Re: FPGA for PCI based servo control board

[jmkasunich-at-ra.rockwell.com]

Johhn Sheahan wrote:

> John Kasunich wrote:

>> fact that the question was PCI related, but I was responding to
>> the fact that you seemed to consider PCI compatibility the sole
>> criterion for choosing the FPGA.

> que?  My only point here was 5V pci can be driven with
> a 3v3 rail.

> personally - I'm still of the opinion a PCI interface is
> inappropriate for  motor control. I think this should be
> outside the PC and use something fast and serial to
> communicate with it.  but thats just my opinion - with
> the particular constraints I have.

OK, maybe messages from somebody else got merged with yours
in my head.

>> It sure is.  I design VFDs and other industrial equipment.  ALL
>> of our stuff runs on 5V, even the newest designs.  We are using
>> some very recent Hitachi micros with built in PWM generators,
>> A/Ds, and so on, and they are 5V parts.  Regardless of where
>> the PC world goes, 5V is still the universal standard in the
>> industrial world, and I see no reason to change that.

> my recent designs has been partly fast comms - 1.5V core in
> virtexII pro (with limited 3V3 and decent 2v5 IO)
> and partly 5v logic doing random stuff.
> comms is not 5v. Its fast becoming not 3v3 any more.

What kind of products are you designing?  When I think of
fast comms, I think ethernet, ATM, T1 lines, ASDL, and
so on.  That's still computer stuff, not industrial stuff.

> for me - I'd be choosing modern fpgas that fit and route
> well - with flexible io pinout - and are cheap.
> which means low voltage core today.  The older stuff
> is just horrible by comparison.

A lot of this is perspective.  I'm coming at this strictly
from a hobby point of view.  I want simple circuits, parts
that I can solder, and non-critical layout.  Ideally I'd
be able to use a two layer board, since they are much
cheaper in small quantities.  These priorities are completely
different than those in commercial development of high tech
products, where high clock speeds, low voltages, and fine
pitch parts are the norm.

Even the stuff I design at work is much lower tech than
what you work on.  As I mentioned, we use 5V only.  Boards
are four layer, not six or more.  The biggest part in our
most recent design is 100 pins.  Much of it is analog,
with 1206 and 0805 parts.  I know the bleeding edge folks
are using 0402s and such, but for our kind of products
it just doesn't make sense.  Our designs have expected
annual volumes of a few hundred to a few thousand.  The
real money in our products is the power components, not
the PC boards.  Just like the real money in a CNC machine
is the iron, motors, screws, and so on.

> Just spent 2 days juggling RTL to partition a design
> into available PLCC84 devices. ended up with XC95108's.
> Bad choice, pinout cannot be maintained when its full
> and the design tweaks.

Some designs are more partitionable (is that a word?)
than others.  The CNC project has natural breaks, either
by axis or by function.

> I usually interface to higher voltage/current (ie the
> outside world) with discrete tansistors - so  fpga IO
> voltage  is not really an issue here. And since all
> the logic is inside the fpga - there -is- nothing else
> to interface to.

We definitely work in different worlds.  About 95% of a
drive's electronics is interface.  The micro handles all
of the logic, and includes RAM, flash, etc. on one chip
(that's the 100 pin one).  The rest of the stuff on the
boards is current and voltage feedback, IGBT gate drivers,
fault detection, and user I/O, both analog and digital.
Power supplies are a big part of it too.  All of the
digital stuff is 5V, but we have +/-12V for analog,
+/-20V for current sensors, isolated 24V for digital
I/O, and 6 isolated supplies for the gate drives.

> for a hobby project - the package is really more of
> an issue.  I dislike soldering pin pitch less than
> ~0.8mm by hand, which is a much bigger restriction
> on device choices today.
>
> john

Exactly.  It seems everybody comes into this with a
different view, based on their own experiences and skills.
My FPGA work was 6 years or so ago, on a 6K gate device.
Since then, I have worked only on board and system level
design.  Folks who do high gate count designs today
probably consider my stuff as obsolete as 7400 series
logic.  But I would argue that for hobby projects,
"obsolete" technologies aren't all bad.

John Kasunich