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`timescale 1ns / 1ps
module cdc_bus_pulse
(
src_clk, src_din, src_req,
dst_clk, dst_dout, dst_pulse
);
/* This module is based on design suggested on page 27 of an article titled
"Clock Domain Crossing (CDC) Design & Verification Techniques Using SystemVerilog"
by Clifford E. Cummings (Sunburst Design, Inc.)
*/
//
// Parameters
//
parameter DATA_WIDTH = 32; // width of data bus
//
// Ports
//
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
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