AM79C875KC datasheet(21/48 Pages) AMD

21 / 48 page

Am79C875

Auto-Negotiation and Miscellaneous

Functions

Auto-Negotiation

The object of the Auto-Negotiation function is to deter-

mine the abilities of the devices sharing a link. After ex-

changing abilities, the NetPHY™ 4LP device and

remote link partner device acknowledge each other

and make a choice of which advertised abilities to sup-

port. The Auto-Negotiation function facilitates an or-

dered resolution between exchanged abilities. This

exchange allows both devices at either end of the link

to take maximum advantage of their respective shared

abilities.

The NetPHY™ 4LP device implements the transmit

and receive Auto-Negotiation algorithm as defined in

IEEE 802.3u, Section 28. The Auto-Negotiation algo-

rithm uses a burst of link pulses called Fast Link Pulses

(FLP). The burst of link pulses are spaced between 55

and 140 µs so as to be ignored by the standard

10BASE-T algorithm. The FLP burst conveys informa-

tion about the abilities of the sending device. The

receiver can accept and decode an FLP burst to learn

the abilities of the sending device. The link pulses

transmitted conform to the standard 10BASE-T tem-

plate. The NetPHY™ 4LP device can perform auto-

negotiation with reverse polarity link pulses. The

NetPHY™ 4LP device supports Next Page advertise-

ment.

The NetPHY™ 4LP device uses the Auto-Negotiation

algorithm to select the type connection to be estab-

lished according to the following priority: 100BASE-TX

full duplex, 100BASE-T4, 100BASE-TX half-duplex,

10BASE-T full duplex, 10BASE-T half-duplex. The

device does not support 100BASE-T4 connections.

The Auto-Negotiation algorithm is initiated when one or

the following events occurs: Auto-Negotiation enable

bit is set, or reset, or soft reset, or transition to link fail

state (when Auto-Negotiation enable bit is set), or

Auto-Negotiation restart bit is set. The result of the

Auto-

Negotiation process can be read from the sta-

tus register for the port of interest (Diagnostic Register,

The NetPHY™ 4LP device supports Parallel Detection

for remote legacy devices that do not support the Auto-

Negotiation algorithm. In the case that a 100BASE-TX

only device is connected to the remote end, the Net-

PHY™ 4LP device will see scrambled idle symbols and

establish a 100BASE-TX only connection. If NLPs are

seen, the NetPHY™ 4LP device will establish a

10BASE-T connection.

Loopback Operation

A local loopback and a remote loopback are provided

for diagnostic testing. Local loopback can be achieved

by writing to Register 0, bit 14 (LPBK). Remote loop-

back can be achieved by writing to Register 24, bit 3.

The local loopback routes transmitted data at the out-

put of the NRZ-to-NRZI conversion module back to the

receiving path’s clock and data recovery module for

connection to PCS in 5-bit symbol format. This loop-

back is used to check all the device’s connection at the

5-bit symbol bus side and the operation of the analog

phase locked loop. In the local loopback mode, the SDI

output is forced to high and the TX± outputs are

tri-stated.

The remote loopback routes receiving data at the out-

put of the clock and data recovery module to the trans-

mitting path’s NRZI-to-MLT-3 conversion module. This

loopback is used to check the device’s connection on

the media side and the operation of its internal adaptive

equalizer, digital phase locked loop, and digital wave

shape synthesizer. During the remote loopback mode,

the SDI output is forced to low.

Power Savings Mechanisms

The NetPHY™ 4LP device has three mechanisms for

reducing power: Selectable 1.25:1 transmit transformer

ratio, Unplugged, and Power Down.

Selectable Transformer

The TX outputs can drive either a 1:1 transformer or a

1.25:1 transformer. The latter can be used to reduce

transmit power further. The TP1_1 pin must be pulled

low at reset to select 1.25:1 transformers. The current

at the TX± pins for a 1:1 ratio transformer is 40 mA for

MLT-3 and 100 mA for 10BASE-T. Using the 1.25:1

ratio reduces the current to 32 mA for MLT-3 and 80 mA

for 10BASE-T.

The cost of using the 1.25:1 option is in impedance

coupling. The reflected capacitance is increased by the

square of the ratio of windings (1.252 = 1.56). Thus, the

reflected capacitance on the media side is roughly

1½ times the capacitance on the board. Extra care in

the layout to control capacitance on the board is re-

quired.

Unplugged

The Unplugged feature reduces power consumption

whenever the PHY is operating. The TX output drivers

limit the drive capability if the corresponding receivers

do not detect a link partner within 4 seconds. This pre-

vents “wasted” power. If the receiver detects the ab-

sence of a link partner, the corresponding transmitter is

limited to transmitting normal link pulses. Any energy

detected by the receiver enables full transmit capabili-

ties. A typical situation encountered is that most un-

used ports still consume power. Up to 25% of repeater

and switch ports are unconnected to allow room for fu-

ture expansion. With NetPHY™ 4LP, unconnected

ports have their receiver disabled until energy is de-

tected. The power savings is most notable on uncon-

AM79C875KC 数据表(PDF) 21 Page - Advanced Micro Devices

AM79C875KC 数据表(HTML) 21 Page - Advanced Micro Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

数据表下载