Functional Description
913
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
Table 15-17. RX MAC Flow Control (continued)
TFE Bit in
EMACFLOWCTL
DUPM Bit in
EMACCFG
Description
1
0
The MAC receiver does not detect the received Pause frames but recognizes such
frames as Control frames.
1
1
The MAC receiver detects or processes the Pause frames and responds to such frames
by stopping the MAC transmitter.
15.3.5 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.
15.3.5.1 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 IEEE 802.3.
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
When a new frame transmission is requested, the MAC transmit module sends out a preamble and SFD,
followed by the data in the TX FIFO. The preamble is defined as 7 bytes of 0xAA and the SFD is defined
as 1 byte of 0xAB pattern.