Pre-fix Tests 1/2/4 in fpga_self_test.v gave false PASS even on broken
silicon:
S-19 Test 1 (CIC): `result_flags[1] <= 1'b1` unconditional, comment
admitted "always true for simple check".
S-20 Test 2 (FFT): `(16'sd100+16'sd100 == 16'sd200) && (...)` —
both predicates compile-time-fold to 1; synth reduces to a
constant write.
S-21 Test 4 (ADC): PASS once N samples land, regardless of value.
A stuck-at-0 / stuck-at-MAX / dead LVDS link still PASSed
provided adc_valid_in toggled.
Fixes:
Test 1: drive impulse {5,0,0,0,0,0,0} through registered integrator
y[n]=y[n-1]+x[n]; require accumulator==5 after step
response. Real adder + register path; sign-extension
exercised. Detail = 0xC1 on fail.
Test 2: real radix-2 butterfly with twiddle multiply across 4 FSM
states. A=8, B=4 (real), W=2+3j -> WB=(8,12), A'=(16,12),
B'=(0,-12). Forces synth to instantiate signed multiplier
(DSP slice) + 17-bit signed add/sub. Detail = 0xF2 on fail.
Test 4: track min/max across 256-sample capture, require
(max - min) > ADC_RANGE_THRESHOLD (10 LSB). Catches stuck-at
faults. Does NOT distinguish AD9484 format mismatches
(audit's per-mode mean check requires SPI, impossible per
AUDIT-C13). Detail = 0xAD on fail.
Tests:
- tb_fpga_self_test.v existing Group 1-4 (16 PASS) still pass: varied
ADC counter input gives range >> 10.
- New Group 5: drive constant 0 -> expect Test 4 FAIL + detail=0xAD.
- New Group 6: drive constant 0x7FFF -> expect Test 4 FAIL + detail=0xAD.
- Regression: 41/41 PASS; fpga_self_test 22/22 (was 16/16).
