walter:I do not think ibis should be spending effort trying to legislate
models. Other standards committees already do that. For example tx models for
pci express have required preset taps and the model maker documents them.
From: Walter Katz <wkatz@xxxxxxxxxx>
To: C. Kumar <kumarchi@xxxxxxxxx>; Arpad_Muranyi@xxxxxxxxxx;
ibis@xxxxxxxxxxxxx
Sent: Thursday, June 11, 2015 9:37 PM
Subject: [ibis] Re: Flexibility of a specification
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{}#yiv6981662461 Kumar, What in the spec says that a Tx model make must give
the user the ability to turn the Tx optimization off. Is there a standard way
that a User or an EDA tool turn Tx optimization off? Do you think there should
be a standard way (e.g. a new Reserved Parameter) that can turn Tx optimization
off? Walter
From: "" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "kumarchi@xxxxxxxxx"
for DMARC)
Sent: Thursday, June 11, 2015 8:49 PM
To: wkatz@xxxxxxxxxx; Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Subject: Re: [ibis] Re: Flexibility of a specification The tx model
optimization can be always turned off ; you can do presets or anything you
want including inputting what ever coefficients you want. The eda tool can
automate the process if you wish so I do not know what you are referring to
here From: Walter Katz <wkatz@xxxxxxxxxx>
To: C. Kumar <kumarchi@xxxxxxxxx>; Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Sent: Thursday, June 11, 2015 6:44 PM
Subject: [ibis] Re: Flexibility of a specification Kumar, I am sure there are
cases where the Tx can optimize it’s tap coefficients to open the eye at the
input to the Rx works, and works well. But there are cases were it does not. Do
you not agree that we should be able to run Tx models without letting the Tx
automatically change its tap coefficients so that the user and EDA tool can
have control on what the Tx model does? Saying it another way, do you think an
EDA tool should be able to run simulations where the Tx model does what the Tx
silicon does? Walter From: C. Kumar [mailto:kumarchi@xxxxxxxxx]
Sent: Thursday, June 11, 2015 6:30 PM
To: wkatz@xxxxxxxxxx; Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Subject: Re: [ibis] Re: Flexibility of a specification I have equally data
supporting my view. we have been very successful with that. You cannot go and
make blanket statements. From: Walter Katz <wkatz@xxxxxxxxxx>
To: C. Kumar <kumarchi@xxxxxxxxx>; Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Sent: Thursday, June 11, 2015 6:17 PM
Subject: [ibis] Re: Flexibility of a specification Kumar, Not true. Optimizing
the Tx to open the eye at the input to the Rx is just wrong at date rates above
6 Gbps, and I have the data to prove it. It probably is OK when we first put
AMI together and the state of the art was 3 Gbps and there was just a two tap
Tx and a one tap DFE and very limited CTLE. Walter From: C. Kumar
[mailto:kumarchi@xxxxxxxxx]
Sent: Thursday, June 11, 2015 6:12 PM
To: wkatz@xxxxxxxxxx; Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Subject: Re: [ibis] Re: Flexibility of a specification Walter: Tx optimization
is very useful and in the absense of backchannel support works very nicely with
proper constraints on post cursor. also rx devices are not so brittle that they
need eda tool to figure out their adaptation settings. The devices are very
sophisticate, robust and will not under wide variety of starting points From:
Walter Katz <wkatz@xxxxxxxxxx>
To: Arpad_Muranyi@xxxxxxxxxx; ibis@xxxxxxxxxxxxx
Sent: Thursday, June 11, 2015 5:50 PM
Subject: [ibis] Re: Flexibility of a specification Arpad, You are correct. If
we are not talking about communication from the Rx to the Tx, then the EDA tool
is controlling the configuration of the Tx and the Rx. This is one of the modes
that we want the optimization BIRD to support. Currently EDA tools need to do a
blind search (or we prefer an intelligent blind search) to run the simulations
with a range of Tx tap equalizations. This is why the original concept of the
Tx model optimizing itself was always fundamentally flawed. We cannot do
anything with old models but we could (should) add a reserved parameter to the
Tx so that the Tx DLL can be run without risking the Tx changing its
equalization based on the Impulse Response of the channel that is input to the
Tx (as I proposed in the Redriver Flow clarification BIRD that is currently
tabled in IBIS_AMI. Walter From: ibis-bounce@xxxxxxxxxxxxx
[mailto:ibis-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Thursday, June 11, 2015 5:28 PM
To: ibis@xxxxxxxxxxxxx
Subject: [ibis] Re: Flexibility of a specification Walter, Regarding: “…in no
way precludes the Rx from setting the Tx equalization. Thus the CTLE can be
optimized with whatever Tx equalization the Rx model chooses. I would expect
that an Rx optimization algorithm will set the Tx equalization to off (or the
minimal amount of Tx equalization that will allow the Rx to evaluate an eye),
then optimize its CTLE and DFE, and then make incremental changes to the Tx to
fine tune the channel. So for those of us evaluating the various equalization
messaging methods one would want one that is capable of setting the Tx
