CS8900A
Crystal LAN™ ISA Ethernet Controller
3.11.2 Transmitter
are tested by the squelch circuit. Any signal with
amplitude less than the squelch threshold (either
positive or negative, depending on polarity) is re-
jected.
When configured for 10BASE-T operation,
Manchester encoded data from the ENDEC is fed
into the transmitter’s predistortion circuit where
initial wave shaping and preequalization is per-
formed. The output of the predistortion circuit is
fed into the transmit filter where final wave shaping
occurs and unwanted noise is removed. The signal
then passes to the differential driver where it is am-
plified and driven out of the TXD+/TXD- pins.
3.11.3.2 Extended Range
The CS8900A supports an Extended Range feature
that reduces the 10BASE-T receive squelch thresh-
old by approximately 6 dB. This allows the
CS8900A to operate with 10BASE-T cables that
are longer than 100 meters (100 meters is the max-
imum length specified by the Ethernet standard).
The exact additional distance depends on the qual-
ity of the cable and the amount of electromagnetic
noise in the surrounding environment. To activate
this feature, the host must set the LoRxSquelch bit
(Register 13, LineCTL, Bit E).
In the absence of transmit packets, the transmitter
generates link pulses in accordance with section
14.2.1.1. of the Ethernet standard. Transmitted link
pulses are positive pulses, one bit time wide, typi-
cally generated at a rate of one every 16 ms. The
16 ms timer starts whenever the transmitter com-
pletes an End-of-Frame (EOF) sequence. Thus,
there is a link pulse 16 ms after an EOF unless there
is another transmitted packet. Figure 14 diagrams
the operation of the Link Pulse Generator.
3.11.4 Link Pulse Detection
To prevent disruption of network operation due to
a faulty link segment, the CS8900A continually
monitors the 10BASE-T receive pair (RXD+/
RXD-) for packets and link pulses. After each
packet or link pulse is received, an internal Link-
Loss timer is started. As long as a packet or link
pulse is received before the Link-Loss timer finish-
es (between 25 and 150 ms), the CS8900A main-
tains normal operation. If no receive activity is
detected, the CS8900A disables packet transmis-
sion to prevent "blind" transmissions onto the net-
work (link pulses are still sent while packet
transmission is disabled). To reactivate transmis-
sion, the receiver must detect a single packet (the
packet itself is ignored), or two link pulses separat-
If no link pulses are being received on the receiver,
the 10BASE-T transmitter is internally forced to an
inactive state unless bit DisableLT in register 19
(Test Control register) is set to one.
3.11.3 Receiver
The 10BASE-T receive section consists of the re-
ceive filter, squelch circuit, polarity detection and
correction circuit, and link pulse detector.
3.11.3.1 Squelch Circuit
The 10BASE-T squelch circuit determines when
valid data is present on the RXD+/RXD- pair. In-
coming signals passing through the receive filter
Time
Link
Link
Pulse
Pulse
Packet
Packet
Less Than
16ms
16ms
16ms
Figure 14. Link Pulse Transmission
CIRRUS LOGIC PRODUCT DATA SHEET
36
DS271PP3
CIRRUS [ CIRRUS LOGIC ]
