[Fpga-synth] Try out spartan3e projects, questions

[Eric Brombaugh]

Dave Manley wrote:
> 
> I'll chime in with one comment: the max FPGA frequency is no where near 
> the max frequency of modern processors.  I haven't looked at DSPs in a 
> long time, but assume some must be able to run in the GHz range, while 
> most FPGAs are going to be limited to a few hundred MHz at most (with 
> any significant amount of logic).  In terms of clock speed only, what is 
> the fastest DSP out there?  I see some AD Blackfin rated at 750MHz.

True, but it depends on what type of FPGA and DSP your talking about. A 
Spartan 3E probably tops out around 250MHz if you're lucky, but that's a 
$15 FPGA. I'm using some high-end Virtex5 parts that can run > 500MHz 
internally and have 640 MAC units. Lets see - that works out 320GOPS 
max. Name me a DSP that can get anywhere close to that? Of course, 
that's cheating a bit - that part costs $2500. :)

The big advantage of FPGAs is parallelism - even on a low-end part you 
can clock all those registers, multipliers and RAMS simultaneously. Even 
high-end DSPs with multiple execution units (normally no more than 8 or 
so) have bus bottlenecks that will prevent you from keeping those 
resources busy all the time.

The big advantage of DSPs is flexibility - it takes only fractions of a 
second to reconfigure a DSP for a completely different task (ie, load a 
program), or to compile & debug code. The FPGA configuration process is 
slower and the compile times can take hours.

There's a lot of overlap though and you can use both technologies to 
accomplish many of the same things. If your only tool is a hammer...

Eric