Intel 82555 manual 100BASE-TX Adapter Mode Operation, 100BASE-TX Transmit Clock Generation

Networking Silicon — 82555

4.0100BASE-TX Adapter Mode Operation

4.1100BASE-TX Transmit Clock Generation

A 25 MHz crystal or a 25 MHz oscillator is used to drive the 82555’s X1 and X2 pins. The 82555 derives its internal transmit digital clocks from this crystal or oscillator input. The Transmit Clock signal is a derivative of the 25 MHz internal clock. The accuracy of the external crystal or oscillator must be ± 0.0005% (50 PPM).

4.2100BASE-TX Transmit Blocks

The transmit subsection of the 82555 accepts nibble-wide data on the TXD[3:0] lines when TXEN is asserted (high). The transmit subsection passes data unconditionally to the 4B/5B encoder as long as TXEN is active.

The 4B/5B encoder accepts nibble-wide data (4 bits) from the MAC and compiles it into 5-bit-wide parallel symbols. These symbols are scrambled and serialized into a 125 Mbps bit stream, converted by the analog transmit driver into a MLT-3 waveform format, and transmitted onto the Unshielded Twisted Pair (UTP) or Shielded Twisted Pair (STP) wire.

4.2.1100BASE-TX 4B/5B Encoder

The 4B/5B encoder complies with the IEEE 802.3u 100BASE-TX standard. Four bits are encoded according to the transmit 4B/5B lookup table. The lookup table matches a 5-bit code to each 4-bit code.

The table below illustrates the 4B/5B encoding scheme associated with the given symbol.

Table 2. 4B/5B Encoder

Symbol	5B Symbol Code	4B Nibble Code
0	11110	0000
1	01001	0001
2	10100	0010
3	10101	0011
4	01010	0100
5	01011	0101
6	01110	0110
7	01111	0111
8	10010	1000
9	10011	1001
A	10110	1010
B	10111	1011
C	11010	1100
D	11011	1101

Datasheet 13