LAN89218 datasheet(61/148 Pages) MICROCHIP

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DS60001255B-page 61

LAN89218

4.0

INTERNAL ETHERNET PHY

4.1

Top Level Functional Description

Functionally, the internal PHY can be divided into the following sections:

• 100Base-TX transmit and receive

• 10Base-T transmit and receive

• Internal MII interface to the Ethernet Media Access Controller

• Auto-negotiation to automatically determine the best speed and duplex possible

• Management Control to read status registers and write control registers

4.2

100Base-TX Transmit

The data path of the 100Base-TX is shown in Figure 4-1. Each major block is explained below.

4.2.1

4B/5B ENCODING

The transmit data passes from the MII block to the 4B/5B encoder. This block encodes the data from 4-bit nibbles

to 5-bit symbols (known as “code-groups”) according to Table 4-1. Each 4-bit data-nibble is mapped to 16 of the 32

possible code-groups. The remaining 16 code-groups are either used for control information or are not valid.

The first 16 code-groups are referred to by the hexadecimal values of their corresponding data nibbles, 0 through F.

The remaining code-groups are given letter designations with slashes on either side. For example, an IDLE code-

group is /I/, a transmit error code-group is /H/, etc.

The encoding process may be bypassed by clearing bit 6 of register 31. When the encoding is bypassed the 5th

transmit data bit is equivalent to TX_ER.

FIGURE 4-1:

100BASE-TX DATA PATH

MAC

Driver

MLT-3

Converter

NRZI

Converter

4B/5B

Encoder

Magnetics

CAT-5

RJ45

100M

PLL

Internal

MII 25 MHz by 4 bits

TX_CLK

25 MHz

by 5 bits

NRZI

MLT-3

Scrambler

and PISO

125 Mbps Serial

MII

25 MHz

by 4 bits

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