Sensors

Complete guide to the H7-Digital onboard sensors and external sensor connections for GPS and compass modules.

Table of Contents

  1. Onboard Sensors
    1. IMU: ICM-42688-P
      1. High-Resolution Sampling: ArduPilot vs Betaflight
    2. Barometer: DPS368
  2. External Sensors
    1. GPS Module (Not Included)
    2. External Compass (Not Included)
  3. Sensor Calibration
    1. When to Calibrate
    2. Accelerometer Calibration
    3. Compass Calibration (ArduPilot)
  4. Vibration Isolation
    1. Why Vibration Matters
    2. Soft Mounting Recommendations
    3. Checking Vibration Levels
  5. Troubleshooting Sensor Issues
    1. IMU Not Detected
    2. Barometer Not Detected
    3. GPS No Fix / No Satellites
    4. Compass Interference / Poor Calibration
    5. High Vibration Levels
  6. Related Documentation
  7. External Resources
  8. Support

Onboard Sensors

IMU: ICM-42688-P

The H7-Digital features the TDK InvenSense ICM-42688-P, a high-performance 6-axis MEMS motion tracking device combining a 3-axis gyroscope and 3-axis accelerometer. The ICM-42688-P is considered one of the best IMUs for flight control, offering exceptional noise performance and stability with an isolated 3.3V power rail.

For complete technical specifications, hardware capabilities, and firmware configuration details, see Specifications - IMU.

Connection:

  • SPI Bus: SPI1 (PA5=SCK, PA6=MISO, PA7=MOSI)
  • Chip Select: PB0 (GYRO_CS)
  • Interrupt: PB1 (GYRO_EXTI)
  • Clock Input: PE5 (GYRO_CLKIN - Betaflight only)

Isolated Power Rail: The ICM-42688-P uses a dedicated ultra-low-noise 3.3V LDO regulator, isolated from MCU digital switching noise. This ensures clean gyroscope readings and reduces false vibrations in sensor data. See Power System for details.

External Clock (Betaflight): The H7-Digital routes the gyro external clock (CLKIN) to PE5 for ultra-precise timing. Betaflight uses this for reduced gyro jitter and improved loop timing. ArduPilot does not currently support external gyro clocking.

Understanding Programmable Range Modes:

The ICM-42688-P offers eight programmable gyroscope ranges (±15.625 to ±4000 dps) and four accelerometer ranges (±2g to ±32g). However, both Betaflight and ArduPilot use fixed settings:

  • Gyroscope: ±2000 dps (both firmwares)
  • Accelerometer: ±16g (both firmwares)

Why ±2000 dps / ±16g? This is the industry-standard “sweet spot” that provides 2-4x safety margin above typical flight maneuvers while maintaining excellent resolution (0.0610 dps/LSB gyro, 0.488 mg/LSB accel in standard 16-bit mode) for precise PID control. Lower ranges risk saturation during aerobatics; higher ranges sacrifice resolution with no practical benefit.

The Resolution vs. Range Trade-off:

The ICM-42688-P uses 16-bit ADCs (-32,768 to +32,767 range). Increasing the full-scale range decreases resolution per bit:

Resolution values below are for standard 16-bit mode. ArduPilot uses high-resolution FIFO mode (19-bit gyro, 18-bit accel) which provides 8x/4x better resolution at the same ±2000 dps / ±16g ranges. See High-Resolution Sampling section below.

Gyro Range Resolution (dps/LSB) Firmware Use Trade-off
±125 dps 0.0038 (16x better) ❌ Not used Saturates during basic maneuvers
±500 dps 0.0153 (4x better) ❌ Not used Saturates during fast rolls/flips
±1000 dps 0.0306 (2x better) ❌ Not used Marginal for aggressive flight
±2000 dps 0.0610 ✅ Standard Optimal balance
±4000 dps 0.1225 (2x worse) ❌ Not used Unnecessary - halves resolution

Why Both Firmwares Chose ±2000 dps:

  • ✅ Racing quads peak at ~1500 dps (extreme flips) - 33% safety margin
  • ✅ Autonomous aircraft stay below 500 dps - 4x safety margin
  • ✅ Maintains excellent resolution for precise PID control
  • ✅ Industry standard - tuning parameters designed for this range
  • ✅ Proven with billions of flight hours

Accelerometer: Why ±16g Standard:

Resolution values below are for standard 16-bit mode. ArduPilot’s high-resolution mode provides 18-bit accel (0.122 mg/LSB at ±16g).

