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).
Closes AUDIT-C15. The 200T XDC `adc_or_p/n` PACKAGE_PIN was a TODO
placeholder that blocked all 200T synth/impl with unplaced-IO errors.
Pins U20/V20 are L11P/L11N_T1_SRCC_14 — same T1 clock tile as adc_dco_p
on L12_MRCC (W19/W20), so OR captures with the same IBUFDS->BUFIO->IDDR
source-synchronous topology as adc_d_p[*]. Free-pair confirmed by Vivado
get_package_pins query against xc7a200tfbg484-2.
Adds DIFF_TERM TRUE on adc_or_n (was only on the p-side; explicit on
both is safer). Adds input_delay constraints mirroring adc_d_p
(max 1.0 ns / min 0.2 ns on both edges).
Header pin counts updated: Bank 14 21/50 used, total 184/285.
This is the FPGA-team RECOMMENDATION for the production PCB (NEW
design); the PCB designer must route AD9484 OR+ -> U20 and OR- -> V20.
Validation:
- read_xdc + link_design -part xc7a200tfbg484-2 -> READ OK on both
xc7a200t_fbg484.xdc and adc_clk_mmcm.xdc; no PACKAGE_PIN errors.
- ./run_regression.sh --quick: 29/29 PASS (RTL untouched).
Build-blocking fixes surfaced by gpu-server synth:
1. radar_system_top_50t.v wrapper was missing adc_or_p/n ports and the
u_core instantiation left them unconnected. Every XDC line in the 50T
anchor block (PACKAGE_PIN M6/N6, IOSTANDARD, DIFF_TERM, set_input_delay)
therefore matched no ports and emitted CRITICAL WARNINGs, leaving the
overrange pin effectively tied off. Added the two inputs and wired them
through to the core.
2. adc_clk_mmcm.xdc used foreach / unset — Vivado's XDC parser only
accepts a restricted Tcl subset and rejected them as
[Designutils 20-1307]. Moved the clk_mmcm_out0 ↔ USB-clock false paths
into each board XDC (ft_clkout for 50T, ft601_clk_in for 200T) where
the clock name is already known.
The AD9484 OR (overrange) LVDS pair is routed on the 50T main board to
xc7a50t-ftg256 bank-14 pins M6/N6 but was previously left unconnected at
the top level. Plumb it through the full stack so saturation at the raw
ADC boundary shows up in the existing overflow aggregation:
- ad9484_interface_400m: add adc_or_p/n inputs, IBUFDS + IDDR capture of
both phases in the BUFIO domain, re-register into the clk_400m BUFG
domain, OR rise|fall into adc_overrange_400m output.
- radar_receiver_final: stickify adc_overrange_400m in clk_400m, CDC to
clk_100m via a 2FF ASYNC_REG chain (same reasoning as F-1.2's
cdc_cic_fir_overrun — single-bit, latched low→high, GPIO-class
diagnostic), OR into the existing ddc_overflow_any aggregation.
- radar_system_top: expose adc_or_p/n top-level ports and pass through.
- xc7a50t_ftg256.xdc: anchor M6/N6 as LVDS_25 DIFF_TERM, with the same
DCO-relative input-delay constraints as adc_d_p[*].
- xc7a200t_fbg484.xdc: IOSTANDARD/DIFF_TERM set; PACKAGE_PIN left as a
documented TODO — the 200T dev-board schematic has not been checked
and the 200T build will need the anchor filled in before place/route.
- Expand ft601_be from [1:0] to [3:0] across RTL, top-level, testbenches,
and XDC (uncomment be[2:3] pin assignments B21/A21)
- Fix NCO XSim testbench: correct reset check (0x7FFF not 0), add pipeline
warmup and sample skip for DSP48E1 quadrature test
- All local regression tests pass (39/39 USB, 10/10 integration, all co-sim)
