Cirrus Logic AN83 - Manual (Page 35)

AN83
AN83REV3 35
Layout Considerations for the CS8900A
The CS8900A is a mixed signal device having dig-
ital and analog circuits for an Ethernet communica-
tion. While doing the PCB layout and signal
connections, it is important to take the following
precautions:
- Provide a low inductive path to reduce power
and ground connection noise.
- Provide proper impedance matching especially
to the Ethernet analog signals.
- Provide low inductive path, wider and short
traces, for all analog signals.
It is important that a PCB designer follow sugges-
tions made in this document for proper and reliable
operation of the CS8900A. These guidelines will
also benefit the design with good EMI test results.
General Guidelines
Figure 24 shows component placement for an ISA
COMBO Ethernet adapter card using a CS8900A.
The placement of the CS8900A should be such that
the routes of the analog signals and the digital sig-
nals are not intermixing. No signal should route
beneath the CS8900A on any plane.
Power Supply Connections
The CS8900A has 3 analog and 4 digital power pin
pairs (Vcc and GND). Additional ground connec-
tions are provided. Each power pin pair should be
connected to a 0.1 µF bypass capacitor. Connect
the extra ground pins directly the ground plane.
Two Layered Printed Circuit Board (PCB)
A two layered PCB has signal traces on the compo-
nent and solder side of the PCB. Fill unused areas
with copper planes. Typically, planes on the com-
ponent side of the PCB are connected to ground
and those on the solder side are connected to VCC
or +5 volts.
Provide each pair of power pin with a 0.1 µF bypass
capacitor. Place each bypass capacitor as close as
possible to the corresponding power pin pair. Con-
nect the capacitor to the pads of the power pins by
short, wide traces, the other end of these traces
should be connected to VCC and GND planes. Fig-
ure 19 and Figure 20 illustrate ground and power
(Vcc) plane connections, respectively.
Multi-layered Printed Circuit Board
A multi-layered printed circuit board (PCB) typi-
cally has separate ground and power (Vcc) planes.
Multi-layered PCBs are required when the compo-
nent and trace density is high. Often discrete com-
ponents like resistors and capacitors are placed on
the solder side of a printed circuit board.
For a multi-layer PCB with all components on one
side of the board, follow the power connection
guide lines as explained in “Two Layered Printed
Circuit Board (PCB)” on page 35. Instead of con-
necting the ground and Vcc to the copper fills on
the component and solder side of the board, con-
nect them to the internal ground and Vcc planes.
Figures 27 through 30 show the four layers of the
four-layer card.
For a multi-layered board the discrete components
are to be placed on the solder side of the PCB, by-
pass capacitors for the CS8900A can be placed on
the solder side of the PCB. Each bypass capacitor
should be placed beneath the CS8900A and closest
to its corresponding power pin pair. Figures 31
and 32 illustrate the placement and routing of one
bypass capacitor.
Routing of the Digital Signals
Most of the digital signals from the CS8900A go to
the ISA bus connector. Route these signals directly
to the connector. Isolate the digital signals from
analog signals.
Routing of the Analog Signals
Routing of the clock signals: Place the
20.000 MHz crystal within one inch of XTL1 (pin
#97) and XTL2 (pin #98) pins of the CS8900A.