AM79C874 datasheet(21/60 Pages) AMD

21 / 60 page

Am79C874

P R E L I M INARY

Figure 3.

TX± and RX± Termination for 100BASE-TX and 10BASE-T

Clock/Data Recovery

The equalized MLT-3 signal passes through a slicer cir-

cuit which then converts it to NRZI format. The Net-

PHY-1LP device uses an analog phase-locked loop

(APLL) to extract clock information from the incoming

NRZI data. The extracted clock is used to re-time the

data stream and set the data boundaries. The transmit

clock is locked to the 25-MHz clock input, while the re-

ceive clock is locked to the incoming data streams.

When initial lock is achieved, the APLL switches to lock

to the data stream, extracts a 125 MHz clock from it and

use that for bit framing to recover data. The recovered

125 MHz clock is also used to generate the 25 MHz

RX_CLK. The APLL requires no external components

for its operation and has high noise immunity and low

jitter. It provides fast phase align (lock) to data in one

transition and its data/clock acquisition time after

power-on is less than 60 transitions.

The APLL can maintain lock on run-lengths of up to 60

data bits in the absence of signal transitions. When no

valid data is present, i.e., when the SD is de-asserted,

the APLL switches back to lock with TX_CLK, thus pro-

viding a continuously running RX_CLK.

The recovered data is converted from NRZI-to-NRZ

and then to a 5-bit parallel format. The 5-bit parallel

data is not necessarily aligned to 4B/5B code-group’s

symbol boundary. The data is presented to PCS at re-

ceive data register output, gated by the 25-MHz

RX_CLK.

PLL Clock Synthesizer

The NetPHY-1LP device includes an on-chip PLL clock

synthesizer that generates a 125 MHz and a 25 MHz

clock for the 100BASE-TX or a 100 MHz and 20 MHz

clock for the 10BASE-T and Auto-Negotiation opera-

tions. Only one external 25 MHz crystal or a signal

source is required as a reference clock.

After power-on or reset, the PLL clock synthesizer is

defaulted to generating the 20 MHz clock output and

will stay active until the 100BASE-X operation mode is

selected.

RX+

TX+

RX-

TX-

0.1

µF

0.1

µF

VDD

(Note 1)

470 pF, 2 kV

(chassis ground)

(8)

(7)

TX+ (1)

(5)

(4)

TX- (2)

RX+ (3)

RX- (6)

Ω

Isolation

Transformer with

common-mode

chokes

RJ45

Connector

1:1 or 1.25:1

1:1

0.1

µF

(Note 2)

22235I-5

Notes:

1. 49.9

W if a 1:1 isolation transformer is used or 78.1 W if a 1.25:1 isolation transformer is used.

2. 49.9

W is normal, but 54.9 W can be used for extended cable length operation.

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