ENC424J600-IPT datasheet(74/168 Pages) MICROCHIP

74 / 168 page

ENC424J600/624J600

DS39935C-page 72

 2010 Microchip Technology Inc.

If the Least Significant bit in the first byte of this address

is set (i.e., the byte is odd), the address is a Multicast

destination.

From

the

example,

01-00-BA-BE-F0-0D and 33-45-DE-AD-BE-EF are

Multicast addresses. Multicast frames are designated

for use by a selected group of Ethernet nodes. The

Multicast address, FF-FF-FF-FF-FF-FF, is reserved; it

is known as the Broadcast address and is directed to all

nodes on the network.

ENC424J600/624J600 devices incorporate several

packet filters which can be configured to accept or dis-

card Unicast, Multicast and/or Broadcast frames. For

details about these and other receive filters, refer to

Section 10.0 “Receive Filters”

. When transmitting

frames, the host controller is responsible for writing the

desired destination address into the transmit buffer.

6.1.3

SOURCE ADDRESS

The source address is a 6-byte field containing the

MAC address of the node which transmitted the Ether-

net frame. Every Ethernet device must have a globally

unique MAC address. Each ENC424J600/624J600

device has a unique address which is loaded into the

MAADR registers on power-up. This value can be used

as is, or the registers may be reconfigured with a

different address.

6.1.4

TYPE/LENGTH

The type/length field is a 2-byte field indicating the pro-

tocol to which the frame belongs. Applications using

standards such as Internet Protocol (IP) or Address

Resolution Protocol (ARP) should use the type code

specified in the appropriate standards document.

Alternately, this field can be used as a length field when

implementing proprietary networks. Typically, any

value of 1500 (05DCh) or smaller is considered to be a

length field and specifies the amount of non-padding

data which follows in the data field.

6.1.5

DATA

The data field typically consists of between 0 and

1500 bytes of payload data for each frame.

ENC424J600/624J600 devices are capable of trans-

mitting and receiving frames larger than this when the

Huge Frame Enable bit, HFRMEN (MACON2<2>), is

set. However, these larger data frames violate Ethernet

specifications and will likely be dropped by most

Ethernet nodes.

6.1.6

PADDING

The padding field is a variable length field appended to

meet IEEE 802.3 specification requirements when

transmitting small data payloads. As mentioned, the

minimum Ethernet frame size is 64 bytes. Removing

the 18 bytes of address and type information, and the

terminating 4-byte CRC, leaves a minimum of 46 bytes.

Smaller frames must be padded to fill this space.

When transmitting frames, ENC424J600/624J600

devices can automatically generate zero padding if the

PADCFG<2:0> bits (MACON2<7:5>) are configured to

do so. Otherwise, the application must append the

appropriate padding. The device will not prevent the

transmission of these “runt” frames if the host

commands such an action, but the frame is likely to be

dropped by other nodes.

When receiving frames, ENC424J600/624J600 devices

accept and write all padding to the receive buffer.

Frames shorter than the required 64 bytes can optionally

be filtered by the Runt Error Reject filter, described in

Section 10.4 “Runt Error Rejection Filter”

6.1.7

CRC

The CRC is a 4-byte field containing a standard 32-bit

CRC calculated over the destination, source, type, data

and padding fields. It allows for the detection of

transmission errors.

When transmitting frames, ENC424J600/624J600

devices can automatically generate and append a valid

CRC if the PADCFG<2:0> bits are configured to do so.

Otherwise, the host controller must generate and

append this value. It is strongly recommended that the

PADCFG bits be configured so that the hardware

automatically manages this field.

When

receiving

frames,

ENC424J600/624J600

devices accept and write the CRC field to the receive

buffer. Frames with invalid CRC values can be

discarded by the CRC Error Rejection filter, described

in Section 10.3 “CRC Error Rejection Filter”.

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