[SI-LIST] Re: Series termination
- From: steve weir <weirsp@xxxxxxxxxx>
- To: vinayak.agrawal@xxxxxx, Vinayak <vinayak.agrawal@xxxxxx>
- Date: Tue, 28 Dec 2004 02:39:52 -0800
A little clarification, I should have said that the ground and Vcc need to
be tightly coupled at the I/O pad. They can be decoupled everywhere else.
Steve.
At 02:18 AM 12/28/2004 -0800, steve weir wrote:
>Vinayak,
>
>I strongly disagree that what you have described looks like the SATA
>spec. The SATA spec drivers are all balanced 50 ohm + 50 ohm outputs at
>all times. The current steered model maintains this by shunt, while the
>voltage driven models do it in series. You should be copying the lower
>part of Figure 29. If you doubt this, build a model and short the low side
>50 ohm resistor. You aren't going to like the results and neither will
>your customers.
>
>Ultimately, from the package boundary to the driver high side, the return
>reference and Vcc must be tightly coupled. You can see this by building a
>fairly simple SPICE model and inserting a series resistor or inductor in
>the coupling path.
>
>The concept in the old slow days was that Vcc and ground were tightly
>coupled inside and outside the package. The return current divided going
>back into the package across both rails, which inside the package were a
>near AC short. However, fast forward to today, and that model has serious
>issues. Any apparent lumped inductance of any size is going to cause the
>kind of severe SSO that has Lee pounding the table, and Chris constantly
>warns of when signal returns are not designed correctly. If you build a
>6Gbps I/O the way you are talking, you will become a poster child for the
>problems Lee has been writing about in EETimes.
>
>At high edge rates, you need to carry I/O from the pad through the package
>as a well designed transmission line. The reference for that line should
>be carefully selected to be relevant at the pad. This reference needs to
>be identified so that at the package / PCB interface, the transmission line
>is simply continued. Logic common is usually a good choice. You will
>still need to have adequate bypassing inside the IC to maintain stability
>of the opposing rail.
>
>Steve.
>
>At 02:59 PM 12/28/2004 +0530, Vinayak AGRAWAL wrote:
> >Thanks Vinu and Patrick
> >Unfortunatly I have termination resistance specs and not return loss
> >specs. I'm not using a current steering topology and the termination
> >resistors are on-chip. The problem is that I'm an ASIC designer, I can
> >get package data but I don't have any experince of board design and I
> >don't know how to take board effects into account.
> >
> >The output circuit may seem unusual but I don't think it is too unusual.
> >You can find something similar on page 91 of SATA (Serial ATA) 1.0a
> >specification (you can download from
> >http://www.serialata.org/docs/serialata10a.ZIP, or see the attached
> >page): there are two figures, first is a current steering driver (that
> >obviously has a parallel termination) and the second is similar to I'm
> >trying to design. This driver works effectively as two single-ended
> >drivers (properly synchronised to remove skew).
> >
> >
> >Another doubt I have is does 50ohm termination in a series-temrinated
> >single-ended driver really work as a 50ohm termination? Assuming signal
> >trace runs over a ground plane, then the return current for high speed
> >signals on this trace will run directly underneath the trace on the
> >ground plane, AND I ASSUME to the package and to the driver. But on a
> >high going transition the driver must get the return current through Vdd
> >(or Vtt). Which means it'll have to go through either the supply-ground
> >bypass capacitor or some other way. Either way there must be
> >siginificant inductances in the path (I'm sure I'm missing something,
> >but don't know exactly what) which means termination is not 50ohms, but
> >is 50ohms in series with an inductance (and may be some transmission
> >lines as well if the bypass capacitor is away from the chip). Will there
> >not be siginificant amount of reflections in such a case?
> >
> >Regards
> >Vinayak
> >
> >
> >
> >
> >Vinu Arumugham wrote:
> >
> > > While I am no expert, this looks like an unusual output structure.
> > > You may be familiar with the OIF CEI standard. These standards for
> > > 6/11 Gbps signaling, specify the common mode and differential return
> > > loss that TX and RX devices must meet with respect to ground. Most
> > > high speed differential interconnects at the system level are
> > > referenced only to ground. If your structure is able to meet
> > > specifications such as CEI, including package and package/board
> > > interface effects, they should be fine.
> > >
> > > Thanks,
> > > Vinu
> > >
> > >
> >
> >
> >
> >-- Binary/unsupported file stripped by Ecartis --
> >-- Type: image/jpeg
> >-- File: sata.jpg
> >
> >
> >-- Binary/unsupported file stripped by Ecartis --
> >-- Type: application/pdf
> >-- File: sata.pdf
> >
> >
> >------------------------------------------------------------------
> >To unsubscribe from si-list:
> >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //www.freelists.org/archives/si-list
> >or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> >Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
