incutec-OpenFC-Lite-Mini/hardware/research/imu-selection

IMU Selection Investigation: OpenFC-Lite-Mini Rev 2

Status: open · Started: 2026-06-02

The Rev 1 IMU (ST LSM6DSV16XTR) is populated for development only. The Betaflight team considers it unflyable; bench/flight data agrees. This folder collects datasheets, sourcing data, and test evidence to pick the Rev 2 IMU.

The footprint and its ST/TDK compatibility are described in hardware/docs/DESIGN.md (IMU section). IMU swap is a part-population change once a part is chosen. Pin-by-pin verification against each candidate family is in §8c below.

imu-selection/
├── README.md          ← this file (single source of truth for the investigation)
├── datasheets/        ← collected datasheets, named by part
└── (add scope captures, logs, FFTs here as they come in)

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1. Hard requirements

#	Requirement	Why
R1	Betaflight driver on current master	No driver = no flyable firmware. Authoritative list below.
R2	Drops into LGA-14 (2.5×3 mm), pins 2/3 GND, 10/11 NC	Existing footprint; pin-1 orientation must match.
R3	Flyable: low in-band gyro noise, robust to electrical + vibration noise	The whole reason DSV16X is being replaced.
R4	Sourceable at volume (LCSC/JLC preferred, real stock)	Selling the board; rest of BOM < $5.
R5	Price target: ideally ≤ ~$3-4 to keep the "Lite" price point	IMU currently dominates BOM cost.
R6	VDDIO works at 1.8 V analog / 3.3 V IO	Board runs IMU off +1.8V_GYRO (U6 NCV8187) + 3.3V IO.

A part failing R1 or R2 is out unless someone commits to adding the driver / reworking the footprint.

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2. Betaflight supported-IMU list (authoritative)

From drivers/accgyro/accgyro.h (gyroHardware_e) + sensors/acceleration.h on current master. Only these have flyable drivers today.

• InvenSense MPU: MPU6000, MPU6050, MPU6500, MPU9250
• InvenSense ICM-206xx: ICM20601, ICM20602, ICM20608G, ICM20649, ICM20689
• InvenSense ICM-426xx / 40609: ICM42605, ICM42688P, ICM42622P, ICM42686P, ICM40609D
• InvenSense IIM industrial (426xx driver): IIM42652, IIM42653
• InvenSense ICM-456xx: ICM45605, ICM45686
• Bosch: BMI160, BMI270
• ST: LSM6DSO, LSM6DSV16X, LSM6DSK320X, L3GD20 (gyro-only)
• Virtual: SITL/HITL

NOT on master (would need a driver): ICM-56686 (RPi is adding it, not merged), BMI323, BMI088, ICM-42670-P, LSM6DSV320X (the V high-g, ≠ the K).

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3. Candidate matrix

Pricing/stock from LCSC paste 2026-06-02 (EUR). "BF" = on master. Footprint column flags anything that isn't a clean drop-in.

Part	BF	Price (qty)	LCSC stock	Footprint	Gyro noise mdps/√Hz	Verdict
ICM-42688-P	✓ king	€12.15 / €10.49 lt	0 (10k @7-9d other)	LGA-14 2.5×3 ✓	2.8	Best-flying, too expensive + tight supply
ICM-42622P	✓	not in paste, source	?	LGA-14 ✓	~2.8 (class)	strong reports, find pricing/stock
IIM-42652	✓	(see paste; IIM-42653 €13.15)	42653: 4,969	LGA-14 ✓	3.8	Industrial temp, pricey
ICM-45686	✓	€18.20 / €6.23 lt	0 (353 other)	LGA-14 3×2.5 ⚠ rotated	~	Newer; verify pin-1, costly
ICM-45605	✓	€6.35	0	LGA-14	-	budget 456xx, 0 stock
ICM-42605	✓	€5.60 / €4.90 lt	10,000 ✓	LGA-14 2.5×3 ✓	(class)	In stock, supported: solid fallback
BMI270	✓	€3.19 / €1.36 lt	0 (50k @6-8d other)	LGA-14 2.5×3	7 (perf)	Cheap + available + only AC-PSRR spec'd ⚠ verify Bosch pinout
LSM6DSO	✓	€3.70	6,312 ✓	LGA-14 2.5×3 ✓	(older)	cheap, available, older gen
LSM6DSV16X	✓	€4.07	4,350	LGA-14 2.5×3 ✓	2.8	Current Rev 1: unflyable, being replaced
LSM6DSK320X	✓	not released	0	LGA-14 2.5×3 ✓	3.8	BF/STM target; not orderable, samples gated
ICM-56686	✗	(RPi sharing ds)	-	LGA-14 2.5×3	2.9	No BF driver yet (RPi adding); 29.3kHz MEMS
BMI323	✗	€3.28	2,920	LGA-14	-	No BF driver; Bosch filters undocumented
BMI088	✗	€8.16	3,468	LGA-16 3×4.5 ✗	14	No driver, wrong footprint, 2.4V floor: out
LSM6DSV320X	✗	€10.95	0	LGA-14L	-	V variant, no BF driver (≠ DSK)

