RE: Fw: PCI Clock

["Clark, Raymond J (WBST)" <Raymond.Clark@usa.xerox.com>]

Anecdotal information regarding cheating on the spec.

My PC at home had a sound card by a major vendor who was gaining fast on the
leader.  The PC randomly crashes, some months alot, some less.  I had tried
everything.  Following the discussion on this subject a few months ago, I
pulled out all my cards and found (1) The clock trace was too long, (2)
there were almost NO decoupling caps on the board (perhaps 1 or 2).

I put in the same model, plug compatable card made by a MORE major vendor
who bought the first vendor about 2-3 years ago.  It was a new PWBA design,
and minor changes to other components.  It had very few decoupling
capacitors, but at least you ran out of thumbs counting them (>2).  The
clock trace is closer to being in spec (but still too long), but there is a
series resistor about 1.5 inches up, and a ground trace paired with it.
There has been a 10x reduction in crashing.  Given that my "problem" waxed
and waned, I cannot be sure that this was the cause.  But I am suspicious.

I wish vendors would follow the spec.  Also a few months ago someone posted
data on the cost differential for a cheap 4 layer board, I think it was $2
for a small PCI board.  Of course decoupling caps are pretty cheap.  I would
gladly pay $5 or $10 more to get something that works vs. something that
makes my system unreliable.  That might add up to another $50 for the whole
system, a small price to pay.  What good is a computer that crashes all the
time?  Vendors could make it a sales point - "Our boards meet the spec,
don't crash your system like those OTHER guys..."

My perspective as a frustrated consumer who knows better.

--Ray Clark


-----Original Message-----
From: Venkateshwarlu V [mailto:venkateshwarluv@myw.ltindia.com]
Sent: Thursday, April 11, 2002 4:30 AM
To: dinesh@cosystems.com
Cc: pci-sig@znyx.com
Subject: Re: Fw: PCI Clock


** Proprietary **

Hi,
PCI spec says SKEW should not be more than 1ns.It is required to provide
good shielding(guard) for clock to reduce the skew.
      Series Resistor increases the rise time, inturn affects skew.It's
better to provide end termination resistor(equal to characteristic
impedance) .
  Experts comments pls.

Thanks,
Venkat.V
Design Engineer(FPGA),
Larsen & Toubro Limited,
MYSORE
INDIA

>>> "dinesh" <dinesh@cosystems.com> 04/11/02 01:56PM >>>
Hi,

 Putting a serice  resistor of appropriate value at a distance of 2.5" may
help in handling reflection related issues on clock line while extending it
beyond 2.5".

Any comments on this point from Exp. Group ?

Din

----- Original Message -----
From: arvind v <v_arv@lycos.com>
To: <pci-sig@znyx.com>
Sent: Thursday, April 11, 2002 1:06 PM
Subject: PCI Clock


> Hi All,
>
> In one of my designs in Processor  pmc form factor, I need to run pci
clock(66MHz) to processor for trace length more than 7 inches (limitation of
the design).
>
> As per PCI Specs , on an expansion board I can run only 2.5 inches.
>
> What are the implications of this long length , any ideas to over come?
>
> Thanks in advance
> Arvind.
>
>
> See Dave Matthews Band live or win a signed guitar
>
http://r.lycos.com/r/bmgfly_mail_dmb/http://win.ipromotions.com/lycos_020201

/splash.asp
>
>