Accel Range Resolution (mg/LSB) Use Case Trade-off
±8g 0.244 (2x better) ❌ Risk of saturation during hard maneuvers
±16g 0.488 ✅ Standard Covers aerobatics + crash detection
±32g 0.977 (2x worse) ❌ Unnecessary - halves resolution

Typical flight: 1-4g normal, 4-8g aerobatics, 10-20g crashes. The ±16g range handles all scenarios with good resolution.

For driver source code references, firmware implementation details, and the official datasheet, see Specifications - IMU Driver References.

Additional Reading:

High-Resolution Sampling: ArduPilot vs Betaflight

The ICM-42688-P features an industry-first 20-bit FIFO format that provides higher resolution data than standard 16-bit ADC output. ArduPilot and Betaflight use this capability differently based on their design priorities.

ArduPilot: High-Resolution FIFO Mode Enabled ✅

ArduPilot enables the ICM-42688-P’s 20-bit FIFO format (FIFO_HIRES_EN bit) for enhanced resolution:

Data Structure (20 bytes per sample):

  • 19-bit gyroscope (16-bit MSB + 3-bit fractional) = 0.0076 dps/LSB at ±2000 dps
  • 18-bit accelerometer (16-bit MSB + 2-bit fractional) = 0.122 mg/LSB at ±16g
  • Header, temperature (16-bit), timestamp (16-bit)

Resolution Improvement:

  • Gyro: 16-bit → 19-bit = 8x better resolution
  • Accel: 16-bit → 18-bit = 4x better resolution

Sampling Strategy:

  1. Hardware samples at 8 kHz with 19/18-bit resolution
  2. Data buffered in 2048-byte FIFO (~105 high-res samples)
  3. Software reads FIFO, averages 8 samples → 1 kHz backend (default)
  4. EKF runs at 400 Hz on downsampled data

Why High-Res Mode Benefits ArduPilot:

  • Precision hover: At 10 dps slow drift, 19-bit = 0.076% error vs 16-bit = 0.6% error (8x improvement)
  • EKF fusion: Cleaner sensor data improves GPS/optical flow fusion for position hold
  • Anti-aliasing: 8 kHz → 1 kHz downsampling prevents 600-4000 Hz vibration aliasing
  • CPU headroom available: 400 Hz main loop uses ~30-50% CPU, +15-20% overhead acceptable

Trade-offs:

  • ⚠️ +0.5-1ms latency from FIFO buffering
  • ⚠️ +15-20% CPU overhead for bit extraction and averaging
  • ⚠️ More complex driver implementation

Betaflight: Standard 16-bit Direct Register Mode

Betaflight reads directly from 16-bit output registers without using FIFO or high-resolution mode:

Data Structure (12 bytes per sample):

  • 16-bit gyroscope = 0.0610 dps/LSB at ±2000 dps
  • 16-bit accelerometer = 0.488 mg/LSB at ±16g
  • Direct register read: GYRO_DATA_X1 (0x25), ACCEL_DATA_X1 (0x1F)

Sampling Strategy:

  1. Hardware samples at 8 kHz (native ODR)
  2. Data-ready interrupt triggers immediate register read
  3. Simple int16_t conversion, minimal processing
  4. All filtering (AAF, notch, lowpass) runs at 8 kHz
  5. PID loop locked to gyro rate (8 kHz)

Why Standard Mode Benefits Betaflight:

