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
