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Functional Description
910
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
15.3.4.1 Transmit (TX) Control Path
The DMA controller is used for all Ethernet transmissions. The Ethernet frames are read from memory and
transferred to the TX FIFO by the DMA. When the MAC is available, the frame is transferred from the
FIFO. When the end-of-frame (EOF) is transferred, the MAC notifies the DMA the status of the
transmission.
The TX FIFO has a depth of 2KB. The FIFO fill level has the capability of triggering the DMA to initiate a
burst transfer. The DMA also transfers start-of-frame (SOF), end-of-frame (EOF), CRC and pad-insertion
information to the TX/RX Controller so that this information can be passed to the MAC when it is ready for
transmission from the TX FIFO.
Data can be transmitted to the MAC in threshold mode or store-and-forward mode. If the TTC field is
configured in the Ethernet MAC DMA Operation Mode (EMACDMAOPMODE) register at offset 0xC18 and
the TSF bit in the same register is 0x0, then the TX Controller is operating in threshold mode. In this
mode, the data is transferred to the MAC when the number of bytes in the FIFO crossed the value
configured in the TTC bit field or when the end-of-frame is written before the threshold is crossed. In store-
and forward mode, the TTC bit field is configured and the TSF bit is set. Data is transferred to the MAC
only when one or more of the following conditions are true:
•
A complete frame is stored in the FIFO
•
The TX FIFO becomes almost full
•
The TX FIFO does not have space to accommodate the requested burst length
With these conditions, the TX Controller continues store-and-forward mode even if the Ethernet frame
length is bigger than the TX FIFO size.
The TX FIFO can be flushed of all contents by setting the FTF bit in the EMACDMAOPMODE register.
This bit is self-clearing and initializes the FIFO pointers to the default state. If the FTF bit is set during a
frame transfer from the TX Controller to the MAC, then the TX Controller stops further transfer. Early
termination of the transfer causes a underflow event and this status is communicated to the DMA.
15.3.4.1.1 Transmit Operation
During a transmit, single-packet or double-packets can reside in the buffer. The following describes the
details of each:
•
Single-packet transmit: During single packet transmission, the DMA controller fetches data from the
CPU memory and forwards it to the TX FIFO and continues to receive data until the end-of-frame is
transferred. The data is transmitted from the TX FIFO to the MAC by the TX/RX Controller when the
threshold level is crossed or a full packet of data is received into the TX FIFO. When the TX/RX
Controller receives acknowledgment from the MAC that it has received the EOF, it notifies the DMA so
another transmit can begin.
•
Two-packet transmit: Because the DMA must update the descriptor status before releasing it to the
CPU, there can be at most two frames inside a transmit FIFO. The second frame is fetched by the
DMA and put into the TX FIFO only if the OSF bit is set in the EMACDMAOPMODE register at offset
0xC18. If this bit is not set, the next frame is fetched from memory only after the MAC has completely
processed the frame and the DMA has released the descriptors.
•
If the OSF bit is set, the DMA starts fetching the second frame immediately after completing the
transfer of the first frame to the FIFO. It does not wait for the status to be updated. The TX/RX
Controller receives the second frame into the FIFO while transmitting the first frame. As soon as the
first frame has been transferred and the status is received from the MAC, the TX/RX Controller sends
the acknowledgment to the DMA. If the DMA has already completed sending the second packet to the
TX/RX Controller, it must wait for the status of the first packet before proceeding to the next frame.
15.3.4.1.2 Collision and Retransmission
If a collision occurs at the MAC application interface while the TX/RX Controller is transferring data to the
MAC, the transmission is aborted and the MAC indicates a retry attempt by giving a collision status before
the EOF is transferred to the TX/RX Controller from the DMA. This enables the TX/RX Controller to retry
transmission of the frame data from the FIFO.