Datasheet

 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
SA
Padding
FCS
(1)
Number Field Comments
6
46-1500
4
DA
Destination Address,
such as Multicast, Broadcast or Unicast
Source Address
Packet Payload
Frame Check Sequence – CRC
Type/Length
Data
of Bytes
6
2
Type of Packet or the Length of the Packet
(with optional padding)
7
Filtered Out by the Module
SFD
1
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/