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4.3.6. Physical Requirements

4.3.6.1. Routing and Layout Recommendations for Four-Layer Motherboards

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4.3.6.2. Motherboard Impedance

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4.3.7. Connector Pin Assignments

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4.4. Expansion Board Specification

4.4.1. Board Pin Assignment

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NOTE:

If the M66EN pin is implemented, the number of ground pins for a Universal board is 17 and the number of ground pins for a 3.3V board is 21.

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4.4.2. Power Requirements

4.4.2.1. Decoupling

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4.4.2.2. Power Consumption

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35 While the primary goal of the PCI 5V to 3.3V transition strategy is to spare vendors the burden and expense of implementing 3.3V parts that are "5V tolerant," such parts are not excluded. If a PCI component of this type is used on the Universal Board, its I/O buffers may optionally be connected to the 3.3V rail rather than the "I/O" designated power pins; but high clamp diodes must still be connected to the "I/O" designated power pins. (Refer to the last paragraph of Section 4.2.1.2. - "Clamping directly to the 3.3V rail with a simple diode must never be used in the 5V signaling environment.") Since the effective operation of these high clamp diodes may be critical to both signal quality and device reliability, the designer must provide enough extra "I/O" designated power pins on a component to handle the current spikes associated with the 5V maximum AC waveforms (Section 4.2.1.3.).

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