AM79C874VC datasheet(22/60 Pages) AMD

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Am79C874

P R E L I M INARY

10BASE-T Block

The Ne tPHY-1L P tran sceive r incor p orate s th e

10BASE-T physical layer functions, including clock re-

covery (ENDEC), MAUs, and transceiver functions.

The NetPHY-1LP transceiver receives 10-Mbps data

from the MAC, switch, or repeater across the MII at 2.5

million nibbles per second (parallel), or 10 million bits

per second (serial). It then Manchester encodes the

data before transmission to the network.

Refer to Figure 4 for the 10BASE-T transmit and re-

ceive data paths.

Figure 4.

10BASE-T Transmit /Receive Data Paths

Twisted Pair Transmit Process

In 10BASE-T mode, Manchester code will be gener-

ated by the 10BASE-T core logic, which will then be

synthesized through the output waveshaping driver.

This will help reduce any EMI emission, eliminating the

need for an external filter. Data transmission over the

10BASE-T medium requires use of the integrated

10BASE-T MAU and uses the differential driver cir-

cuitry on the TX± pins.

TX± is a differential twisted-pair driver. When properly

terminated, TX± meets the transmitter electrical re-

quirements for 10BASE-T transmitters as specified in

IEEE 802.3, Section 14.3.1.2. The load is a twisted pair

cable that meets IEEE 802.3, Section 14.4.

The TX± signal is filtered on the chip to reduce har-

monic content per Section 14.3.2.1 (10BASE-T). Since

filtering is performed in silicon, TX± can be connected

directly to a standard transformer. External filtering

modules are not needed

Twisted Pair Receive Process

In 10BASE-T mode, the signal first passes through a

third order Elliptical filter, which filters all the noise from

the cable, board, and transformer. This eliminates the

need for a 10BASE-T external filter. A Manchester de-

coder and a Serial-to-Parallel converter then follow to

generate the 4-bit nibble in MII mode.

RX+ ports are differential twisted-pair receivers. When

properly terminated, each RX+ port meets the electri-

cal requirements for 10BASE-T receivers as specified

in IEEE 802.3, Section 14.3.1.3. Each receiver has in-

ternal filtering and does not require external filter mod-

ules or common mode chokes.

Signals appearing at the RX± differential input pair are

routed to the internal decoder. The receiver function

meets the propagation delays and jitter requirements

specified by the 10BASE-T standard. The receiver

squelch level drops to half its threshold value after un-

squelch to allow reception of minimum amplitude sig-

nals and to mitigate carrier fade in the event of worst

case signal attenuation and crosstalk noise conditions.

Twisted Pair Interface Status

The NetPHY-1LP transceiver will power up in the Link

Fail state. The Auto-Negotiation algorithm will apply to

allow it to enter the Link Pass state. A link-pulse detec-

tion circuit constantly monitors the RX± pins for the

presence of valid link pulses. In the Link Pass state, re-

ceive activity which passes the pulse width/amplitude

requirements of the RX± inputs cause the PCS Control

block to assert Carrier Sense (CRS) signal at the MII

interface.

Collision Detect Function

Simultaneous activity (presence of valid data signals)

from both the internal encoder transmit function and

the twisted pair RX± pins constitutes a collision,

thereby causing the PCS Control block to assert the

COL pin at the MII.

Collisions cause the PCS Control block to assert the

Carrier Sense (CRS) and Collision (COL) signals at the

MII. In the Link Fail state, this block would cause the

PCS Control block to de-assert Carrier Sense (CRS)

and Collision (COL).

Jabber Function

The Jabber function inhibits the 10BASE-T twisted pair

transmit function of the NetPHY-1LP transceiver device

if the TX± circuits are active for an excessive period

(20-150 ms). This prevents one port from disrupting the

network due to a stuck-on or faulty transmitter condi-

tion. If the maximum transmit time is exceeded, the

data path through the 10BASE-T transmitter circuitry is

disabled (although Link Test pulses will continue to be

Clock

Data

Manchester

Encoder

Clock

Data

Manchester

Decoder

Squelch

Circuit

RX Driver

TX Driver

Loopback

(Register 0)

22235I-6

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