The previous attempt put BUFIO inside u_core/gen_ft_bufr, but the pad
(ft_clkout) and its inferred IBUF live at the top wrapper level. Vivado
shape-packs IBUF↔BUFIO into the same IOB tile, and it couldn't do that
across the wrapper→u_core hierarchy boundary — producing CRITICAL
WARNING [12-1411] "Illegal to place BUFIO on TIEOFF site" and WNS=-5.737
(worse than the CLOCK_DEDICATED_ROUTE=FALSE baseline).
Fix: instantiate IBUF+BUFIO+BUFR explicitly in radar_system_top_50t.v
and pass the BUFR output into u_core.ft601_clk_in. radar_system_top.v
now does a pass-through wire assign for USB_MODE=1 (no BUFG) so the
clock net doesn't get double-buffered.
Jason
813ee4c962