AN2692 Freescale Semiconductor / Motorola, AN2692 Datasheet - Page 3

• Manufacturer Part Number: AN2692
• Description: MC9S12NE64 Integrated Ethernet Controller
• Manufacturer: Freescale Semiconductor / Motorola

inspecting the incoming Ethernet packet destination hardware address. Depending on the destination
hardware address of the incoming packet, the device will either ignore the message or accept it for further
processing.
Ethernet Packet
Figure 2
Ethernet packet is encapsulated by a media access controller (MAC) header and frame check sequence
(FCS) trailer.
It is important to understand the format and content of Ethernet packets that are transferred between
devices in a typical LAN because Ethernet devices operate on these data fields. The data fields are:
Freescale Semiconductor
•
MAC Header
–
–
–
–
–
illustrates the specific structure of an Ethernet packet.
Preamble — Seven bytes of an alternating pattern of 1s and 0s that indicates that a frame is
coming and provides synchronization.
Start of frame delimiter (SFD) — One byte that follows the preamble and consists of an
alternating pattern of 1s and 0s that ends with two consecutive 1s.
Destination MAC address (or hardware MAC address) — 6-byte value that indicates the MAC
hardware address of the network device that should receive the Ethernet packet.
Source MAC address (or source hardware address) — 6-byte value that indicates the MAC
hardware address of the network device that is sending the Ethernet packet.
Length/type — 2-byte value that indicates the number of data bytes that are encapsulated by
the Ethernet packet if the length/type value is less-than or equal-to 1500. If the length/type field
is greater than 1536, the length/type field identifies the type (or Ethertype) of the packet. The
Ethertype identifies the higher-level protocol used to create the encapsulated data portion of
an Ethernet packet.
Figure 2. Format and Content of Ethernet Packets
ENCAPSULATED DATA
MC9S12NE64 Integrated Ethernet Controller, Rev. 0.2
(LAYER 3 AND UP)
MAC HEADER
MAC TRAILER
46 TO 1500
BYTES
7 BYTES
1 BYTES
6 BYTES
6 BYTES
2 BYTES
4 BYTES
START DELIMITER
MAC ADDRESS
MAC ADDRESS
FRAME CHECK
LENGTH/TYPE
DESTINATION
SEQUENCE
PREAMBLE
SOURCE
DATA
FIELD
FIELD
PAD
Figure 2
10101010
10101011
shows that the data field of an
Introduction
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