ENC28J60-CSLASHSS datasheet(34/96 Pages) MICROCHIP

34 / 96 page

ENC28J60

DS39662B-page 32

Preliminary

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 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.

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 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

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.

5.1.4

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 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.

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 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

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.

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.

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 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”).

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.

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