MC9S12NE64_06 Datasheet, PDF (331/554 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
The frame length is defined to be 64 bytes at minimum and 1518 bytes at maximum, excluding the
preamble and SFD. Transmission and reception of each byte of data is performed one nibble at a time
across the MII interface with the order of nibble as shown in Figure 11-23
First Nibble
LSB
First
Bit
D0
D1
D2
Second Nibble
D3
D4
D5
MSB
D6
D7
LSB
D0
D1
D2
D3
MSB
MII Nibble
Figure 11-23. MII Nibble/Byte-to-Byte/Nibble Mapping
11.4.1.1 Preamble and SFD
The preamble is a 56-bit field that consists of a fixed pattern of alternating 1s and 0s.
1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010
The left-most 1 value represents the byte LSB and the right-most 0 value represents the byte MSB.
The SFD field is the sequence 10101011 and immediately follows the preamble pattern. The preamble and
SFD are used to allow the Ethernet interfaces on the network to synchronize themselves with the incoming
data stream before the data fields arrive.
The EMAC does not require any preamble before the SFD byte. If a preamble is detected, the preamble
must be a valid preamble pattern until the SFD or else the frame is dropped.
11.4.1.2 Address Fields
Each frame contains two address fields: the destination address field and the source address field, in that
order. The destination address field specifies the network node(s) for which the frame is intended. The
source address field specifies the network node that sent the frame.
A 48-bit address is written as 12 hexadecimal digits with the digits paired in groups of two, representing
a byte of information. The byte order of transmission on the network is from the most- to least-significant
byte. The transmission order within the byte, however, is starting from the least-significant bit (LSB) of
the byte through the most-significant bit (MSB). For example, an Ethernet address that is written as the
MC9S12NE64 Data Sheet, Rev. 1.1
Freescale Semiconductor
331