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Re: XSBI: cls51 PCS output timing


I asked my apps engineer and looked in the OIF document that the XSBI
was based on.  The numbers look reasonable.  The only thing I question
is why have a difference between the PCS output and PMA output.

- Board traces can be long.  Customers will push the limit.
  We have seen 15 inches used.  Board trace on the system board, 
  board trace on a module.  If you delay the clock by trace, this 
  also adds ~5" trace length.  There is variability between layers 
  which makes for skew.  With +/- 10% FR4, the skew is about 15% in
  time which could be 350+ ps.

- clock jitter (175 ps according to the specification)

- connector will add noise, crosstalk, and skew.  (50-100ps)

- clock duty cycle if inverting the clock. (160ps with 40/60 duty cycle)

Total board skew could be 350+175+100+160=785ps by inverting the clock.
Total board skew with etch delay could be 350+175+100=625ps.

With VERY optimistic numbers (5" clock delay+ 2.5" trace delay)
and not inverting the clock:

Getting board skew to 600ps can be a challenge depending on your 
situation.  Especially in production.

/Brian Cruikshank

Jscquake@xxxxxxx wrote:
> Hello,
> Below is a msg string in regards to PCS and XSBI timing, specifically the
> setup and hold numbers that are currently in Draft 2.0. I would like to
> solicit
> the system board designers inputs on the discussion below. Thanks in
> advance for your assistance.
> Justin Chang
> Quake Technologies, Inc.
> 50 Airport Parkway, San Jose, CA. 95110
> Tel: 408-437-7723 email: justin@xxxxxxxxxxxxx
> Fax: 408-437-4923 internet:
> ------
> Justin,
> On the PCS output it's not the 600ps board spec that's tight. It's the
> 1100ps PCS output spec.
> As you point out, we want more setup/hold time for the PCS in the other
> direction. Similarly we need to allow more slack to the PCS in the transmit
> direction.
> I would like to get numbers from some board/connector people. I don't think
> they need 500ps-600ps.
> Henning
> -----Original Message-----
> From: Jscquake@xxxxxxx [mailto:Jscquake@xxxxxxx]
> Sent: 7. december 2000 03:10
> To: henning.lysdal@xxxxxxxxx; stuart_robinson@xxxxxxxxxxxxxxxx
> Cc: steen.christensen@xxxxxxxxx
> Subject: Re: cls51 PCS output timing
> Hello Henning, Stuart,
> If the 600ps is tight then the return path PMA to PCS is even tighter. That
> is spec'd with a 500ps margin ...
> PMA output 1500-400=1100ps
> PCS setup 600ps
> ----
> total left for margin is 500ps
> The point of the asymmetry was to give more setup and hold for the CMOS
> PCS IC. I agree that we should hear from the system designers on this one.
> The place where we could cut back are
> 1) setup and hold for the PMA input (better technologies) ... change to
> +/-200ps
> 2) setup and hold for PCS (overly generous w/ 300ps?) ... change to 200ps
> as well ... this would make everything symmetric
> Total budget in both directions would be 1550-800=750ps for the board
> designer (connector and traces). Thoughts?
> I think I would put this out on the reflector?
> Justin
> In a message dated 12/1/00 3:09:10 AM Pacific Standard Time,
> henning.lysdal@xxxxxxxxx writes:
> > Our CMOS designers have informed me that the PCS output timing is pretty
> > strict.
> >
> > Looking at the numbers:
> > Period approx. 1.5ns (LAN mode)
> > PCS output data valid: 1100ps (1500ps - Tcq_min-Tcq-max, from table 51-3)
> > PMA input valid requirement: 500ps (tsetup+thold, from table 51-4)
> >
> > Margin for board and connectors: 600ps
> >
> > I think we should get a realistic estimate from a system implementer on
> the
> > required margin for the board before we finalize these numbers. If not,
> we
> > end up putting tough restrictions on the PCS design for no appearant
> reason.
> >