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Fix CDC timing violations: add synchronizers for all inter-clock crossings

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Resolve all 4 inter-clock timing violations found in Vivado synthesis
attempt #11 (WNS was -2.552 ns). Changes:

- Add reset synchronizer for clk_120m_dac domain (2-FF chain)
- Add Gray-code CDC for chirp_counter (6-bit, 120MHz->100MHz)
- Add single-bit CDC for new_chirp_frame (3-stage, 120MHz->100MHz)
- Add 2-stage input synchronizers for valid signals in USB module
  (clk_100m->ft601_clk_in) with data capture on rising edge
- Fix ft601_clk_out multi-driven net (removed duplicate assignment)
- Update XDC: set_max_delay -datapath_only for CDC, false_path for reset

Result: Vivado attempt #12 passes with 0 errors, 0 timing violations,
and 'All user specified timing constraints are met.' (WNS +0.983 ns)
This commit is contained in:
Jason
2026-03-15 17:58:14 +02:00
parent eefaf94e9e
commit ffed7c1623
2 changed files with 121 additions and 23 deletions
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9_Firmware/9_2_FPGA/usb_data_interface.v
+65 -13
View File
@@ -58,6 +58,57 @@ reg [31:0] data_buffer;
reg [31:0] ft601_data_out;
reg ft601_data_oe; // Output enable for bidirectional data bus
// ========== CDC INPUT SYNCHRONIZERS (clk domain -> ft601_clk_in domain) ==========
// The valid signals arrive from clk_100m but the state machine runs on ft601_clk_in.
// Even though both are 100 MHz, they are asynchronous clocks and need synchronization.
// 2-stage synchronizers for valid signals
reg [1:0] range_valid_sync;
reg [1:0] doppler_valid_sync;
reg [1:0] cfar_valid_sync;
// Synchronized data captures (registered in ft601_clk_in domain)
reg [31:0] range_profile_cap;
reg [15:0] doppler_real_cap;
reg [15:0] doppler_imag_cap;
reg cfar_detection_cap;
wire range_valid_ft;
wire doppler_valid_ft;
wire cfar_valid_ft;
always @(posedge ft601_clk_in or negedge reset_n) begin
if (!reset_n) begin
range_valid_sync <= 2'b00;
doppler_valid_sync <= 2'b00;
cfar_valid_sync <= 2'b00;
range_profile_cap <= 32'd0;
doppler_real_cap <= 16'd0;
doppler_imag_cap <= 16'd0;
cfar_detection_cap <= 1'b0;
end else begin
// Synchronize valid strobes
range_valid_sync <= {range_valid_sync[0], range_valid};
doppler_valid_sync <= {doppler_valid_sync[0], doppler_valid};
cfar_valid_sync <= {cfar_valid_sync[0], cfar_valid};
// Capture data on rising edge of synchronized valid
if (range_valid_sync[0] && !range_valid_sync[1])
range_profile_cap <= range_profile;
if (doppler_valid_sync[0] && !doppler_valid_sync[1]) begin
doppler_real_cap <= doppler_real;
doppler_imag_cap <= doppler_imag;
end
if (cfar_valid_sync[0] && !cfar_valid_sync[1])
cfar_detection_cap <= cfar_detection;
end
end
// Rising-edge detect on synchronized valid (pulse in ft601_clk_in domain)
assign range_valid_ft = range_valid_sync[0] && !range_valid_sync[1];
assign doppler_valid_ft = doppler_valid_sync[0] && !doppler_valid_sync[1];
assign cfar_valid_ft = cfar_valid_sync[0] && !cfar_valid_sync[1];
// FT601 data bus direction control
assign ft601_data = ft601_data_oe ? ft601_data_out : 32'hzzzz_zzzz;
@@ -74,13 +125,14 @@ always @(posedge ft601_clk_in or negedge reset_n) begin
ft601_rd_n <= 1;
ft601_oe_n <= 1;
ft601_siwu_n <= 1;
ft601_clk_out <= 0;
// NOTE: ft601_clk_out is driven by the clk-domain always block below.
// Do NOT assign it here (ft601_clk_in domain) — causes multi-driven net.
end else begin
case (current_state)
IDLE: begin
ft601_wr_n <= 1;
ft601_data_oe <= 0; // Release data bus
if (range_valid || doppler_valid || cfar_valid) begin
if (range_valid_ft || doppler_valid_ft || cfar_valid_ft) begin
current_state <= SEND_HEADER;
byte_counter <= 0;
end
@@ -102,10 +154,10 @@ always @(posedge ft601_clk_in or negedge reset_n) begin
ft601_be <= 2'b11; // All bytes valid for 32-bit word
case (byte_counter)
0: ft601_data_out <= range_profile;
1: ft601_data_out <= {range_profile[23:0], 8'h00};
2: ft601_data_out <= {range_profile[15:0], 16'h0000};
3: ft601_data_out <= {range_profile[7:0], 24'h000000};
0: ft601_data_out <= range_profile_cap;
1: ft601_data_out <= {range_profile_cap[23:0], 8'h00};
2: ft601_data_out <= {range_profile_cap[15:0], 16'h0000};
3: ft601_data_out <= {range_profile_cap[7:0], 24'h000000};
endcase
ft601_wr_n <= 0;
@@ -120,15 +172,15 @@ always @(posedge ft601_clk_in or negedge reset_n) begin
end
SEND_DOPPLER_DATA: begin
if (!ft601_txe && doppler_valid) begin
if (!ft601_txe && doppler_valid_ft) begin
ft601_data_oe <= 1;
ft601_be <= 2'b11;
case (byte_counter)
0: ft601_data_out <= {doppler_real, doppler_imag};
1: ft601_data_out <= {doppler_imag, doppler_real[15:8], 8'h00};
2: ft601_data_out <= {doppler_real[7:0], doppler_imag[15:8], 16'h0000};
3: ft601_data_out <= {doppler_imag[7:0], 24'h000000};
0: ft601_data_out <= {doppler_real_cap, doppler_imag_cap};
1: ft601_data_out <= {doppler_imag_cap, doppler_real_cap[15:8], 8'h00};
2: ft601_data_out <= {doppler_real_cap[7:0], doppler_imag_cap[15:8], 16'h0000};
3: ft601_data_out <= {doppler_imag_cap[7:0], 24'h000000};
endcase
ft601_wr_n <= 0;
@@ -143,10 +195,10 @@ always @(posedge ft601_clk_in or negedge reset_n) begin
end
SEND_DETECTION_DATA: begin
if (!ft601_txe && cfar_valid) begin
if (!ft601_txe && cfar_valid_ft) begin
ft601_data_oe <= 1;
ft601_be <= 2'b01;
ft601_data_out <= {24'b0, 7'b0, cfar_detection};
ft601_data_out <= {24'b0, 7'b0, cfar_detection_cap};
ft601_wr_n <= 0;
current_state <= SEND_FOOTER;
end