ENC28J60
5.1.2
DESTINATION ADDRESS
5.1.5
DATA
The destination address field is a 6-byte field filled with
the MAC address of the device that the packet is directed
to. If the Least Significant bit in the first byte of the MAC
address is set, the address is a multicast destination. For
example, 01-00-00-00-F0-00 and 33-45-67-89-AB-CD
are multicast addresses, while 00-00-00-00-F0-00 and
32-45-67-89-AB-CD are not.
The data field is a variable length field anywhere from 0
to 1500 bytes. Larger data packets will violate Ethernet
standards and will be dropped by most Ethernet nodes.
The ENC28J60, however, is capable of transmitting and
receiving larger packets when the Huge Frame Enable
bit is set (MACON3.HFRMEN = 1).
5.1.6
PADDING
Packets with multicast destination addresses are
designed to arrive and be important to a selected group
of Ethernet nodes. If the destination address field is the
reserved multicast address, FF-FF-FF-FF-FF-FF, the
packet is a broadcast packet and it will be directed to
everyone sharing the network. If the Least Significant
bit in the first byte of the MAC address is clear, the
address is a unicast address and will be designed for
usage by only the addressed node.
The padding field is a variable length field added to
meet IEEE 802.3 specification requirements when
small data payloads are used. The destination, source,
type, data and padding of an Ethernet packet must be
no smaller than 60 bytes. Adding the required 4-byte
CRC field, packets must be no smaller than 64 bytes. If
the data field is less than 46 bytes long, a padding field
is required.
When transmitting packets, the ENC28J60 automatically
generates zero padding if the MACON3.PADCFG<2:0>
bits are configured to do so. Otherwise, the host control-
ler should manually add padding to the packet before
transmitting it. The ENC28J60 will not prevent the
transmission of undersize packets should the host
controller command such an action.
The ENC28J60 incorporates receive filters which can
be used to discard or accept packets with multicast,
broadcast and/or unicast destination addresses. When
transmitting packets, the host controller is responsible
for writing the desired destination address into the
transmit buffer.
5.1.3
SOURCE ADDRESS
When receiving packets, the ENC28J60 automatically
rejects packets which are less than 18 bytes; it is
assumed that a packet this small does not contain even
the minimum of source and destination addresses, type
information and FCS checksum required for all pack-
ets. All packets 18 bytes and larger will be subject to
the standard receive filtering criteria and may be
accepted as normal traffic. To conform with IEEE 802.3
requirements, the application itself will need to inspect
all received packets and reject those smaller than 64
bytes.
The source address field is a 6-byte field filled with the
MAC address of the node which created the Ethernet
packet. Users of the ENC28J60 must generate a
unique MAC address for each controller used.
MAC addresses consist of two portions. The first three
bytes are known as the Organizationally Unique
Identifier (OUI). OUIs are distributed by the IEEE. The
last three bytes are address bytes at the discretion of
the company that purchased the OUI.
When transmitting packets, the assigned source MAC
address must be written into the transmit buffer by the
host controller. The ENC28J60 will not automatically
transmit the contents of the MAADR registers which
are used for the unicast receive filter.
5.1.7
CRC
The CRC field is a 4-byte field which contains an indus-
try standard 32-bit CRC calculated with the data from
the destination, source, type, data and padding fields.
5.1.4
TYPE/LENGTH
When receiving packets, the ENC28J60 will check the
CRC of each incoming packet. If ERXFCON.CRCEN is
set, packets with invalid CRCs will automatically be
discarded. If CRCEN is clear and the packet meets all
other receive filtering criteria, the packet will be written
into the receive buffer and the host controller will be
able to determine if the CRC was valid by reading the
receive status vector (see Section 7.2 “Receiving
Packets”).
The type/length field is a 2-byte field which defines
which protocol the following packet data belongs to.
Alternately, if the field is filled with the contents of
05DCh (1500) or any smaller number, the field is
considered a length field and it specifies the amount of
non-padding data which follows in the data field. Users
implementing proprietary networks may choose to treat
this field as a length field, while applications implement-
ing protocols such as the Internet Protocol (IP) or
Address Resolution Protocol (ARP), should program
this field with the appropriate type defined by the
protocol’s specification when transmitting packets.
When transmitting packets, the ENC28J60 will auto-
matically generate a valid CRC and transmit it if the
MACON3.PADCFG<2:0> bits are configured to cause
this. Otherwise, the host controller must generate the
CRC and place it in the transmit buffer. Given the com-
plexity of calculating a CRC, it is highly recommended
that the PADCFG bits be configured such that the
ENC28J60 will automatically generate the CRC field.
DS39662B-page 32
Preliminary
© 2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
