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Jason
6f5ff792fa
The AD9484 is SDR LVDS — datasheet p.5 lists "Output (LVDS—SDR)" as the
only output mode and p.16 confirms "data outputs are valid on the rising
edge of DCO." DCO runs at fs (400 MHz), one new sample per period, held
stable across the period. There is no DDR mode and no SPI access (CSB is
tied to +1V8 on the production board, RADAR_Main_Board.sch:46719).
ad9484_interface_400m.v previously instantiated an IDDR per data bit and
alternated Q1/Q2 via a `dco_phase` FSM, expecting to demux a "DDR" stream
into 400 MSPS. Because the chip is SDR, both Q1 and Q2 represent the same
sample, and the alternation produced approximately
[s_{-1}, s_1, s_1, s_3, s_3, s_5, …]
— odd-sample duplication with even-sample loss, equivalent to
decimate-by-2 followed by ZOH-upsample-by-2. In the frequency domain
that's a fold around fs/4 = 100 MHz; our 120-150 MHz IF lands at
50-80 MHz, so the DDC's 120 MHz NCO mixes the wrong frequency and the
matched filter sees baseband 40-70 MHz off where it expects.
The bug was hidden by tb/ad9484_interface_400m_stub.v, which has always
done single-rising-edge SDR-correct capture, so all iverilog regression
ran against the correct semantics — only the synthesizable Xilinx-
primitive path was wrong. This bug only fires on real silicon.
Fix:
- ad9484_interface_400m.v: drop IDDR + dco_phase; capture each data bit
with a single (* IOB = "TRUE" *) negedge-clocked IFF on adc_dco_bufio.
Falling DCO sits 1.25 ns inside AD9484's stable window, giving ~0.4 ns
setup margin against tPD = 0.85 ns. Same pattern on the OR (overrange)
path. Output FSM now emits one Q per BUFG cycle = clean 400 MSPS.
- tb_ad9484_xsim.v: add Test Group 8 (AUDIT-C4) that drives a 64-sample
counter ramp synchronously with rising DCO, captures the output, and
asserts (a) consecutive deltas equal +1 for ≥ (captured-6) of the
stream, (b) zero duplicate samples (catches DDR-style demux), (c) zero
unexpected jumps (catches DDR-style sample drops). This locks in SDR
semantics so any future regression that reintroduces a DDR demux on
this chip fails loudly.
- ad9484_interface_400m_stub.v: comment-only update — the stub already
does correct SDR capture; document AUDIT-C4 + why iverilog regression
was silent on the synth-path bug.
- xc7a200t_fbg484.xdc: fix stale "DDR class" comment near the OR pair
(now "SDR LVDS").
Verification: bash run_regression.sh — 42 passed, 0 failed, 1 skipped
(the skip is the T-6 drift cosim, which needs scipy from the dev group;
CI installs it via uv sync --group dev). Test Group 8 in the xsim TB
runs against the real UNISIM primitives and is exercised separately on
the Vivado host (run_xfft_xsim.sh-style flow).