AXI Stream FIFO#

The AXI Stream FIFO core is a simple FIFO (First Input First Output) with AXI streaming interfaces, supporting synchronous and asynchronous operation modes. It can be used to mitigate data rate differences or transfer an AXI stream to a different clock domain.

Features#

Supports Intel/Altera and AMD Xilinx devices
Configurable data width and depth
Supports asynchronous (double clocked) mode
Supports TLAST to indicate packet boundary
Supports FULL/EMPTY and ALMOST_FULL/ALMOST_EMPTY status signals
Supports zero-deep implementation

Files#

Name	Description
library/util_axis_fifo/util_axis_fifo.v	Verilog source for the peripheral.

Configuration Parameters#

Name	Description	Default Value	Choices/Range
`DATA_WIDTH`	Data width of AXI streaming interface.	`64`	8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096
`ADDRESS_WIDTH`	Width of the address, defines the depth of the FIFO.	`5`	From 0 to 4096.
`ASYNC_CLK`	Clocking mode. If set, the FIFO operates on asynchronous mode.	`1`	Synchronous (0), Asynchronous (1)
`M_AXIS_REGISTERED`	Add and additional register stage to the AXI stream master interface.	`True`
`ALMOST_EMPTY_THRESHOLD`	Defines the offset (in data beats) between the almost empty and empty assertion.	`16`
`ALMOST_FULL_THRESHOLD`	Defines the offset (in data beats) between the almost full and full assertion.	`16`
`TLAST_EN`	Enable `TLAST` logical port on the AXI streaming interface.	`False`
`TKEEP_EN`	Enable `TKEEP` logical port on the AXI streaming interface.	`True`
`REMOVE_NULL_BEAT_EN`	REMOVE_NULL_BEAT_EN Enable.	`False`

Interface#

s_axis

Slave AXI stream.

Physical Port	Logical Port	Direction	Dependency
`s_axis_valid`	`TVALID`	in
`s_axis_ready`	`TREADY`	out
`s_axis_data`	`TDATA`	in [63:0]
`s_axis_tlast`	`TLAST`	in	`TLAST_EN = 1`
`s_axis_tkeep`	`TKEEP`	in [7:0]	`TKEEP_EN = 1`

m_axis

Master AXI stream.

Physical Port	Logical Port	Direction	Dependency
`m_axis_valid`	`TVALID`	out
`m_axis_ready`	`TREADY`	in
`m_axis_data`	`TDATA`	out [63:0]
`m_axis_tlast`	`TLAST`	out	`TLAST_EN = 1`
`m_axis_tkeep`	`TKEEP`	out [7:0]	`TKEEP_EN = 1`

Ports

Physical Port	Direction	Description
`m_axis_aclk`	in	Master AXI stream clock signal Bus `m_axis` is synchronous to this clock domain.
`m_axis_aresetn`	in	Master AXI stream reset signal (active low) Bus `m_axis` is synchronous to this reset signal.
`m_axis_level`	out [4:0]	Indicates how much data is in the FIFO
`m_axis_empty`	out	If set, the FIFO is empty
`m_axis_almost_empty`	out	If set, the FIFO is almost empty
`s_axis_aclk`	in	Slave AXI stream clock signal Bus `s_axis` is synchronous to this clock domain.
`s_axis_aresetn`	in	Slave AXI stream reset signal (active low) Bus `s_axis` is synchronous to this reset signal.
`s_axis_room`	out [4:0]	Indicates how much space (in data beats) is in the FIFO
`s_axis_full`	out	If set, the FIFO is full
`s_axis_almost_full`	out	If set, the FIFO is almost full

Detailed Description#

The util_axis_fifo is a generic First Input First Output module, that can be used to control clock and data rate differences or to do data buffering on a AXI4 stream based data path. FIFO’s write interface is an AXI4 slave streaming interface, and the FIFO’s read interface is an AXI4 master streaming interface. The depth of the FIFO is defined by the equation, which is a function of the ADDRESS_WIDTH and DATA_WIDTH parameters:

FIFO depth in bytes = DATA_WIDTH/8 * 2 ^ ADDRESS_WIDTH

The FIFO has three different status indicator ports on both side, which provides information about the state of the FIFO for both the source and destination logic:

FULL or EMPTY - If these outputs are asserted, the FIFO is full or empty. In case of a full FIFO, all the write operations are suspended. In case of an empty FIFO, all the read operations are suspended.
ALMOST_EMPTY/ALMOST_FULL - It can be used to foresee a potential FULL or EMPTY state, asserting before the EMPTY/FULL before a predefined number of words. The offset between ALMOST_EMPTY and EMPTY, and between ALMOST_FULL and FULL can be set by using the parameters ALMOST_EMPTY_THRESHOLD and ALMOST_FULL_THRESHOLD.
S_AXIS_ROOM - Indicate how many words can be written in the FIFO at the current moment, until the FIFO becomes FULL.
M_AXIS_LEVEL - Indicate how many words can be read from the FIFO at the current moment, until the FIFO becomes EMPTY.

Important

In case of asynchronous mode, because of the delays introduced by the clock domain crossing logic, the ROOM and LEVEL indicators can not reflect the actual state of the FIFO in real time. Source and destination logic should take this into account when controlling the data stream into and from the FIFO. Carefully adjusting the ALMOST_EMPTY/ALMOST_FULL indicators can provide a safe operating margin.