Datasheet

 2006-2012 Microchip Technology Inc. . DS39662E-page 31
ENC28J60
5.0 ETHERNET OVERVIEW
Before discussing the use of the ENC28J60 as an
Ethernet interface, it may be helpful to review the
structure of a typical data frame. Users requiring more
information should refer to the IEEE 802.3 standard
which is the basis for the Ethernet protocol.
5.1 Packet Format
Normal IEEE 802.3 compliant Ethernet frames are
between 64 and 1518 bytes long. They are made up of
five or six different fields: a destination MAC address, a
source MAC address, a type/length field, data payload,
an optional padding field and a Cyclic Redundancy
Check (CRC). Additionally, when transmitted on the
Ethernet medium, a 7-byte preamble field and Start-of-
Frame (SOF) delimiter byte are appended to the
beginning of the Ethernet packet. Thus, traffic seen on
the twisted-pair cabling will appear as shown in
Figure 5-1.
5.1.1 PREAMBLE/START-OF-FRAME
DELIMITER
When transmitting and receiving data with the
ENC28J60, the preamble and Start-of-Frame delimiter
bytes will automatically be generated or stripped from
the packets when they are transmitted or received. The
host controller does not need to concern itself with
them. Normally, the host controller will also not need to
concern itself with padding and the CRC which the
ENC28J60 will also be able to automatically generate
when transmitting and verify when receiving. The
padding and CRC fields will, however, be written into
the receive buffer when packets arrive, so they may be
evaluated by the host controller if needed.
FIGURE 5-1: 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
Preamble
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.