fix(fpga): F-0.9 option A — BUFIO+BUFR for 50T ft_clkout (SRCC pin)

C4 is an SRCC pin (IS_CLK_CAPABLE=1, IS_MASTER=0 in the Vivado device
model), not an MRCC as earlier comments claimed. SRCC cannot drive BUFG
through dedicated routing, so the previous CLOCK_DEDICATED_ROUTE=FALSE
override forced fabric routing and burned ~5 ns on the ft_clkout path
(WNS -5.362 ns in the d36a4c9 build).

Swap to BUFIO + BUFR for USB_MODE=1 (50T/FT2232H): SRCC → BUFIO → BUFR
is the standard 7-series path for regional clock distribution. All
ft_clkout-domain logic (FT2232H FSM, toggle CDCs, USB FIFO flops) is
contained in bank 35 / one clock region, so regional distribution is
sufficient. USB_MODE=0 (200T/FT601) keeps the BUFG because D17 is a
proper MRCC pin.

Removed CLOCK_DEDICATED_ROUTE=FALSE from both the XDC and the build
script — no longer needed with dedicated BUFIO/BUFR routing.

9_Firmware/9_2_FPGA/constraints/xc7a50t_ftg256.xdc

@@ -98,24 +98,20 @@ set_input_jitter [get_clocks adc_dco_p] 0.05
 # FT2232H 60 MHz CLKOUT (Bank 35, MRCC pin C4)
 # --------------------------------------------------------------------------
 # The FT2232H provides a 60 MHz clock in 245 Synchronous FIFO mode.
-# Pin C4 is IO_L12N_T1_MRCC_35 (N-type of MRCC pair). Vivado requires
-# CLOCK_DEDICATED_ROUTE FALSE for clock inputs on N-type MRCC pins
-# (Place 30-876). The schematic routes CLKOUT to C4; this cannot be
-# changed without a board respin. The clock still uses an IBUFG and
-# reaches the clock network — the constraint only disables the DRC check.
+# Pin C4 is IO_L12N_T1_SRCC_35 (SRCC, not MRCC — confirmed via Vivado
+# device model: IS_CLK_CAPABLE=1, IS_MASTER=0). SRCC pins cannot drive
+# BUFG through dedicated routing, which is why the earlier build needed
+# CLOCK_DEDICATED_ROUTE=FALSE and burned ~5 ns in fabric routing.
+#
+# Fix: radar_system_top.v now instantiates BUFIO+BUFR instead of BUFG
+# for USB_MODE=1 (50T). SRCC → BUFIO → BUFR is the standard 7-series
+# path for regional clock distribution; all ft_clkout-domain logic is
+# contained in bank 35 / one clock region, so regional is sufficient.
+# The CLOCK_DEDICATED_ROUTE=FALSE override is no longer needed.
 set_property PACKAGE_PIN C4 [get_ports {ft_clkout}]
 set_property IOSTANDARD LVCMOS33 [get_ports {ft_clkout}]
 create_clock -name ft_clkout -period 16.667 [get_ports {ft_clkout}]
 set_input_jitter [get_clocks ft_clkout] 0.2
-# N-type MRCC pin requires dedicated route override (Place 30-876).
-# Audit F-0.4: the literal net name `ft_clkout_IBUF` exists post-synth but
-# the XDC scan happens before synthesis, when the IBUF net does not yet
-# exist — Vivado reported `No nets matched 'ft_clkout_IBUF'` + CRITICAL
-# WARNING. Use -hierarchical -filter + -quiet so the constraint matches
-# post-synth without warning during pre-synth XDC scan. The TCL duplicate
-# at scripts/50t/build_50t.tcl:119 remains as belt-and-suspenders.
-set_property -quiet CLOCK_DEDICATED_ROUTE FALSE \
-    [get_nets -quiet -hierarchical -filter {NAME =~ *ft_clkout_IBUF}]
 
 # ============================================================================
 # RESET (Active-Low)

9_Firmware/9_2_FPGA/radar_system_top.v

@@ -327,10 +327,27 @@ BUFG bufg_120m (
     .O(clk_120m_dac_buf)
 );
 
-BUFG bufg_ft601 (
-    .I(ft601_clk_in),
-    .O(ft601_clk_buf)
-);
+// USB clock buffering:
+//   USB_MODE=0 (200T/FT601): pin is MRCC (D17) → BUFG, global clock network.
+//   USB_MODE=1 (50T/FT2232H): pin is SRCC (C4, non-MRCC) → BUFIO + BUFR for
+//     regional dedicated routing. SRCC can drive BUFIO/BUFR but not BUFG
+//     directly (the "poor placement IO→BUFG" CLOCK_DEDICATED_ROUTE=FALSE
+//     override was burning ~5 ns in fabric routing on the ft_clkout path).
+//     All ft_clkout-domain logic (FT2232H FSM, FIFO flops, toggle CDCs) is
+//     contained in bank 35 / one clock region, so regional distribution
+//     is sufficient. See UG472 §3 BUFIO/BUFR.
+generate if (USB_MODE == 1) begin : gen_ft_bufr
+    wire ft_clk_bufio;
+    BUFIO bufio_ft (.I(ft601_clk_in), .O(ft_clk_bufio));
+    BUFR #(.BUFR_DIVIDE("BYPASS"), .SIM_DEVICE("7SERIES")) bufr_ft (
+        .I(ft_clk_bufio),
+        .O(ft601_clk_buf),
+        .CE(1'b1),
+        .CLR(1'b0)
+    );
+end else begin : gen_ft_bufg
+    BUFG bufg_ft601 (.I(ft601_clk_in), .O(ft601_clk_buf));
+end endgenerate
 `endif
 
 // Reset synchronization (clk_100m domain)

9_Firmware/9_2_FPGA/scripts/50t/build_50t.tcl

@@ -111,12 +111,10 @@ set_property SEVERITY {Warning} [get_drc_checks NSTD-1]
 set_property SEVERITY {Warning} [get_drc_checks UCIO-1]
 set_property SEVERITY {Warning} [get_drc_checks PLIO-9]
 
-# FT2232H CLKOUT on C4 (N-type MRCC) — override dedicated clock route check.
-# The schematic routes the FT2232H 60 MHz clock to the N-pin of a differential
-# MRCC pair. Vivado Place 30-876 requires this property to allow placement.
-# The clock still reaches the clock network via IBUFG — this only suppresses
-# the DRC that demands the P-type pin.
-set_property CLOCK_DEDICATED_ROUTE FALSE [get_nets {ft_clkout_IBUF}]
+# FT2232H CLKOUT routing: C4 is SRCC, not MRCC. Earlier builds used
+# CLOCK_DEDICATED_ROUTE=FALSE + implicit BUFG, which forced fabric routing
+# and burned ~5 ns. radar_system_top.v now uses BUFIO+BUFR for USB_MODE=1
+# (regional distribution from SRCC) so the override is no longer required.
 
 # ---- Run implementation steps ----
 opt_design -directive Explore