SparkFun ICM-20948 Arduino Library

The SparkFun ICM-20948 Arduino Library provides an interface for the TDK InvenSense ICM-20948 9-Degree Of Freedom (9DoF) Inertial Measurement Unit (IMU) sensor. It is specifically designed for use with the SparkFun 9DoF IMU Breakout - ICM-20948 (Qwiic).

Key features include support for the InvenSense Digital Motion Processor (DMP™) as of version 1.2, which allows for advanced sensor fusion and motion processing directly on the chip.

The output data rate of the DMP is determined by a combination of two settings:

1. The raw sensor rate, set via setSampleRate().
2. The DMP Output Data Rate (ODR) registers, set via setDMPODRrate().

To ensure correct operation, other settings must be adjusted to match the sample rate. Refer to Examples 9 and 10 in the library repository for implementation details.

The DMP is not active by default. To use it, the library must perform a specific sequence of SPI/I2C transactions to upload the DMP firmware into the sensor's memory banks and then trigger the start command. This process involves:

1. Bank Switching: The ICM-20948 uses multiple register banks. The library must switch between Bank 0, Bank 1, and Bank 2 to access specific configuration registers.
2. Memory Bank Selection: The DMP firmware is loaded into specific memory banks (e.g., Bank 0x01, 0x04) using the Memory Bank Select (Reg 0x7E) and Memory Start Address (Reg 0x7C) registers.
3. Firmware Upload: The library writes the DMP firmware data to these memory locations.
4. Initialization: Once the firmware is loaded, the sensor is configured (e.g., setting accelerometer and gyroscope scales) and the DMP is started.

DMP (Digital Motion Processor) support is disabled by default to save program memory (it occupies 14kBytes). To enable it, you must manually edit the library source files.

1. Locate ICM_20948_C.h in the src/util/ folder of the library.
2. Uncomment line 29 to define ICM_20948_USE_DMP.

File Path: src/util/ICM_20948_C.h

// Change this:
//#define ICM_20948_USE_DMP

// To this:
#define ICM_20948_USE_DMP

Similar to the compass, the B2S (likely referring to specific sensor orientation/mounting) requires mount matrix configuration. This process involves:

1. Selecting the correct memory bank (e.g., writing 0x0D to register 0x7E).
2. Setting the memory start address via register 0x7C.
3. Writing the specific matrix index (e.g., B2S_MTX_00, B2S_MTX_11).
4. Writing the 32-bit matrix value to register 0x7D.

Note: Configuration often requires toggling PWR_MGMT_1 (Register 0x06) to specific modes (like 0x21 for 'Turn on low power; Auto clock best available') between writes to ensure stability.

To ensure accurate compass readings, you must configure the Compass Mount Matrix. This tells the DMP how the magnetometer is oriented relative to the ICM-20948 sensor. The configuration involves writing to the memory start address (Register 0x7C) and then setting the specific matrix registers (Register 0x7D) with the appropriate 32-bit values.

Example SPI sequence for setting Compass Mount Matrix 11 to 0xF6666667:

1. Write 0x7C to PWR_MGMT_1 (Bank 0) to set the memory start address.
2. Write the address for CPASS_MTX_11 (e.g., 0x84 for CPASS_MTX_12 in the log, though specific offsets vary).
3. Write 0x7D to set the matrix value.
4. Provide the 4 bytes of the 32-bit value (e.g., F6 66 66 67).

Once the DMP is running, data is stored in a FIFO buffer. To retrieve it, follow this pattern:

1. Check Data Readiness: Monitor DATA_RDY_STATUS (Reg 0x8A) or check the DMP_INT_STATUS (Reg 0x18) via interrupts.
2. Check FIFO Count: Read FIFO_COUNTH (Reg 0x70) to determine how many bytes are available in the FIFO.
3. Read FIFO: Read the data from FIFO_R_W (Reg 0x72). The data will arrive in a structured format defined by your DATA_OUT_CTL settings (e.g., a Quat6 header followed by the quaternion values and a footer).