LM3S8938 Datasheet, PDF (441/550 Pages) List of Unclassifed Manufacturers – Microcontroller

LM3S8938 Microcontroller
function of these pins is programmable via the PHY layer MR23 register. Refer to page 480 for
additonal details on how to program these LED functions.
17.2.3 MAC Configuration/Operation
17.2.3.1 Ethernet Frame Format
Ethernet data is carried by Ethernet frames. The basic frame format is shown in Figure
17-3 on page 441.
Figure 17-3. 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 by the physical layer signaling circuitry 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 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. The first of two octets is most significant.
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 standard. The MAC module assumes
type interpretation if the value of the Length/Type field is greater than 1500 decimal.
■ Data
The data field is a sequence of 0 to 1500 octets. Full data transparency is provided so any values
can appear in this field. A minimum frame size is required to properly meet the IEEE standard.
If necessary, the data field is extended by appending extra bits (a pad). The pad field can have
a size of 0 to 46 octets. The sum of the data and pad lengths must be a minimum of 46 octets.
The MAC module automatically inserts pads if required, though it can be disabled by a register
June 14, 2007
441
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