CS8920A datasheet(35/144 Pages) CIRRUS

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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.

Transmitter

When configured for 10BASE-T operation, Man-

chester encoded data from the ENDEC is fed

into the transmitter’s pre-distortion circuit where

initial wave shaping and pre-equalization is per-

formed. 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

TXD+/TXD- pins.

In the absence of transmit packets, the transmit-

ter generates link pulses in accordance with

section 14.2.1.1 of the Ethernet standard. Trans-

mitted link pulses are positive pulses, one bit

time wide, typically generated at a rate of one

every 16 ms. The 16 ms timer starts whenever

the transmitter completes an End-of-Frame

(EOF) sequence. Thus, there is a link pulse 16

ms after an EOF unless there is another transmit-

ted packet. Figure 3.11 diagrams the operation of

the Link Pulse Generator.

If no link pulses are being received on the re-

ceiver, the 10BASE-T transmitter is internally

forced to an inactive state unless bit DisableLT

in register 19 (Test Control register) is set to one.

Receiver

The 10BASE-T receive section consists of the

receive filter, squelch circuit, polarity detection

and correction circuit, and link pulse detector.

Squelch Circuit: The 10BASE-T squelch circuit

determines when valid data is present on the

RXD+/RXD- 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.

Extended Range: The CS8920A supports an Ex-

tended Range feature that reduces the 10BASE-T

receive squelch threshold by approximately 6

dB. This allows the CS8920A to operate with

10BASE-T cables that are longer than 100 me-

ters (100 meters is the maximum length specified

by the Ethernet standard). The exact additional

distance depends on the quality of the cable and

the amount of electro-magnetic noise in the sur-

rounding environment. To activate this feature,

the host must set the LoRxSquelch bit (Register

13, LineCTL, Bit E).

Auto-Negotiation

The CS8920A supports Auto-Negotiation, the

mechanism that allows the two devices on either

end of a 10Base-T link segment to share infor-

mation and automatically configure both devices

for maximum performance. When configured for

Auto-Negotiation, the CS8920A will detect and

automatically operate full-duplex, if the device

on the other end of the link segment also sup-

ports full-duplex and Auto-Negotiation. The

CS8920A Auto-Negotiation capability is fully

compliant with the relevant portions of section

28 of the IEEE 802.3u standard.

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 de-

vice. To

remain interoperable with existing

10Base-T devices, the CS8920A also supports

the reception of 10Base-T compliant link integ-

rity test pulses, refered to as Normal Link Pulses

(NLP). Devices that respond to the CS8920A

with a Normal Link Pulse, cause the CS8920A

to operate as a 10Base-T half-duplex device.

CS8920A

DS238PP2

CS8920A Datasheet(PDF) 35 Page - Cirrus Logic

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