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The Boundaries of Simulation Space

Although it is certainly possible to acquire models for a thousand nets and simulate every one of them prior to releasing a design to manufacturing, this approach does not improve corporate profitability. One of the more critical system design exercises is establishing the boundaries of simulation space; that is, what criteria determine whether a net needs to be simulated or whether some other method of analysis is more appropriate. Simulation is expensive and should only be used when there are strong economic and technical motivations for doing so. Once you answer this critical question of whether or not to simulate, then you can get about the tasks of actually running the simulations and interpreting their results.

An excellent place to begin the decision-making process is to compile a comprehensive list of all nets in the design and some relevant information associated with each interface or group of nets. The spreadsheet in Table 1.2 forms a skeleton that allows the signal integrity engineer to view the relevant electrical parameters at one glance and make a decision regarding the level of analysis necessary to ensure reliability over the lifetime of the product.

Table 1.2. Analysis Decision Matrix

Parameter

I2C

PCI-X

DDR2

PCIe

Units

Engineer

Net count

Data rate

Gbps

IO power supply voltage

V

IO circuit technology

Input setup time

ps

Input hold time

ps

Input minimum edge rate

V/ns

Input high threshold

V

Input low threshold

V

Output rise time

ps

Output fall time

ps

Output maximum edge rate

V/ns

Output high impedance

Ω

Output low impedance

Ω

Output high level

V

Output low level

V

Pin capacitance

pF

System clock skew and jitter

ps

Net characteristic impedance

Ω

Termination

Ω

Maximum net length

in.

Number of loads

The simplest case might be an interface that a trusted colleague has analyzed in the past and others have used successfully time and again. In this case, no simulation is required—providing that the combination of the electrical parameters is close enough to the one that was analyzed in the past.

In order of increasing complexity, the next case is the interface for which a design guide or specification exists. If a third party analyzed the interface and published a set of rules that, when followed, will guarantee sufficient operating margins, then simulation is not necessary and the job of the signal engineer defaults to describing design constraints to the CAD system and checking that they are met.

Some interfaces may not require simulation but do require rudimentary hand calculation, such as the value of termination resistors, stub length as a function of rise time, or RC time constant of a heavily loaded reset net. In fact, there is a strong case to be made that every interface deserves a basic set of hand calculations since it is possible to run simulations and still not understand the physics!

Finally, if an interface passes through each of the previous three filters, it is time to assemble the models and fire up the simulator. Not surprisingly, the fastest interfaces require the most accurate models and intense analysis effort. Operating margin estimates are another indicator of effort required; the lower the margins, the more effort. The closer the sorting and analysis process occurs to the beginning of the project, the higher the likelihood of success. If the signal integrity engineer and the person drawing the schematics can agree on a naming convention that involves adding a prefix to the net name of each net in an interface, this will facilitate tracking coverage of all nets in a design.

The goal is to evaluate each net in the system and make a data-driven decision about how best to utilize the skills at your disposal.

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