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Re: comments on Clause 47


Thank you for the feedback.  However, I don't agree with your
argument that the opportunity for DC coupling should be the 
reason for setting maximum voltages, since XAUI is explicitly 
AC coupled.  Quoting clause 47.3.1, "The XAUI is a low swing 
AC coupled differential interface.  AC coupling allows for 
interoperability between components operating from different 
supply voltages...."  (Your own calculations, below, for 
terminal voltages are correct only for AC coupling.)

I do agree that, for the reasons you list, DC coupling is a
worthy goal.  The problem, as implied by the quote above from 
Draft 2.0, is standardizing the DC voltage for interoperability
of parts.  Also, fixing the maximum voltage does not guarantee 
that a receiver wanting to see, for example, 0.5 volts might be
driven by a transmitter at 1.5 volts.  Of course if a system
developer 'owned' the whole link, and could specify the
components at both ends, nothing would preclude designing a 
DC coupled link, since interoperability would not be required.  
But that is not an issue for the 802.3ae standard, I think.


Tord Haulin wrote:
> Mike, Dawson,
> The proposed limits on XAUI transmitter terminal voltages were chosen
> such that the ESD protections would not be forward biased on a 1.8V
> supply receiver without coupling capacitors. If you plan to use a
> XAUI transmitter with 50ohm resistors to a 3.3V supply and an output
> amplitude of 800mVpp is generated, the nominal transmitter terminal
> voltages will be 3.1V and 2.7V. (2.9V and 2.1V for 1.6V amplitude)
> I don't see the reason for going higher than 3.3V if you use a 3.3V
> + 5% supply and return loss compliant precision 50 ohm resistors.
> Remember that the spec point is meant to apply for a nominal 100 ohm
> test load resistor.
> Because of fewer pins than XGMII, XAUI can be a preferred choice also
> for short on-board links. Then, AC coupling capacitors are an
> un-necessary
> burden creating board layout and transmission problems. Already at
> 10Gbit,
> a 24 port switch can have 768 XAUI links with 1536 capacitors. With some
> foresight, XAUI specifications can be geared towards the anticipated
> trends
> in supply and signal voltages rather than towards older style
> signal-voltages
> that might be used.
> The anticipation when the 0V and 2V limits(-0.3 and 2.3 today) were
> first proposed, was to get feedback from implementers using voltages
> other than today's mainstream technology, such that the limits would
> not be made wider than what the implementations considered today demand.
> Best regards  Tord.
> -----Original Message-----
> From: Kesling, Dawson W [mailto:dawson.w.kesling@xxxxxxxxx]
> Sent: Tuesday, January 02, 2001 19:59
> To: Reflector, Higher Speed Study Group
> Subject: RE: comments on Clause 47
> Mike,
> Some comments on your first point.
> The 2.3V upper limit was reached by compromise in November at Tampa.
> Approval was overwhelming, but not unanimous. Several parties other than
> yourself have verbally suggested the same increase that you are
> suggesting,
> though none have submitted comments. The motivation behind the limit was
> mainly to put some reasonable limit on the DC voltage that the AC
> coupling
> must withstand. Some parties were concerned that a 3.3V limit might rule
> out
> certain AC coupling approaches. The main proponents of a lower number
> have
> since agreed that 3.3V would probably be acceptable. However, there may
> be
> some parties who are designing receivers capable of tolerating the 2.3V
> limit without requiring coupling capacitors; these parties might object
> to
> raising the number to 3.3V. If the goal is to allow 3.3V operation, why
> not
> propose 3.3V + 5% for power supply variation? One comment has been
> submitted
> proposing 3.4V. On the low side, I am not aware of any objections to
> reducing the lower limit below -0.3V. But again why not allow margin for
> termination impedance variations and other factors that could increase
> the
> swing slightly?
> -Dawson
> -----Original Message-----
> From: Mike Jenkins [mailto:jenkins@xxxxxxxx]
> Sent: Thursday, December 28, 2000 1:45 PM
> To: Reflector, Higher Speed Study Group
> Subject: comments on Clause 47
> All,
> I have two comments on the XAUI driver electrical specs in clause 47.
> Ballot comments will be submitted, but I want to air them here on the
> reflector in case I'm missing some piece of history on these issues.
> 1) Section 47.3.3  Driver characteristics:  (Table 47-1)
>         Present maximum driver voltage is 2.3 volts.  I'm unaware
>         of the origin of this number, but it seems to pointlessly(?)
>         exclude devices with higher supply voltages.  I propose
>         this spec be changed to 3.3 volts.
>         Also the minimum driver voltage is listed as -0.3 volts.
>         I propose that this value be changed to -0.4 volts.  The
>         motivation is to not preclude any driver that swings
>         around ground.  The max dif'l swing of 1600 mVp-p implies
>         a max single-ended zero-peak voltage of 400 mV, which
>         would be the max swing below ground.
> 2) Section Driver template:  (Table 47-3)
>         Present values for X1 and X2 are 0.325 and 0.450 UI,
>         respectively.  I understand these are typos and comments
>         have likely been submitted.  I propose (as I assume other
>         comments will) that X1 be 0.175 UI, half the peak-peak TX
>         jitter spec.  Several values have been proposed for X2,
>         including X1+0.19 UI (the Fibre Channel value), as well as
>         a larger value.  I propose X2 = X1+0.205 UI = 0.380 UI.
>         The derivation for this is attached, and is the closest
>         I can come to the original Fibre Channel methodology.
>         (The difference between X2=X1+0.19 and X2=X1+0.205 is
>         about 5 ps, so this is not much of a big deal either way.)
> Thanks in advance to anyone who can provide (helpful) feedback.
> Regards,
> Mike
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  Mike Jenkins               Phone: 408.433.7901            _____
>  LSI Logic Corp, ms/G715      Fax: 408.433.7461        LSI|LOGIC| (R)
>  1525 McCarthy Blvd.       mailto:Jenkins@xxxxxxxx        |     |
>  Milpitas, CA  95035      |_____|
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 Mike Jenkins               Phone: 408.433.7901            _____     
 LSI Logic Corp, ms/G715      Fax: 408.433.7461        LSI|LOGIC| (R)   
 1525 McCarthy Blvd.       mailto:Jenkins@xxxxxxxx        |     |     
 Milpitas, CA  95035      |_____|