CS8904
Crystal LAN™ Quad Ethernet Transceiver
3.3.1 10BASE-T Filters
less than the squelch threshold (either positive or
negative, depending on polarity) is rejected.
Each of the CS8904’s 10BASE-T transceivers
include integrated low-pass transmit and receive
filters, eliminating the need for external filters or a
filter/transformer hybrid. On-chip filters are active
(gm/c) implementations of fifth-order Butterworth
low-pass filters. Internal tuning circuits keep the
gm/c ratio tightly controlled, even when large
temperature, supply, and IC process variations
occur. The nominal 3 dB cutoff frequency of the
filters is 16 MHz, and the nominal attenuation at 30
MHz (3rd harmonic) is -27 dB.
3.3.3.2 Auto-Negotiation and Link Pulses
The CS8904 supports Auto-Negotiation, the
mechanism that allows the two devices on either
end of a 10BASE-T link segment to share
information and automatically configure both
devices for maximum performance. The CS8904
Auto-Negotiation capability is fully compliant with
the relevant portions of clause 28 of the Ethernet
standard (ISO/IEC 8802-3:1995(u)).
Auto-Negotiation encapsulates information within
a burst of closely spaced link integrity test pulses,
referred to as a Fast Link Pulse (FLP) Burst. The
FLP Burst consists of a series of link integrity pulses
which form an alternating clock / data sequence.
Extraction of the data bits from the FLP Burst yields
a Link Code Word which identifies the capability of
the remote device. To remain interoperable with
existing 10Base-T devices, the CS8904 also
supports the reception of 10Base-T compliant link
integrity test pulses, referred to as Normal Link
Pulses (NLP).
3.3.2 Transmitter
During transmission, Manchester encoded data
from the ENDEC is fed into the transmitter’s pre-
distortion circuit where initial wave shaping and
pre-equalization is performed. The output of the
pre-distortion 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 amplified and driven
out of the Tx+/Tx- pins.
In the absence of transmit packets, the transmitter
generates Normal Link Pulses (NLP) in accordance
with clause 14.2.1.1 of the Ethernet standard
(ISO/IEC 8802-3:1996).
When configured for Auto-Negotiation, the
CS8904 will detect and automatically operate full-
duplex if the device on the other end of the link
segment also supports full-duplex and Auto-
Negotiation. If the remote device supports Auto-
Negotiation, but only advertises half duplex
capability, the CS8904 will operate half duplex.
If no link pulses or Ethernet frames are being
received on the receiver, the 10BASE-T transmitter
is internally forced to an inactive state.
Once
Auto-Negotiation
has
completed
3.3.3 Receiver
successfully, the CS8904 will send Normal Link
Pulses. The CS8904 Normal Link Pulse operation
is fully compliant with clause 14.2.1.1 of the
Ethernet specification (ISO/IEC 8802-3:1996).
The 10BASE-T receive section consists of the
receive filter, squelch circuit, polarity detection and
correction circuit, and link pulse detector.
3.3.3.1 Squelch Circuit
The CS8904 supports parallel detection. Devices
that respond to the CS8904’s attempt to Auto-
Negotiate with Normal Link Pulses cause the
CS8904 to respond with Normal Link Pulses and to
operate as a 10BASE-T half-duplex device.
The 10BASE-T squelch circuit determines when
valid data is present on the Rx+/Rx- pair. Incoming
signals passing through the receive filter are tested
by the squelch circuit. Any signal with amplitude
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