Re: XAUI IO specs
> I agree that Ali's example was incorrect, but I am surprised by your
> assertions about Vcm. Vcm is, in my understanding, a part of the
> measurement of a differential signal just as much as Vdm is (since
> I don't have subscripts I'm making cm and dm lower case). It is
> at any given moment the average between Vo+ and Vo-. Thus it is
> a measured value rather than a fixed reference point. It would be
> a constant if you had a perfectly balanced differential output, but
> of course we don't ever have prefect balance so it moves around
> with respect to a reference such as ground.
> Vcm is important to receiver performance so most specs of differential
> signaling do specifiy DC and AC amplitude for it.
I think we are actually in some agreement with each other. In my
statements I tried to emphasize that the Vcm number does exist as
a receiver specification, but not as part of a measurement of the
differential operating output characteristics of a driver.
VCM however does exist as part of the single-ended or common mode
specifications of the driver.
The DC offset between a driver and receiver is a more than real issue
as Ali pointed out. It cannot, however, be used as part of the specification
of the differential output amplitude. These are totally different
numbers and they cannot be mixed in the same breath. The primary
issue I had with Ali's original numbers were:
1. They did not agree with themselves
2. They attempted to define a differential measurement in terms or
two single ended measurments.
I also stated in my response that the presence of a common mode operating
range for the receiver is a requirement. I know the JEDEC spec for
differential HSTL references this, and I suspect the LVDS standard does
So yes, I agree that a VCM does exist, however, I do not agree that
it is part of the measurment or specification of a differential value.
It is a standalone specification in and of itself. A differential
value exists only in terms of signal relative to signal.
> -----Original Message-----
> From: Ed Grivna [mailto:elg@xxxxxxxxxxx]
> Sent: Thursday, April 27, 2000 2:01 PM
> To: stds-802-3-hssg@xxxxxxxx; kdemsky@xxxxxxxxxx; t11_2@xxxxxx;
> jenkins@xxxxxxxx; rtaborek@xxxxxxxxxxxxx; Ali.Ghiasi@xxxxxxxxxxx
> Cc: Ali.Ghiasi@xxxxxxxxxxx
> Subject: Re: XAUI IO specs
> Hi Ali,
> I hate to say it, but your explaination disagrees with itself, which leaves
> me and others just as confused as when you started. Please see my comments
> > Hi
> > Several question have been raised about XAUI "Hari" specifications and
> > what was some of the intentions:
> > o. Based on my understanding and everyone who participated during
> > conference call the Vp-p differential max defined as 800 mV.
> These are differential signals. The only way a differntial signal
> should ever be specified is in terms of signal RELATIVE to signal.
> A Vp-p differential number identifes the p-p amplitude that would
> be seen when probing a differential signal using a differntial
> probe and receiving a pulse sequence containing a mix of logic
> zeros and logic ones. A Vp measurement documents the difference
> in voltage between the true and complement signals when driving
> A logic-1 or A logic-0 (not when driving both).
> > - This means each side of the driver has a swing of 400 mV.
> This statement; i.e., "a swing of 400 mV" means that you are making
> a single-ended measurement. Thats OK, but any single-ended measurement
> must be made from a reference voltage. If done correctly (i.e., with
> a stable reference and clean signaling, the measurement of "swing"
> should be the same regardless of the reference.
> If your output has swing of 400 mV, then the difference in amplitude
> between driving a logic-1 and drving a logic-0 is 400 mV.
> > - Logic 1 would defined as Vo+ at (VCM+400 mV) and Vo- at (VCM-400
> Here you are making a masurement relative to some arbitrary level called
> VCM. In this measurement you state that a when a logic-1 is driven, the
> true output (+) measures 400 mV higher than this arbitrary reference.
> Exactly where this reference comes from I do not know. Since I
> believe you are describing an output you should be able to pick some
> arbitrary reference point for your measurement. I would probably pick
> ground for ease of measurement, though many CML drivers are internally
> refrenced to VCC. Im guessing that if you pick just he right VCM
> value, you are assuming that it will be in the middle of the HIGH and LOW
> measurements for that outout of the differential driver.
> > - Logic 0 would defined as Vo+ at (VCM-400 mV) and Vo- at (VCM+400
> Aha! I was right (actually I peeked ahead). Here you state that when
> driving a logic-0, the true output measures -400 mV relative to this
> arbitrary reference. Now, if this is the case, then "SWING" is the
> between these two. Since one is +400 and the other is -400 (both measured
> on the sme pin relative to this arbitrary reference voltage) then the
> SWING (logic-0 to a logic-1) is 800 mV.
> You also have the exact opposite taking place with the complementary output
> of the driver. It also has a SWING of 800 mV. If you were to then
> probe these two points with a differential probe, you would measure +800 mV
> when driving a logic-1 as shown here.
> Vo+ = VCM+400 mV
> Vo- = VCM-400 mV
> Vdiff = Vo+ - (-Vo)
> = (VCM + 400 mV) - (VCM - 400 mV)
> = VCM + 400 mV - VCM -(-400 mV)
> = VCM - VCM + 400 mV -(-400 mV)
> = 0 + 400 mV + 400 mV
> = 800 mV
> When driving a logic-0, the differential probe would measure a -800 mV.
