Re: [SI-LIST] : High Speed Design by Rules of Thumb vs. Simulation

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From: S. Weir (weirsp@a.crl.com)
Date: Sat Feb 19 2000 - 14:18:45 PST


Abe,

This all sounds like "motherhood and apple pie" to me. Unless I missed
something, it sounds like:

1. We trade economy in the design and validation processes against economy
and risk further down the product life cycle.
2. Success requires care and diligence in the trade-offs, and validation of
the results.

I think those who are successful are those who are either very lucky, or
those who get it and do the job. It's a subset of TQM.

Regards,

Steve.
At 08:37 AM 2/19/2000 -0800, you wrote:

>Dear Scholars:
>
> Before initiating a simulation, it is advisable for a SI engineer to
>devote some thought to the question:
>
> Is simulation necessary for this design?
>
> The following paragraphs explore answers to above and related
>questions.
>
> To my knowledge, high PCB speed designs performed without simulation
>usually rely strongly on rules of thumb. That is, many critical
>considerations such as the optimum trace lengths and separation,
>stackup, routing topologies, decoupling, and termination are decided
>upon by utilizing rules of thumb. These rules may be based on
>designer's past experience or originate from technical publications. For
>instance, there are documents by Micron, ATI, HP and Intel which provide
>detailed guidelines for design of a high speed bus or even a complete
>computer motherboard.
>
> Another approach aimed at eliminating or minimizing the need for
>simulation is "scaling". It allows known results at one frequency to be
>extended to another frequency, provided scaling factor requirements are
>met by certain parameters of the topology traces, drivers and receivers.
>
>
> The prime motivation towards designing by rules of thumb is of course
>to save money, by minimizing the need for SI analysis. However, high
>speed PCB designs without simulation can increase possibility of
>failures due to signal quality problems, or being too conservative (i.e
>excessive termination, stackup, decoupling , etc., than optimally
>needed) thereby greater manufacturing cost. Furthermore, a designer may
>be often forced to make compromises or violate a rule of thumb. For
>instance, the rule that a series terminator should be positioned very
>close to the driver is frequently difficult to fulfill due to the PCB
>space constraints. In general, as edge rates are decreasing and
>solution space narrowing, it is becoming more difficult and risky to
>design merely based on such guidelines.
>
> A powerful approach for appraisal of the need or lack thereof for
>simulation is to compare trace and stub lengths to the critical length
>Lc ( where, Lc = Tr/6D, with Tr representing signal rise time and D
>propagation delay). When segments lengths in a topology are less than
>Lc usually no simulation is required, and vice versa. Nevertheless,
>there are noteworthy exceptions to this rule. As an example, the signal
>behavior for certain cases ( such as a well matched network topology)
>with long traces may be readily predictable without simulation by
>applying the reflection coefficient formulas and associated transmission
>line concepts. On the other hand, a net involving a strong low
>impedance driver and a capacitive load may need to be simulated even for
>trace lengths shorter than Lc.
>
> Speed and accuracy are among desired goals of a SI analysis.
>Creating and maintaining a versatile library of high quality calibrated
>models can aid efficiency. Formulating and writing a detailed plan
>prior to start of a simulation task can also prove beneficial.
>
> In conclusion, high speed digital designs performed solely based on
>rules of thumb (without simulation) are accompanied by increased risk of
>failing due to SI deficiencies, or being too conservative hence more
>costly. Simulation, although not always necessary, can significantly
>enhance the chance of producing an optimally functioning device at the
>first attempt. The critical line length offers a logical means for
>evaluating the need for simulation. To maximize efficiency, it is
>recommended to develop an extensive library of validated models, and to
>contrive an effective plan before launching the simulation.
>
> Your comments are genuinely appreciated.
>
> Respectfully,
>
> Abe Riazi
> SI Engineer
> Anigma, Inc.
> ariazi@anigm.com
> ARIAZI@prodigy.net
>
>
>
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