[ibis-macro] Re: Analog Buffer Model Inside DLL

  • From: James Zhou <james.zhou@xxxxxxxxxx>
  • To: "twesterh@xxxxxxxxxx" <twesterh@xxxxxxxxxx>, "ibis-macro@xxxxxxxxxxxxx" <ibis-macro@xxxxxxxxxxxxx>
  • Date: Fri, 14 Dec 2012 12:31:11 -0800

Hi Todd,

My reading of Greg's original question is that only the on-chip (or possibly 
also on package) analog components would be included in the DLL. In this 
situation, the model creator knows which S-parameter (or any other format 
chosen by model creator) file to use under a given set of AMI parameters. It 
does not need to be passed in the AMI function calls. They are "logistically 
internal" to the model (they are likely to be some external files shipped with 
and managed by the DLL).

Once  the response of these on-chip analog circuitry are included in the DLL, 
they will not be included in the channel impulse response (which might be a 
real scary thing to do). However the S-parameters of the external channel 
network will still be processed by the EDA tool.

As scary as it may sound, this practice actually makes things much easier and 
clear for all parties involved.  It does not invalidate anything in IBIS 5.1, 
it only requires a small but important enhancement in IBIS 5.1 to allow the 
waveform generator (aka AMI  block) to have arbitrary output impedance, most 
likely provided in a snp file (which data is already available in existing 
models)

In IBIS AMI the signal source (aka AMI block) is most often considered as the 
digital block or some sort, but as Walter often says " a nose by any other name 
still smells" , the waveform generator (aka signal source, aka AMI block) could 
be anything as long as it provides an output waveform UNDER A KNOWN LOADING 
CONDITION.

It is a standard practice in network cascading to have arbitrary known loading 
conditions at network interfaces . It is neither necessary nor beneficial to 
impose any restrictions on the input/output impedances of the network 
components.

My recommendations are the following:
(1) REMOVE THE RESTRICTIONS ON THE OUTPUT IMPEDANCE OF THE WAVEFORM GENERATOR 
(aka the DLL block) , AND ALLOW IT TO HAVE ANY KNOWN OUTPUT IMPEDANCE SUPPLIED 
IN A SNP FILE.
(2) REMOVE THE RESTRICTIONS ON THE INPUT IMPEDANCE OF THE ANALOG CHANNEL (this 
restriction is caused by the isolation requirement of IBIS 5.1, which should be 
lifted in future releases)

On isolation:
Some postings seem to suggest that the signal source (aka AMI block) and the 
analog channel must be isolated in order to correctly account for the impedance 
conditions. It's worth noting that isolation is an exceptional and very 
restrictive condition which is not necessary at all. By lifting this 
restriction from IBIS 5.1, things will become much clear and easier to 
understand with little or no burden on implementation.  Detailed technical 
information can be provided if there is any interest to pursue this further.

James Zhou


From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of Todd Westerhoff
Sent: Friday, December 14, 2012 5:23 AM
To: ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL

James,

Maybe I’m missing something ... how would the algorithmic model get the 
S-parameters for the channel network to be cascaded, given that only an impulse 
response is passed in?

Todd.


Todd Westerhoff
VP, Software Products
Signal Integrity Software Inc. • www.sisoft.com
6 Clock Tower Place • Suite 250 • Maynard, MA 01754
(978) 461-0449 x24  •  twesterh@xxxxxxxxxx

“I want to live like that”
                                             -Sidewalk Prophets

From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of James Zhou
Sent: Thursday, December 13, 2012 6:28 PM
To: michael.mirmak@xxxxxxxxx; gedlund@xxxxxxxxxx; mike@xxxxxxxxxxx; 
ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL

Considering Greg's original question (put analog buffer inside dll), even 
though IBIS 5.1 does not support this practice, technically it is a network 
cascade problem that had been solved time ago in network theory using 
S-parameters.

Although "nasty things" could happen if someone "puts the analog buffer model 
inside the DLL", it can be easily avoided by the time-tested S-parameter 
network cascading formula, in which case the impulse response CAN also be made 
correct.

Many of the IBIS AMI models  I have seen so far treat the AMI-to-analog 
interface as perfectly matched  (which is referred to as "isolated" in some 
communications). This makes the network cascading problem easier. However this 
is only a special case of a more general problem of network cascading with 
mismatch, which solutions are readily available and can be derived easily using 
S-parameter theory.

