On Wed, 2007-06-20 at 15:34 -0700, Tobias Gogolin wrote:
> Thanks Herbert for this very complete answer!
> Part of the reason for me asking was that I was comparing Altera
> EP1C12 with 12k LU?
> to a Xilinx Spartan 3E at 500k gates?
> and Actel ProAsic3 at 400k Gate functions...
> And Lattice I don't remember, exactly but I think the number was in
> the Altera Range
> And at least for Altera and Xylinx I remember that the CMOS feature
> size was similar, well at least the voltage should be a good indicator
> for that right?
> 1.5 -1.8 Volt, which brings me to another question: They probably like
> a similar switching power supply like for the CPU's on modern
> motherboards, right? Any recommendations as far an easy to implement
> solution there (ok other than a 1.5 V battery)?

Most of the FPGA tools have a "power estimator" tool that will tell you
how many watts you need to supply.

If you're relatively slow then power will be pretty minimal, say a watt
at most. If you're running fast (and have alot of code) you can easily
get into the 10s of watts range on some of the larger parts.

As for power supply, each will need a few rails. At minimum there's
usually a core supply in the 1-2V range, then there's at least one IO
supply for each IO standard you need to drive (i.e. if you are driving
3.3 and 2.5V parts you'd have a supply for each, and the appropriate
banks voltage inputs tied to the appropriate supply). Some parts have
additional supplies for other things (i.e. the configuration section
usually needs it's own voltage, usually a "standard" one like 3.3V).

The only supply that you have to be really careful of is the core
supply. The FPGA can source 20A or more of this supply if you run enough
at a fast enough speed. Most vendors give "recommended" supply designs,
and that's what I've always followed (sometimes with modifications to
reduce size or capacity if I know I won't be drawing that much power).
You are correct in that they are very closely related to CPU supplies,
and most of the reference supply designs I've seen used chips meant for
the CPU market (IIRC the supply I used for a Virtex 4 part actually has
VID pins that are designed to change the voltage level; these pins
usually connect directly to a CPU which is how the CPU can set it's
voltage).

Note that I usually deal with very large FPGAs, for the mid range it's
likely things are "simpler", although power wise I don't see things
being that different (unless you're barely using any of the capacity of
the FPGA I doubt you'd get away with using a linear supply for Vcore).
The IO and other low amperage supplies I usually either already have on
my board, or just throw a linear part down.

Good luck! TTYL

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