LM3S6911 Datasheet, PDF (387/514 Pages) Texas Instruments – Stellaris LM3S6911 Microcontroller

Stellaris® LM3S6911 Microcontroller
15.2.1.1
Ethernet Frame Format
Ethernet data is carried by Ethernet frames. The basic frame format is shown in Figure
15-3 on page 387.
Figure 15-3. Ethernet Frame
Preamble
7
Bytes
SFD Destination Address
Source Address
Length/
Type
1
Byte
6
Bytes
6
Bytes
2
Bytes
Data
46 - 1500
Bytes
FCS
4
Bytes
The seven fields of the frame are transmitted from left to right. The bits within the frame are
transmitted from least to most significant bit.
■ Preamble
The Preamble field is used to synchronize with the received frame’s timing. The preamble is 7
octets long.
■ Start Frame Delimiter (SFD)
The SFD field follows the preamble pattern and indicates the start of the frame. Its value is
1010.1011.
■ Destination Address (DA)
This field specifies destination addresses for which the frame is intended. The LSB (bit 16 of DA
oct 1 in the frame, see Table 15-1 on page 388) of the DA determines whether the address is an
individual (0), or group/multicast (1) address.
■ Source Address (SA)
The source address field identifies the station from which the frame was initiated.
■ Length/Type Field
The meaning of this field depends on its numeric value. This field can be interpreted as length
or type code. The maximum length of the data field is 1500 octets. If the value of the Length/Type
field is less than or equal to 1500 decimal, it indicates the number of MAC client data octets. If
the value of this field is greater than or equal to 1536 decimal, then it is type interpretation. The
meaning of the Length/Type field when the value is between 1500 and 1536 decimal is unspecified
by the IEEE 802.3 standard. However, the Ethernet Controller assumes type interpretation if the
value of the Length/Type field is greater than 1500 decimal. The definition of the Type field is
specified in the IEEE 802.3 standard. The first of the two octets in this field is most significant.
■ Data
The data field is a sequence of octets that is at least 46 in length, up to 1500 in length. Full data
transparency is provided so any values can appear in this field. A minimum frame size of 46
octets is required to meet the IEEE standard. If the frame size is too small, the Ethernet Controller
automatically appends extra bits (a pad), thus the pad can have a size of 0 to 46 octets. Data
padding can be disabled by clearing the PADEN bit in the Ethernet MAC Transmit Control
(MACTCTL) register.
For the Ethernet Controller, data sent/received can be larger than 1500 bytes without causing
a Frame Too Long error. Instead, a FIFO overrun error is reported using the FOV bit in the
April 05, 2010
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