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4. Datasheet spec comparison

Mined from the PDFs in datasheets/. "n/s" = not specified in datasheet.

Spec	ICM-42688-P	IIM-42652	ICM-56686	BMI270	BMI088	LSM6DSV16X	LSM6DSK320X
Gyro noise (mdps/√Hz)	2.8	3.8	2.9 / 2.5(20b)	7 / 10	14	2.8	3.8
Accel noise (µg/√Hz)	65/70	70	55/70	160	190	60/100	60/100/1000(hi-g)
Gyro FS max (dps)	2000	2000	4000	2000	2000	4000	4000
Accel FS max (g)	16	16	32	16	24	16	320 (hi-g)
Max gyro ODR (kHz)	8	8	6.4	6.4	2	7.68	7.68
MEMS resonant freq	n/s	n/s	29.3 kHz	n/s	n/s	n/s	n/s
Anti-alias filter	AAF 42-3979Hz + notch	AAF 42-3979Hz	AAF+LPF	LPF	LPF 5-532Hz	analog AAF + LPF1	analog AAF + LPF1
VDD / VDDIO (V)	1.71-3.6 / 1.71-3.6	1.71-3.6 / 1.71-3.6	1.71-3.6 / 1.08-3.6	1.71-3.6 / 1.2-3.6	2.4-3.6 / 1.2	1.71-3.6 / 1.08	1.71-3.6 / 1.08
SPI max (MHz)	24	24	24	10	10	10	10
Package	LGA-14 2.5×3	LGA-14 2.5×3	LGA-14 2.5×3	LGA-14 2.5×3	LGA-16 3×4.5	LGA-14 2.5×3	LGA-14 2.5×3
PSRR / supply-noise spec	10mVpp tol.	10mVpp tol.	10mV typ/50max, to 1MHz	AC PSRR: gyro 0.40dps/50mV, accel <8mg/50mV, 100Hz-1MHz	DC drift only	n/s	n/s
CLKIN	yes 31-50k	yes	yes 20-40k	no	no	no	no

Headline findings

1. The DSK320X is NOT a quieter gyro than the DSV16X. On paper it's worse: 3.8 vs 2.8 mdps/√Hz, identical accel noise. Neither ST datasheet documents a MEMS resonant-freq change or sensor-level vibration-immunity improvement. The DSK's only vibration-relevant edge is the ±320 g high-g channel (clip resistance under hard impact) + quad-channel UI/EIS/OIS/high-g filtering, not a lower-noise or more supply-robust gyro core. This directly contradicts the framing that the DSK "fixes the noise": the fix, if real, is in MEMS hardware ST does not document, or in the high-g anti-clip path.

2. Neither ST part specs any PSRR / supply-noise rejection. This is the standout gap versus the observed failure (sensitivity to electrical noise). You cannot design a supply-ripple budget against a number that doesn't exist.