  • Minimum latency: Direct read <0.5ms vs FIFO ~1ms (critical for racing)
  • CPU efficiency: 8 kHz PID + BiDir DShot + dynamic notch needs every cycle
  • Resolution sufficient: At 500-1500 dps racing, 0.061 dps/LSB = 0.012-0.036% accuracy (exceeds human perception)
  • Hardware AAF: Configurable 258-1962 Hz anti-alias filter provides clean data before sampling

Trade-offs:

  • ⚠️ Lower resolution at slow speeds (0-100 dps) - less critical for manual control
  • ⚠️ No FIFO buffering (must read every sample immediately)

Technical Comparison

Aspect ArduPilot (High-Res FIFO) Betaflight (Standard Direct)
Data Source FIFO buffer (20-byte packets) Direct registers (12-byte read)
Gyro Resolution 19-bit (0.0076 dps/LSB) 16-bit (0.0610 dps/LSB)
Accel Resolution 18-bit (0.122 mg/LSB) 16-bit (0.488 mg/LSB)
FIFO Enable FIFO_HIRES_EN bit set No FIFO used
Sampling 8 kHz → avg → 1 kHz backend 8 kHz native (no downsampling)
Latency 0.5-1ms (FIFO buffering) <0.5ms (immediate read)
CPU Overhead +15-20% (bit extraction, averaging) Baseline (most efficient)
Anti-Aliasing Downsampling (8 kHz → 1 kHz) Hardware AAF (258-1962 Hz)
Best Use Case 0-100 dps slow precision 500-1500 dps fast maneuvers
Optimized For Autonomous, GPS fusion, hover FPV racing, freestyle, acro

When High Resolution Actually Matters:

Motion Type 16-bit Accuracy 19-bit Accuracy Benefit
Hover (0-10 dps) 0.6% error 0.076% error 8x improvement - significant
Cruise (100-500 dps) 0.012-0.06% error 0.0015-0.008% error Marginal benefit
Racing (500-1500 dps) 0.004-0.012% error 0.0005-0.0015% error No perceptible benefit

Conclusion:

Both implementations are correct for their target applications:

  • ArduPilot: High-res mode provides superior slow-motion sensing for autonomous operations where precision position hold and GPS fusion are critical. The additional latency and CPU cost are acceptable when main loop runs at 400 Hz.

  • Betaflight: Standard mode minimizes latency and CPU load for manual FPV control where every millisecond matters and racing speeds (500-1500 dps) already provide 0.012% accuracy with 16-bit data - far exceeding what human pilots can perceive or utilize.

The H7-Digital’s ICM-42688-P supports both modes, allowing the same hardware to excel at autonomous missions (ArduPilot) and FPV racing (Betaflight).

Driver Source Code:

Mounting Orientation:

The IMU is rotated in firmware to match the H7-Digital board orientation:

ArduPilot Default Rotation:

AHRS_ORIENTATION = 10    # ROTATION_ROLL_180_YAW_90

Betaflight Default Rotation:

GYRO_1_ALIGN CW90_DEG

Runtime adjustment via CLI (if needed):
  set align_board_roll = 180
  set align_board_pitch = 0
  set align_board_yaw = 90

Hardware rotation is configured in config.h via GYRO_1_ALIGN. Runtime adjustments can be made via CLI commands shown above.

If you mount the flight controller in a non-standard orientation, adjust the rotation parameters above. For ArduPilot, see the complete AHRS_ORIENTATION enum list. For Betaflight, set custom degrees via CLI or leave as default and physically mount the board correctly.

Barometer: DPS368

The H7-Digital includes the Infineon DPS368 digital barometric pressure sensor for altitude estimation. With ±0.02m (2cm) resolution and IPX8 waterproof rating, the DPS368 provides precise altitude measurements for flight control and position hold modes.

For complete technical specifications, measurement rates, and accuracy details, see Specifications - Barometer.

Connection:

  • I2C Bus: I2C1 (internal, PB8=SCL, PB9=SDA)
  • I2C Address: 0x76
  • I2C Speed: 400kHz (ArduPilot), 800kHz (Betaflight default)

Driver Naming: Both firmwares use the DPS310 driver for the DPS368 sensor. The DPS310 and DPS368 are register-compatible - the DPS368 is the newer variant with improved IPX8 waterproofing. See Specifications - Barometer Driver Compatibility for driver details and source code references.

