bba_top derived its EXI clock from HALF, defaulting to 3.4 MHz against the
real GC's ~27 MHz. At that rate the whole CDC response latency fits inside
one bit time, so the suite passed green on a design that failed on console --
this is how both the wrong-header-format and the TX-pop-threshold bugs
reached hardware with everything green.
HALF is now EXI_HALF-configurable (default 8 for fast regression). At the
real rate, EXI_HALF=1, every read comes back shifted right by ONE BIT:
04 02 02 00 -> 02 01 01 00, NWAYS 0x17 -> 0x0B. That is a bit-level sampling
misalignment, not the byte-level latency chased earlier: at 54 MHz capture
there are only 2 ticks per EXI bit and the 2-stage FFSynchronizer on EXI CLK
costs 2 ticks, a whole bit.
Recorded as an open bug in CLAUDE.md with the measurements.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
RoflinandClaude Opus 5
7df0c97c78