Thank you Mike. I will add this to the agenda
for the upcoming IBIS-ATM meeting on Tuesday.
Arpad
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From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx]
On Behalf Of Mirmak, Michael
Sent: Friday, July 17, 2015 12:04 PM
To: IBIS-ATM (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
