Cirrus Logic CRYSTAL LAN CS890, Техническое справочное руководство

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76 AN83REV1
CS8900 Technical Reference Manual
all the register and bit definitions in the
CS8900 are also byte swapped. Infor-
mation that is normally appears at bits
[0:7] will now appear on bits [8:15], and
information that usually appears on bits
[8:15] will now appear on bits [0:7].
5.5 Checklist for signal connections
to the CS8900
Please refer to the datasheet for the
CS8900 for the pin assignment and pin
descriptions of various signals discussed
in this section.
Clock: There are two options for the
clock connection to the CS8900. You
may connect a 20.000 MHz crystal be-
tween XTL1 (pin 97) and XTL2 (pin 98)
pins of the CS8900. Or, if there a 20
MHz clock available in the system, it
can be connected to the XTL1 (pin 97)
pin of the CS8900. It is important that
this clock be TTL or CMOS with 40/60
duty cycle and +/- 50 ppm accuracy.
SBHE* signal: It is recommended that
the CS8900 be used in 16-bit mode.
After a hardware or software reset, the
CS8900 comes up as an 8-bit device. A
transition on SBHE* signal (pin 36)
makes the CS8900 function as a 16-bit
device. After this transition, the SBHE*
can be kept low. For a 16-bit access of
the CS8900, the SBHE* and address
line SA0 (pin 37) must be low. Un-
aligned word accesses to the CS8900 are
not supported. In a system, the SBHE*
line can be connected to address line
SA0. In such a case, after a hardware or
software reset, do a dummy read from an
odd address to provide transition on the
SBHE* line. For memory mode, there is
one more alternative for the SBHE*
connection. For a memory mode opera-
tion, if a CHIPSEL* pin is controlled by
an external chip select, the CHIPSEL*
can be connected to the SBHE*. In this
case, after a hardware and software re-
set, do a dummy access to the CS8900
and ignore data.
5.6 Is the EEPROM necessary?
The CS8900 has an interface for a serial
EEPROM. Most of the networking
applications use this EEPROM to store
IEEE MAC (Media Access Control) ad-
dress. Since the CS8900 supports 1K or
2K bits of EEPROM, the EEPROM is
also used to store information such as
hardware configuration, software driver
configuration, etc. Any location in the
EEPROM can be read or written through
the CS8900.
You will require EEPROM if the IO ad-
dress for the CS8900 has to be other
then 0300h, or the only mode supported
by the CS8900 is memory mode. For all
other cases an EEPROM is optional.
Some of the software drivers supplied by
Crystal Semiconductor assume that there
is an EEPROM connected to the
CS8900.
We recommend that the system store the
individual IEEE MAC address in a non-
volatile memory somewhere in the sys-
tem, and that the end-user of the system
not be allowed to create an arbitrary ad-
dress. In a LAN, the existence of net-
work nodes that use the same MAC ad-
dress will cause severe network prob-
lems including destruction of data and
failure of various network nodes.