AM79C875 datasheet(15/48 Pages) AMD

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Am79C875

TXD[1:0]

Transmit Data

TXD[1:0] transitions synchronously with respect to

REF_CLK. When TX_EN is asserted, TXD[1:0] is ac-

cepted for transmission by the PHY. TXD[1:0] is

ignored by the PHY while TX_EN is deasserted.

TXD[1:0] in 100 Mbps Mode

TXD[1:0] provides valid data for each REF_CLK period

while TX_EN is asserted. Refer to Figure 3.

TXD[1:0] in 10 Mbps Mode

As the REF_CLK frequency is ten times the data rate

in 10 Mbps mode, the value on TXD[1:0] is valid such

that TXD[1:0] may be sampled every tenth cycle, re-

gardless of the starting cycle within the group and yield

the correct frame data.

Bit Ordering

Transmission and reception of each octet is done a di-

bit at a time with the order of di-bit transmission and

reception as shown in Figure 4.

REF_CLK

TX_EN

TXD[1]

TXD[0]

Preamble

SFD

Data

22236G-5

Figure 3. 100 Mbps Transmission

First Bit

First

Nibble

Second

Nibble

MAC's Serial Bit Stream

LSB

MSB

TXD[0]/RXD[0]

TXD[1]/RXD[1]

Di-Bit

Stream

22236G-6

Figure 4. Bit Ordering

100BASE-X Block

The functions performed by the device include encod-

ing of MII 4-bit data (4B/5B), decoding of received code

groups (5B/4B), generating carrier sense and collision

detect indications, serialization of code groups for

transmission, de-serialization of serial data from recep-

tion, mapping of transmit, receive, and carrier sense at

the RMII interface, and recovery of clock from the in-

coming data stream. It offers stream cipher scrambling

and descrambling capability for 100BASE-TX applica-

tions.

In the transmit data pa th for 100 Mbps , the

NetPHY™ 4LP transceiver receives 2-bit wide data

across the RMII at 50 million di-bits per second. For

100BASE-TX applicati o ns, it encodes and

scrambles the data, serializes it, and transmits an

MLT-3 data stream to the media via an isolation trans-

former. For 100BASE-FX applications, it encodes and

serializes the data and transmits a Pseudo-ECL

(PECL) data stream to the fiber optic transmitter.

The NetPHY™ 4LP transceiver receives an MLT-3

data stream from the network for 100BASE-TX. It then

recovers the clock from the data stream, de-serializes

the data stream, and descrambles/decodes the data

stream (5B/4B) before presenting it at the RMII inter-

face.

For 100BASE-FX operation, the NetPHY™ 4LP re-

ceives a PECL data stream from the fiber optic trans-

ceiver and decodes that data stream. 100BASE-FX

operation is possible only on Port 3.

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