LSM6DSOX 6-Axis IMU (U1)¶

Overview¶

U1 is an ST LSM6DSOXTR (LCSC C481766) — a 6-axis MEMS inertial measurement unit combining a 3-axis accelerometer and 3-axis gyroscope on the same die, plus an on-chip "machine learning core" that the firmware does not currently use. The chip provides the regulator's motion, vibration, and orientation information.

Marine alternator regulators have historically not bothered with motion sensing. Including one here lets the firmware do things that are otherwise impossible:

Detect that the boat is heeled or pitching and gate certain charging decisions accordingly (a hard heel angle suggests a working sail, which is a useful signal for power-management strategy).
Detect engine vibration to confirm the engine is actually running, independent of the alternator stator pulse train.
Detect installation-time orientation to know which axis is "vertical" for future tilt-based features.
Vibration-fingerprint specific failures (bearing wear, belt slap, mounting looseness) over time using the on-chip MLC.

Why the LSM6DSOX specifically¶

Driver	Why this part
Combined accel + gyro	A single I²C address picks up everything we need; avoids a second part.
3.3 V single-supply	No level shifters or analog supply needed.
Industrial temperature range (−40 °C to +85 °C)	Survives engine-room ambient.
Pin-compatible with LSM6DSO, future-proof against supply	If the OX variant goes EOL, the older DSO drops in.
On-chip ML core	Not used today; reserved for future vibration-classification features.
Low quiescent (≈ 0.55 mA in normal mode)	Acceptable on a continuous-on monitor.

Pin Configuration¶

Pin	Name	Net	Notes
1	SDO/SA0	GND	I²C address bit 0 = 0 → 7-bit address 0x6A
2	SDX	GND	Auxiliary SPI interface — tied off, unused
3	SCX	GND	Auxiliary SPI interface — tied off, unused
4	INT1	(unconnected)	Interrupt output — reserved for future use
5	VDDIO	3V3	I/O supply
6, 7	GND	GND	Ground
8	VDD	3V3	Core supply
9	INT2	(unconnected)	Interrupt output — reserved for future use
10	OCS_AUX	(unconnected)	Aux interface chip select — unused (must be floating per datasheet)
11	SDO_AUX	(unconnected)	Aux interface data — unused
12	CS	3V3	Tied high — selects I²C mode (low would select SPI)
13	SCL	I2C_SCL	Shared I²C clock
14	SDA	I2C_SDA	Shared I²C data

I²C Bus Membership¶

The LSM6DSOX shares the main I²C bus with two other devices:

Address	Device	Function
0x40	INA228 (U20)	Battery shunt current + bus voltage
0x6A	LSM6DSOX (U1)	6-axis IMU
0x76	BMP388 (U14)	Pressure / temperature

Bus pull-ups are provided once on the board (R92 / R93 = 2.2 kΩ each — see BMP388 doc for the rationale).

The bus runs at 400 kHz fast mode in firmware. The LSM6DSOX supports up to 1 MHz (high-speed mode), but moving from 400 kHz to 1 MHz would require characterizing the bus rise time with all three devices loaded and is not worth the firmware refactor for the IMU's data rate.

See CLAUDE.md "I²C Clock at 800 kHz" memory note — the bus was experimentally run at 800 kHz during development with zero error counters but stays at 400 kHz in production for spec compliance.

Power & Decoupling¶

VDD (pin 8) and VDDIO (pin 5) are both supplied from the 3.3 V rail. Decoupling is standard: 100 nF × 2 on the supply pins, placed within 2 mm of the package. The chip's internal LDOs regulate the analog and digital sub-rails — no external analog supply is needed.

Address Selection¶

SDO/SA0 (pin 1) is grounded → 7-bit I²C address = 0x6A.

If a future revision needs a second LSM6DSOX (highly unlikely), pin 1 could be tied to VDDIO instead, giving 0x6B. Both addresses are reserved for the LSM6DSOX family by ST.

SPI Mode (Unused)¶

CS (pin 12) is tied to 3V3 to force I²C mode. If the firmware ever needs SPI for higher data-rate gyro sampling (e.g., 6 kHz ODR), CS could be re-routed to an ESP32 GPIO and the bus reorganized — this requires a board respin and is not currently planned.

Interrupt Outputs¶

INT1 (pin 4) and INT2 (pin 9) are routed as bare pads in the netlist but not connected in V9. They would normally be used for:

INT1: data-ready, wake-on-motion, double-tap, free-fall, tilt
INT2: same set, programmable to different events

Polling at firmware loop rate is adequate for the current feature set. If future work needs sub-millisecond wake-on-motion (e.g., crash detection), INT1 should be routed to an ESP32 GPIO in a later revision.

Auxiliary SPI Interface¶

SDX (pin 2), SCX (pin 3), OCS_AUX (pin 10), and SDO_AUX (pin 11) are the LSM6DSOX's optional secondary SPI interface for daisy-chaining external sensors (a magnetometer, typically). All four are tied off as per datasheet recommendation when unused — SDX and SCX to GND, OCS_AUX and SDO_AUX left floating.

Layout Considerations¶

The LSM6DSOX should be mounted to the PCB itself, not on a flex pigtail. Mechanical compliance between the IMU and the structure being measured (the regulator → enclosure → bulkhead → engine bed) is what the sensor sees; any additional compliance washes out the signal.
Orientation on the board is locked in firmware. The PCB silkscreen marks the +X axis to make assembly orientation unambiguous.
The chip is sensitive to thermal gradients. Avoid placing high-dissipation devices (the LMR36510, the TPS48000 power MOSFETs) within 5 mm of the package. The V9 layout places U1 in a low-dissipation corner of the board.

Specifications Summary¶

Parameter	Value	Source
Accelerometer range	±2 / ±4 / ±8 / ±16 g (selectable)	datasheet
Accelerometer noise	60 µg/√Hz	datasheet
Accelerometer ODR	12.5 Hz – 6.66 kHz	datasheet
Gyroscope range	±125 / ±250 / ±500 / ±1000 / ±2000 dps	datasheet
Gyroscope noise	4 mdps/√Hz	datasheet
Gyroscope ODR	12.5 Hz – 6.66 kHz	datasheet
Operating voltage	1.71 V – 3.6 V	datasheet
Supply current (combined accel + gyro, normal mode)	~0.55 mA at 3.3 V	datasheet
Operating temperature	−40 °C to +85 °C	datasheet
Package	14-pin PQFN, 2.5 × 3 × 0.86 mm	datasheet