[SI-LIST] Re: Hyperlynx vs. Signal Explorer
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <twesterh@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 11 Jan 2007 17:33:23 -0700
To whomever is still tuned in to this exceedingly long and meandering
thread -
Gotta go with Todd. That's been my experience, also. Can you live without
post-route? The faster I go, the less I can do without it. The rule sets
get tighter and tighter, and harder and harder for CAD to implement. My
vision of SI perfection and routing reality do not overlay exactly on
occasion. Pre-route is by far the most extensive, just like Chris is talking
about, but there's almost always something that you catch in post-route.
Pre-route can be months, post-route a few days, but they are important days.
Shoehorning a box of widgets into a thimble of space is by definition
iterative, and that's why in my opionion post-route is necessary. If you
have figured out a way around this write a book, and I will certainly buy
it.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Todd Westerhoff
Sent: Thursday, January 11, 2007 4:53 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Hyperlynx vs. Signal Explorer
Chris,
I agree with your point, in theory. In practice, however, I have found
post-route simulation to be useful. The trick is - post-route analysis
shouldn't become an end to itself. It should be a logical (and efficient)
extension of your pre-route work.
In theory, you can do you your pre-route studies and tell the PCB designers
what you want. If they design to those rules, you should have no problems.
However, what usually happens is that your initial pre-route studies are
based on an estimate of how the board will be routed, and the PCB designer
comes back to you halfway through routing and tells you they can't meet the
match length requirements because one of the other components just got moved
and now interferes with the routing path, or the terminators had to be moved
to make room for the decoupling caps, or whatever. My point is - you almost
never get a board routed with your first set of rules - the design process
iterates.
This is not necessarily a big deal. If you're organized, you can quickly
update your pre-route studies to accommodate the change, run the simulations
and determine what impact they have on interface timing. You'll then know
if you can afford the hit, or if you need to change your routing rules,
which require another round of simulations and timing analysis. At some
point, you'll either have updated rules, be willing to live with the margins
you've got, or have one of those famous discussions with the project manager
(at which point having your analytical ducks in order is definitely
advised).
I believe that high speed design is a triad involving timing analysis,
signal integrity and design decisions/rules (By 'decisions', I mean things
like "do we use termination, what values, and where", and by 'rules', I mean
things like pin ordering, stub lengths & lengths/length matching). Changing
any leg of the triad impacts the others, typically requires re-analysis and
may/may not require a shift in the design strategy. I maintain that high
speed design is iterative and having a good process for quickly "turning the
crank" when there is a change in the timing model/signal integrity/physical
design is a key to success. Knowing your experience level, I'm sure you're
quite good at this.
Here's my point - if it were possible to take the work you've done in
setting up your pre-route analysis, and quickly & easily leverage that to
perform post-route SI & timing, I'd expect you wouldn't have strong
objections. If it cost you nothing other than computer time (and not weeks
of it), you'd probably run it to see what would happen ... and you might
find (as others have) that the routing variations in the actual board didn't
fall quite the way you expected, that your clocks are quite as centered as
you thought they would be, and there's some margin to be had, even if only a
little bit, by moving your clocks.
There's a premise here - it's only worth it if post-route analysis doesn't
become a project unto itself. It shouldn't be a big, separate, deal - it
should be a natural (and automated) extension of your pre-route work.
That's not necessarily easy (and highly tool dependent) - but, in my
experience, it's been worth it.
My $0.015.
Todd.
Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@xxxxxxxxxx
www.sisoft.com
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