MPC8540 datasheet(1145/1302 Pages) FREESCALE

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Introduction

MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

Freescale Semiconductor

21-5

The LLC defines service access for protocols that conform to the open system interconnection

(OSI) model for network protocols. However, many protocols do not obey the rules for those

layers and additional information must be added to the LLC in order to provide information

regarding those protocols. Protocols that fall into this category include IP and IPX. The method

used to provide this additional protocol information is called a subnetwork access protocol

(SNAP) frame. A SNAP encapsulation is indicated by the DSAP and SSAP addresses being set to

0xAA and the LLC control field being set to 0x03. If that address is seen, a SNAP header follows.

The SNAP header is five bytes long. The first three bytes consist of the organization code (SNAP

OUI), which is assigned by the IEEE. The last two bytes become the type value set from the

original Ethernet specifications if SNAP OUI = 0, or they become a SNAP protocol identifier if

SNAP OUI is nonzero.

The FEC allows the flexibility to accelerate the identification and retrieval of all the standard and

non-standard protocols mentioned above. Figure 21-4 depicts the block diagram of the FEC.

Figure 21-4. FEC Block Diagram

21.1.1 10/100 Fast Ethernet Controller Overview

The FEC is designed to support 10/100 Mbps Ethernet/802.3 networks and contains the following

components:

•

Ethernet media access controller (MAC)

•

First-in first-out (FIFO) controller

•

Direct memory access (DMA) controller

Address/Data

filtering

MAC Layer

DMA

Tx system CLK

data path

Pack

words

Unpack

words

Rx FIFO

1 kbyte

FIFO

control

Tx FIFO

1 kbyte

Tx/Rx descriptors

Tx/Rx data

Tx/Rx status

Block

interface

Clocks

MII Mgmt

To PHY

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