IBIS Macromodel Task Group

Meeting date: 28 September 2021

Members (asterisk for those attending):
Achronix Semiconductor:       Hansel Dsilva
Amazon:                       John Yan
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
Mathworks (SiSoft):         * Walter Katz
                              Mike LaBonte
Micron Technology:          * Randy Wolff
                              Justin Butterfield
Missouri S&T                  Chulsoon Hwang
Siemens EDA (Mentor):       * Arpad Muranyi
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:

- None.

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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 September 21st
meeting.  Randy moved to approve the minutes.  Bob seconded the motion.
There were no objections.

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New Discussion:

PAMn BIRD213.1 draft 6:
Arpad said he had two suggested changes:
1.  In the Definition of the Issue section, the enumerated list of values of n
    should include 4 (i.e., PAM4).  Walter agreed and added 4 to the list.
2.  In the Solution Requirements section, the last bullet refers to "legacy PAM4
    keywords".  Arpad said the term "legacy" is nebulous because it is relative
    to a particular version of IBIS.  Walter agreed and removed the word legacy.
    
Walter noted that once Arpad's BIRD214 (bit_time -> symbol_time) is accepted
into IBIS 7.1, several sections referring to bit_time will be rewritten to use
symbol_time.  Walter suggested we table this BIRD until IBIS 7.1 is approved.
Walter said he would then update the BIRD's language so that it is relative to
IBIS 7.1.

Randy asked if we will have to add something to the descriptions of the existing
PAM4 keywords to acknowledge the overlap with the new PAMn keywords.  Walter
said he did not think this would be a significant effort, and he suggested we
deal with it when the BIRD is updated relative to IBIS 7.1.

GDDR6X update:
Walter shared his GDDR6X presentation and said it contained some of his initial
thoughts as he considered Arpad and Randy's "GDDR6X IBIS Modeling" presentation
from the DesignCon IBIS Summit.

slide 2: GDDR6X Supporters
- Micron
- Nvidia
- Mentor
Arpad noted that the Verilog-A model he had developed for the multi-level Tx
should be recognized by any EDA tool that supports the multi-lingual [External
Model] keyword in IBIS.

slide 3:  What is GDDR6X?
- Memory Interface
- DQ/DQS Clock forwarded
- DQ is single-ended PAM4
- Approx 20 Gbps
- Loss at Nyquist ~10 dB
- Loss at 1/(Rise Time) ~50 dB

Walter said his assumptions about losses were based on some experience with
Ethernet backplanes.  He said at these data rates it's hard to get a channel
that is better than 10 dB, and you can do much worse.  He said for this memory
interface he's guessing 10 dB at Nyquist and about 60 dB at the edge rate.  He
said an 8" trace on FR4 would give you this type of loss.  Walter asked if
anyone had any comments on these rough assumptions, and no one offered any.

slide 4:  What are the issues?
- PAM4 Vertical non-linearities
  - 4 Levels not uniformly spaced
- 12 different transitions have different edge rates and skews
- Drive impedance at the 4 levels may differ

Walter noted that the level spacing non uniformity could be an issue for
differential PAM4 but is likely a bigger issue for single-ended PAM4.

slide 5: Rising and Falling Edge Simulation Results
- Level 0, 2, 3 are uniformly spaced.
- Level 1 is off by about 10 mV

slide 6: SPICE vs AMI Comparison (Ideal Tx)
The slide shows an eye diagram presented at the summit.  Walter noted that with
an ideal AMI Tx you're going to get uniform spacing between the levels.
However, the level 1 from the SPICE simulation is off by about 10mV.  This non-
linearity could be caused by many things.  The AMI simulation results are going
to be optimistic in this case.

slide 7: SPICE vs AMI Comparison (Full swing IBIS buffer Tx model)
This slide from the summit presentation shows better agreement with SPICE,
but there are still issues modeling the non-linearities and transition
timing.

slide 8: SPICE vs multi-edge AMI Comparison (Full swing IBIS buffer Tx model)
This slide from the summit presentation shows better agreement with SPICE in
terms of some edge shapes, but there are still issues modeling transition
timing.

Walter said slides 6 through 8 show that there are issues you can't model
with AMI_Init statistical processing.  You'd be optimistic because of the
non-linearities you can't model, and the only way to deal with it would be
margining (for example, reducing the predicted eye opening by 5 or 10 mV
of noise).

slide 9:  Various level transitions
This slide from the summit presentation shows IBIS Tx buffer rising and falling
transition waveforms extracted from the SPICE model for all 12 possible state
transitions.

Walter wondered how important the edge rate differences in these transitions
would be after you go through a channel that is 60 dB down at the edge rate.
He suggested that instead of extracting the transitions when driving a simple
resistive load (typical IBIS waveform extraction fixture), the SPICE simulation
be redone using 8" of FR4 or some non-low-loss material.  He predicted that the
edges would all look similar at the Rx end of the channel.  Since AMI only cares
about the step response at the receiver, he wondered if the multiple transition
waveforms would add any value for AMI.

slide 10: AMI Statistical Analysis
Walter suggested that the different skews and slew rates for the 12 transitions
could be modeled with statistical AMI Reserved parameters.  In this case, for
example, a 5ps jitter.   Vertical non-linearity would also have to be handled
statistically, in this case a 10mV noise margin.

slide 11: AMI Time domain Analysis
Walter proposed that Tx AMI_GetWave could modify the input stimulus waveform to
account for some of the non-linearities.  The ideal input stimulus waveform
could be modified so that the transition timing and level for each transition
could be modeled.  The transitions' edge timings and levels could be stored as 
tables in the model and utilized for each type of transition.

Walter said he would prefer to avoid the additional complications of introducing
new [Rising Waveform]s and [Falling Waveform]s for all of the transitions.  He
said we could leave it to the Tx AMI model maker to model the relevant effects.

Randy said he was also resistant to the idea of adding new I-V and V-T tables to
the IBIS [Model].  He said doing so might yield a good model for this particular
implementation of a driver with two pullup and two pulldown legs, but it may not
be what other driver architectures would require.

Bob noted that Fangyi, et. al., had submitted a paper on single-ended PAM4 in 
AMI for the upcoming IBIS Summit in Japan. (November 12th)

Arpad asked how the AMI_GetWave approach could capture the interaction between a
driver attempting to drive a certain level into the channel with reflections,
crosstalk, etc.  He said that interaction is lost with an AMI_GetWave approach
with a "hardcoded" output voltage level.  Walter said we are stuck with that
limitation in IBIS today.  We know the impedance of a driver in the high state
and in the low state, but IBIS provides no information about what the impedance
is during a transition between the two.

Walter said he thought the device designers would work very hard to make the
impedances the same at all 4 output levels.  He said this issue isn't a big deal
for differential because you care about the differential impedance, but for
single-ended you could create a PAM4 driver with a huge vertical non-linearity.
He said he expected device makers to work very hard to avoid that.

Walter asked about any rough deadlines for us to deliver a solution.  When will
GDDR6X users need to verify their system designs?  Will we need to update IBIS?
Walter said he hoped the modeling issues could all be addressed with Model
Specific parameters or hard coded in the .dll.  He said we could put the onus
on the model maker to model these effects, and we should start conversations
with EDA vendors, device makers, and end users to find an approach that works
for all of them.

- Walter: Motion to adjourn.
- Randy: Second.
- Arpad: Thank you all for joining.

AR: Walter to send his GDDR6X presentation to the ATM list.

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Next meeting: 05 October 2021 12:00pm PT
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IBIS Interconnect SPICE Wish List:

1) Simulator directives