3. BMI270 is the only candidate with a frequency-resolved AC PSRR: gyro 0.40 dps per 50 mV ripple (100 Hz-1 MHz). InvenSense parts at least give a max-tolerable-ripple (10 mV). If supply-coupled noise is the real culprit, Bosch + InvenSense give you a spec; ST gives you nothing.

4. Vibration-aliasing immunity: ICM-56686 is the only part publishing a MEMS drive frequency (29.3 kHz, far above any propwash band, good anti-alias margin). BMI088 is the only one with explicit "drone/robotics vibration suppression + ±24 g anti-clip" language, but it's the worst on noise, wrong footprint, no driver. Out on R1/R2.

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5. External input and flight data

Betaflight-ecosystem consensus

• LSM6DSV16X is considered unflyable: "too sensitive to electrical noise." Tested on 5+ FCs across brands, all unacceptable; it will not be approved for ecosystem hardware.
• LSM6DSK320X is the recommended successor, developed together with ST; mass production targeted June 2026. Described as worth waiting for, drop-in on the LSM16 design.
• ICM-42688 "still king" but TDK supply is hard. ICM-42622 also rated very highly.
• Bosch BMI270: internal filters undocumented, "not great but cheap and works", pin-for-pin swappable later. Recommended only as a cheap interim.
• Clone IMUs (HXY ICM-42688P etc.) rejected on principle: unknown provenance.
• ⚠ Bias to note: Betaflight and ST have a working relationship, and ST has a commercial interest in the DSK320X winning the comparison. Weigh accordingly.

Counter-argument: the failure may be PDN-specific

• This board's IMU supply is unusually good: NCV8187 ~90 dB PSRR, and running the IMU at 1.8 V means a large LDO dropout → even better rejection. The board's electrical environment is likely better than the cheap FCs the DSV16X was condemned on, so those failures may be PCB/PDN-specific, not intrinsic to the part. (For comparison, at least one well-regarded reference design uses a ~75 dB PSRR IMU LDO, worse than ours.)
• DSK320X has different MEMS hardware. DSV320X is €5-6 at DigiKey (the V high-g).
• One proposed strategy: ship the Lite with BMI270 (cheap, holds the price point) now; a premium board with a higher-end IMU later.

Flight data: hover A/B against an ICM reference board

• Our board (DSV16X): prefilter gyro shows ~20 dB more <100 Hz broadband noise than an ICM reference board, on a hover test. Would need a hard detune to fly.
• Comparison is apples-to-apples: both FCs MCU-down, gyro-up → noise is not ESC-side.
• Adding a cap on 5 V often helps in this failure mode → points at supply-coupled noise.
• ⚠ Counter-datapoint: a 1408/3750kv/6S 3" test quad on DSV16X flew perfectly once a suitable tune/filter preset was applied, i.e. the issue may be tune/filtering, not raw sensor. Sample size 1. Worth resolving before committing.

ICM-56686

• Betaflight support for the ICM-56686 (positioned as an ICM-42688 alternative) is reportedly in development elsewhere; no driver on master yet. TDK cleared the datasheet for sharing → in datasheets/.

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5b. What actually matters in FPV (not bench noise density)

FPV is not a clean environment: the gyro lives in heavy mechanical vibration (motors, props, frame/arm resonances, often 0.5-4 kHz) plus ESC switching/EMI. Bench noise density is nearly the wrong figure of merit. Proof: the DSV16X has the best noise density of every candidate here (2.8 mdps/√Hz, tied with the 42688) and is still unflyable. Flyability is set by how the part handles out-of-band energy, not its quiet-room floor:

1. Vibration rectification / aliasing. Out-of-band vibration (kHz) folds into the control band as a DC offset shift + low-frequency broadband, exactly the <100 Hz broadband signature in the hover A/B data (§5). Governed by the MEMS drive/sense resonant frequency and the analog anti-alias filter ahead of the ADC (not the digital LPF, which is too late). → ICM-56686 is the only part publishing its resonant freq (29.3 kHz, big margin). ST parts publish neither resonant freq nor rectification behavior.
2. Clipping headroom under vibration. Instantaneous rate/accel spikes that exceed FS clip, and clipping rectifies into DC + harmonics. This, not "more range", is why DSK320X has a ±320 g accel channel and BMI088 has ±24 g. A ±16 g accel can saturate on a rough quad.
3. Supply / EMI rejection. ESC switching coupling into a part with unbounded PSRR. ST specs none; BMI270 specs AC PSRR; InvenSense specs a max tolerable ripple.