I have recently run into a case where the AMI channel simulation generated 
incorrect amplitude. The EDA tool vendor  contributed the cause to 
misinterpretation of AMI to Tx analog interface impedance in the model. From 
users point of view, this kind of problems can be very difficult and 
time-consuming to debug. It would be very helpful to further clarify these 
issues in the specification by some working examples as Michael has suggested.

James Zhou



From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx> 
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Mirmak, Michael
Sent: Thursday, December 13, 2012 11:51 AM
To: gedlund@xxxxxxxxxx<mailto:gedlund@xxxxxxxxxx>; 
mike@xxxxxxxxxxx<mailto:mike@xxxxxxxxxxx>; 
ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL

(putting on my user hat)

I would have to agree with Greg here.  We are seeing cases where the 
relationship between the data in the traditional IBIS [Model] – including 
C_comp, I-V tables and V-t tables – and the algorithmic model – the .ami file 
and accompanying DLL -- isn’t consistently defined or implemented.

It’s understandable that complex, non-LTI behaviors in the traditional [Model] 
data would be undesirable to include alongside an algorithmic model (for 
example, a non-linear I-V curve set or, if traditional IBIS supported it, a 
complex, voltage-dependent buffer capacitance representation).  However, the 
discussion here implies that realistic, simple traditional [Model] data should 
always be present, not zeroed out as Greg is seeing.

If the language in the specification says the data is “used” for analog channel 
characterization, perhaps that language isn’t strong enough to ensure that both 
the right data and the right processing algorithms are used to combine them?

Alternately, could we define some simple tests of analog and algorithmic model 
interaction to show that “everything’s working OK?”  For example, in 
traditional IBIS, one of the key tests we have used to show how C_comp 
functions is to ask a user to:

-          Simulate an IBIS driver into a resistive load

-          Simulate the same driver into a lengthy (hopefully mismatched) 
transmission line

-          Alter the C_comp (even setting it to zero) in the driver model, 
re-run both tests above, and observe the behavior

We would expect to see the resistive load cases not differ much at all, due to 
the way C_comp double-counting is avoided in most tools.  However, there should 
be a fairly big difference in the t-line cases due to reflections interacting 
differently with the changed C_comp.

Do we have, or can we produce, a set of similar tests that would demonstrate 
that a traditional + algorithmic model set is working correctly, together, in a 
given simulation environment?


-          MM

From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx> 
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Gregory R Edlund
Sent: Thursday, December 13, 2012 9:56 AM
To: mike@xxxxxxxxxxx<mailto:mike@xxxxxxxxxxx>; 
ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL


Mike & Others,

I didn't mean to propose any changes to the existing IBIS 5.1 flow.  I was 
responding to a question that arose within the company about the validity of an 
AMI model that has no analog buffer model in the .ibs file (empty tables and 
zero C_comp).  The vendor claims that information is stored in the DLL.  Maybe 
it is, but the model is not IBIS 5.1 compliant nor will it produce a physical 
simulation with any simulator on the market.

I think we need to focus our efforts on building momentum behind IBIS 5.1, and 
that means building confidence among the user community by establishing a base 
of high-quality models.

Greg Edlund
Senior Engineer
Signal Integrity and System Timing
IBM Systems & Technology Group
3605 Hwy. 52 N  Bldg 050-3
Rochester, MN 55901



[cid:image001.gif@01CDD9EA.6E1C3750]Mike LaBonte ---12/12/2012 04:48:48 PM---I 
understood Greg's proposal as simply implementing "forward" simulation of 
analog effects within a

From: Mike LaBonte <mike@xxxxxxxxxxx<mailto:mike@xxxxxxxxxxx>>
To: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>>
Cc: "ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>" 
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>
Date: 12/12/2012 04:48 PM
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL
Sent by: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>

________________________________



I understood Greg's proposal as simply implementing "forward" simulation of 
analog effects within a DLL, such that it only affects the output of each 
stage, no chance to create or respond to backward signals. It's easy to see 
that this is going to be a problem. Vladimir's proposal amounts to embedding 
s-parameter models within DLLs, to be selected and either passed to the EDA 
tool or combined with the channel response right within the DLL. These are 
completely different proposals.

The latter idea could be seen as a combination of Fangyi's proposal that the 
DLL be able to make choices based on inputs then communicate them back out, and 
the idea that the analog model should be s-parameters, resulting in the ability 
to simply embed the s-parameter data sets in the DLL, which communicates the 
correct set back out based on settings. One observation is that we now seem to 
have more proposals putting the analog model in the AMI model than proposals 
keeping it in the legacy IBIS domain.

