IBIS Macromodel Task Group

Meeting date:15 June 2021

Members (asterisk for those attending):
Achronix Semiconductor        Hansel Dsilva
ANSYS:                      * Curtis Clark
                            * Wei-hsing Huang
Cadence Design Systems:     * Ambrish Varma
                              Ken Willis
                              Jared James
Google:                       Zhiping Yang
Intel:                        Michael Mirmak
                              Kinger Cai
                              Alaeddin Aydiner
Keysight Technologies:      * Fangyi Rao
                            * Radek Biernacki
                              Ming Yan
                              Todd Bermensolo
                            * Rui Yang
Luminous Computing            David Banas
Marvell                       Steve Parker
Micron Technology:          * Randy Wolff
                              Justin Butterfield
Missouri S&T                  Chulsoon Hwang
Siemens EDA (Mentor):       * Arpad Muranyi
SiSoft (Mathworks):         * Walter Katz
                              Mike LaBonte
Teraspeed Labs:             * Bob Ross
Zuken USA:                    Lance Wang

The meeting was led by Arpad Muranyi.  Curtis Clark took the minutes.

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Opens:
  
- None.

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Review of ARs:

- Walter to send out BIRD211.2_draft11 with changes discussed in the meeting.
  - Done.
  
- Walter to send out additional draft(s) after review of Bob's comments.
  - Done.

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Call for patent disclosure:

- None.

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Review of Meeting Minutes:

Arpad asked for any comments or corrections to the minutes of the June 8th
meeting.  Randy moved to approve the minutes.  Walter seconded the motion.
There were no objections.

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New Discussion:

BIRD211.2:
The group reviewed the comments Ambrish had sent in reply to Walter's latest
draft of BIRD211, which was submitted to the Open Forum as BIRD211.2.

Comment 1:
Ambrish asked, if the Init flow is LTI, why do we need both the 'Combined' and
'Upstream' options for Tx_Impulse_Input?

Fangyi said Init can optimize the equalization based on the input impulse
response it receives.  If the value is 'Upstream', then the Tx wants to optimize
based on the upstream channel.  Arpad said the Tx2 Init gets different input
data in the 'Combined' flow vs. the 'Upstream' flow.  Curtis said Ambrish's
"LTI" assumption is not true if the Init function optimizes based on the input
impulse response.

Walter gave an example.  He said a hardware Tx2 might have an AGC that tries to
amplify the input signal to roughly .3V.  The AMI model for such a device would
specify 'Upstream' so it could optimize based on the upstream response.  If
the EDA tool were to pass that model a 'Combined' response that included the
effects of a -20dB downstream channel, then the model would mistakenly choose a
huge gain setting trying to overcome that 20dB downstream channel loss.

Ambrish asked if everyone else in attendance was okay with the 4 different
values for Tx_Impulse_Input.  There were no objections.  No changes to the BIRD
were recommended.

Comment 2:
Ambrish said the parameter definition for Tx_Impulse_Input is written with
respect to repeaters.  He said we need to add language to clarify the situation
for the regular single-channel flow.

Fangyi suggested adding language stating that for the terminal Tx the upstream
cumulative response is the unit impulse response.  Ambrish agreed.  Curtis asked
if Ambrish had found the term "direct Downstream channel" confusing, and he
noted that "Downstream" was not in the Redriver sense of the upstream and
downstream channels.  Ambrish said this text had not caused confusion.

Comment 3:
Ambrish noted that we had explicitly mentioned the possibility of the EDA tool
adding a unit impulse response to the aggressor columns in order to determine
the model's equalization's response.  In addition, if 'Separate' were used for
the terminal Tx then the new 'aggressors + 2' column would also contain a unit
impulse response.  If there are 2 columns with a unit impulse response, which
one should the model modify?

Fangyi agreed it was possible to have two columns containing the unit impulse
response.  Curtis said the model knows which one to modify based on the position
of the column.  The 'aggressors + 2' column is the newly defined column and it
is not modified.  A unit impulse response that is found in the legacy aggressor
columns is modified by the model.  Ambrish said we should add text explicitly
stating that if the EDA tool adds a unit impulse response as an aggressor for
the purpose of determining the filter response, then it must increase the value
of the "aggressors" argument to the AMI_Init function by 1.

Comment 4:
Ambrish asked if step 6b in the time domain reference flow on page 8 had an
issue with double counting the effects of the channel.

Fangyi said no, the "Tx filter impulse response" in 6b does not contain the
channel.  Ambrish asked what it was and how it is determined.  Fangyi said it's
determined by passing the unit impulse response into the model as an aggressor
(as discussed in comment 3 above).  Ambrish said it wasn't immediately clear
what "Tx filter impulse response" meant when he had read it in 6b, though he
acknowledged it was described in an earlier section of the text about passing in
the unit impulse response.  He suggested qualifying that phrase with "as
determined by the EDA tool."

Comment 5:
Ambrish asked if the second channel should consider the upstream IRs in the
Retimer flow.  Fangyi said no, the channels are completely decoupled in the
Retimer flow.  You're resampling in the Retimer and the output passed to the
downstream channel is a digital stimulus.

Comment 6:
Ambrish suggested that the Redriver flow section had too many notes and
repetitive statements.  Fangyi suggested we might change the order and first
introduce the four new flows and then state that the 'Downstream' flow replaces
the original Redriver flow.  Ambrish agreed, and said we shouldn't talk about
a "changed flow" because it causes confusion.  Walter said he had no objection
to changing the order of this section.  Bob said we have to make it clear the
existing flow is being replaced.

Comment 7:
Ambrish said he thought that combining the statistical and time domain flows
into one section caused confusion and unnecessarily tied the two together.
He said in IBIS 7.0 there was a clear separation between the two.  Ambrish also
asked what the new term "initialization flow" meant.  Arpad asked whether the
intent was to contrast it with the statistical flow, where we execute Init and
use the results, and time domain flow, where we execute Init but then don't use
the results directly and instead proceed to time domain.

Walter noted that he had combined the statistical and time domain flows, which
share a common initialization flow.  However, he had separated the Redriver and
Retimer flows, which he thought made them clearer.  He said if someone wanted
to rewrite the BIRD to turn the 3 distinct flows into 6 distinct flows, to
separate statistical and time domain for all three existing flows, he had
no objection.  Fangyi said his BIRD210 had separated them, and his recollection
was that Ambrish had said that wasn't readable either.

Arpad noted that there were many changes proposed, and he asked if this would
require a new BIRD211.3.  If so, we won't be able to vote on it at the next
Open Forum meeting.  Ambrish said he didn't understand the hurry.  Arpad said
this BIRD is all that is holding up IBIS 7.1, and we've been working on it for
years.

Walter moved that ATM recommend that IBIS 7.1 proceed without BIRD211.  Fangyi
seconded.  No one objected.

Bob noted that this means we won't proceed with the scheduled Open Forum vote
on BIRD211.2.  Randy agreed and said he would not send out the email to the Open
Forum asking for votes on BIRD211.2.

Bob said we would still introduce BIRD211.2 at the next Open Forum meeting.
Walter said we could then move to table it there until ATM has had time to
prepare a BIRD211.3.

- Ambrish: Motion to adjourn.
- Fangyi: Second.
- Arpad: Thank you all for joining.

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Next meeting: 22 June 2021 12:00pm PT
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IBIS Interconnect SPICE Wish List:

1) Simulator directives