Consequence for this investigation: rank candidates on (1) anti-alias/resonant-freq margin, (2) clip headroom, (3) PSRR, and above all on empirical flight/blackbox data, since datasheets barely document any of the three. The noise-density column in §4 is informational, not a selection criterion. This is why BF qualifies gyros by flying them, and why a worse-on-paper part (DSK320X) can outfly the DSV16X.

5c. Prior diagnostic campaign, firmware A/B builds (sister board)

A documented DSV16X diagnostic effort already ran on a sister board carrying the same LSM6DSV16XTR (late April 2026): one firmware variant per hypothesis, each with a blackbox test plan. Results live in the blackbox logs, still to be recovered.

Firmware change	Hypothesis	Symptom targeted
DRDY PULSED→LATCHED	RP2350 @150 MHz EXTI+SPI-DMA latency exceeds the 75 µs DRDY pulse under heavy IRQ load (PIO DSHOT/UART, SD blackbox) → missed gyro samples → sample-rate drift	200 Hz oscillation under throttle, fine at hover
accel LPF2 250 Hz→~22 Hz	motor-harmonic vibration >250 Hz leaks into accel → corrupts IMU-fusion gravity vector	"gravity-vector wandering" in ANGLE mode
on-board chirp PID identifier (USE_SYSID)	(tooling) sysid raw emits a versioned FRF CSV for frequency-response analysis	n/a

Why this matters for IMU selection: the two leading in-house hypotheses for the DSV16X misbehaviour are firmware/platform, not "bad MEMS":

1. a DRDY timing bug specific to RP2350 under load (missed samples → the 200 Hz throttle oscillation), and
2. an accel-filter config leaking vibration into fusion (gravity wander).

Both are fixable without changing the sensor. This strongly reinforces §6: the part may be partly mis-integrated on RP2350, not intrinsically unflyable. Note these symptoms (200 Hz under-throttle oscillation; gravity wander) are distinct from the hover A/B result (<100 Hz broadband prefilter noise at hover), so there may be ≥2 independent problems, only one of which the sensor swap would fix.

The blackbox logs from these builds plus their baseline, and the sysid raw FRF CSV, are the highest-value missing evidence (see §8).

5d. RESOLVED on the sister board, empirical

The firmware A/B campaign concluded the DSV16X problem is not firmware-fixable: it's a MEMS hardware property. The sister board's firmware config records the swap rationale: the LSM6DSV16XTR was found to have a MEMS resonance vulnerability in the ~25 kRPM motor band on that airframe (the internal gyro saturating from acoustic/vibration energy at the resonator's drive frequency, producing bursts of false rotation reads up to chip full-scale ±2000 dps). Swapping to the ICM-42688-P (drop-in pin-compatible, confirmed by multiple BF targets that support both chips on the same pads) eliminated the resonance issue, and the sister board's next PCB revision is designed with the ICM-42688-P only.

What this settles, and what it does NOT.

Settled: swapping to ICM-42688-P made the board flyable, and the sister board's next revision standardizes on it. That's a solid empirical result for part selection.

