Beats without addresses

AXI-Stream carries a flow of payload beats. There are no addresses, so position in the stream gives each beat its meaning.

The source offers a beat with TVALID. The sink shows it can accept with TREADY. The beat transfers on the edge where both are 1.

The signals

TSTRB, TID, TDEST and TUSER also exist in the standard. They mark sparse byte lanes, stream IDs, destinations and user sideband. This topic leaves them out of scope.

The two equations

Two Boolean relations identify transfers and packet ends. They require the temporal offer/hold rules as well.

fire    = TVALID && TREADY
pkt_end = fire && TLAST

A packet ends on the beat where TVALID, TREADY and TLAST are all 1. Count transfers, not levels: only edges with fire = 1 move data.

Read each row as one clock edge. Only the last two rows move data, and only the very last closes a packet. The testbench below checks all eight input combinations against these columns.

The source asserts TVALID when a beat is available without waiting for TREADY. Once offered, TVALID remains asserted and TDATA's valid bytes, TKEEP and TLAST stay stable until acceptance, including the accepting edge. The sink may wait for TVALID before asserting TREADY. See Rules That Keep Flow Control Safe for the shared handshake rules.

Common mistakes

Testbench

The two equations plus a testbench that checks all eight input combinations.

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module axis_fire (
  input  logic tvalid,
  input  logic tready,
  input  logic tlast,
  output logic fire,
  output logic pkt_end
);
  assign fire = tvalid && tready;
  assign pkt_end = fire && tlast;
endmodule

module tb_axis_fire;
  logic tvalid, tready, tlast, fire, pkt_end;
  axis_fire dut (.*);
  int errors = 0;
  initial begin
    for (int i = 0; i < 8; i++) begin
      tvalid = i[2]; tready = i[1]; tlast = i[0]; #1;
      if (fire !== (tvalid && tready)) errors++;
      if (pkt_end !== (fire && tlast)) errors++;
    end
    if (errors == 0) $display("PASS: fire and packet-end equations hold for all 8 input combinations");
    else $display("FAIL: %0d errors", errors);
    $finish;
  end
endmodule

Related topics

• A Stream Pipeline Register

• Packets, TLAST, TKEEP and Monitors