Cirrus Logic CRYSTAL LAN CS890, Техническое справочное руководство

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AN83REV1 9
CS8900 Technical Reference Manual
2.0 Hardware Design
This section give design guidance for both
embedded and adapter card designs, includ-
ing recommendations for dealing with the
upper ISA address lines (LA[20:23]),
choosing transformers, and laying out the
board.
2.1 Ethernet Hardware Design for
Embedded Systems and Mother-
boards
This section describes the hardware design
of a four-layer, 10BASE-T solution in-
tended for use on PC motherboards, or in
other embedded applications. The goal of
this design is minimal board space and
minimal material cost. Therefore, a number
of features (BootPROM, AUI, 10BASE-2)
are not supported in this particular PCB
design. An example of this circuit is in-
cluded in this technical reference manual,
and is implemented in an ISA form factor.
This same circuit can be implemented di-
rectly on the processor PCB.
2.1.1 General Description
The small footprint, high performance and
low cost of the CS8900 Ethernet solution,
makes the CS8900 an ideal choice for em-
bedded systems like personal computer
(PC) mother boards. The very high level of
integration in the CS8900 results in a very
low component count Ethernet design.
This makes it possible to have a complete
solution fit in an area of 1.5 square inches.
Since the analog filters are integrated on
the CS8900, card may more easily be made
compliant with FCC part 15 class (B).
2.1.2 Board Design Considerations
2.1.2.1 Crystal Oscillator
The CS8900, in this reference design, uses
a 20.000 MHz crystal oscillator. The crys-
tal has a maximum load capacitance of 18
pF. The rest of the oscillator circuitry is
built internally to the CS8900. Please note
that the crystal must be placed very close to
XTL1 and XTL2 pins of the CS8900.
This crystal oscillator can be eliminated if
accurate clock signal (20.00 MHz +/-
0.01% and 45-55 duty cycle) available in
the system.
2.1.2.2 ISA Bus Interface
The CS8900 has a direct ISA bus interface.
Note that the ISA bus interface is simple
enough to allow the CS8900 to interface
with variety of micro-processors directly or
with the help of simple programmable logic
like a PAL or a GAL.
This reference design actually has the form
factor of an ISA adapter card. In this de-
sign, all the ISA bus connections from the
CS8900 are directly routed to the ISA con-
nector. The pin-out of the CS8900 is such
that if the CS8900 is placed as shown in
Figure 2.1.1, there will be almost no cross-
over of the ISA signals.
2.1.2.3 External Decode Logic
The CS8900 can be accessed in I/O mode
or memory mode. For this reference de-
sign, in memory mode the CS8900 is in the
conventional or upper memory of the PC.