NOT settled: the root cause. The "MEMS resonance in the ~25 kRPM band" wording is a hypothesis writeup, not a measured result, and it does not survive scrutiny as a single cause:

• The hover A/B noise is <100 Hz broadband at HOVER (low RPM/vibration). A high-RPM resonance should be throttle-dependent and absent at hover, which is what the diag build describes ("200 Hz under throttle, fine at hover"). So there are ≥2 distinct symptoms, not one.
• "~25 kRPM band" is physically shaky: 25 kRPM ≈ 417 Hz fundamental; a MEMS gyro drive resonance is ~20-25 kHz (~50× higher). 417 Hz vibration doesn't excite a 20 kHz drive mode directly. The plausible mechanism is aliasing (out-of-band energy folding in through an inadequate AAF), a filtering/architecture issue, not "resonator saturation."
• "5 V cap helps" + the "electrical noise" reports + ST publishing no PSRR → an independent supply/PSRR contributor.
• A tune/filter preset made one build fly perfectly → tune/filtering is a large lever; a hard MEMS-saturation defect wouldn't yield to a preset.
• The ICM swap is a confounded experiment: it changed sensor + driver maturity + AAF/notch behavior + CLKIN jitter removal simultaneously. It does not isolate resonance as the cause.

Working model: multi-causal, at least: (1) anti-alias/filtering inadequacy → aliased broadband; (2) supply/PSRR sensitivity; (3) RP2350 DRDY/sample-timing under load; (4) MEMS resonance/clipping at high vibration; (5) driver/tune immaturity. The ICM-42688-P is better on several of these at once, which is why it works, but the dominant factor is not isolated.

Consequence for selection: because the win is partly filtering/driver/PSRR (not purely a MEMS-element property), a same-family ICM-42605 is a strong bet (shares AAF+notch+driver), and the BMI270's real AC-PSRR spec regains relevance if supply coupling is a major contributor. Only the actual logs/FFT (hover vs throttle, ± a supply-ripple injection test) can decompose it.

6. Working hypothesis

The DSV16X's in-flight noise is most plausibly supply/electrical-coupling + tune, not a raw-noise-density deficit (its density spec is best-in-class, tied with the 42688). Evidence:

• 20 dB excess is broadband <100 Hz = classic aliased/rectified noise, not white density.
• "Adding a cap on 5 V helps" + "sensitive to electrical noise" = PDN/supply path.
• A good preset made it fly fine on our board (better PDN than the FCs BF condemned).
• ST publishes no PSRR, so the part's supply robustness is genuinely unknown/unbounded, consistent with it being fragile on noisy boards even if fine on a clean one.

This means the decision is partly political (BF approval / the 180-brand ecosystem rejects DSV16X regardless of our data) and partly technical (what actually flies clean on our PDN). Those can have different answers.

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7. Decision options

Reframed by §5d: the failure is MEMS resonance saturation, and ICM-42688-P is the empirically proven fix on the sister board. The 426xx family shares the same gyro sense-element architecture, so a cheaper family member is the likely, but not yet proven, way to keep the fix at a Lite price. A different-MEMS part (BMI270) is now a real risk, not a safe interim.

• A: ICM-42688-P (proven). ← lowest technical risk. The exact part that eliminated the resonance on the sister board, whose next revision standardizes on it. Drop-in (§4), uses wired CLKIN (GPIO15). Cost is the only problem: €10-12, 0 LCSC stock (10k other @7-9d). Kills the sub-$5 "Lite" price point. Right call for a premium OpenFC; painful for Lite.
• B: ICM-42605 (cheap same-family bet). ← leading for Lite (2026-06-02). Same 426xx gyro architecture as the 42688 → most likely shares the resonance immunity (same sense element, different binning/noise grade). BF-supported, 10,000 in LCSC stock @ €4.90-5.60, clean 2.5×3 footprint, uses the wired CLKIN. Risk: the resonance immunity is not yet confirmed on the 42605 specifically: verify by flying it (it shares the 42688 driver, so it's a config one-liner: USE_GYRO_SPI_ICM42605). If it holds, this is the answer for Lite.
• C: ICM-42622P (highly rated, same family), but €15 / out of stock at last check. Out on price/supply right now.
• D: BMI270 (cheapest, now higher risk). €1.36-3.19, 50k stock, BF-supported, footprint drop-in (§7b), only candidate with a real AC-PSRR spec. But: different Bosch MEMS: we now know the DSV16X failure was MEMS-resonance, and there is no evidence BMI270 survives this airframe's resonance band. Bench noise density (7) is irrelevant (§5b); the resonance question is not. Only choose if 42605 also fails and price dominates, and flight-test it against the same airframe first.
• E: DSK320X, BF/STM blessed, drop-in, but not orderable (samples gated), and per §4 not a lower-noise gyro. Track for the premium SKU; not actionable for Rev 2 now.
• F: Two-SKU strategy. Premium OpenFC = ICM-42688-P (proven). Lite = ICM-42605 if it flies clean, else BMI270 as the cost-floor fallback. Aligns the premium board with the sister board's next revision.

