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Am79C875
15
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.
0
0
0
0
0
0
0
0
0
0
0
0
1
A
B
C
D
E
F
0
0
A
B
C
D
E
F
0
0
REF_CLK
TX_EN
TXD[1]
TXD[0]
Preamble
SFD
Data
1
1
1
1
1
1
1
1
1
1
1
1
1
22236G-5
Figure 3. 100 Mbps Transmission
D0 D1 D2 D3 D4 D5 D6 D7
First Bit
First
Nibble
Second
Nibble
MAC's Serial Bit Stream
D0
D1
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 path 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 applications, 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.