PIC18F97J60 Datasheet, PDF (236/474 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1 Mbit Flash Microcontrollers with Ethernet

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18.5.1.2 Destination Address
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 des-
tination. 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.
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 Ethernet module incorporates receive filters which
can be used to discard or accept packets with
Multicast, Broadcast and/or Unicast destination
addresses. When transmitting packets, the application
is responsible for writing the desired destination
address into the transmit buffer.
18.5.1.3 Source Address
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 Ethernet module must generate a
unique MAC address for each and every
microcontroller used.
MAC addresses consist of two portions. The first three
bytes are known as the Organizationally Unique Identi-
fier (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
application. The module will not automatically transmit
the contents of the MAADR registers which are used
for the Unicast receive filter.
18.5.1.4 Type/Length
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
implementing 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.
18.5.1.5 Data
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 Ethernet module, however, is capable of transmit-
ting and receiving larger packets when the Huge Frame
Enable bit, HFRMEN, is set (MACON3<2> = 1).
18.5.1.6 Padding
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 Ethernet module
automatically generates zero padding if the
PADCFG2:PADCFG0 bits (MACON3<7:5>) are config-
ured for this. Otherwise, the user application will need to
add any padding to the packet before transmitting it. The
module will not prevent the transmission of undersized
packets should the application command such an action.
When receiving packets, the module automatically
rejects packets which are less than 18 bytes. All pack-
ets, 18 bytes and larger, will be subject to the standard
receive filtering criteria and may be accepted as normal
traffic. Since the module only rejects packets smaller
than 18 bytes, it is important that the firmware check
the length of every received packet and reject packets
which are smaller than 64 bytes to meet IEEE 802.3
specification requirements.
18.5.1.7 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.
When receiving packets, the Ethernet module will
check the CRC of each incoming packet. If the CRCEN
bit 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 application will be
able to determine if the CRC was valid by reading the
receive status vector (see Section 18.5.3 “Receiving
Packets”).
When transmitting packets, the module automatically
generates a valid CRC and transmits it if the
PADCFG2:PADCFG0 bits are configured for this. Other-
wise, the user application must generate the CRC and
place it in the transmit buffer. Given the complexity of
calculating a CRC, it is highly recommended to allow the
module to automatically calculate and include the CRC.
DS39762A-page 234
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© 2006 Microchip Technology Inc.