Achronix Speedster22i 10G/40G/100G Ethernet - Manual (Page 45)

UG029, September 6, 2013 45
Table 23 – FIFO Sections
Configuration
Register
Behavior
RX_FIFO_SECTIONS[15:0]
Section Available: Legacy purpose only.
Not available in this implementation.
RX_FIFO_SECTIONS[31:16]
Section Empty: If this level is reached during receive of
frames, pause frames will be generated to stop the remote
side from transmitting further data.
If set to 0, no automatic pause frame generation is done.
This function can be used only when link pause mode is
enabled. It has no effect otherwise.
TX_FIFO_SECTIONS[15:0]
Section Available: If the defined amount of data is written
into the TX FIFO, transmission will start. Transmission will
also start, when a complete frame has been written, even if
this watermark is not reached (e.g. a short frame).
As the TX FIFO must not dry out during transmission (this
would lead to a corrupted frame sent), this can help to adapt
a slower user application to the MAC. Or it can give the user
application more flexibility inserting wait states during write,
without disturbing the transmission of a frame. If set to 0,
the FIFO operates in store-and-forward mode.
TX_FIFO_SECTIONS[31:16]
Section Empty: Legacy purpose only.
Not available in this implementation.
Flow Control Interface
The MAC supports two modes of flow control:

Link Pause Flow Control: This is the standard IEEE 802.3 defined pause frame to
allow pausing the remote device connected to the link (link local pause). Upon
reception (if enabled by configuration) the transmitter is paused for the time received
in the pause frame.

Priority Flow Control (PFC): The PFC mechanism refines the pause mechanism
allowing for up to 8 traffic classes which can be paused individually. In this mode the
application will get an indication on the pause state of each class. The transmitter is
not affected and it is the responsibility of the application to stop scheduling frames for
transmission to a congested traffic class.
Only one mode can be active at a time: If a frame of the other mode is received it is treated as
a regular command frame. Depending on COMMAND_CONFIG(CMD_FRM_ENA) it is then
either discarded or forwarded to the user application but has no effect within the MAC.
See the COMMAND_CONFIG register for configuration options (see section
“COMMAND_CONFIG Register Bit Definitions” on page 67).
Link Pause Flow Control Frames
The IEEE 802.3 defined pause frame has the following format: