Intel 41110 manual Power Distribution and Decoupling, Trace Impedance

General Routing Guidelines

6.4Power Distribution and Decoupling

Have ample decoupling to ground, for the power planes, to minimize the effects of the switching currents. Three types of decoupling are: the bulk, the high-frequency ceramic, and the inter-plane capacitors.

•Bulk capacitance consist of electrolytic or tantalum capacitors. These capacitors supply large reservoirs of charge, but they are useful only at lower frequencies due to lead inductance effects. The bulk capacitors can be located anywhere on the board.

•For fast switching currents, high-frequency low-inductance capacitors are most effective. Place these capacitors as close to the device being decoupled as possible. This minimizes the parasitic resistance and inductance associated with board traces and vias.

•Use an inter-plane capacitor between power and ground planes to reduce the effective plane impedance at high frequencies. The general guideline for placing capacitors is to place high- frequency ceramic capacitors as close as possible to the module.

6.4.1Decoupling

Inadequate high-frequency decoupling results in intermittent and unreliable behavior.

A general guideline recommends that you use the largest easily available capacitor in the lowest inductance package. For specific decoupling requirements for a 41110 application please refer to Chapter 4.

6.5Trace Impedance

All signal layers require controlled impedance 60 Ω +/- 15%, microstrip or stripline for add-in card applications. Selecting the appropriate board stack-up to minimize impedance variations is very important. When calculating flight times, it is important to consider the minimum and maximum trace impedance based on the switching neighboring traces. Use wider spaces between traces, since this can minimize trace-to-trace coupling, and reduce cross talk.

When a different stack up is used the trace widths must be adjusted appropriately. When wider traces are used, the trace spacing must be adjusted accordingly (linearly).

It is highly recommended that a 2D Field Solver be used to design the high-speed traces. The following Impedance Calculator URL provide approximations for the trace impedance of various topologies. They may be used to generate the starting point for a full 2D Field solver.

http://emclab.umr.edu/pcbtlc/

The following website link provides a useful basic guideline for calculating trace parameters:

http://www.ultracad.com/calc.htm

Intel® 41110 Serial to Parallel PCI Bridge Design Guide 25