Post-route WNS = -5.355 ns on path group ft_clkout, net
u_core/gen_ft2232h.usb_inst/ft_data_TRI[0]_repN_1
FT2232H 245-sync FIFO input setup (t_su = 11.667 ns on a 16.667 ns
CLKOUT) leaves the FPGA only ~5 ns from clock edge to pad. Without
IOB=TRUE, the output / tristate FFs live in fabric and FF→OBUFT
routing eats 2–3 ns, forcing Vivado to replicate the tristate
driver (ft_data_TRI[*]_repN) and still miss timing.
The FSM in usb_data_interface_ft2232h.v already registers
ft_data_out / ft_data_oe / ft_{rd,wr,oe}_n at the output boundary
in the ft_clk domain, so packing them into the IOB is safe with
no RTL change.
Jason
94bf6944a3