Mike,
The biggest thing for me in [Terminator Model] is that it has this series RC
between the signal and ground which is not available in any other model
types in IBIS. This seems to be the biggest feature that draws people to
want to use it.
I hear you about the desire for simplicity, but I wonder, is an [External
Model] with an IBIS-ISS subcircuit containing that RC circuit (or any other
termination topology for that matter) too complicated? This is available
in IBIS since BIRD160 has been approved (v6.0), and you could use it today
without any modifications to the spec...
Thanks,
Arpad
============================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] ;
On Behalf Of Mirmak, Michael
Sent: Wednesday, June 14, 2017 5:15 PM
To: IBIS-ATM (ibis-macro@xxxxxxxxxxxxx) <ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Terminators, Input models, and BIRD158
IBIS prohibits the Terminator-specific keywords ([Rgnd], [Rpower], [Rac], and
[Cac]) under most other model types, including Model_type Input.
The nominal reason for this is that Terminators are not truly "buffers" and
therefore do not really have input logic thresholds, while Input and related
Model_types do and therefore require Vinh and Vinl.
Bluntly put, this is a rule observed "only in the breach" for quite a few
IBIS-AMI models today. Using a simple pair of keywords to describe a buffer's
impedance is highly convenient, and the original IBIS input logic thresholds
have little applicability to current SerDes buffer designs. The terminator
keywords also have application in single-ended input models.
BIRD158.5 certainly provides a comprehensive solution for complex analog buffer
impedances modeled under IBIS-AMI. However, there will (often?) be cases where
the linearity of a given buffer's impedance, particularly early in the design
cycle, may be assumed and generating a Touchstone file to represent it may be
undesirable. Further, the BIRD158.5 solution is *only* available for IBIS-AMI
models, not standard IBIS models.
Would there be significant opposition to a BIRD relaxing the Input Model_type
keyword rules to permit [Rac], [Cac], [Rgnd] and [Rpower]?
Thanks in advance.
- MM
