DM9102DEP datasheet(49/70 Pages) DAVICOM

49 / 70 page

Final

Version: DM9102D-DS-F01

May 10, 2006

7.3 Network Function

7.3.1 Overview

This chapter will introduce the normal state machine

operation and MAC layer management like collision back-off

algorithm. In transmit mode, the DM9102D initiates a DMA

cycle to access data from a transmit buffer. It prefaces the

data with the preamble, the SFD pattern, and it appends a

32-bit CRC. In receive mode, the data is de-serialized by

receive mechanism and is fed into the internal FIFO. For

detailed process, please see below.

7.3.2 Receive Process and State Machine

a. Reception Initiation

As a preamble being detected on receive data lines, the

DM9102D synchronizes itself to the data stream during the

preamble and waits for the SFD. The synchronization

process is based on byte boundary and the SFD byte is

10101011. If the DM9102D receives a 00 or a 11 after the

first 8 preamble bits and before receiving the SFD, the

reception process will be terminated.

b. Address Recognition

After initial synchronization, the DM9102D will recognize the

6-byte destination address field. The first bit of the

destination address signifies whether it is a physical address

(=0) or a multicast address (=1). The DM9102D filters the

frame based on the node address of receive address filter

setting. If the frame passes the filter, the subsequent serial

data will be delivered into the host memory.

c. Frame Decapsulation

The DM9102D checks the CRC bytes of all received frames

before releasing the frame along with the CRC to the host

processor.

7.3.3 Transmit Process and State Machine

a. Transmission Initiation

Once the host processor prepares a transmit descriptor for

the transmit buffer, the host processor signals the DM9102D

to take it. After the DM9102D has been notified of this

transmit list, the DM9102D will start to move the data bytes

from the host memory to the internal transmit FIFO. When

the transmit FIFO is adequately filled to the programmed

threshold level, or when there is a full frame buffered into the

transmit FIFO, the DM9102D begins to encapsulate the

frame. The transmit encapsulation is performed by the

transmit

state

machine,

which

delays

the

actual

transmission onto the network until the network has been

idle for a minimum inter frame gap time.

b. Frame Encapsulation

The transmit data frame encapsulation stream consists of

two parts: Basic frame beginning and basic frame end. The

former contains 56 preamble bits and SFD, the later, FCS.

The basic frame read from the host memory includes the

destination address, the source address, the type/length

field, and the data field. If the data field is less than 46 bytes,

the DM9102D will pad the frame with pattern up to 46 bytes.

c. Collision

When concurrent transmissions from two or more nodes

occur

(termed;

collision),

the

DM9102D

halts

the

transmission of data bytes and begins a jam pattern

consisting of AAAAAAAA. At the end of the jam

transmission, it begins the backoff wait time. If the collision

was detected during the preamble transmission, the jam

pattern is transmitted after completing the preamble. The

backoff process is called truncated binary exponential

backoff. The delay is a random integer multiple of slot times.

The number of slot times of delay before the Nth

retransmission attempt is chosen as a uniformly distributed

random integer in the range:

≤ r < 2k

k = min ( n, N ) and N=10

7.3.4 Physical Layer Overview

The DM9102D supports 100Mbps and 10Mbps operation. It

provides a direct interface either to Unshielded Twisted Pair

cable UTP5 for 100BASE-TX fast Ethernet, or UTP5/UTP3

cable for 10BASE-T Ethernet. In physical level operation, it

consists of the following functions:

－PCS: 4B5B encode/decode, scramble/de-scramble, and

data serialize/parallelize

－NRZ/NRZI, MLT-3 encoder/decoder and driver

－MANCHESTER encoder/decoder

－ 10BASE-T filter, driver/receiver, and MANCHESTER

encoder/decoder

－Auto. MDI/MDIX detection

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