Datasheet

16.3 Functional Description
Note: A 12.4-kΩ resistor should be connected between the ERBIAS and ground. The 12.4-kΩ
resistor should have a 1% tolerance and should be located in close proximity to the ERBIAS
pin. Power dissipation in the resistor is low, so a chip resistor of any geometry may be used.
The functional description of the Ethernet Controller is discussed in the following sections.
16.3.1 MAC Operation
The following sections decribe the operation of the MAC unit, including an overview of the Ethernet
frame format, the MAC layer FIFOs, Ethernet transmission and reception options, packet timestamps,
and LED indicators.
16.3.1.1 Ethernet Frame Format
Ethernet data is carried by Ethernet frames. The basic frame format is shown in Figure
16-3 on page 601.
Figure 16-3. Ethernet Frame
Preamble SFD Destination Address Source Address
Length/
Type
FCSData
7
Bytes
6
Bytes
6
Bytes
2
Bytes
1
Byte
4
Bytes
46 - 1500
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 16-3 on page 603) 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
601June 18, 2012
Texas Instruments-Production Data
Stellaris
®
LM3S8962 Microcontroller
NRND: Not recommended for new designs.