ICM20948 DMP代码详解（23）

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上一回解析完了inv_icm20948_wakeup_mems函数，本回回到inv_icm20948_initialize_lower_driver函数中，继续往下解析。为了便于理解和回顾，再次贴出inv_icm20948_initialize_lower_driver函数源码，在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948DataBaseDriver.c中，如下：

/** Should be called once on power up. Loads DMP3, initializes internal variables needed 
*   for other lower driver functions.
*/
int inv_icm20948_initialize_lower_driver(struct inv_icm20948 * s, enum SMARTSENSOR_SERIAL_INTERFACE type, 
	const uint8_t *dmp3_image, uint32_t dmp3_image_size)
{
	int result = 0;
	static unsigned char data;
	// set static variable
	s->sAllowLpEn = 1;
	s->s_compass_available = 0;
	// ICM20948 do not support the proximity sensor for the moment.
	// s_proximity_available variable is nerver changes
	s->s_proximity_available = 0;

	// Set varialbes to default values
	memset(&s->base_state, 0, sizeof(s->base_state));
	s->base_state.pwr_mgmt_1 = BIT_CLK_PLL;
	s->base_state.pwr_mgmt_2 = BIT_PWR_ACCEL_STBY | BIT_PWR_GYRO_STBY | BIT_PWR_PRESSURE_STBY;
	s->base_state.serial_interface = type;
	result |= inv_icm20948_read_mems_reg(s, REG_USER_CTRL, 1, &s->base_state.user_ctrl);

	result |= inv_icm20948_wakeup_mems(s);

	result |= inv_icm20948_read_mems_reg(s, REG_WHO_AM_I, 1, &data);

	/* secondary cycle mode should be set all the time */
	data = BIT_I2C_MST_CYCLE|BIT_ACCEL_CYCLE|BIT_GYRO_CYCLE;

	// Set default mode to low power mode
	result |= inv_icm20948_set_lowpower_or_highperformance(s, 0);
	
	// Disable Ivory DMP.
	if(s->base_state.serial_interface == SERIAL_INTERFACE_SPI)   
		s->base_state.user_ctrl = BIT_I2C_IF_DIS;
	else
		s->base_state.user_ctrl = 0;

	result |= inv_icm20948_write_single_mems_reg(s, REG_USER_CTRL, s->base_state.user_ctrl);

	//Setup Ivory DMP.
	result |= inv_icm20948_load_firmware(s, dmp3_image, dmp3_image_size);
	if(result)
		return result;
	else
		s->base_state.firmware_loaded = 1;
	result |= inv_icm20948_set_dmp_address(s);
	// Turn off all sensors on DMP by default.
	//result |= dmp_set_data_output_control1(0);   // FIXME in DMP, these should be off by default.
	result |= dmp_icm20948_reset_control_registers(s);
	
	// set FIFO watermark to 80% of actual FIFO size
	result |= dmp_icm20948_set_FIFO_watermark(s, 800);

	// Enable Interrupts.
	data = 0x2;
	result |= inv_icm20948_write_mems_reg(s, REG_INT_ENABLE, 1, &data); // Enable DMP Interrupt
	data = 0x1;
	result |= inv_icm20948_write_mems_reg(s, REG_INT_ENABLE_2, 1, &data); // Enable FIFO Overflow Interrupt

	// TRACKING : To have accelerometers datas and the interrupt without gyro enables.
	data = 0XE4;
	result |= inv_icm20948_write_mems_reg(s, REG_SINGLE_FIFO_PRIORITY_SEL, 1, &data);

	// Disable HW temp fix
	inv_icm20948_read_mems_reg(s, REG_HW_FIX_DISABLE,1,&data);
	data |= 0x08;
	inv_icm20948_write_mems_reg(s, REG_HW_FIX_DISABLE,1,&data);

	// Setup MEMs properties.
	s->base_state.accel_averaging = 1; //Change this value if higher sensor sample avergaing is required.
	s->base_state.gyro_averaging = 1;  //Change this value if higher sensor sample avergaing is required.
	inv_icm20948_set_gyro_divider(s, FIFO_DIVIDER);       //Initial sampling rate 1125Hz/19+1 = 56Hz.
	inv_icm20948_set_accel_divider(s, FIFO_DIVIDER);      //Initial sampling rate 1125Hz/19+1 = 56Hz.

	// Init the sample rate to 56 Hz for BAC,STEPC and B2S
	dmp_icm20948_set_bac_rate(s, DMP_ALGO_FREQ_56);
	dmp_icm20948_set_b2s_rate(s, DMP_ALGO_FREQ_56);

	// FIFO Setup.
	result |= inv_icm20948_write_single_mems_reg(s, REG_FIFO_CFG, BIT_SINGLE_FIFO_CFG); // FIFO Config. fixme do once? burst write?
	result |= inv_icm20948_write_single_mems_reg(s, REG_FIFO_RST, 0x1f); // Reset all FIFOs.
	result |= inv_icm20948_write_single_mems_reg(s, REG_FIFO_RST, 0x1e); // Keep all but Gyro FIFO in reset.
	result |= inv_icm20948_write_single_mems_reg(s, REG_FIFO_EN, 0x0); // Slave FIFO turned off.
	result |= inv_icm20948_write_single_mems_reg(s, REG_FIFO_EN_2, 0x0); // Hardware FIFO turned off.
    