> Vo+ = VCM-400 mV
> Vo- = VCM+400 mV
> Vdiff = Vo+ - (-Vo)
> = (VCM - 400 mV) - (VCM + 400 mV)
> = VCM - 400 mV - VCM -(+400 mV)
> = VCM - VCM - 400 mV -(+400 mV)
> = 0 - 400 mV - 400 mV
> = -800 mV
> These are the signal amplitudes that you would see on a differential scope
> (or on a single-ended scope using two probes with the scope set to display
> A-B). When driving a logic-1, the scope would show an amplitude of +800 mV,
> and when driving a logic-0 the scope would show an amplitude of -800 mV.
> The Vp-p of this differential measurement is 1600 mV. However, your opening
> paragraph defines this as being only 800 mV p-p differential.
> IT CANNOT BE BOTH.
> > - If the driver output VDp-p <85 mV you got noise.
> > VCM- Voltage Common Mode
> I do not see where this magical VCM number comes from. These are
> signals. In many cases thay cannot be seen from the local noise EXCEPT as
> a differential signal. The only way they can be specified or measured is as
> a differntial signal. This introduction of an undefined an non-measureable
> VCM signal does nothing to improve the measurement, and definately does not
> clarify the issue.
> > VDp-p Voltage Differential p-p
> A measurement of a differential signal when drving only a steady-state
> signal only shows you peak voltage. It does not show p-p. This would
> show as a horizontal line on the scope. A horizontal line shows
> the peak voltage when driving either a logic-1 OR a logic-0. To see
> the p-p you need to see BOTH a logic-1 AND a logic-0 (this requires
> at least one transition in the data stream).
> > o. During the time we defined Hari, we explicitly left out reference to
> > VCM. This was due to interoperablity of different logic voltage levels
> NOT CORRECT. VCM does not belong anywhere with respect to the MEASUREMENT
> of a differential signal. A differential signal can and should only
> be specified as signal relative to signal. The common mode spec you
> are refereing to is a receiver specification indicating just how
> much common mode range (or rejection) that receiver has.
> > and technologies. For example most of the existing SerDes today, it is
> > not possible to connect the driver directly to the same part receiver.
> Sure you can. You just need to do it through DC-blocking caps. These
> caps remove the DC-content (or offset) from the source signal. This
> is exactly what a differntial measurement does; i.e., it subtracts out any
> present offset and only shows how one signal changes relative to another
> > At the same time there are some applications such as large
> > switches which may be very desirable not to use AC-coupling caps.
> You are correct. You also have some PHYs that are powerd by 5V, some at
> 3.3V, some at 2.5V, 2.2V, 2.0V, and hopefully soon 1.8V. Since many
> of these outputs are referenced to VCC (thats how CML works), the DC
> content of the signals will vary as the power supply shifts.
> > In some of these applications you may end up using several thousands of
> > caps on a backplane. In another standard I proposed the followings an is
> > incorporated in to the specifications:
> > - If the transmitter does not provide VCM of 0.75 V, then must add
> AC caps
> > - If the receiver can not tolerate 0.75 V of VCM, then it must add
> AC caps.
> Ali, you're acting like this VCM is some fixed voltage. It is anything but.
> Probe some of these differential signals once, but do it as two
> single-ended measurements. You will see all kinds of hash on the signals.
> If you probe these same signals differentially, most (hopefully) of this
> hash will dissapear.
> All that hash is what the common mode rejection of the receiver is supposed
> to take care of. Each receiver will have a center to its common mode
> operating range. So long as the received signals remain within that range,
> they are received just fine. As soon as they go outside that range, you
> may start to see some nasties. This center-point in the common mode
> operating range is where PHY vendors either provide internal DC-restoration
> or they recommend an external biasing to that same point.
> For most receivers, this point is NOT at 0.75V.
> > Overall the above definition of VCM would not burden the implementation
> > as it is optional and only high applications need it.
> I most heartily beg to differ. The specification of a VCM in reference to
> a differntial signal is an abomination. It totally confuses the entire
> concept of differntial signaling. DIFFERENTIAL SIGNALS ARE NOT MEASURED
> RELATIVE TO A REFERENCE. While you may have some desire to allow a
> DC-connection between transmitters and receivers, please do not push this
> desire into totally unrelated areas such as how to measure or
> define a differential signal. Especially when the explinations do not
> resolve to the common practive of how these signals are specified in
> EITHER NCITS T11 (Vp-p differntial) or IEEE 802.3 (Vp differential)
> > o. I have also had several comments on the drive amplitude. If we decide
> > to define copper twin-ax cable of length 10-15 m, we would need to
> > increase the amplitude somewhat. IB drive amplitude are higher than Hari.
> > One option here might be:
> > - Drive hari levels pre-emphasis Off
> > - Drive IB levels turn pre-emphasis On
> Since your present drive amplitiude numbers basicaly match that of the
> existing T11 Fibre Channel and IEEE 802.3z Gigabit Ethernet specifications,
> and in fact exceed the minimum (1100 mVp-p differential) by 500mV, you
> have already specified a 3.2 dB amplitude increase over the present
> minimum definitions. This additional 3+ dB will give you a significant
> increase in your cable length.
> > I hope this answers some of your question.
> Actually, it doesn't (at least not for me). If possible, could you
> please review my responses and comment on same. The area of
> differential signaling can at times be confusing.
> Ed Grivna
> Cypress Semiconductor
> (952) 851-5046
> > Thanks,
> > Ali Ghiasi
> > Sun Microsystems
> > (650)786-3310