I2C Speed (Betaflight): Betaflight defaults to 800kHz I2C bus speed for faster sensor reads and reduced loop jitter. If experiencing I2C sensor detection issues, reduce speed to 400kHz via CLI: set i2c1_clockspeed_khz = 400. See Pinout Reference for details.

Calibration: Barometers measure relative altitude changes accurately but require ground-level calibration for absolute altitude. Always set ground elevation before flight or use GPS altitude for absolute reference.

External Sensors

Bench Testing with USB Power: The GPS and compass connectors receive 4.5V when powered via USB alone (no battery required). This allows you to test GPS lock, compass calibration, and firmware sensor detection during bench setup - useful for indoor configuration before installing sensors on the aircraft.

GPS Module (Not Included)

An external GPS module is recommended for autonomous flight modes, position hold, and return-to-launch functionality.

Connection:

  • Connector: GPS/Compass (6-pin JST-SH)
  • UART: UART8 (PE0=RX, PE1=TX)
  • I2C: I2C4 (PB6=SCL, PB7=SDA for compass)
  • Power: 5V (4.5V via USB - see USB power note above)

Firmware-Specific Configuration:

Feature ArduPilot Betaflight
Serial Port SERIAL8 (UART8) UART8
Protocol GPS (default) Set in Ports tab
Baud Rate 115200 (auto-detect) 115200 (UBLOX), 9600 (NMEA)
DMA Enabled Enabled

ArduPilot Parameters:

SERIAL8_PROTOCOL = 5     (GPS)
SERIAL8_BAUD = 115       (115200 for most GPS modules)
GPS_TYPE = 1             (Auto-detect u-blox/NMEA)

Betaflight Configuration:

  • Ports Tab: Enable GPS on UART8, set baud to 115200
  • Configuration Tab: GPS protocol (UBLOX, NMEA, etc.)

Recommended GPS Modules:

Model GPS Chipset Compass Notes
Holybro M10 Micro u-blox M10 IST8310 Recommended for ArduPilot - ArduPilot Dev Review
Matek M10-5883 u-blox M10 QMC5883L Latest generation, excellent accuracy
Beitian BN-880 u-blox M8N QMC5883L Budget option, widely available

GPS Generation Comparison: u-blox M10 (newest, 2020+) offers faster fix times and better multipath rejection than M9 (2019) and M8 (2014). All three generations are supported by ArduPilot and Betaflight. M10 modules are recommended for new builds but M8/M9 remain excellent choices.

Mounting Recommendations:

  • Location: Mount away from motors and ESCs (reduce electromagnetic interference)
  • Height: Elevated above frame (better satellite visibility)
  • Orientation: Arrow pointing forward (verify in ground station)
  • Distance from compass: Keep 10+ cm from high-current wires and motors

External Compass (Not Included)

An external compass (magnetometer) is highly recommended for ArduPilot autonomous flight.

Why External Compass?

The H7-Digital does not have an onboard compass. External compasses are preferred because:

  • ✅ Can be mounted away from electromagnetic interference
  • ✅ Better heading accuracy (less distortion from motors/ESCs/wires)
  • ✅ Essential for compass-dependent flight modes (Loiter, Auto, Guided)
  • ✅ Reduces compass calibration issues

Connection:

  • Connector: GPS/Compass (6-pin JST-SH)
  • I2C Bus: I2C4 (PB6=SCL, PB7=SDA)
  • I2C Speed: 400kHz (ArduPilot), 800kHz (Betaflight default)
  • Power: 5V (4.5V via USB - see USB power note above)

Firmware-Specific Configuration:

Feature ArduPilot Betaflight
I2C Bus I2C4 I2CDEV_4 (MAG_I2C_INSTANCE)
I2C Speed 400kHz 800kHz (default)
Detection Auto on boot Auto on boot (limited)
Required Recommended for autonomous modes Not used for stabilization
Allow Arming Without compass (ALLOW_ARM_NO_COMPASS) N/A