	s->base_state.lp_en_support = 1;
	
	if(s->base_state.lp_en_support == 1)
		inv_icm20948_set_chip_power_state(s, CHIP_LP_ENABLE, 1);

	result |= inv_icm20948_sleep_mems(s);   
        
	return result;
}

当前来到以下一行：

	result |= inv_icm20948_read_mems_reg(s, REG_WHO_AM_I, 1, &data);

再次读一下WHO_AM_I寄存器。这一句代码似乎没有什么作用。

接下来是：

	/* secondary cycle mode should be set all the time */
	data = BIT_I2C_MST_CYCLE|BIT_ACCEL_CYCLE|BIT_GYRO_CYCLE;

BIT_I2S_MST_CYCLE、BIT_ACCEL_CYCLE、BIT_GYRO_CYCLE三个宏都是在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Defs.h中定义，如下：

#define REG_LP_CONFIG           (BANK_0 | 0x05)
#define BIT_I2C_MST_CYCLE               0x40
#define BIT_ACCEL_CYCLE                 0x20
#define BIT_GYRO_CYCLE                  0x10

对应芯片手册中的内容如下：

这段代码的意思是设置LP_CONFIG寄存器中的I2C_MST_CYCLE、ACCEL_CYCLE、GYRO_CYCLE位，也即设置i2C master、加速度计、陀螺仪为占空比模式。当然，这里还没有实际设置到寄存器中，只是先进行了变量赋值。

接下来是：

	// Set default mode to low power mode
	result |= inv_icm20948_set_lowpower_or_highperformance(s, 0);

inv_icm20948_set_lowpower_or_highperformance函数在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Setup.c中，代码如下：

int inv_icm20948_set_lowpower_or_highperformance(struct inv_icm20948 *s, uint8_t lowpower_or_highperformance)
{
	s->go_back_lp_when_odr_low = 0;
	if(lowpower_or_highperformance)
		return inv_icm20948_enter_low_noise_mode(s);
	else
		return inv_icm20948_enter_duty_cycle_mode(s);
}

inv_icm20948_set_lowpower_or_highperformance函数一上来先将s->go_back_lp_when_odr_low设置为0。go_back_lp_when_odr_low是struct inv_icm20948中的成员，定义如下：

typedef struct inv_icm20948 {
    ……

	uint8_t go_back_lp_when_odr_low; // set to 1 when we forced a switch from LP to LN mode to be able to reach 1kHz ODR, so we will need to go back to LP mode ASAP
    ……
} inv_icm20948_t;

接下来根据第2个参数uint8_t lowpower_or_highperformance，选择不同分支。

1）非0分支 —— inv_icm20948_enter_low_noise_mode函数

inv_icm20948_enter_low_noise_mode函数也在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948DataBaseDriver.c中，代码如下：

int inv_icm20948_enter_low_noise_mode(struct inv_icm20948 *s)
{
	/* secondary cycle mode should be set all the time */
	unsigned char data  = BIT_I2C_MST_CYCLE;

	s->base_state.chip_lp_ln_mode = CHIP_LOW_NOISE_ICM20948;
	return inv_icm20948_write_mems_reg(s, REG_LP_CONFIG, 1, &data);
}

上边提到了BIT_I2C_MST_CYCLE，这里是实际将值BIT_I2C_MST_CYCLE设置到了LP_CONFIG寄存器，也即设置i2C master为占空比模式。同时将s->base_state.chip_lp_ln_mode设置为CHIP_LOW_NOISE_ICM20948。

CHIP_LOW_NOISE_ICM20948为枚举值，在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948.h中，定义如下：

typedef enum {
	CHIP_LOW_NOISE_ICM20948,
	CHIP_LOW_POWER_ICM20948,
} chip_lp_ln_mode_icm20948_t;

2）0分支 —— inv_icm20948_enter_duty_cycle_mode函数

inv_icm20948_enter_duty_cycle_mode函数也在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948DataBaseDriver.c中，代码如下：

int inv_icm20948_enter_duty_cycle_mode(struct inv_icm20948 * s)
{
	/* secondary cycle mode should be set all the time */
	unsigned char data  = BIT_I2C_MST_CYCLE|BIT_ACCEL_CYCLE|BIT_GYRO_CYCLE;

	s->base_state.chip_lp_ln_mode = CHIP_LOW_POWER_ICM20948;
	return inv_icm20948_write_mems_reg(s, REG_LP_CONFIG, 1, &data);
}

这个函数其实就是上边那段代码加上实际写入寄存器。

	/* secondary cycle mode should be set all the time */
	data = BIT_I2C_MST_CYCLE|BIT_ACCEL_CYCLE|BIT_GYRO_CYCLE;

估计是老版本中直接是在inv_icm20948_initialize_lower_driver函数中设置的，后来多了个Low Noise（LN）模式，就单独封了inv_icm20948_set_lowpower_or_highperformance函数，但是忘记把上边这一段代码删掉了。从这里以及整体工程代码的混乱格式就能感觉出来，要么这个DMP库代码赶时间，对于代码规范做得不够细致；要么这个项目是Invensense找的外包做的，实现了功能就得，对于其它不大要求。总之，以Linux内核代码规范标准来评判，整个工程代码在格式上“一门都没有”。

书归正题，此处由于传给inv_icm20948_set_lowpower_or_highperformance函数的实参是0，因此走的是0分支。

	// Set default mode to low power mode
	result |= inv_icm20948_set_lowpower_or_highperformance(s, 0);

至此，inv_icm20948_set_lowpower_or_highperformance函数就解析完了。下一回继续解析inv_icm20948_initialize_lower_driver函数中的其余部分。