Recommendation: Lite-Mini Rev 2 → ICM-42605 (in stock, cheap, same family as the proven fix, wired CLKIN usable), with ICM-42688-P as the drop-in premium/no-compromise option on the identical footprint. Flight-test the 42605 against a ~25 kRPM-band airframe before committing: that's the one open risk, and it's cheap to close.

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7b. BMI270 footprint verification (DONE 2026-06-02)

Checked BMI270 pinout vs the actual board net on U9 (netlist export of OpenFC.kicad_sch). Board pins wired for the LSM6DSV16X: 1 MISO, 2 GND, 3 GND, 4 INT, 5 +3.3V, 6 GND, 7 GND, 8 +1.8V_GYRO, 9 CLKIN(GPIO15), 10 NC, 11 NC, 12 CS, 13 SCK, 14 MOSI.

BMI270 is a drop-in, with two caveats:

• Pins 2/3 = ASDx/ASCx (aux/OIS), hard-grounded on our board. Bosch documents "VDDIO or DNC", not GND. The shared 20×20 FC footprint grounds these and BMI270 ships on it across many BF boards (works empirically), but it is outside Bosch's stated spec: verify on a sample; worst case a DNC cut on 2/3.
• Pin 9 = our CLKIN net (GPIO15). On BMI270 pin 9 is INT2 (no clock input). No regression (ST part also has no CLKIN); just don't drive a clock on GPIO15 in firmware. Aligns with the Rev 2 "drop CLKIN" plan.
• All other pins (SPI 1/12/13/14, VDD 8 @1.8V, VDDIO 5 @3.3V, INT1 4, GND 6/7, NC 10/11) match 1:1.

8. Open verification tasks

• BMI270 LGA-14 pinout vs our footprint → drops in; caveats on pins 2/3 (grounded aux, out-of-spec but field-proven) and pin 9 (CLKIN→INT2, harmless). See §7b.
• ICM-45686 footprint is LGA-14 3×2.5 (rotated vs our 2.5×3): check pin-1 / land.
• Source ICM-42622P: LCSC/DigiKey/Mouser pricing, stock, lead time.
• DSK320X availability + price: DigiKey/Mouser/STM samples; realistic date beyond "June."
• CLKIN decision: Rev 2 plan drops CLKIN (GPIO15); if a TDK part (42605/42688/42622/ 45686/56686) is chosen, keep CLKIN for jitter removal. ST/Bosch don't use it.
• Get the actual FFT/blackbox behind the hover A/B (§5) into this folder: quantify the 20 dB, identify the noise frequency signature (aliased vs supply tone vs propwash).
• Resolve the contradiction: if a tune made DSV16X fly clean on our board, characterize why BF's boards failed: PDN measurement on a noisy reference FC vs ours.
• Confirm +1.8V_GYRO rail is fine for each finalist's VDD (all OK except BMI088's 2.4V).
• Decide whether ICM-56686 is worth waiting for its in-development driver.

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8b. Log comparison: PIDtoolbox method + first run (2026-06-02)

How PIDtoolbox (PTB) works (Brian White / bw1129, MATLAB or compiled standalone): decodes BF blackbox → modules: Spectral Analyzer (gyro/dterm PSD in dB + Freq×Throttle spectrogram), Step Response (setpoint→gyro deconvolution), time-domain plots.