Common Compass ICs:

  • QMC5883L - Most common in GPS combo modules, cheap but with low performance
  • HMC5883L - Discontinued; all modules marketed as HMC today are clones or QMC5883L
  • LIS3MDL - Modern, STMicroelectronics part with good performance
  • IST8310 - High accuracy, good interference resistance

ArduPilot Configuration:

COMPASS_ENABLE = 1
COMPASS_USE = 1
COMPASS_EXTERNAL = 1
COMPASS_AUTODEC = 1      (Auto magnetic declination)

ArduPilot will auto-detect most I2C compass modules on boot.

Betaflight:

  • Betaflight does not use compass for stabilization
  • Compass support is limited to GPS rescue mode heading (optional)
  • Compass auto-detection may be less reliable than ArduPilot

Sensor Calibration

When to Calibrate

Accelerometer:

  • ✅ First-time setup
  • ✅ After firmware update
  • ✅ After physical impact or crash
  • ✅ If horizon drifts in level flight

Compass (ArduPilot only):

  • ✅ First-time setup
  • ✅ After moving compass location
  • ✅ If heading drifts or “toilet bowling” occurs (spiraling in position hold)
  • ✅ When flying in new geographic location (magnetic declination changes)

Barometer:

  • Auto-calibrated on boot (ArduPilot)
  • Set ground elevation in ground station before flight

Accelerometer Calibration

ArduPilot (via Mission Planner):

  1. Connect to flight controller via USB or telemetry
  2. Go to Initial Setup → Mandatory Hardware → Accel Calibration
  3. Click Calibrate Accel
  4. Follow on-screen prompts to position aircraft in 6 orientations:
    • Level
    • Left side
    • Right side
    • Nose down
    • Nose up
    • Upside down
  5. Keep aircraft perfectly still during each measurement
  6. Click Done when complete

Betaflight (via Configurator):

  1. Place flight controller on level surface
  2. Go to Setup Tab
  3. Click Calibrate Accelerometer
  4. Keep board perfectly still during calibration (~5 seconds)
  5. Verify horizon is level in artificial horizon display

Level Surface Required: Calibrate accelerometer on a perfectly level surface (use spirit level if available). Poor accelerometer calibration causes drift, toilet bowling, and unstable flight.

Compass Calibration (ArduPilot)

Large Vehicle Method (Recommended for quads):

  1. Connect to Mission Planner
  2. Go to Initial Setup → Mandatory Hardware → Compass
  3. Select Onboard Mag Calibration
  4. Arm throttle and take off
  5. Fly in large circles for 30-60 seconds
  6. Perform aggressive yaw maneuvers (rotate aircraft on all axes)
  7. Land and disarm - calibration saves automatically

Manual Method (Ground calibration):

  1. Go to Initial Setup → Mandatory Hardware → Compass
  2. Click Start (Live Calibration or Onboard Calibration)
  3. Rotate aircraft slowly in all directions:
    • Rotate 360° around each axis (pitch, roll, yaw)
    • Move smoothly and continuously
    • Cover all orientations (imagine aircraft inside a sphere)
  4. Continue until progress bars fill or “Calibration Successful” appears
  5. Click Done

Interference Check: After calibration, check compass health in Flight Data screen. “Compass variance” should be < 0.5. Higher values indicate magnetic interference from motors, ESCs, or wires.

Vibration Isolation

Why Vibration Matters

Excessive vibration causes:

  • ❌ Poor gyro data quality
  • ❌ False vibration detection in logs
  • ❌ Ineffective dynamic notch filters
  • ❌ Reduced flight performance
  • ❌ “Toilet bowling” in GPS modes (vibration interferes with accelerometer)

Soft Mounting Recommendations

Good Vibration Isolation:

  • ✅ Use soft-mount grommets or standoffs (included with board)
  • ✅ Ensure grommets are not over-compressed
  • ✅ Avoid metal-to-metal contact between FC and frame
  • ✅ Use 3M double-sided foam tape for additional damping (optional)

Avoid:

