am79c960 Advanced Micro Devices, am79c960 Datasheet - Page 125

• Manufacturer Part Number: am79c960
• Description: Pcnettm-isa Single-chip Ethernet Controller
• Manufacturer: Advanced Micro Devices

Recommendation for Reducing
Noise Injection
DECOUPLING LOW-PASS R/C
FILTER DESIGN
The PCnet-ISA controller is an integrated, single-chip
Ethernet controller, which contains both digital and ana-
log circuitry. The analog circuitry contains a high speed
Phase-Locked Loop (PLL) and Voltage Controlled
Oscillator (VCO). Because of the mixed signal charac-
teristics of this chip, some extra precautions must be
taken into account when designing with this device.
Described in this section is a simple decoupling low-
pass R/C filter that can significantly increase noise im-
munity of the PLL circuit, thus, prevent noise from
disrupting the VCO. Bit error rate, a common measure-
ment of network performance, as a result can be
drastically reduced. In certain cases the bit error rate
can be reduced by orders of magnitude.
Implementation of this filter is not necessary to achieve
a functional product that meets the IEEE 802.3 specifi-
cation and provides adequate performance. However,
this filter will help designers meet those specifications
with more margin.
Digital Decoupling
The DVSS pins that are sinking the most current are
those that provide the ground for the ISA bus output sig-
nals since these outputs require 24 mA drivers. The
DVSS10 and DVSS12 pins provide the ground for the
internal digital logic. In addition, DVSS11 provides
ground for the internal digital and for the Input and
I/O pins.
The CMOS technology used in fabricating the
PCnet-ISA controller employs an n-type substrate. In
this technology, all VDD pins are electrically connected to
each other internally. Hence, in a 4-layer board, when
decoupling between VDD and critical VSS pins, the spe-
cific VDD pin that you connect to is not critical. In fact, the
VDD connection of the decoupling capacitor can be
made directly to the power plane, near the closest VDD
pin to the VSS pin of interest. However, we recommend
that the VSS connection of the decoupling capacitor be
made directly to the VSS pin of interest as shown.
APPENDIX B
Am79C960
AMD recommends that at least one low-frequency bulk
decoupling capacitor be used in the area of the
PCnet-ISA controller. 22 F capacitors have worked
well for this. In addition, a total of 4 or 5 0.1 F capaci-
tors have proven sufficient around the DVSS and DVDD
pins that supply the drivers of the ISA bus output pins.
Analog Decoupling
The most critical pins are the analog supply and ground
pins. All of the analog supply and ground pins are lo-
cated in one corner of the device. Specific requirements
of the analog supply pins are listed below.
AVSS1 and AVDD3
These pins provide the power and ground for the
Twisted Pair and AUI drivers. Hence, they are very
noisy. A dedicated 0.1 F capacitor between these pins
is recommended.
AVSS2 and AVDD2
These pins are the most critical pins on the PCnet-ISA
controller because they provide the power and ground
for the PLL portion of the chip. The VCO portion of the
PLL is sensitive to noise in the 60–200 kHz. range. To
prevent noise in this frequency range from disrupting the
VCO, AMD strongly recommends that the low-pass filter
shown below be implemented on these pins. Tests us-
ing this filter have shown significantly increased noise
immunity and reduced Bit Error Rate (BER) statistics in
designs using the PCnet-ISA controller.
via to VSS plane
via to VDD plane
VDD Pin
VSS Pin
PCnet-ISA
B-1