• Set debug_mode = GYRO_SCALED so unfiltered gyro is logged (PTB then shows gyro prefilt vs gyro, and computes filter phase latency). Modern BF (2026.x) logs gyroUnfilt[] natively regardless of debug_mode, our logs have it, so prefilter analysis works anyway.
• Log at 2 kHz (spectrograms reach ~1 kHz). Fly throttle sweeps (idle→full over 5-10 s, ×2-3).
• Reading Freq×Throttle: diagonal line rising with throttle = motor noise (tracks RPM); horizontal line at fixed Hz = frame resonance or electrical (throttle-independent); <20 Hz = real craft movement (ignore for noise); 20-100 Hz should be quiet (propwash/tune).
• Bad-gyro test: one axis much noisier <200 Hz; rotate FC 90°: if the noisy axis follows, gyro suspect.

Why prefilter (gyroUnfilt) is the right signal here: it's the raw sensor before BF's filter stack, so comparing it removes the filter-tune confound and isolates the sensor.

Reproduce without MATLAB: analysis/compare_gyro.py LOG_A.csv LOG_B.csv labelA labelB (numpy/scipy/matplotlib) emits psd_overlay.png, spectrogram_<label>_<axis>.png, and a band-RMS table. PTB GUI crashed on macOS 2026-06-02; this is the fallback + it's repo-committed.

First run: LOG00083 vs LOG00090 (both built for OPENFC_ECO, the OpenFC-ECO-era firmware target since superseded by OPENFC_LITE_MINI_RP2350A; BF 2026.6.0-alpha, 8 kHz loop, logged ≈1.88 kHz → 940 Hz Nyquist). Inconclusive on sensor, the pair is not controlled:

• LOG83 = 135 s full sweep, gyro LPF off; LOG90 = only 6 s, sparse throttle, dyn-LPF 250/500.
• Which log is which sensor is unknown (BF header doesn't name the gyro; same firmware hash runs either chip via autodetect). The LOG#→sensor mapping must be recovered from test notes.
• Prefilter PSD floors nearly overlap in the noise-dominated bands (motor hump ~200-400 Hz,

  400 Hz). The big 0-20 Hz "RMS" differences are flight movement, not noise (135 s aggressive vs 6 s), not a sensor signal.

• LOG83 spectrogram = clean, healthy: motor band tracks throttle, no fixed horizontal lines, no saturation bursts. LOG90 looks hotter in 200-500 Hz but is too short/sparse to trust.

Verdict: these two logs cannot decide DSV16X vs ICM. Need a matched pair: same craft, both throttle sweeps, ≥30 s each, same filter config (or compare prefilt only), and a known LOG#→sensor map. Then the Freq×Throttle + prefilt-PSD overlay will show whether the DSV16X has (a) RPM-tracking diagonal excess (vibration/alias), (b) fixed-Hz lines at idle (electrical/PSRR), or (c) saturation bursts (resonance/clipping), decomposing §5d's multi-causal model.

8c. Universal drop-in verification (footprint + nets, working tree 2026-06-03)

Footprint lib:LGA-14_L3.0-W2.5-P0.50-BR. U9 net map (current schematic): 1 MISO · 2 GND · 3 GND · 4 INT · 5 +3.3V(VDDIO) · 6 GND · 7 GND · 8 +1.8V_GYRO(VDD) · 9 CLKIN(GPIO15) · 10 NC · 11 NC · 12 CS · 13 SCK · 14 MOSI. Checked against datasheet pinouts:

Pin	Board net	ST LSM6D* (DSV16X / DSK320X / DSO)	TDK ICM-426xx / IIM-426xx	Bosch BMI270
1	MISO	SDO ✓	AP_SDO ✓	SDO ✓
2	GND	SDx: "VDDIO or GND" ✓	RESV: NC/GND ✓	ASDx (aux, unused→input): table prefers VDDIO/DNC, GND benign ✓
3	GND	SCx: "VDDIO or GND" ✓	RESV: NC/GND ✓	ASCx (aux, unused→input): GND benign ✓
4	INT	INT1 ✓	INT1 ✓	INT1 ✓
5	+3.3V	VDDIO ✓	VDDIO ✓	VDDIO ✓
6	GND	GND ✓	GND ✓	GNDIO ✓
7	GND	GND ✓	RESV→GND ✓	GND ✓
8	+1.8V	VDD ✓	VDD ✓	VDD ✓
9	CLKIN	INT2 (idle, ok)¹	INT2/FSYNC/CLKIN ✓ (uses it)	INT2 (idle, ok)¹
10	NC	OCS_Aux NC ✓	RESV NC ✓	OCSB DNC ✓
11	NC	SDO_Aux NC ✓	RESV NC ✓	OSDO DNC ✓
12	CS	CS ✓	AP_CS ✓	CSB ✓
13	SCK	SCL/SCLK ✓	AP_SCLK ✓	SCK ✓
14	MOSI	SDA/SDI ✓	AP_SDI ✓	SDI ✓

