Datasheet

ManualsBrandsMicrochip ManualsIntegrated Circuits (ICs)TX Ethernet Transceiver Ethernet/MII/RMII VQFN 114mA

 2013-2015 Microchip Technology Inc. DS00001987A-page 17

LAN8740A/LAN8740Ai

3.0 FUNCTIONAL DESCRIPTION

This chapter provides functional descriptions of the various device features. These features have been categorized into

the following sections:

• Transceiver

• Auto-Negotiation

• HP Auto-MDIX Support

• MAC Interface

• Serial Management Interface (SMI)

• Interrupt Management

• Configuration Straps

• Miscellaneous Functions

• Application Diagrams

3.1 Transceiver

3.1.1 100BASE-TX TRANSMIT

The 100BASE-TX transmit data path is shown in Figure 3-1. Each major block is explained in the following subsections.

3.1.1.1 100BASE-TX Transmit Data Across the MII/RMII Interface

For MII, the MAC controller drives the transmit data onto the TXD bus and asserts TXEN to indicate valid data. The data

is latched by the transceiver’s MII block on the rising edge of TXCLK. The data is in the form of 4-bit wide 25 MHz data.

For RMII, the MAC controller drives the transmit data onto the TXD bus and asserts TXEN to indicate valid data. The

data is latched by the transceiver’s RMII block on the rising edge of REF_CLK. The data is in the form of 2-bit wide 50

MHz data.

3.1.1.2 4B/5B Encoding

The transmit data passes from the MII/RMII 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 3-1. Each 4-bit data-nibble is mapped to 16 of the 32 pos-

sible 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.

FIGURE 3-1: 100BASE-TX TRANSMIT DATA PATH

MAC

Driver

MLT-3

Converter

NRZI

Converter

4B/5B

Encoder

CAT-5RJ45

25 MHz by

5 bits

NRZI

MLT-3MLT-3

MLT-3

Scrambler

and PISO

MII/RMII

25 MHz

by 4 bits

Ext Ref_CLK (for RMII only)

PLL

MII 25 MHz by 4 bits

RMII 50 MHz by 2 bits

MLT-3

Magnetics

125 Mbps Serial

TX_CLK

(for MII only)