Mike


On Wed, Dec 12, 2012 at 5:14 PM, Muranyi, Arpad 
<Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>> wrote:
Todd,



The original question from Greg was:



“What nasty things are likely to happen if someone puts the analog buffer model 
inside the DLL?

At the very least, the impulse response will be incorrect.  Are there any 
circumstances under

which this can work correctly?



This spawned a bunch of replies ranging from “why not”

to “impossible”, mostly on theoretical bases.  You

may be right that this may not be practical at the

moment, but that’s not what the question was about.



Whether it is worth solving or not could be discussed

in a separate thread.  Who knows, we might find out

that it is worth considering it…



Thanks,



Arpad

=======================================================





From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>]
Sent: Wednesday, December 12, 2012 4:07 PM

To: Muranyi, Arpad
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: Re: [ibis-macro] Re: Analog Buffer Model Inside DLL



... and revisiting one of the fundamental assumptions on which all of IBIS-AMI 
is based.



Which, based on past experience, is expensive, both in terms of time and effort.



I therefore assert that this is a problem that isn't worth solving.



Todd.





--



Todd Westerhoff

VP, Software Products

SiSoft

6 Clock Tower Place, Suite 250

Maynard, MA 01754

(978) 461-0449 x24<tel:%28978%29%20461-0449%20x24>

twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>

www.sisoft.com<http://www.sisoft.com/>





“I want to live like that"

                                             -Sidewalk Prophets



On Dec 12, 2012, at 5:01 PM, "Muranyi, Arpad" 
<Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>> wrote:
Todd,



I did not suggest that this was possible under the

umbrella of the current v5.1 IBIS specification.



In one of my responses I stated:



“We would also need to revisit the AMI flow a

little bit, but that’s just detail.”



which implies a spec change, as far as I can tell…



Thanks,



Arpad

===================================================



From: Todd Westerhoff [mailto:twesterh@xxxxxxxxxx]
Sent: Wednesday, December 12, 2012 3:56 PM
To: Muranyi, Arpad
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: Re: [ibis-macro] Re: Analog Buffer Model Inside DLL



Arpad,



We have a published spec that says the analog model is to be included in the 
calculation of that channel impulse response.



Anything else is not compliant.



Todd.



--



Todd Westerhoff

VP, Software Products

SiSoft

6 Clock Tower Place, Suite 250

Maynard, MA 01754

(978) 461-0449 x24<tel:%28978%29%20461-0449%20x24>

twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>

www.sisoft.com<http://www.sisoft.com/>





“I want to live like that"

                                             -Sidewalk Prophets



On Dec 12, 2012, at 4:21 PM, "Muranyi, Arpad" 
<Arpad_Muranyi@xxxxxxxxxx<mailto:Arpad_Muranyi@xxxxxxxxxx>> wrote:
Todd,



This is not super-theoretical-science in the clouds.



It can be done just as practically and “easily” as the

EQ, DFE and CDR AMI algorithms can be done in the DLL-s.

It involves the same kind of algorithm knowledge, and

programming skill.



The question is, do we want to let model makers to do

this, and possibly get a bunch of bad models from

inexperienced people, or should we, the EDA vendors

still have the opportunity to sell our expertise and

provide higher quality and more reliable solutions to

our customers and AMI model users.



Thanks,



Arpad

==========================================================



From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx> 
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Todd Westerhoff
Sent: Wednesday, December 12, 2012 2:54 PM
To: Dmitriev-Zdorov, Vladimir; 'Gregory R Edlund'
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL



Vladimir,



Not clear to me how you propose to handle the reflections associated with 
discontinuities at the point where the TX and RX analog circuits interface to 
the channel.  More importantly, even if it’s theoretically possible, that 
doesn’t make it practical.



I’ll admit I’m guessing here, but I expect Greg wants to solve a problem, not 
just establish that it should be possible to solve it.



Todd.



Todd Westerhoff

VP, Software Products

Signal Integrity Software Inc. • www.sisoft.com<http://www.sisoft.com/>

6 Clock Tower Place • Suite 250 • Maynard, MA 01754

(978) 461-0449 x24<tel:%28978%29%20461-0449%20x24>  •  
twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>



“I want to live like that”

                                             -Sidewalk Prophets



From: Dmitriev-Zdorov, Vladimir [mailto:vladimir_dmitriev-zdorov@xxxxxxxxxx]
Sent: Wednesday, December 12, 2012 2:39 PM
To: twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>; 'Gregory R Edlund'
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: RE: [ibis-macro] Re: Analog Buffer Model Inside DLL



In an abstract/theoretical way, it is still possible that AMI DLL correctly 
takes care of the “impulse response” by adding its internal model to it. Then 
however it should not be an ‘impulse response’, but 2- or 4-port S-parameters 
representing the core portion of the channel, which does not include analog 
models. Each model then can ‘append’ its analog part to the S-parameters, and 
restore the resulting impulse response, if needed for equalization. Instead of 
returning the updated impulse response, the Init function (or how we call it) 
will return the updated touchstone file, which then is passed to the Rx model, 
with the same purpose.



