CS8904 datasheet(13/34 Pages) CIRRUS

13 / 34 page

DS191PP2

CS8904

Crystal LAN™ Quad Ethernet Transceiver

CIRRUS LOGIC ADVANCED PRODUCT DATABOOK

3.2.1 Encoder

The encoder converts NRZ data from the digital

controller and a 10 MHz Transmit Clock signal into

a serial stream of Manchester data. The Transmit

Clock is produced by an on-chip oscillator circuit

that is driven by either an external 20 MHz quartz

crystal or a TTL-level CMOS clock input. The

encoded signal is routed to the 10BASE-T

transceiver.

3.2.2 Carrier Detection

The internal Carrier Detection circuit informs the

digital controller that valid receive data is present

by asserting the Carrier Detect (CD) signal as soon

it detects a valid Ethernet preamble. During normal

packet reception, Carrier Detect remains asserted

while the frame is being received, and is de-

asserted after the last low-to-high transition of the

End-of-Frame (EOF) sequence. Whenever the

receiver is idle (no receive activity), Carrier Detect

remains de-asserted.

3.2.3 Clock and Data Recovery

When the receiver is idle, the phase-lock loop

(PLL) is locked to the internal clock signal. The

assertion of the Carrier Sense signal interrupts the

PLL. When it restarts, it locks on the incoming

data. The receive clock is then compared to the

incoming data at the bit cell center and any phase

difference is corrected. The PLL remains locked as

long as the receiver input signal is valid. Once the

PLL has locked on the incoming data, the ENDEC

converts the Manchester data to NRZ and passes

the decoded data and the recovered clock to the

digital controller for further processing.

3.3 10BASE-T Transceiver

The

CS8904

includes

integral

10BASE-T

transceivers that are compliant with the relevant

portions of clause 14 of the Ethernet standard

(ISO/IEC 8802-3:1996). It includes all analog and

digital circuitry needed to interface the CS8904

directly to a simple isolation transformer (see

Figure 2 for a connection diagram). Figure 5

provides a block diagram of one of the 10BASE-T

transceivers.

Auto-Negotiation

and Link Pulse

TX Pre-

Distortion

TX Filters

Filter Tuning

RX Filters

TX Drivers

Rx+

Rx-

Tx+

Tx-

ENDEC

Auto-Negotiation and Link

Control and Status

10BASE-T Transceiver

Comparator

Figure 5. 10BASE-T Transceiver

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