AM79C875KC datasheet(20/48 Pages) AMD

20 / 48 page

Am79C875

mode filtering blocks the DC component of the code

and the DC offset of the differential receive input can

wander. The shift in the signal levels causes increase

in error rates. A DC restoration circuit is needed to

compensate for the attenuation of DC components.

The NetPHY™ 4LP device implemented a patent-

pending DC restoration circuit which, unlike the tra-

ditional implementation, does not need the feedback

information from the slicer and clock recovery. The

baseline wander correction circuit is not required and,

therefore, is bypassed when the port is 10BASE-T.

Clock/Data Recovery

The equalized MLT-3 signal is converted into NRZI for-

mat. The NetPHY™ 4LP device uses an analog phase

locked loop (APLL) to extract clock information of the

incoming NRZI data which is used to re-time the data

stream and set data boundaries. The receive clocks

are locked to the incoming data streams. PPM should

be between 50 and 100.

When initial lock is achieved, the APLL switches to

lock-to-data stream, extracts a 125-MHz clock. The re-

covered 125 MHz clock is also used to generate the 25-

MHz RX_CLK. The APLL requires no external compo-

nents 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, the APLL switches back to lock

with the TX_CLK, providing 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.

TX+

TX–

SDI/MLT3EN

RX+

RX–

TX+ (1)

RX+ (3)

TX– (2)

RX– (6)

RJ45

Connector

Isolation

Transformer with

common-mode

chokes

0.1 µF

470 pF

1:1

1:1 or 1.25:1 *

(8)

(7)

(5)

(4)

(chassis ground)

1 k

(Note 3)

VDD

0.1 µF

470 pF

Ω

Ω (Note 4)

(Note 4)

22236G-9

Notes:

2. The isolation transformers include common-mode chokes.

3. Consult magnetics vendors for appropriate termination schemes.

4. 50

Ω if a 1:1 isolation transformer is used or 78 Ω if a 1.25:1 isolation transformer is used.

5. 50 (49.9)

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

Figure 7. TX± and RX± Termination for 100BASE-TX and 10BASE-T

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