All,
It was an editorial oversite that PAM4 Thresholds were not required for
Input buffers, and not allowed for Output buffers. I think that this can
be handled by the Editorial Committee. I will make the following motion
at the next IBIS-ATM meeting:
"IBIS-ATM considers not limiting the PAM4 Thresholds to Rx AMI models an
editorial oversight and requests that the Editorial Committee clarify this
in the IBIS 6.1 specification."
The next problem is how the IBIS Parser shall handle this. Currently, the
AMI Parser does not know if a .ami file is for a Tx or an Rx. This means
that in the currently released parser a .ami model used in a Output model
will not generate an error if it has a Reserved Parameter
Rx_Receiver_Sensitivity, and a .ami model used in a Input model will not
generate an error if it has a Reserved Parameter Tx_Dj.
The approved PAM4 BIRD175.3 states that PAM4_Upper_Threshold and
PAM4_Lower_Threshold are required for an Input .ami model when the model
has a PAM4 Modulation. The IBIS AMI Parser currently does not know if a
.ami file is an Input or an Output, therefor it would not know how to
enforce this rule. I consider it a bug that the IBIS Parser does not use
the information that the .ami file is called from an Input or an Output
[Model]. It should pass that information to the IBIS AMI Parser to check
this rule as well as generating an error when Tx only AMI parameters are
used in Rx AMI models, and Rx only AMI parameters are used in Tx AMI
models.
This is consistent with the BIRD 178 logic that is intended to be in IBIS
7.0.
We use Tx and Rx in the AMI world, the IBIS world uses Input, Output, I/O,
Open and 3-state.
IBIS Model_type AMI Model Type
Input Rx
Input_ECL Rx
Output Tx
3-state Tx
Open_drain Tx
Open_sink Tx
Open_source Tx
Output_ECL Tx
3-state_ECL Tx
I/O Tx and Rx
I/O_open_drain Tx and Rx
I/O_open_sink Tx and Rx
I/O_open_source Tx and Rx
I/O_ECL Tx and Rx
Terminator NA
Series NA
Series_switch NA
IBIS 6.0 does not support AMI models in:
I/O Tx and Rx
I/O_open_drain Tx and Rx
I/O_open_sink Tx and Rx
I/O_open_source Tx and Rx
I/O_ECL Tx and Rx
AMI models are forbidden in:
Terminator NA
Series NA
Series_switch NA
The BIRD 178 logic (most likely will be in IBIS 7.0) essentially says:
if a .ami file is Tx if it is in:
Input Rx
Input_ECL Rx
if a .ami file is Rx if it is in:
Output Tx
3-state Tx
Open_drain Tx
Open_sink Tx
Open_source Tx
Output_ECL Tx
3-state_ECL Tx
if a .ami file is Rx if it is in an Exectuable_Rx subparameter in:
I/O Tx and Rx
I/O_open_drain Tx and Rx
I/O_open_sink Tx and Rx
I/O_open_source Tx and Rx
I/O_ECL Tx and Rx
if a .ami file is Rx if it is in an Exectuable_Tx subparameter in:
I/O Tx and Rx
I/O_open_drain Tx and Rx
I/O_open_sink Tx and Rx
I/O_open_source Tx and Rx
I/O_ECL Tx and Rx
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Mirmak, Michael
Sent: Friday, July 17, 2015 1:04 PM
To: IBIS-ATM (ibis-macro@xxxxxxxxxxxxx) <ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] PAM4 thresholds and directionality
A recent IBIS Editorial Task Group discussion highlighted a potential
ambiguity regarding PAM4 thresholds.
Currently, BIRD175.3 and the draft IBIS 6.1 specification contain the
following language for the PAM4_UpperThreshold, PAM4_CenterThreshold, and
PAM4_LowerThreshold Reserved Parameters:
If the Reserved AMI Parameter Modulation lists "PAM4" (either as a Value
or as a List selection), PAM4_UpperThreshold and PAM4_LowerThreshold are
required.
The surrounding text clearly implies that the Thresholds for PAM4 are
intended for use with input buffers. However, there is no specific
statement of this, and the line above would result in the parser requiring
thresholds for any buffers that use PAM4 modulation, including both TX and
RX buffers.
BIRD178.2 addresses the issue of directionality explicitly, adding a table
that shows the direction associated with the PAM4 Thresholds and other
parameters that would guide the parser as to when the parameter was
appropriate (the BIRD may also be revised to add a direction Descriptor
for each Reserved Parameter). However, BIRD178.2 is not intended for
inclusion in IBIS 6.1. This leaves a point of potential confusion for the
parser, for EDA tools, and for IBIS users.
The Editorial Task Group discussed several options to resolve this,
including:
- Restricting the requirement for the PAM4 thresholds to buffers
of Model_type Input, I/O, I/O_open_sink, etc. (potentially prohibiting
thresholds with Output Model_types)
- Maintaining the requirement for thresholds regardless of
direction, but allowing EDA tools to ignore them for TX buffers
- Adding BIRD178.2 to IBIS 6.1, assuming the BIRD is approved, to
handle directionality for all Reserved Parameters
All of these would require changes to the specification language that are
somewhat beyond mere editorial polishing.
We would like to request that discussion of this issue be added to the
agenda for the next IBIS ATM Task Group meeting. Comments are certainly
welcome beforehand. Thanks in advance.
- MM
