Altera Triple Speed Ethernet MegaCore Function - Manual (Page 51)

Transmit Thresholds
Figure 4-6: Transmit FIFO Thresholds
Network
Switch Fabric
Frame Buffer n
Frame Buffer n - 1
Frame Buffer k
Frame Buffer 1
The remaining
unwritten entries in
the FIFO buffer
before it is full.
Almost full
The remaining
unread entries in
the FIFO buffer
before it is empty.
Almost empty
An early indication
that the FIFO
buffer is getting
full.
Section Empty
Sufficient unread
entries in the FIFO
buffer for the
transmitter to start
transmission.
Section full
Frame Buffer 2
Table 4-7: Transmit Thresholds
Description Register Name Threshold
The number of unread entries in the FIFO buffer before the buffer
is empty. When the level of the FIFO buffer reaches this threshold,
the MAC function asserts the
ff_tx_a_empty
signal. The MAC
function stops reading from the FIFO buffer and sends the Ethernet
frame with GMII / MII/ RGMII error to avoid FIFO underflow.
tx_almost_empty Almost
empty
The number of unwritten entries in the FIFO buffer before the buffer
is full. When the level of the FIFO buffer reaches this threshold, the
MAC function asserts the
ff_tx_a_full
signal. The MAC function
deasserts the ff_tx_rdy signal to backpressure the Avalon-ST
transmit interface.
tx_almost_full Almost full
An early indication that the FIFO buffer is getting full. When the
level of the FIFO buffer reaches this threshold, the MAC function
deasserts the
ff_tx_septy
signal. This threshold can serve as a
warning about potential FIFO buffer congestion.
tx_section_empty Section
empty
This threshold indicates that there are sufficient entries in the FIFO
buffer to start frame transmission.
Set this threshold to 0 to enable store and forward on the transmit
path. When you enable the store and forward mode, the MAC
function forwards each frame as soon as it is completely written to
the transmit FIFO buffer.
tx_section_full
Section full
Altera Corporation Functional Description
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4-15 Transmit Thresholds
UG-01008
2014.06.30