====================================================================== IBIS INTERCONNECT TASK GROUP http://www.ibis.org/interconnect_wip/ Mailing list: ibis-interconnect@freelists.org Archives at http://www.freelists.org/archive/ibis-interconn/ ====================================================================== Attendees from March 9 Meeting (* means attended at least using audio) ANSYS Curtis Clark* Cadence Design Systems Bradley Brim* Cisco David Siadat Intel Corp. Michael Mirmak* Keysight Technologies Radek Biernacki* Mentor Graphics Arpad Muranyi* Micron Technology Justin Butterfield*, Randy Wolff* SAE ITC Maureen Lemankiewicz, Logen Johnson Signal Integrity Software Walter Katz*, Mike LaBonte* Teraspeed Labs Bob Ross* University of Aveiro in Portugal Wael Dghais Michael Mirmak convened the meeting. No patents were declared. Michael called for comments on the minutes of both the Feb. 4 and March 2 meetings. No comments were made. Brad Brim moved to approve the minutes of both the Feb. 24 and March 2 meetings. Curtis Clark seconded the motion. The minutes were approved without objection. No opens were raised. Brad summarized offline discussions regarding the shortcut approach. He noted that Walter Katz made several good points regarding mistakes that can be made with a subcircuit wrapper and also a shortcut. Walter agreed with Brad’s summary, stating that both approaches can be abused. Radek Biernacki noted that there may be additional restrictions like global node 0, but in terms in functionality, there should be no difference between a wrapper and the shortcut. Brad replied that, if you use the wrapper, the designer will know exactly how it will be used in the simulator (it’s a netlist). Radek responded that we are trying to control netlisting of the subcircuit. Brad replied that we want to know the correct connections for *every* circuit simulator out there. We put a wrapper around the data when we want to be sure. Brad and Walter agreed that “node” and “terminal” are not synonymous. Next time, Brad will complete his presentation and the team will resolve the shortcut approach. Walter summarized changes he proposed on the reflector to the terminal description in the Interconnect proposal. An S-parameter can be used directly, as a shortcut if the signal_name on the reference terminal shall be a signal_name on any of the port terminals. “Return port” rather than “return terminal” should be used; Eric Bogatin does not like the word “reference”, as opposed to “terminal”. The second rule is that all of the ports shall reference the same signal_name as the reference terminal. These rules are “good” rules for S-parameters generally, but the same errors being checked here could be made within an IBIS-ISS circuit. Arpad Muranyi asked about the phrase, “Terminal N+1 (reference terminal) shall be connected to a Pin with a model_name of POWER or GND”. How can one connect reference of two-terminal differential interconnect? How about 3-terminal power/ground/signal? Walter replied that, if you can’t satisfy the rule, then you cannot use a wrapper. Arpad suggested his second example violates this rule. Radek reviewed his very simple RL example (a series RL path with no reference terminal). If the purpose of a shortcut is to have a single S-element in the specification, rather than in the ISS subcircuit separate file, then having an extra rule is superficial. The N+1 approach makes sense here. If you use only N terminals, you must use global 0 and we don’t want that. Walter replied that the shortcut is a way to address how S-parameters are being used today. Brad used the analogy of a small town in Montana, with a bypass that supports cars but not trucks. Cars can go quickly through, trucks must take a longer route. The solution is by definition and applies to the shortcut; not all circuits can be described through the shortcut. Arpad asked about a 3 pin, 3 pad circuit with separate reference, in an s6p. Walter asked how one would measure this circuit. Where would you put the probes? Brad suggested that one cannot measure a “reference” with respect to global node 0. Your “universe” has 6 nodes, because a reference would use a 7th node. Radek replied that this is a separate issue. If you have 2N terminals, you can have 2N-1 ports, and everything is fine. Bob Ross asked if there is a ground (e.g., for s6p) assumed for S-parameter extraction. Is the ground implied? Walter replied that the problem is the use of words like “ground”; think about the second terminal of probe. Arpad asked if it is possible to remove the phrase “shall be connected to a Pin with a model_name of POWER or GND”. Radek replied that, if Walter’s example is understood, it can be removed. Arpad asked whether this approach is for the EDA vendor, or the model-maker. Radek replied that the model-maker provides all the details. Michael asked if the reference terminal, node, and port clarifications were already done for Touchstone and IBIS-ISS already. Arpad responded that ports are ports in Touchstone; no connectivity is defined. Radek added that IBIS-ISS took its connectivity from commercial SPICE. Arpad asked whether connectivity assumes the same (reference) nodes on both sides. Walter replied that there are two uses of S-parameters. This describes traditional use in signal integrity, not power integrity, where you supply a specific voltage to each buffer. Arpad asked, if we cannot use the same reference on both sides, then how can we instance an S-parameter with one reference? Radek replied that this question is asked all the time, and the 2N reference is allowed but not recommended. Arpad followed up by asking if the 2N reference isn’t recommended, why isn’t ground allowed? Radek replied that one can be limited in available data and be effectively underdetermined. If you use network analyzer, you have test fixtures at opposite ends. Walter added that return terminals get “sucked up” by node zero, and everything is handled properly. Arpad asked whether the “shall be” phrase can removed and the issue left up to the EDA vendor implementing the circuit. Walter and Radek replied that this is up to the model maker, not the EDA vendor. Walter added that accurate power integrity analyses cannot use the N+1 method. Arpad asked whether the number of ports must equal the number of pins and the number of pads. Radek summarized several points, which will be addressed next time in detail: 1) N-port usage is redundant. This has been discussed extensively and is the focus of meeting 2) What is syntax for unconnected terminals? Something unconnected should be open-circuit. The meaning of the termination may change with context. 3) The reference terminal must not be unconnected; the reference must be explicitly identified. Arpad stated that, if “N+1 Terminals” language remains in specification, then he accepts a requirement of connecting POWER/GND as a reference terminal. Bob asked about RLC circuits. Brad replied that converting RLC to S-parameters causes loss of information. Bob suggested that in a circuit with two ports, two terminals plus reference, which side contains the reference? Radek responded that two ports with four terminals is a problem in terms of under-determination. For three terminals, you are covered. Arpad asked for confirmation that a 3-pin, 3-pad package uses 5 port data. Radek confirmed this, stating that one port is the reference. Bob added that one never get 5 ports in practice. Brad asked how many ports one would see: 4, 5, or 6? Brad proposed that the team next time discuss the intent of the shortcut. He suggested that the objective was to automate (the “A” in “EDA” is for automation) so that wrapping the S-parameters in a subcircuit isn’t needed. The shortcut approach will not work without some restrictions. Arpad moved to adjourn. Brad seconded the motion. The meeting adjourned.