equalization to either off, low, or one of a number of presets, and then be
able to change the Tx equalization incrementally.” this sounds like you have
the Backchannel BIRD (147) or its derivativesin mind which is still in the
future (if it will ever happen). I wastalking about the existing spec and
existing models, and this was asimple illustration with one example from the
spec for why I think thespec is inflexible. Aside from that, you missed the
point of the thoughts I had in mindwhen I started this thread. The point I am
trying to make is that wewill continually run into limitations if we use the
spec to describewhat and how people should do. Consider plain old SPICE. Does
theSPICE language tell you that you can only write subcircuits with acertain
predetermined “footprint”? Does the SPICE language tell youthat you can only
simulate the subcircuits you are allowed to write ina certain order? Obviously
not. That’s what I am suggesting we shouldalso do in IBIS-AMI (or IBIS in
general). Give people a language, letthem write any function they want, and
give them a simulator that allowsthem to execute these functions in any order.
Disclaimer: There are my personal views, not my employer’s views. Imay say
different things when I represent my employer’s views. Thanks,
Arpad========================================================================
From: ibis-bounce@xxxxxxxxxxxxx [mailto:ibis-bounce@xxxxxxxxxxxxx] On Behalf Of
Walter Katz
Sent: Thursday, June 11, 2015 4:01 PM
To: ibis@xxxxxxxxxxxxx
Subject: [ibis] FW: Re: Flexibility of a specification All, I have seen 18
e-mails yesterday on IBIS-AMI. They indicate the following things to me:1.
There are many of us (users and EDA vendors) frustrated with the quality of
AMI models delivered by IC Vendorsa. Models are not interoperableb.
Models do not work on all EDA platformsc. Models do not work on all
computer hardwared. Models are delivered DOAe. Models do not comply
with the standard2. There are many users frustrated with the fact that
EDA platforms do not give the same result3. The IBIS spec does not have a
cookbook that tells model makers how to write models4. Some of the
e-mails are written by people who do not understand what AMI modeling is and
what it is not5. … What is IBIS-AMI There is the IBIS 6.0 specification
and a paper we recently gave at DesignCon
http://www.designcon.com/santaclara/scheduler/session/understanding-ibis-ami-simulations.
But let me describe here how simple it is. AMI models are executable code that
links in directly to the EDA simulation tool as a Dynamically7 Loaded Library
(DLL on Windows or a Shared Object on Linux). There are three functions in the
DLL: AMI_Init, AMI_GetWave and AMI_Close. · The AMI_Init function
initializes the function and can output results that an EDA tool can do
statistical analysis. o The input to Tx AMI_Init is an Impulse Response of
the channel and configuration of the model as described in an ASCII .ami file
that is supplied with the DLL. o There is no magic to create an Impulse
Response of the channel. It can be done by simulating a Step Response with the
IBS model for the Tx and Rx and the interconnect (channel) between the Tx and
Rx. The time derivative of this Step Response is the Impulse Response. There
are other methods of generating this Impulse Response in the frequency domain
as well. They should give the identical result (within the numerical accuracy
of each of the methods). This Impulse Response (or a Pulse Response that can be
trivially generated from an Impulse Response) is exactly the input to Stat-Eye,
COM, or any SerDes MatLab based tool.o There is no magic to create the
configuration that controls the operation of the DLL. This is explained in the
IBIS 6.0 spec.o The output of the Tx AMI_Init function is an Impulse Response
modified by the Tx equalization (usually but not necessarily an FFE filter)o
The input to the Rx AMI_Init function is the Impulse Response output of the Tx
AMI_Init function, and the configuration of the Rx model as described in the Rx
.ami file.o Both the Tx and Rx .ami files describe the controls that the user
has in configuring the silicon itself, and should correspond either directly or
with some mapping software into the registers that the hardware used to
configure the silicon.o The output of the Rx AMI_Init function is an Impulse
Response at the Rx latch (decision point) that the EDA tool can use to generate
a Statistical Eye, and information that the EDA tool can use to generate a
Clock PDF function.o The EDA tool is responsible for generating bathtub
curves, BER, and other analysis of the Rx output.· The AMI_GetWave
function is used to do time domain simulations. The input to the Tx AMI_GetWave
function is a stimulus waveform with a well-defined format, the output of the
Tx_GetWave function is a waveform which includes the Tx equalization. This is
convolved with the Impulse Response of the channel to generate the waveform
input to Rx AMI_GetWave. The output of Rx AMI_GetWave is a waveform at the Rx
latch (decision point), and clock times that can be used to sample the
waveform. The EDA tool is responsible for analyzing the Rx outputs.·
The AMI_Close function is used when the simulation is complete to free memory.
