Datasheet

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 previous 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”.