fpga: lib: Add AXI-Stream splitter (axis_split)

The axis_split module takes a single AXI-Stream input and duplicates it
onto multiple AXI-Stream outputs. This block correctly handles the
somewhat tricky flow-control logic so that the AXI-Stream handshake
protocol is honored at all top-level ports.

2 files changed, 129 insertions, 0 deletions

diff --git a/fpga/usrp3/lib/axi/Makefile.srcs b/fpga/usrp3/lib/axi/Makefile.srcs
index 28f63104d..bf8b3dbd2 100644
--- a/fpga/usrp3/lib/axi/Makefile.srcs
+++ b/fpga/usrp3/lib/axi/Makefile.srcs
@@ -33,4 +33,5 @@ axis_shift_register.v \
 axis_upsizer.v \
 axis_downsizer.v \
 axis_width_conv.v \
+axis_split.v \
 ))
diff --git a/fpga/usrp3/lib/axi/axis_split.v b/fpga/usrp3/lib/axi/axis_split.v
new file mode 100644
index 000000000..26759eaa2
--- /dev/null
+++ b/fpga/usrp3/lib/axi/axis_split.v
@@ -0,0 +1,128 @@
+//
+// Copyright 2020 Ettus Research, a National Instruments Brand
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+//
+// Module:  axis_split
+//
+// Description:
+//
+//   This module takes a single AXI-Stream input and duplicates it onto
+//   multiple AXI-Stream outputs. This block correctly handles the somewhat
+//   tricky flow-control logic so that the AXI-Stream handshake protocol is
+//   honored at all top-level ports.
+//
+//   The internal buffering is finite, so if the data from any of the output
+//   ports can't be consumed then the flow-control logic will cause the input
+//   to stall (i.e., s_axis_tready will deassert).
+//
+// Parameters:
+//
+//   DATA_W    : The bit width of tdata for all ports.
+//   NUM_PORTS : The number of output ports on which to duplicate the input.
+//   INPUT_REG : Set to 1 to add an input register stage to break combinatorial
+//               paths. Set to 0 to allow combinatorial input paths.
+//
+
+
+module axis_split #(
+  parameter DATA_W     = 32,
+  parameter NUM_PORTS  = 4,
+  parameter INPUT_REG  = 0
+) (
+  input wire clk,
+  input wire rst,
+
+  // Input AXI-Stream
+  input  wire [DATA_W-1:0] s_axis_tdata,
+  input  wire              s_axis_tvalid,
+  output wire              s_axis_tready,
+
+  // Output AXI-Streams
+  output wire [DATA_W*NUM_PORTS-1:0] m_axis_tdata,
+  output wire [       NUM_PORTS-1:0] m_axis_tvalid,
+  input  wire [       NUM_PORTS-1:0] m_axis_tready
+);
+
+  // Output of the input-register stage
+  wire [DATA_W-1:0] reg_tdata;
+  wire              reg_tvalid;
+  wire              reg_tready;
+
+  // Input to the Output FIFO stage
+  wire [DATA_W*NUM_PORTS-1:0] fifo_tdata;
+  wire [       NUM_PORTS-1:0] fifo_tvalid;
+  wire [       NUM_PORTS-1:0] fifo_tready;
+
+  // Indicates all output FIFOs are ready for a transfer
+  wire all_fifo_tready;
+
+
+  //---------------------------------------------------------------------------
+  // Optional Input Register
+  //---------------------------------------------------------------------------
+
+  if (INPUT_REG) begin : gen_input_reg
+    axi_fifo_flop2 #(
+      .WIDTH (DATA_W)
+    ) axi_fifo_flop2_i (
+      .clk      (clk),
+      .reset    (rst),
+      .clear    (1'b0),
+      .i_tdata  (s_axis_tdata),
+      .i_tvalid (s_axis_tvalid),
+      .i_tready (s_axis_tready),
+      .o_tdata  (reg_tdata),
+      .o_tvalid (reg_tvalid),
+      .o_tready (reg_tready),
+      .space    (),
+      .occupied ()
+    );
+  end else begin : gen_no_input_reg
+    assign reg_tdata     = s_axis_tdata;
+    assign reg_tvalid    = s_axis_tvalid;
+    assign s_axis_tready = reg_tready;
+  end
+
+
+  //---------------------------------------------------------------------------
+  // Forking Logic
+  //---------------------------------------------------------------------------
+
+  assign all_fifo_tready = &fifo_tready;
+
+  // Data transfer occurs when we have valid data on the input and all output
+  // FIFOs are ready to accept data. Note that having tvalid depend on tready
+  // is normally not allowed, but the FIFO has been chosen to tolerate this.
+  assign reg_tready  = all_fifo_tready;
+  assign fifo_tvalid = { NUM_PORTS {all_fifo_tready & reg_tvalid} };
+  assign fifo_tdata  = { NUM_PORTS {reg_tdata} };
+
+
+  //---------------------------------------------------------------------------
+  // Output FIFOs
+  //---------------------------------------------------------------------------
+
+  genvar i;
+  for (i = 0; i < NUM_PORTS; i = i + 1) begin : gen_ports
+
+    // We use axi_fifo_short specifically because it can tolerate tvalid
+    // de-asserting at any time. This is normally not allowed by AXI-Stream.
+    axi_fifo_short #(
+      .WIDTH (DATA_W)
+    ) axi_fifo_short_i (
+      .clk      (clk),
+      .reset    (rst),
+      .clear    (1'b0),
+      .i_tdata  (fifo_tdata[i*DATA_W+:DATA_W]),
+      .i_tvalid (fifo_tvalid[i]),
+      .i_tready (fifo_tready[i]),
+      .o_tdata  (m_axis_tdata[i*DATA_W+:DATA_W]),
+      .o_tvalid (m_axis_tvalid[i]),
+      .o_tready (m_axis_tready[i]),
+      .space    (),
+      .occupied ()
+    );
+  end
+
+endmodule