AM79C971VCW datasheet(233/265 Pages) AMD

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APPENDIX B

Am79C971

B-1

Recommendation for Power and Ground

Decoupling

The mixed analog/digital circuitry in the Am79C971 de-

vice makes it imperative to provide noise-free power

and ground connections to the device. Without clean

power and ground connections, a design may suffer

from high bit error rates or may not function at all.

Hence, it is highly recommended that the guidelines

presented here are followed to ensure a reliable de-

sign.

Decoupling/Bypass Capacitors

Adequate decoupling of the power and ground pins and

planes is required by all Am79C971 designs. This in-

cludes both low-frequency bulk capacitors and high fre-

quency capacitors. It is recommended that at least

one low-frequency bulk (e.g., 22

µF) decoupling ca-

pacitor be used in the area of the Am79C971 device.

The bulk capacitor(s) should be connected directly to

the power and ground planes. In addition, at least

eight high frequency decoupling capacitors (e.g.,

0.1

µF multilayer ceramic capacitors of dielectric type

XDR) should be used around the periphery of the

PCnet-PCI II device to prevent power and ground

bounce from affecting device operation. To reduce the

inductance between the power and ground pins and

the capacitor, the pins should be connected directly to

the capacitors, rather than through the planes to the ca-

pacitors. The suggested connection scheme for the ca-

pacitors is shown in the figure below. Note also that the

traces connecting these pins to the capacitors should

be as wide as possible to reduce inductance (15 mils is

desirable).

Figure 1.

Connection Scheme for Capacitors

Am79C971

Best

Not Recommended

19364D-1

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