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Pipeline CFAR noise computation: break critical path for timing closure

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Split ST_CFAR_THR into two pipeline stages (THR + MUL) to fix Build 23
timing violation (WNS = -0.309 ns). The combinational path from
leading_sum through GO/SO cross-multiply into alpha*noise DSP was too
long for 10 ns.

New pipeline:
  ST_CFAR_THR: register noise_sum_comb (mode select + cross-multiply)
  ST_CFAR_MUL: compute alpha * noise_sum_reg in DSP
  ST_CFAR_CMP: compare + update window (unchanged)

3 cycles per CUT instead of 2 (~85 us vs 70 us per frame, negligible).
All detection results identical: 23/23 CFAR standalone, 22/22 full
regression, 3/3 real-data co-sim (5137/5137 exact match) PASS.
This commit is contained in:
Jason
2026-03-20 05:24:08 +02:00
parent f71923b67d
commit 0745cc4f48
1 changed files with 34 additions and 10 deletions
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9_Firmware/9_2_FPGA/cfar_ca.v
+34 -10
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@@ -18,8 +18,11 @@
* process each Doppler column independently:
* a) Read 64 magnitudes from BRAM for one Doppler bin (ST_COL_LOAD)
* b) Compute initial sliding window sums (ST_CFAR_INIT)
* c) Slide CUT through all 64 range bins (ST_CFAR_PROC)
* - 2 sub-cycles per CUT: THRESHOLD compute, then COMPARE + window update
* c) Slide CUT through all 64 range bins:
* - 3 sub-cycles per CUT:
* ST_CFAR_THR: register noise_sum (mode select + cross-multiply)
* ST_CFAR_MUL: compute alpha * noise_sum_reg in DSP
* ST_CFAR_CMP: compare CUT magnitude against threshold + update window
* d) Advance to next Doppler column (ST_COL_NEXT)
*
* CFAR Modes (cfg_cfar_mode):
@@ -44,8 +47,9 @@
* typically clutter).
*
* Timing:
* Phase 2 takes ~(66 + T + 2*64) * 32 ≈ 7000 cycles per frame @ 100 MHz
* = 70 µs. Frame period @ PRF=1932 Hz, 32 chirps = 16.6 ms. Fits easily.
* Phase 2 takes ~(66 + T + 3*64) * 32 ≈ 8500 cycles per frame @ 100 MHz
* = 85 µs. Frame period @ PRF=1932 Hz, 32 chirps = 16.6 ms. Fits easily.
* (3 cycles per CUT due to pipeline: THR → MUL → CMP)
*
* Resources:
* - 1 BRAM18K for magnitude buffer (2048 x 17 bits)
@@ -113,7 +117,8 @@ localparam [3:0] ST_IDLE = 4'd0,
ST_BUFFER = 4'd1,
ST_COL_LOAD = 4'd2,
ST_CFAR_INIT = 4'd3,
ST_CFAR_THR = 4'd4, // Compute threshold
ST_CFAR_THR = 4'd4, // Register noise_sum (mode select + cross-multiply)
ST_CFAR_MUL = 4'd8, // Compute alpha * noise_sum_reg in DSP
ST_CFAR_CMP = 4'd5, // Compare + update window
ST_COL_NEXT = 4'd6,
ST_DONE = 4'd7;
@@ -171,8 +176,9 @@ reg [1:0] r_mode;
reg r_enable;
reg [15:0] r_simple_thr;
// Threshold pipeline register
reg [PROD_WIDTH-1:0] noise_product;
// Threshold pipeline registers
reg [SUM_WIDTH-1:0] noise_sum_reg; // Stage 1: registered noise_sum_comb output
reg [PROD_WIDTH-1:0] noise_product; // Stage 2: alpha * noise_sum_reg
reg [MAG_WIDTH-1:0] adaptive_thr;
// Init counter for computing initial lagging sum
@@ -279,6 +285,7 @@ always @(posedge clk or negedge reset_n) begin
leading_count <= 0;
lagging_count <= 0;
init_idx <= 0;
noise_sum_reg <= 0;
noise_product <= 0;
adaptive_thr <= 0;
r_guard <= 4'd2;
@@ -422,13 +429,30 @@ always @(posedge clk or negedge reset_n) begin
end
// ================================================================
// ST_CFAR_THR: Compute adaptive threshold for current CUT
// ST_CFAR_THR: Register noise estimate (mode select + cross-multiply)
// ================================================================
// Register the alpha * noise product. Result used next cycle.
// Pipeline stage 1: register the combinational noise_sum_comb
// output. This breaks the critical path:
// leading_sum → cross-multiply (GO/SO) → mux → alpha*noise DSP
// into two shorter paths:
// Cycle 1: leading_sum → cross-multiply → mux → noise_sum_reg
// Cycle 2: noise_sum_reg → alpha * noise_sum_reg → noise_product
ST_CFAR_THR: begin
cfar_status <= {4'd4, 1'b0, col_idx[2:0]};
noise_product <= r_alpha * noise_sum_comb;
noise_sum_reg <= noise_sum_comb;
state <= ST_CFAR_MUL;
end
// ================================================================
// ST_CFAR_MUL: Compute alpha * noise_sum_reg in DSP
// ================================================================
// Pipeline stage 2: multiply registered noise sum by alpha.
// This is a clean registered-input → DSP path.
ST_CFAR_MUL: begin
cfar_status <= {4'd4, 1'b1, col_idx[2:0]};
noise_product <= r_alpha * noise_sum_reg;
state <= ST_CFAR_CMP;
end