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Jason 1342ce7b9d docs(fpga-xdc): PR-AD.b — close H-1/H-2/H-4 (FT601 XDC documentation rot)
H-1: ft601_rd_n/ft601_oe_n IOB-packing block claimed "constant-1, USB read
not implemented" but the driver has had a full RD_OE_ASSERT/READING/DEASSERT
read FSM since the host-command path was added. Rewrote the comment to
describe the real FSM-driven traffic and explain that -quiet is now for
post-synthesis retiming/consolidation cases, not because the registers
don't exist.

H-2: production XDC (create_generated_clock ft601_clk_fwd + set_output_delay)
and FMC dev XDC (set_max_delay -datapath_only) implement two different but
deliberate timing strategies for the same FT601 output ports — production
forwards ft601_clk_out via ODDR, FMC UMFT601X-B has no return path. Added
cross-reference VARIANT STRATEGY blocks in both XDCs explaining the choice
and the no-mixing rule.

H-4: set_false_path -from ft601_txe -to clk_100m registers was undocumented.
Traced both capture paths: ft601_clk_in (write FSM in usb_data_interface.v)
and clk_100m (cdc_single_bit in radar_system_top.v driving status_reg[1]).
Added comment explaining the async port → clk_100m sync-FF path is what
this constraint covers.

Comment-only XDC change; regression 46/0/0 unchanged.
2026-05-18 15:11:31 +05:45
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