Digital Signal Processing Reference
In-Depth Information
If a fly-by trace is used to connect a parallel terminator to a load, has that net
been analyzed for coupling and crosstalk?
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Are any vias shared?
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Vias connecting decoupling capacitors should never be shared: each de-
coupling capacitor should be connected to power and ground through
individual vias.
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15.7
Questions to Ask When Reviewing a Termination Strategy
Has termination located on the ASIC die (ODT) been investigated?
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This is generally superior to termination placed on the circuit board.
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It may not allow as much flexibility in make adjustments.
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If a parallel termination strategy is used, in this application is it more cost-
effective to use a resistor pair, or should a Thevenin resistor/supply be used?
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The two-resistor scheme is simple and does not require creating and rout-
ing a separate supply voltage.
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Often the single resistor Thevenin termination can be placed closer to the
load and is especially attractive in large, densely packed designs.
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Does the simulation model account for the proper location of the series ter-
mination resistors?
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The model should include the actual distance from the I/O pin to the termi-
nation resistor, and the impedance of that trace (which is might be higher
than the remainder of the signal trace) should be included.
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15.8
Questions to Ask When Reviewing PCB Layout Design Rules
Have the recommendations of the ASIC manufacturer been incorporated
into the design rules?
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The quality and practicality of ASIC vendor-supplied signal and power
integrity rules vary greatly. Nevertheless, the ASIC manufacturer's goals
should be observed when developing signal integrity rules for an end
product.
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Has an ASIC manufacturer's reference design been studied?
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Valuable insight regarding the unique layout requirements of an ASIC can
be obtained by reviewing the reference design supplied by the manufac-
turer. Especially valuable are stackup and decoupling (or other power sup-
ply filtering) information, but termination and trace impedance factors can
also be discovered.
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15.9
Questions to Ask When Reviewing ASIC Driver Selection Choices
Have drivers with various drive strengths and rise times been tested?
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