Em 5/3/2011 13:57, Herbert Graf escreveu:
>> 4. How are they used?
> Really there are 2 main uses for programmable logic:
>
> 1) Prototyping digital designs - being user reconfigurable you can put
> your prototype HDL design into these parts and debug any problems. It's
> a step above simulation, and allows you to catch problems that sims are
> not that great at catching.
>
> 2) Implementing designs where going to an ASIC isn't financially viable.
> This means either R&D type products, where the company only needs say
> 100 parts, or commercial products that are "niche" enough not to justify
> putting it in an ASIC (another case is if the turnaround time of
> spinning the ASIC is too long/costly). Examples of this are broadcast
> video. Alot of their widgets use FPGAs, since they don't need that many
> of them, and the profit margins are so high as to warrant use of an
> FPGA.


Don't forget that they are reprogrammable, so they are upgradeable. Some
designs may need updates or functionality changes after deployment, and
some may need to be reconfigured regularly.

If you use an ASIC, you are stuck with the initial design.


>> 6. What is the difference between PAL, CPLD, and FPGA and under what
>> circumstances would you use each of them in?
> PALs are mostly dead, I wouldn't bother looking into them.
>
> CPLDs CAN be thought of as "small" FPGAs, so if your design fits in a
> CPLD you'd probably go with that. Otherwise go with an FPGA.
>
> That said, FPGAs have REALLY encroached the CPLD space, so much so that
> most designers just skip over CPLDs these days (you can buy a tiny FPGA
> for just a buck or 2 in quantity).

FPGAs and CPLDs serve the same functions, but they have different
architectures, "logical blocks" (FPGA) versus "macrocells" (CPLDs),
something like the difference between Von Neuman and Harward architectures.

FPGAs are usually much larger than CPLDs and some load their "program"
at power-up (boot-up) from an external memory. CPLDs are (re-)programmed
with a programmer.
So FPGAs fit much larger designs and may be updated by software.


Isaac

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