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//
// FIFO backed by an off chip ZBT/NoBL SRAM.
//
// This module and its sub-hierarchy implment a FIFO capable of sustaining
// a data throughput rate of at least int_clk/2 * 36bits and bursts of int_clk * 36bits.
//
// This has been designed and tested for an int_clk of 100MHz and an ext_clk of 125MHz,
// your milage may vary with other clock ratio's especially those where int_clk < ext_clk.
// Testing has also exclusively used a rst signal synchronized to int_clk.
//
// Interface operation mimics a Xilinx FIFO configured as "First Word Fall Through",
// though signal naming differs.
//
// For FPGA use registers interfacing directly with signals prefixed "RAM_*" should be
// packed into the IO ring.
//
module ext_fifo
#(parameter INT_WIDTH=36,EXT_WIDTH=18,DEPTH=19)
(
input int_clk,
input ext_clk,
input rst,
input [EXT_WIDTH-1:0] RAM_D_pi,
output [EXT_WIDTH-1:0] RAM_D_po,
output RAM_D_poe,
output [DEPTH-1:0] RAM_A,
output RAM_WEn,
output RAM_CENn,
output RAM_LDn,
output RAM_OEn,
output RAM_CE1n,
input [INT_WIDTH-1:0] datain,
input src_rdy_i, // WRITE
output dst_rdy_o, // not FULL
output [INT_WIDTH-1:0] dataout,
output src_rdy_o, // not EMPTY
input dst_rdy_i // READ
);
wire [EXT_WIDTH-1:0] write_data;
wire [EXT_WIDTH-1:0] read_data;
wire full1, empty1;
wire almost_full2, full2, empty2;
wire [INT_WIDTH-1:0] data_to_fifo;
wire [INT_WIDTH-1:0] data_from_fifo;
// FIFO buffers data from UDP engine into external FIFO clock domain.
fifo_xlnx_512x36_2clk_36to18 fifo_xlnx_512x36_2clk_36to18_i1 (
.rst(rst),
.wr_clk(int_clk),
.rd_clk(ext_clk),
.din(datain), // Bus [35 : 0]
.wr_en(src_rdy_i),
.rd_en(space_avail&~empty1),
.dout(write_data), // Bus [17 : 0]
.full(full1),
.empty(empty1));
assign dst_rdy_o = ~full1;
// External FIFO running at ext clock rate and 18 bit width.
nobl_fifo #(.WIDTH(EXT_WIDTH),.DEPTH(DEPTH))
nobl_fifo_i1
(
.clk(ext_clk),
.rst(rst),
.RAM_D_pi(RAM_D_pi),
.RAM_D_po(RAM_D_po),
.RAM_D_poe(RAM_D_poe),
.RAM_A(RAM_A),
.RAM_WEn(RAM_WEn),
.RAM_CENn(RAM_CENn),
.RAM_LDn(RAM_LDn),
.RAM_OEn(RAM_OEn),
.RAM_CE1n(RAM_CE1n),
.write_data(write_data),
.write_strobe(space_avail & ~empty1 ),
.space_avail(space_avail),
.read_data(read_data),
.read_strobe(data_avail & ~full2),
.data_avail(data_avail),
.upstream_full(almost_full2)
);
// FIFO buffers data read from external FIFO into DSP clk domain and to TX DSP.
fifo_xlnx_512x36_2clk_18to36 fifo_xlnx_512x36_2clk_18to36_i1 (
.rst(rst),
.wr_clk(ext_clk),
.rd_clk(int_clk),
.din(read_data), // Bus [17 : 0]
.wr_en(data_avail & ~full2 ),
.rd_en(dst_rdy_i),
.dout(dataout), // Bus [35 : 0]
.full(full2),
.almost_full(almost_full2),
.empty(empty2));
assign src_rdy_o = ~empty2;
endmodule // ext_fifo
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