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CDK8904-5

Crystal LAN™ Quad Ethernet Transceiver

Cirrus Logic 

CS8904

Crystal LAN™ Quad Ethernet Transceiver

3.3.1 10BASE-T Filters

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.

3.3.2 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).

If no link pulses or Ethernet frames are being

received on the receiver, the 10BASE-T transmitter

is internally forced to an inactive state.

3.3.3 Receiver

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 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

less than the squelch threshold (either positive or

negative, depending on polarity) is rejected.

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).

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.

Once Auto-Negotiation has completed

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 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.

CIRRUS LOGIC ADVANCED PRODUCT DATABOOK

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DS191PP2

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