¹ pin 9 = INT2 on ST/Bosch; harmless as long as firmware does not drive a clock on GPIO15.

Verdict: pins 2/3 at GND are fine for all; footprint is universal

The board grounds pins 2/3. For an unused aux interface this is correct/benign across all families:

• ST LSM6D* (DSV16X / DSK320X / DSO): datasheet sanctions pins 2/3 → "VDDIO or GND". ✓
• TDK ICM-426xx/IIM and ICM-456xx/56xx-gen (confirmed from ICM-56686 pinout: pins 2/3/10/11 = RESV/AUX1, pin 9 = INT2/FSYNC/CLKIN, same universal layout): RESV = "NC or GND". ✓
• Bosch BMI270: GND is safe (mechanism verified in datasheet, 2026-06-03): aux/OIS is disabled by default (PWR_CTRL.aux_en=0; BF never enables it for a bare gyro), so ASDx/ASCx are high-Z inputs: nothing drives them, GND = no contention. The only "pull-up" is a configurable internal pull-up on ASDA (AUX_IF_TRIM.asda_pupsel: off/40k/10k/2k) used for the aux-I2C SDA line; inactive when aux is off (worst case a 40k pull → ~80 µA to GND, harmless). Bosch's "VDDIO or DNC" is conservative guidance for the aux-enabled case; it does not mean GND damages the part. Bosch's own power-off note even says interface pins "must be kept close to GNDIO potential." The only failure case is firmware enabling the aux I2C master (ASCx becomes a driven clock → GND short), which BF does not do. GND is the correct universal tie (VDDIO fails TDK RESV; NC fails ST's "needs a level"). One firmware caveat: never enable the BMI270 aux interface.

So the footprint is a genuine universal drop-in for every ST LSM6D*, every TDK ICM-426xx/IIM, the 456xx/56xx-generation TDK parts, and the BMI270: no schematic change needed.

Open item

• ICM-45686/45605 physical orientation: pin functions match (per 56686), but LCSC lists the 45686 as LGA-14 3×2.5 vs our 2.5×3 → check body/pin-1 rotation against the footprint before committing. (Functional pinout is fine; this is purely a package-outline check.)
• Keep pin 9 (GPIO15→CLKIN) wired: revisit the Rev 2 "drop CLKIN" note. Harmless for ST/Bosch (INT2, idle), and gives TDK 426xx/456xx sample-clock jitter removal (relevant to the noise work).

9. Datasheet index

File	Part	Class	BF driver
datasheets/ICM-42688-P.pdf	TDK ICM-42688-P	consumer flagship	✓
datasheets/IIM-42652.pdf	TDK IIM-42652	industrial-temp 426xx	✓
datasheets/ICM-56686.pdf	TDK ICM-56686	AR/VR-OIS (29.3kHz MEMS)	✗ (RPi adding)
datasheets/BMI270.pdf	Bosch BMI270	consumer/wearable	✓
datasheets/BMI088.pdf	Bosch BMI088	drone/robotics high-vib	✗
datasheets/LSM6DSV16X.pdf	ST LSM6DSV16X	consumer/OIS (Rev 1 part)	✓
datasheets/LSM6DSK320X_rev0.1.pdf	ST LSM6DSK320X	high-end dual-accel	✓

Still to collect: ICM-42622P, ICM-42605, ICM-45686, ICM-45605, LSM6DSO datasheets.