//======================================================================
//
// cdc_bus_pulse.v
// ---------------
// Clock Domain Crossing handler for the Cryptech Novena
// FPGA framework design.
//
// This module is based on design suggested on page 27 of the
// paper 'Clock Domain Crossing (CDC) Design & Verification Techniques
// Using SystemVerilog' by Clifford E. Cummings (Sunburst Design, Inc.)
//
//
// Author: Pavel Shatov
// Copyright (c) 2015, NORDUnet A/S All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// - Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// - Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// - Neither the name of the NORDUnet nor the names of its contributors may
// be used to endorse or promote products derived from this software
// without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
// TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
//======================================================================
module cdc_bus_pulse
#(parameter DATA_WIDTH = 32) // width of data bus
(
input wire src_clk, // source domain clock
input wire [DATA_WIDTH-1:0] src_din, // data from source clock domain
input wire src_req, // start transfer pulse from source clock domain
input wire dst_clk, // destination domain clock
output wire [DATA_WIDTH-1:0] dst_dout, // data to destination clock domain
output wire dst_pulse // transfer done pulse to destination clock domain
);
//
// Source Side Registers
//
reg src_ff = 1'b0; // transfer request flag
reg [DATA_WIDTH-1:0] src_latch = {DATA_WIDTH{1'bX}}; // source data buffer
//
// Source Request Handler
//
always @(posedge src_clk)
//
if (src_req) begin // transfer request pulse?
src_ff <= ~src_ff; // toggle transfer request flag...
src_latch <= src_din; // ... and capture data in source buffer
end
//
// Source -> Destination Flag Sync Logic
//
/* ISE may decide to infer SRL here, so we explicitly instantiate slice registers. */
wire flag_sync_first; // first FF output
wire flag_sync_second; // second FF output
wire flag_sync_third; // third FF output
wire flag_sync_pulse; // flag toggle detector output
FDCE ff_sync_first
(
.C(dst_clk),
.D(src_ff), // capture flag from another clock domain
.Q(flag_sync_first), // metastability can occur here
.CLR(1'b0),
.CE(1'b1)
);
FDCE ff_sync_second
(
.C(dst_clk),
.D(flag_sync_first), // synchronize captured flag to remove metastability
.Q(flag_sync_second), // and pass it to another flip-flop
.CLR(1'b0),
.CE(1'b1)
);
FDCE ff_sync_third
(
.C(dst_clk),
.D(flag_sync_second), // delay synchronized flag in another flip-flip, because we need
.Q(flag_sync_third), // two synchronized flag values (current and delayed) to detect its change
.CLR(1'b0),
.CE(1'b1)
);
// when delayed flag value differs from its current value, it was changed
// by the source side, so there must have been a transfer request
assign flag_sync_pulse = flag_sync_second ^ flag_sync_third;
//
// Destination Side Registers
//
reg dst_pulse_reg = 1'b0; // transfer done flag
reg [DATA_WIDTH-1:0] dst_latch = {DATA_WIDTH{1'bX}}; // destination data buffer
assign dst_pulse = dst_pulse_reg;
assign dst_dout = dst_latch;
//
// Destination Request Handler
//
always @(posedge dst_clk) begin
//
dst_pulse_reg <= flag_sync_pulse; // generate pulse if flag change was detected
//
if (flag_sync_pulse)
dst_latch <= src_latch;
/* By the time destination side receives synchronized flag
* value, data should be stable, we can safely capture and store
* it in the destination buffer.
*/
end
endmodule
//======================================================================
// EOF cdc_bus_pulse.v
//======================================================================