  • ❌ Hard-mounting FC directly to carbon frame
  • ❌ Over-tightening mounting screws (compresses grommets too much)
  • ❌ Mounting near high-vibration sources (ESC, motors)

Checking Vibration Levels

ArduPilot (via Mission Planner):

  1. Fly and record Blackbox log
  2. Download log via Flight Data → DataFlash Logs
  3. Analyze in Log Review
  4. Check VIBE messages:
    • VibeX, VibeY, VibeZ should be < 30 (ideally < 15)
    • Clip counts should be 0 or very low

Betaflight (via Blackbox Explorer):

  1. Record Blackbox log during flight
  2. Download and open in Blackbox Explorer
  3. Check gyroADC traces:
    • Should be smooth with minimal high-frequency noise
    • Sharp spikes indicate vibration issues
  4. Check Debug → Gyro Scaled for vibration amplitude

Good Vibration Levels: ArduPilot VIBE < 15, Betaflight gyro traces smooth without excessive spikes. If vibration is high, check motor balance, propeller condition, and soft-mount integrity.

Troubleshooting Sensor Issues

IMU Not Detected

Symptoms: “IMU not detected” error, no gyro data in ground station

Check:

  1. ✓ Firmware flashed correctly (verify board target)
  2. ✓ IMU check (no physical damage)
  3. ✓ Power to IMU (check 3.3V isolated rail)
  4. ✓ Re-flash firmware (may fix SPI initialization issues)

Solutions:

  • Reflash latest firmware
  • Check for cold solder joints on IMU (manufacturing defect)
  • Contact support if hardware failure suspected

Barometer Not Detected

Symptoms: “Baro not detected” error, no altitude data

Check:

  1. ✓ I2C1 bus initialized correctly
  2. ✓ Firmware includes DPS368 driver
  3. ✓ No I2C address conflicts

Solutions:

  • Re-flash firmware
  • Contact support if persistent

GPS No Fix / No Satellites

Symptoms: GPS shows “No Fix”, 0 satellites, or HDOP > 2.0

Check:

  1. ✓ Outdoor location (GPS does not work indoors)
  2. ✓ Clear view of sky (no buildings, trees, metal roofs)
  3. ✓ GPS antenna orientation (ceramic patch facing up)
  4. ✓ UART8 configured for GPS protocol
  5. ✓ GPS module powered (4.5V on GPS connector)
  6. ✓ TX/RX not swapped

Solutions:

  • Wait 2-5 minutes for cold start (first GPS lock)
  • Move to open area away from buildings
  • Verify GPS UART configuration (SERIAL8_PROTOCOL = 5)
  • Check GPS LED (should blink when searching, solid when locked)

Compass Interference / Poor Calibration

Symptoms: “Compass variance” errors, toilet bowling, heading drift

Check:

  1. ✓ Compass mounted away from motors/ESCs (> 10cm)
  2. ✓ High-current wires not near compass
  3. ✓ Compass orientation set correctly
  4. ✓ Magnetic declination set (auto or manual)

Solutions:

  • Re-calibrate compass (large vehicle method)
  • Move compass further from interference sources
  • Check for metal screws/fasteners near compass
  • Verify compass orientation matches physical mounting
  • Set COMPASS_AUTODEC = 1 for automatic magnetic declination

High Vibration Levels

Symptoms: VIBE > 30 in logs, poor flight performance, toilet bowling

Check:

  1. ✓ Soft-mount grommets installed and not over-compressed
  2. ✓ Motor balance (check for bent shafts, damaged bearings)
  3. ✓ Propeller condition (no nicks, cracks, or imbalance)
  4. ✓ Frame rigidity (no loose screws, cracked carbon)
  5. ✓ ESC mounting (not touching frame directly)

Solutions:

  • Replace damaged propellers
  • Balance motors (or replace if bearings worn)
  • Use dynamic notch filters (enabled by default in modern firmware)
  • Add additional soft-mount damping (foam tape under FC)
  • Reduce motor/propeller imbalance sources

External Resources

Support

For sensor-related questions:

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