The synchronous RX interface provides packet-oriented data much like the TX AXI4-Stream interface accepts. All signals are synchronous with the
rising-edge of the c0_axi_clk clock. The following figure shows a
sample waveform for two data transactions for 65-byte packets using a 512-bit segmented
bus.
Similar to the TX AXI4-Stream interface, the RX AXI4-Stream interface is divided into 128-bit segments, with multiple
transfers presented in parallel during the same clock cycle. The number of segments is
based on the width of the bus as programmed in the
c0_ctl_rx_axis_width[2:0] field of the CFG_C0_RX_OVERALL_REG
register. The first of the 128-bit transfers occurs on segment 0 (that is,
rx_axis_tdata0), the second on segment 1 (that is,
rx_axis_tdata1), and so forth.
Data is supplied on segment <M> by the ILKNF subsystem on every c0_axi_clk clock cycle when rx_axis_tuser_ena<M> is asserted. This signal qualifies the other
outputs of segment <M> of the RX AXI4-Stream
interface. The RX core logic stores incoming data and does not forward it until it has a
sufficient quantity. Consequently, there can be clock cycles where none of the rx_axis_tuser_ena<M> signals are asserted.
In any cycle where any rx_axis_tuser_ena<M> is asserted, rx_axis_tuser_ena0 is also required to be asserted. Furthermore, segments
are filled in sequence with no gaps between active segments. For example, if rx_axis_tuser_ena<i> is set to 1, then rx_axis_tuser_ena<i - 1> is also set to 1, for i
ranging from:
- 1 to 15 for a 2048-bit bus
- 1 to 11 for a 1536-bit bus
- 1 to 7 for a 1024-bit bus
- 1 to 3 for a 512-bit bus
The RX is similar to the TX, in that rx_axis_tuser_sop<M> identifies the start of a packet on segment
<M> and rx_axis_tuser_eop<M> identifies the
end of a packet on segment <M>. Both rx_axis_tuser_sop<M> and rx_axis_tuser_eop<M> are asserted during the same cycle for packets
that are less than or equal to the segment width.
As in the TX, the first byte of a packet that starts on segment <M> is
supplied on bits [7:0] of rx_axis_tdata<M>, the second byte on bits
[15:8], and so forth.
Portions of packets are written on the bus segments in the full width of the
segment (16 bytes) unless for the last segment of the packet. When rx_axis_tuser_eop<M> is asserted on segment
<M>, the rx_axis_tuser_mty<M> bus indicates
how many bytes in the segment are invalid. The encoding is the same as for tx_axis_tuser_mty<M>.
During the last cycle of a packet, when rx_axis_tuser_eop<M> is asserted with rx_axis_tuser_ena<M>, rx_axis_tuser_err<M> can also be asserted to indicate an error in the
packet ended on segment<M>.
There is no mechanism to backpressure the RX AXI4-Stream interface. The user logic must be capable of receiving data
when rx_axis_tuser_ena<M> is asserted. The
Interlaken flow control mechanism can be used to stop the flow of data, either using the
in-band or out of band protocol, or both.
The data provided by the RX AXI4-Stream
interface is in the same sequence as it is received from the Interlaken bus. Packets can
be interleaved and are distinguished using the channel number presented on rx_axis_tuser_chan<M>.
The RX segmented AXI4-Stream bus can contain more than one SOP and more than one EOP. There is no requirement to accommodate Burst Control Words on the receive side AXI4-Stream interface (they have already been processed) and therefore packets can be packed as efficiently as possible when they arrive.