AM79C971VCW Advanced Micro Devices, AM79C971VCW Datasheet - Page 233

• Manufacturer Part Number: AM79C971VCW
• Description: PCnet-FAST Single-Chip Full-Duplex 10/100 Mbps Ethernet Controller for PCI Local Bus
• Manufacturer: Advanced Micro Devices

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 F) decoupling ca-
pacitor be used in the area of the Am79C971 device.
APPENDIX B
C
A
P
Figure 1. Connection Scheme for Capacitors
C
A
P
Am79C971
Best
Am79C971
Not Recommended
Am79C971
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 F 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).
C
A
P
Am79C971
OK
19364D-1
B-1