接前一篇文章：ICM20948 DMP代码详解（33）
上一回解析了inv_icm20948_initialize_lower_driver函数中设置FIFO_RST和FIFO_CFG寄存器相关的代码，本回继续往下解析inv_icm20948_initialize_lower_driver函数的后续代码。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：
cpp
/** 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;
}当前来到以下代码片段：
cpp
// 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.3）FIFO_EN和FIFO_EN_2寄存器
对应代码片段如下：
cpp
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.REG_FIFO_EN和REG_FIFO_EN_2宏也在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Defs.h中定义，如下：
cpp
#define REG_FIFO_EN (BANK_0 | 0x66)
#define BIT_SLV_0_FIFO_EN 0x01
#define REG_FIFO_EN_2 (BANK_0 | 0x67)
#define BIT_PRS_FIFO_EN 0x20
#define BIT_ACCEL_FIFO_EN 0x10
#define BITS_GYRO_FIFO_EN 0x0E对应芯片手册中的以下内容：
综上，这两句代码的意思是：第1步，设置从设备FIFO（slave FIFO）禁止；第2步，设置加速度计、陀螺仪X、Y、Z轴以及温度传感器不使能。
接下来，来到inv_icm20948_initialize_lower_driver函数的以下代码片段：
cpp
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); lp_en_support在此之前为0，因为在inv_icm20948_initialize_lower_driver函数的一开始，就将s->base_state清零了。见上边代码片段：
那么，在此就将s->base_state.lp_en_support设置为1。
紧接着由于lp_en_support当前值为1了，因此执行判断体中的：
cpp
inv_icm20948_set_chip_power_state(s, CHIP_LP_ENABLE, 1);这个函数前文书多次讲过，使能ICM20948芯片的低功耗模式。详情参见：ICM20948 DMP代码详解（15）-CSDN博客
再往下就来到了inv_icm20948_initialize_lower_driver函数的最后一个函数调用：
cpp
result |= inv_icm20948_sleep_mems(s); 这个函数前文书也讲过，就是将ICM20948芯片置为（最）低功耗模式。这里不再重复深入细节了，只是为了回顾再贴一下代码，在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948DataBaseDriver.c中，如下：
cpp
/** Puts DMP3 (SMARTSENSOR) into the lowest power state. Assumes sensors are all off.
*/
int inv_icm20948_sleep_mems(struct inv_icm20948 * s)
{
int result;
unsigned char data;
data = 0x7F;
result = inv_icm20948_write_mems_reg(s, REG_PWR_MGMT_2, 1, &data);
result |= inv_icm20948_set_chip_power_state(s, CHIP_AWAKE, 0);
return result;
}至此，inv_icm20948_initialize_lower_driver函数就全部解析完了（用了得有小20篇文章），回到实际调用它的位置即icm20948_sensor_setup函数中，下一回继续对于icm20948_sensor_setup后续的内容进行解析。