Does the I2C master and compass ODR need to be set to 68.75Hz or is the accuracy better when it matches the gyro/accel ODR, potentially higher than compass ODR, e.g. 225Hz?

[msrivast]

Not an issue - just a question.

[PaulZC]

Hi @msrivast ,

Honest answer: I don't know! I've included a lot of notes here which should help:

SparkFun_ICM-20948_ArduinoLibrary/examples/Arduino/Example10_DMP_FastMultipleSensors/Example10_DMP_FastMultipleSensors.ino

Lines 264 to 310 in d5ae1eb

	// Normally, when the DMP is not enabled, startupMagnetometer (called by startupDefault, which is called by begin) configures the AK09916 magnetometer
	// to run at 100Hz by setting the CNTL2 register (0x31) to 0x08. Then the ICM20948's I2C_SLV0 is configured to read
	// nine bytes from the mag every sample, starting from the STATUS1 register (0x10). ST1 includes the DRDY (Data Ready) bit.
	// Next are the six magnetometer readings (little endian). After a dummy byte, the STATUS2 register (0x18) contains the HOFL (Overflow) bit.
	//
	// But looking very closely at the InvenSense example code, we can see in inv_icm20948_resume_akm (in Icm20948AuxCompassAkm.c) that,
	// when the DMP is running, the magnetometer is set to Single Measurement (SM) mode and that ten bytes are read, starting from the reserved
	// RSV2 register (0x03). The datasheet does not define what registers 0x04 to 0x0C contain. There is definitely some secret sauce in here...
	// The magnetometer data appears to be big endian (not little endian like the HX/Y/Z registers) and starts at register 0x04.
	// We had to examine the I2C traffic between the master and the AK09916 on the AUX_DA and AUX_CL pins to discover this...
	//
	// So, we need to set up I2C_SLV0 to do the ten byte reading. The parameters passed to i2cControllerConfigurePeripheral are:
	// 0: use I2C_SLV0
	// MAG_AK09916_I2C_ADDR: the I2C address of the AK09916 magnetometer (0x0C unshifted)
	// AK09916_REG_RSV2: we start reading here (0x03). Secret sauce...
	// 10: we read 10 bytes each cycle
	// true: set the I2C_SLV0_RNW ReadNotWrite bit so we read the 10 bytes (not write them)
	// true: set the I2C_SLV0_CTRL I2C_SLV0_EN bit to enable reading from the peripheral at the sample rate
	// false: clear the I2C_SLV0_CTRL I2C_SLV0_REG_DIS (we want to write the register value)
	// true: set the I2C_SLV0_CTRL I2C_SLV0_GRP bit to show the register pairing starts at byte 1+2 (copied from inv_icm20948_resume_akm)
	// true: set the I2C_SLV0_CTRL I2C_SLV0_BYTE_SW to byte-swap the data from the mag (copied from inv_icm20948_resume_akm)
	result = i2cControllerConfigurePeripheral(0, MAG_AK09916_I2C_ADDR, AK09916_REG_RSV2, 10, true, true, false, true, true); if (result > worstResult) worstResult = result;
	//
	// We also need to set up I2C_SLV1 to do the Single Measurement triggering:
	// 1: use I2C_SLV1
	// MAG_AK09916_I2C_ADDR: the I2C address of the AK09916 magnetometer (0x0C unshifted)
	// AK09916_REG_CNTL2: we start writing here (0x31)
	// 1: not sure why, but the write does not happen if this is set to zero
	// false: clear the I2C_SLV0_RNW ReadNotWrite bit so we write the dataOut byte
	// true: set the I2C_SLV0_CTRL I2C_SLV0_EN bit. Not sure why, but the write does not happen if this is clear
	// false: clear the I2C_SLV0_CTRL I2C_SLV0_REG_DIS (we want to write the register value)
	// false: clear the I2C_SLV0_CTRL I2C_SLV0_GRP bit
	// false: clear the I2C_SLV0_CTRL I2C_SLV0_BYTE_SW bit
	// AK09916_mode_single: tell I2C_SLV1 to write the Single Measurement command each sample
	result = i2cControllerConfigurePeripheral(1, MAG_AK09916_I2C_ADDR, AK09916_REG_CNTL2, 1, false, true, false, false, false, AK09916_mode_single); if (result > worstResult) worstResult = result;
	// Set the I2C Master ODR configuration
	// It is not clear why we need to do this... But it appears to be essential! From the datasheet:
	// "I2C_MST_ODR_CONFIG[3:0]: ODR configuration for external sensor when gyroscope and accelerometer are disabled.
	// ODR is computed as follows: 1.1 kHz/(2^((odr_config[3:0])) )
	// When gyroscope is enabled, all sensors (including I2C_MASTER) use the gyroscope ODR.
	// If gyroscope is disabled, then all sensors (including I2C_MASTER) use the accelerometer ODR."
	// Since both gyro and accel are running, setting this register should have no effect. But it does. Maybe because the Gyro and Accel are placed in Low Power Mode (cycled)?
	// You can see by monitoring the Aux I2C pins that the next three lines reduce the bus traffic (magnetometer reads) from 1125Hz to the chosen rate: 68.75Hz in this case.
	result = setBank(3); if (result > worstResult) worstResult = result; // Select Bank 3
	uint8_t mstODRconfig = 0x04; // Set the ODR configuration to 1100/2^4 = 68.75Hz
	result = write(AGB3_REG_I2C_MST_ODR_CONFIG, &mstODRconfig, 1); if (result > worstResult) worstResult = result; // Write one byte to the I2C_MST_ODR_CONFIG register

Best wishes,
Paul