This is all there is. The inputs and outputs requirement are precisely defined
by the IBIS 6.0 standard. All is not perfect however. A model can support just
statistical, just time domain or both statistical and time domain processing.
This is particularly problematic when a Tx model does not support statistical.
There are other special cases – like repeater models and retimer models that
are not discussed here. As for comments about the flexibility of the
specification, both SiSoft and Keysight demonstrated prototype implementations
of PAM4 in their AMI simulators at DesignCon. It only took us a couple of weeks
to resolve our differences and write a BIRD (172) to enable PAM4 AMI modeling.
Dotting the I’s and crossing the T’s took another month or two. The resulting
BIRD is ready to be approved at this week’s Open Forum. IBIS would be well
served by approving this BIRD and including it in IBIS 6.1. IBIS should commit
itself to being responsive to the needs of the IC Vendor, User and EDA Vendor
communities. What are the Problems The biggest problem that we see are
non-compliant models. It is easy to write a parser for the .ibs and .ami ASCII
files to insure that they comply to the IBIS specification, it is harder to
test for the actions of the executable to be compliant. Engineers who develop
IP and use tools such as MatLab often do not have the expertise to write
quality C code, they do not necessarily have both 32 bit and 64 bit
environments to build on, and they use tools to develop their models that might
have samples per bit or block size limitations. Writing bug free software is
not easy, but just like there is no excuse for model writers delivering IBIS
files that do not pass the IBIS parser, there is no excuse for AMI model
writers from running their models on Windows and Linux 32 bit and 64 bit
computers exercising every possible value of each of the AMI parameters that
are input to the model, and making sure that the models to not crash, and give
expected results. It is inexcusable for a model writer to deliver a model
without documenting the inputs and outputs and the models limitations. The next
problem that is tarnishing the AMI modeling committee is that EDA Vendor tools
do not use the AMI models correctly. See for example
http://www.designcon.com/santaclara/scheduler/session/ibis-ami-model-simulations-over-six-eda-platforms.
The bottom line is that Users need to reject IC Vendor parts when the IC
Vendor AMI models do not pass a set of usability requirements. Nothing gets an
IC Vendors attention better than the Golden Rule, he who has the Gold Rules.
Both SiSoft and Cadence have supplied the source code for a program that can
exercise an AMI DLL. It is not big deal to write scripts that can exercise a
DLL with a full set of inputs. One Should Not Confuse Analogue Simulation with
AMI Simulation Generating the Impulse Response is a different problem then
generating the AMI simulation. For example, the following statement is
misleading:“A number of folks that I communicate with have expressed great
frustration with the use of IBIS-AMI models in their simulators of choice. Are
there examples and a user guide which go beyond the specification?”One should
not expect or require that a single simulator can both analyze the channel to
generate the Impulse Response and also do the AMI simulation. These are
separate problems. There are simulators that can do both, but there are
simulators out there that do one well and the other either not at all or
poorly. So a SPICE simulator can be IBIS compliant, but not AMI compliant. It
is certainly valid to use that SPICE simulator to generate the Impulse Response
of the channel, but do not expect it to be able to do the AMI simulation.
Optimization Confusion There were several e-mails relating to optimization that
indicate that even us experts do not agree on what the specification says.
