Table 20-17. RX MAC Flow Control (continued)
DescriptionDUPM bit in
EMACCFG
TFE bit in
EMACFLOWCTL
The MAC receiver detects or processes the Pause frames and responds to
such frames by stopping the MAC transmitter.
11
20.3.4 MAC Operation
The MAC module enables the CPU to transmit and receive data over Ethernet in compliance with
the IEEE 802.3-2008 standard. The MAC supports the interface to the PHY and is comprised of a
receive and transmit module whose features are described in the following sections.
20.3.4.1 MAC Transmit Module
MAC transmission is initiated when the TX/RX Controller transmits data with the start of frame (SOF)
signal asserted. When the SOF signal is detected, the MAC accepts the data and begins transmitting
to the PHY. The time required to transmit the frame data after the application initiates transmission
varies, depending on delay factors like inter-frame gap (IFG) delay, time to transmit preamble or
start of frame data (SFD), and any back-off delays for half-duplex mode. Until then, the MAC does
not accept data received from the TX/RX Controller.
After the end-of-frame (EOF) is transferred to the MAC, the MAC completes the normal transmission
and gives the status of transmission to the TX/RX Controller. If a normal collision (in half-duplex
mode) occurs during transmission, the MAC conveys the transmit status to the TX/RX Controller.
It then accepts and drops all further data until the next SOF is received. The TX/RX Controller should
retransmit the same frame from SOF on observing a retry request (in the transmit status word) from
the MAC. The MAC issues an underflow status if the TX/RX Controller is not able to provide the
data continuously during the transmission. During the normal transfer of a frame from the TX/RX
Controller, if the MAC receives an SOF without getting an EOF for the previous frame, it ignores
the SOF and considers the new frame as a continuation of the previous frame.
If the number of bytes received from memory is less than 60 bytes, zeros are automatically appended
to the transmitting frame to make the data length exactly 46 bytes to meet the minimum data field
requirement of IEEE802.3.
The transmit engine controls the operation of Ethernet frame transmission. Some of the functions
of the transmit module include:
■ Output of (32-bit) Transmit Status (TDES0) to the application at the end of normal transmission
or collision
■ Generating preamble and Start of Frame Data (SFD)
■ Generating jam pattern in half-duplex mode
■ Supporting Jabber time-out
■ Supporting flow control
■ Generating timestamp information for transmission
■ Scheduling frame transmission to satisfy inter-frame gap (IFG) and back-off delays
■ Generating CRC and FCS field for Ethernet Frame
■ Generating pause frames as necessary in full duplex mode
June 18, 20141438
Texas Instruments-Production Data
Ethernet Controller
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