ST STM32F40 Series Reference Manual page 921

Ethernet (ETH): media access control (MAC) with DMA controller
remains full at a configurable number of slot-times (PLT bits in ETH_MACFCR) before
this Pause time runs out, a second Pause frame is transmitted. The process is
repeated as long as the receive FIFO remains full. If this condition is no more satisfied
prior to the sampling time, the MAC transmits a Pause frame with zero pause time to
indicate to the remote end that the receive buffer is ready to receive new data frames.
Single-packet transmit operation
The general sequence of events for a transmit operation is as follows:
1. If the system has data to be transferred, the DMA controller fetches them from the
memory through the AHB Master interface and starts forwarding them to the FIFO. It
continues to receive the data until the end of frame is transferred.
2. When the threshold level is crossed or a full packet of data is received into the FIFO,
the frame data are popped and driven to the MAC core. The DMA continues to transfer
data from the FIFO until a complete packet has been transferred to the MAC. Upon
completion of the frame, the DMA controller is notified by the status coming from the
MAC.
Transmit operation—Two packets in the buffer
1. Because the DMA must update the descriptor status before releasing it to the Host,
there can be at the most two frames inside a transmit FIFO. The second frame is
fetched by the DMA and put into the FIFO only if the OSF (operate on second frame) bit
is set. If this bit is not set, the next frame is fetched from the memory only after the MAC
has completely processed the frame and the DMA has released the descriptors.
2. 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. In the meantime, the second frame is received into the FIFO while the first
frame is being transmitted. As soon as the first frame has been transferred and the
status is received from the MAC, it is pushed to the DMA. If the DMA has already
completed sending the second packet to the FIFO, the second transmission must wait
for the status of the first packet before proceeding to the next frame.
Retransmission during collision
While a frame is being transferred to the MAC, a collision event may occur on the MAC line
interface in Half-duplex mode. The MAC would then indicate a retry attempt by giving the
status even before the end of frame is received. Then the retransmission is enabled and the
frame is popped out again from the FIFO. After more than 96 bytes have been popped
towards the MAC core, the FIFO controller frees up that space and makes it available to the
DMA to push in more data. This means that the retransmission is not possible after this
threshold is crossed or when the MAC core indicates a late collision event.
Transmit FIFO flush operation
The MAC provides a control to the software to flush the Transmit FIFO through the use of Bit
20 in the Operation mode register. The Flush operation is immediate and the Tx FIFO and
the corresponding pointers are cleared to the initial state even if the Tx FIFO is in the middle
of transferring a frame to the MAC Core. This results in an underflow event in the MAC
transmitter, and the frame transmission is aborted. The status of such a frame is marked
with both underflow and frame flush events (TDES0 bits 13 and 1). No data are coming to
the FIFO from the application (DMA) during the Flush operation. Transfer transmit status
words are transferred to the application for the number of frames that is flushed (including
partial frames). Frames that are completely flushed have the Frame flush status bit (TDES0
13) set. The Flush operation is completed when the application (DMA) has accepted all of
921/1422 Doc ID 018909 Rev 4
RM0090