E-mails like the following muddle the landscape and need to be called out and
corrected: I would like to give you an example for the inflexibility of the
IBIS-AMIspecification. The words I used before (“canned”) may not have been
thebest choice, but let’s not argue over that. Multiple SERDES experts would
say these days that there are times when itis better to optimize the Rx CTLE to
the channel’s IR without includingthe Tx EQ effects. Since the CTLE is usually
LTI, its algorithm can verywell be placed in the Rx Init function. However, on
the bottom of pg. 172this is what the spec says: “Under certain circumstances,
for example when the Rx AMI_Init function includes an optimizationalgorithm,
the impulse response presented to the Rx AMI_Init function must include the
Txequalization effects for the optimization to work correctly.” This statement
doesn’t seem to allow the CTLE be optimized without the TxEQ’s effects. Do you
consider this to be a flexible specification? Do yousee a way around this
requirement in the DLL’s algorithms? Do you thinkit is OK for EDA vendors to
ignore this statement in the specification andstill be considered IBIS spec
compliant? Although Cadence and SiSoft disagree on the details of how messages
are defined that get passed back and forth between the Tx and Rx model, we both
agree that the Rx model controls the Tx equalization. The statement “the
impulse response presented to the Rx AMI_Init function must include the Tx
equalization effects for the optimization to work correctly” in no way
precludes the Rx from setting the Tx equalization. Thus the CTLE can be
optimized with whatever Tx equalization the Rx model chooses. I would expect
that an Rx optimization algorithm will set the Tx equalization to off (or the
minimal amount of Tx equalization that will allow the Rx to evaluate an eye),
then optimize its CTLE and DFE, and then make incremental changes to the Tx to
fine tune the channel. So for those of us evaluating the various equalization
messaging methods one would want one that is capable of setting the Tx
equalization to either off, low, or one of a number of presets, and then be
able to change the Tx equalization incrementally. 32 Bit VS 64 Bit This is a
potentially difficult computer science problem that is simply solved by the
model maker supplying both 32 bit and 64 bit versions of the model. Advanced
Features – Supporting Innovation Models that support advanced features do not
work on all simulators. This is not a negative – it is a positive! Take PAM4 as
an example. The authors of the PAM4 BIRD (SiSoft, Keysight, Avago and Xilinx)
are developing (and delivering) PAM4 models to companies that are designing
PAM4 systems as we speak. We initially privately, and then publicly in BIRD 172
defined new AMI Reserved Parameters that have enabled IC Vendors to move
forward developing models, enabled EDA tool vendors to move forward enhancing
their tools, and users to evaluate channels and make engineering decisions
today. Until BIRD 172 is approved, and until IBIS 6.1 (or so) is approved, and
until IBIS supplies a parser that supports these new Reserved Parameters, we
are all supporting these parameters in the Model Specific section of a .ami
file. Converting these IBIS 6.0 models to IBIS 6.1 will simply require that
these Model Specific parameters be move to the Reserved Parameter section of
the IBIS file. Other EDA Vendors are free to implement this PAM4 functionality
as they see fit. So I quote “Only the 2nd can be blamed on the IBIS committee
and only if the complaint is that they don’t move fast enough.” Non-Compliant
AMI Models The only solution to deal with model makers who supply models with
fixed samples per bit or fixed block sizes, is for users to not buy those parts
(remember the Golden Rule). If model makers think it is too much of a burden to
write models at any block size and samples per bit, they should submit a BIRD
that makes Samples_Per_Bit and Block_Size reserved parameters, become dues
paying members of IBIS and vote it in the standard. These problems have been
around from the beginning. We even submitted a BIRD to allow a model to declare
Samples_Per_Bit as a reserved parameter that would have required the EDA tool
to do the “Torque Conversion” in the simulator. Enforcing Better Models – AMI
Model Plug Fests! There were statements like “IBIS should require a Model User
Guide”, “IBIS should supply tools that validate models”, … It is easy to say
that every AMI model should have a Model User Guide, and someone can make such
a motion at this Friday’s Open Forum. Is someone going to write a specification
one what should be in an AMI Model User Guide? Can we write a parser to verify
that a Model User Guide is complete and correct? Who is going to spec a tool to
validate a model? More interesting, who is going to write a spec to validate an
AMI EDA Tool? Imagine organizing an IBIS AMI Plug Fest (like the PCIeG3 Plug
Fests). IBIS Cannot Do Anything, But You Can The IBIS organization is most
likely forbidden to name specific IC Vendors, EDA Vendors or User offenders of
the standard in order to protect the innocence of the guilty, and I think this
will prohibit IBIS from effectively addressing these problems. Ultimately it is
the user of IBIS models that need to stand up and do this. I suspect that
someone could start a Blog somewhere and continue this discussion outside of
the IBIS umbrella. Walter Walter Katzwkatz@sisoft.comPhone 303.449-2308Mobile
303.335-6156
