spartan3
don't have embedded CPUs. If you needed a processor in a Spartan3 you
would have to use a soft CPU, however you want to use one that was much
simpler then the PPC in the Virtex2Ps and 4s. If you implemented a full
PPC you would use so many slices that you would negate the cost advantage
of the Spartan3.

Re: Soft CPU vs Hard CPU's

That is the main reason why Hard CPU's are still used. They are
cheaper.... a Phillips Arm 7 32 bit CPU is about $3.50 in quantity. An
FPGA with a similar size softcpu like a Microblaze is about $10.

Soft CPU's are used for performance. They take advantage of the FPGA's
logic and use parallelizm to speed up operatiations (ie multipuly two
32 bit numbers in a single clk cycle). A hard CPU would take 4 or more
clk cycles to multiply two 32bit numbers.

So SoftCPU's used with FPGA logic can have a higher performance than an
equivelent hardCPU.

So it totally depends on your application to whether a soft or hard CPU
is for you.


Re: Soft CPU vs Hard CPU's
SoftCPUs also can have custom peripherals (ie 5 x UARTs, 6 x SPI bues,
20 x I2C bues).

With a Hard CPU your stuck with what the manufacturer gives you.

Personally SoftCPU's are more fun.... custom home brewed CPU's are

Writing a VHDL model of a CPU is the closest to designing my own CPU
I'll ever get. There is something exciting about that!!!!


Re: Soft CPU vs Hard CPU's
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Er, that is designing a CPU! Probably the large majority of commerical
CPUs are synthesizable, or contain a lot of synthesizable code.


Re: Soft CPU vs Hard CPU's
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True enough, but it seems to me an external (or on-chip) hard CPU
loosely coupled with a "coprocessor" built of FPGA fabric gives you the
best of both worlds for performance/cost and also minimizes power and
you don't need lots of address pins going into the FPGA.


Re: Soft CPU vs Hard CPU's
Spartan 3E should help with this.  The smallest Spartan 3E starts at
under $2 in quantity (and fits a small MicroBlaze system) for an
effective MicroBlaze cost of $.48 [1].  You may need a larger device to
get everything you want, but I did want to point out that the $10 is a
little high if one is going primarily for cost.  It is not fair to
compare the entire cost of the FPGA to just a hard processor and then
use the rest of the FPGA for other stuff.  In this case the FPGA is
doing a lot more than the hard processor.

As far as the benefits of a soft processor.  MicroBlaze is highly
configurable.  You can turn on the features you need: instruction cache,
data cache, FSL, XCL, FPU.  See

for more information on MicroBlaze v4.00.a.

The other huge advantage of soft processors is the FPGA architecture
itself.  EDK has an FSL wizard to allow easy creation of accelerators.
With careful acceleration of one's application, one can obtain
performance that no hard processor can touch.

EDK also includes a lot of other IP allowing one to build a System On a
Chip (SoC) out of the box.  It is a great deal for the price.

Personally, I also find debugging on an FPGA a lot easier.  One can
simulate the EDK system and get a ton of information.  I find ChipScope
a lot more convenient than a logic analyzer and one can dig a lot
deeper.  Tracking down a bus issue on a hard processor can be a pain. On
both sides there are lots of good tools if you want to spend the money,
but I think the costs may be more reasonable on the FPGA side.

If you have the need, Xilinx will sell the MicroBlaze source code for
you to customize as you see fit.  Try doing that at a reasonable price
on a hard processor.

I am not in marketing or sales, but I have no problem listing some of
the advantages of soft processors.

This post is my own opinion, and not an official Xilinx post.  Reverse
domain and remove the NOSPAM from e-mail address to respond by e-mail.

[1] Xilinx. Xilinx Shatters Price/Density Barrier For Low Cost FPGAs
With New Spartan-3E Family Starting At Less Than $2.00.

Eric wrote:

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Re: Soft CPU vs Hard CPU's
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"*500K unit volume, second half 2006" !!!

plus you have to factor in the cost of:

 - the configuration prom
 - extra regulators that you wouldn't need with most micros
 - analog peripherals
 - possibly an oscillator
 - possibly some external RAM

It's not quite a simple as multiple cost-per-LUT by the number of LUTs used.


Re: Soft CPU vs Hard CPU's

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... and neatly avoided ANY mention of the memories needed to actually
RUN the '48c core'.

  Soft CPUs are great for consuming LUTs, and FPGA vendors love them, but
they are very much 'stone soup'.

  Key deployment 'reality check' questions are :

** Is there a key operational plus, to having the CPU on the FPGA ?

** Can I use a smaller FPGA, if I jettison the Soft CPU ?
    [Suddenly that claimed 48c CPU can get very expensive ! ]

** Will I get less EMC, if I chose a uC with on-chip, secure FLASH code,
vs a FPGA ? [ that also needs short lifetime, off chip SDRAM, that must
be loaded from another NV memory -> you have to pay twice to store your
less secure code !]

** Can I afford the IDLE power budget to run the soft CPU ?

  eg take a hard look at the new widely sourced Flash ARMs (for
example), that include Large Secure FLASH, AND Analog peripherals, AND
Oscillators, AND save a LOT of power (wrt Spartan 3E et al). All for
less than the cost of the dual-CODE memory the Soft-CPU needed....


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