The objection here is that Tx must have the complete channel info, with Rx 
analog model,  before its Init function can start thinking about equalization, 
but then ‘appending’ analog models could be either separated from Init, and 
organized as one more function, possibly combined with what Fangyi proposed 
about resolving dependences, or we could still do everything in just one 
function, but perform a few cycles of Initialization, for example: (Tx_Init(), 
Rx_Init()), (Tx_Init(), Rx_Init()) … which resembles “backchannel” 
communication on Init() stage.



Of course, the writer of the AMI model must be able to do some operations with 
touchstone files, such as appending the model to it, and converting it into 
transfer function, finding impulse response by IFFT, etc.



From: ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx> 
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Todd Westerhoff
Sent: Wednesday, December 12, 2012 11:51 AM
To: 'Gregory R Edlund'
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: Analog Buffer Model Inside DLL



Greg,



Ask yourself how the person writing an algorithmic model should accurately 
model the reflections associated with an unspecified channel.  If there’s a way 
to do that, I’d like to hear about it.



Todd.



Todd Westerhoff

VP, Software Products

Signal Integrity Software Inc. • www.sisoft.com<http://www.sisoft.com/>

6 Clock Tower Place • Suite 250 • Maynard, MA 01754

(978) 461-0449 x24<tel:%28978%29%20461-0449%20x24>  •  
twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>



“I want to live like that”

                                             -Sidewalk Prophets



From: Gregory R Edlund [mailto:gedlund@xxxxxxxxxx]
Sent: Wednesday, December 12, 2012 1:41 PM
To: Todd Westerhoff
Cc: ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>
Subject: Re: [ibis-macro] Analog Buffer Model Inside DLL



Todd,

Thanks for the response.

So, there are no "mathematical tricks" one can play in the DLL to account for 
the absence of the analog buffer model in the impulse response?  You can tell I 
haven't taken enough time to think this through all the way.  I'm having a 
knee-jerk reaction to a discussion that's going on internally.  8-)  I'm about 
to dig into the IBIS 5.1 flow material to support my position.  I just wanted 
to make sure I had my ducks in a row and get some outside corroboration.

Anyone else care to chime in?

Greg Edlund
Senior Engineer
Signal Integrity and System Timing
IBM Systems & Technology Group
3605 Hwy. 52 N  Bldg 050-3
Rochester, MN 55901



<image001.gif>Todd Westerhoff ---12/12/2012 12:31:11 PM---Greg, That is not 
possible. The analog model, by definition, interacts with the channel and must 
be

From: Todd Westerhoff <twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>>
To: Gregory R Edlund/Rochester/IBM@IBMUS
Cc: "ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>" 
<ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>
Date: 12/12/2012 12:31 PM
Subject: Re: [ibis-macro] Analog Buffer Model Inside DLL

________________________________



Greg,

That is not possible. The analog model, by definition, interacts with the 
channel and must be included in the impulse response. The equalization, also by 
definition, is considered to be electrically isolated from the channel and is 
thus represented in the DLL.

Putting the analog model in the DLL violates a fundamental assumption of 
IBIS-AMI. You may get good-looking results, but they will be invalid.

Todd.


--

Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24<tel:%28978%29%20461-0449%20x24>
twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx>
www.sisoft.com<http://www.sisoft.com/>


“I want to live like that"
                                             -Sidewalk Prophets


On Dec 12, 2012, at 1:13 PM, Gregory R Edlund 
<gedlund@xxxxxxxxxx<mailto:gedlund@xxxxxxxxxx>> wrote:


What nasty things are likely to happen if someone puts the analog buffer model 
inside the DLL?  At the very least, the impulse response will be incorrect.

Are there any circumstances under which this can work correctly?

Greg Edlund
Senior Engineer
Signal Integrity and System Timing
IBM Systems & Technology Group
3605 Hwy. 52 N  Bldg 050-3
Rochester, MN 55901

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