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//
// Copyright 2014 Ettus Research LLC
// Copyright 2018 Ettus Research, a National Instruments Company
//
// SPDX-License-Identifier: LGPL-3.0-or-later
//
//
// Ultra fast critical path FIFO.
// Only 2 entrys but no combinatorial feed through paths
//
module axi_fast_fifo
#(parameter WIDTH=64)
(
input clk,
input reset,
input clear,
//
input [WIDTH-1:0] i_tdata,
input i_tvalid,
output reg i_tready,
//
output [WIDTH-1:0] o_tdata,
output reg o_tvalid,
input o_tready
);
reg [WIDTH-1:0] data_reg1, data_reg2;
reg [1:0] state;
localparam EMPTY = 0;
localparam HALF = 1;
localparam FULL = 2;
always @(posedge clk)
if (reset | clear) begin
state <= EMPTY;
data_reg1 <= 0;
data_reg2 <= 0;
o_tvalid <= 1'b0;
i_tready <= 1'b0;
end else begin
case (state)
// Nothing in either register.
// Upstream can always push data to us.
// Downstream has nothing to take from us.
EMPTY: begin
if (i_tvalid) begin
data_reg1 <= i_tdata;
state <= HALF;
i_tready <= 1'b1;
o_tvalid <= 1'b1;
end else begin
state <= EMPTY;
i_tready <= 1'b1;
o_tvalid <= 1'b0;
end
end
// First Register Full.
// Upstream can always push data to us.
// Downstream can always read from us.
HALF: begin
if (i_tvalid && o_tready) begin
data_reg1 <= i_tdata;
state <= HALF;
i_tready <= 1'b1;
o_tvalid <= 1'b1;
end else if (i_tvalid) begin
data_reg1 <= i_tdata;
data_reg2 <= data_reg1;
state <= FULL;
i_tready <= 1'b0;
o_tvalid <= 1'b1;
end else if (o_tready) begin
state <= EMPTY;
i_tready <= 1'b1;
o_tvalid <= 1'b0;
end else begin
state <= HALF;
i_tready <= 1'b1;
o_tvalid <= 1'b1;
end
end // case: HALF
// Both Registers Full.
// Upstream can not push to us in this state.
// Downstream can always read from us.
FULL: begin
if (o_tready) begin
state <= HALF;
i_tready <= 1'b1;
o_tvalid <= 1'b1;
end
else begin
state <= FULL;
i_tready <= 1'b0;
o_tvalid <= 1'b1;
end
end
endcase // case(state)
end // else: !if(reset | clear)
assign o_tdata = (state == FULL) ? data_reg2 : data_reg1;
endmodule // axi_fast_fifo
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