[ibis] FW: Re: Flexibility of a specification

  • From: Walter Katz <wkatz@xxxxxxxxxx>
  • To: <ibis@xxxxxxxxxxxxx>
  • Date: Thu, 11 Jun 2015 17:01:01 -0400 (EDT)

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 Vendors

a. Models are not interoperable

b. Models do not work on all EDA platforms

c. Models do not work on all computer hardware

d. Models are delivered DOA

e. Models do not comply with the standard

2. There are many users frustrated with the fact that EDA platforms
do not give the same result

3. The IBIS spec does not have a cookbook that tells model makers
how to write models

4. Some of the e-mails are written by people who do not understand
what AMI modeling is and what it is not

5. .





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-a
mi-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-simul
ations-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-AMI

specification. The words I used before ("canned") may not have been the

best choice, but let's not argue over that.



Multiple SERDES experts would say these days that there are times when it

is better to optimize the Rx CTLE to the channel's IR without including

the Tx EQ effects. Since the CTLE is usually LTI, its algorithm can very

well be placed in the Rx Init function. However, on the bottom of pg. 172

this is what the spec says:



"Under certain circumstances, for example when the Rx AMI_Init function
includes an optimization

algorithm, the impulse response presented to the Rx AMI_Init function must
include the Tx

equalization effects for the optimization to work correctly."



This statement doesn't seem to allow the CTLE be optimized without the Tx

EQ's effects. Do you consider this to be a flexible specification? Do
you

see a way around this requirement in the DLL's algorithms? Do you think

it is OK for EDA vendors to ignore this statement in the specification and

still 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 Katz

<mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx

Phone 303.449-2308

Mobile 303.335-6156



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