ENC424J600-IPT datasheet(73/168 Pages) MICROCHIP

73 / 168 page

 2010 Microchip Technology Inc.

DS39935C-page 71

ENC424J600/624J600

6.0

ETHERNET OVERVIEW

Before discussing the use of ENC424J600/624J600

devices in Ethernet applications, it may be helpful to

review the structure of a typical data frame. For more

detailed information, refer to IEEE 802.3 Standard, which

defines the Ethernet protocol, or to Microchip Application

Note AN1120, “Ethernet Theory of Operation”.

6.1

Frame Format

Ethernet communications utilize a series of frames to

transmit data between nodes. (These frames are also

commonly referred to as “packets”, and in the context of

this document, the two terms will be used interchange-

ably.) Compliant Ethernet frames are between 64 and

1518 bytes long. They consist of five or six different

fields: a destination MAC address, source MAC

address, type/length field, data payload, optional

padding field and a Cyclic Redundancy Check (CRC).

Additionally, when transmitted on the Ethernet medium,

a start of stream/preamble field and a Start-Of-Frame

(SOF) delimiter byte are appended to the beginning of

the Ethernet frame. Thus, traffic seen on the twisted-pair

cabling will appear as shown in Figure 6-1.

6.1.1

START OF STREAM/PREAMBLE

AND START-OF-FRAME DELIMITER

When using ENC424J600/624J600 devices, the start

of stream/preamble and Start-Of-Frame delimiter fields

are automatically generated for transmitted frames and

stripped from received ones. These bytes are not

written to the data buffer and the host controller does

not need to account for these bytes.

6.1.2

DESTINATION ADDRESS

The destination address is a 6-byte field containing the

MAC address of the device to which the frame is

directed. If the Least Significant bit in the first byte of

this address is clear (i.e., the first byte of the address is

even), the address is a Unicast address. For example,

00-00-BA-BE-F0-0D and 32-45-DE-AD-BE-EF Unicast

addresses,

while

01-00-BA-BE-F0-0D

and

33-45-DE-AD-BE-EF are not. Frames with a Unicast

destination are designated for usage by the addressed

node only.

FIGURE 6-1:

ETHERNET PACKET FORMAT

Padding

FCS(1)

Number

Field

Comments

46-1500

Destination Address,

such as Multicast, Broadcast or Unicast

Source Address

Packet Payload

Frame Check Sequence – CRC

Type/Length

Data

of Bytes

Type of Packet or the Length of the Packet

(with optional padding)

Filtered Out by the Module

SFD

Start-Of-Frame Delimiter

(filtered out by the module)

Used in the

Calculation

of the FCS

Note 1: The FCS is transmitted starting with bit 31 and ending with bit 0.

Preamble

Start of Stream/

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