Altera Triple Speed Ethernet MegaCore Function - Manual (Page 50)

Description Register Name Threshold
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_rx_a_full signal. If the user
application is not ready to receive data (
ff_rx_rdy
= 0), the MAC
function performs the following operations:
• Stops writing data to the FIFO buffer.
• Truncates received frames to avoid FIFO buffer overflow.
• Asserts the
rx_err[0]
signal when the
ff_rx_eop
signal is
asserted.
• Marks the truncated frame invalid by setting the rx_err[3]
signal to 1.
If the
RX_ERR_DISC
bit in the
command_config
register is set to 1
and the section-full ( rx_section_full ) threshold is set to 0, the
MAC function discards frames with error received on the Avalon-
ST interface.
rx_almost_full
Almost full
An early indication that the FIFO buffer is getting full. When the
level of the FIFO buffer hits this threshold, the MAC function
generates an XOFF pause frame to indicate FIFO congestion to the
remote Ethernet device. When the FIFO level goes below this
threshold, the MAC function generates an XON pause frame to
indicate its readiness to receive new frames.
To avoid data loss, you can use this threshold as an early warning
to the remote Ethernet device on the potential FIFO buffer
congestion before the buffer level hits the almost-full threshold. The
MAC function truncates receive frames when the buffer level hits
the almost-full threshold.
rx_section_empty
Section
empty
The section-full threshold indicates that there are sufficient entries
in the FIFO buffer for the user application to start reading from it.
The MAC function asserts the
ff_rx_dsav signal when the buffer
level hits this threshold.
Set this threshold to 0 to enable store and forward on the receive
datapath. In the store and forward mode, the
ff_rx_dsav
signal
remains deasserted. The MAC function asserts the
ff_rx_dval
signal as soon as a complete frame is written to the FIFO buffer.
rx_section_full Section full
Functional Description Altera Corporation
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UG-01008
Receive Thresholds 4-14
2014.06.30