ICM20948 DMP代码详解（46）

接前一篇文章：ICM20948 DMP代码详解（45）

上一回讲到了inv_icm20948_setup_compass_akm函数中的以下代码片段：

	/* Set compass in power down through I2C SLV for compass */result = inv_icm20948_execute_write_secondary(s, COMPASS_I2C_SLV_WRITE, s->secondary_state.compass_chip_addr, s->secondary_state.mode_reg_addr, DATA_AKM_MODE_PD);if (result)return result;

本会就对于inv_icm20948_execute_write_secondary函数以及其中调用的inv_icm20948_write_secondary函数代码进行解析。

再次贴出inv_icm20948_execute_write_secondary函数代码，在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948AuxTransport.c中，如下：

int inv_icm20948_execute_write_secondary(struct inv_icm20948 *s, int index, unsigned char addr, int reg, uint8_t v)
{int result = 0;result |= inv_icm20948_write_secondary(s, index, addr, reg, v);result |= inv_icm20948_secondary_enable_i2c(s);inv_icm20948_sleep_us(SECONDARY_INIT_WAIT*1000);result |= inv_icm20948_secondary_disable_i2c(s);result |= inv_icm20948_secondary_stop_channel(s, index);return result;
}

inv_icm20948_write_secondary函数在同文件中，就在inv_icm20948_execute_write_secondary函数的上边，代码如下：

/*
* inv_icm20948_write_secondary(): set secondary registers for writing?.
The chip must be set as bank 3 before calling.
* This is derived from inv_icm20948_write_secondary in linux...
* for now, uses a very simple data struct for the registers
* 
* index gives the mapping to the particular SLVx registers
* addr is the physical address of the device to be accessed
* reg is the device register we wish to access
* len is the number of bytes to be read
* 
*/
int inv_icm20948_write_secondary(struct inv_icm20948 *s, int index, unsigned char addr, unsigned char reg, char v)
{int result = 0;unsigned char data;data = (unsigned char)addr;result |= inv_icm20948_write_mems_reg(s, s->secondary_state.slv_reg[index].addr, 1, &data);data = reg;result |= inv_icm20948_write_mems_reg(s, s->secondary_state.slv_reg[index].reg, 1, &data);data = v;result |= inv_icm20948_write_mems_reg(s, s->secondary_state.slv_reg[index].d0, 1, &data);data = INV_MPU_BIT_SLV_EN | 1;result |= inv_icm20948_write_mems_reg(s, s->secondary_state.slv_reg[index].ctrl, 1, &data);return result;
}

可以对比着前文书对于inv_icm20948_read_secondary函数的解析过程来看。

第1段代码如下：

    data = (unsigned char)addr;result |= inv_icm20948_write_mems_reg(s, s->secondary_state.slv_reg[index].addr, 1, &data);

data是addr，即AK09916芯片的地址0x0C。注意，此处由于是写，因此并没有INV_MPU_BIT_I2C_READ | addr。

第2行代码中的s->secondary_state.slv_reg[index].addr是在前文书解析的inv_icm20948_init_secondary函数中初始化的（参见https://phmatthaus.blog.csdn.net/article/details/142306413）。

注意，此处的index和inv_icm20948_read_secondary函数不同了，对应的实参是在inv_icm20948_setup_compass_akm函数中传入的，为COMPASS_I2C_SLV_WRITE，就是上一回所说的写通道。

/** @brief I2C from secondary device can stand on up to 4 channels. To perform automatic read and feed DMP :
- channel 0 is reserved for compass reading data
- channel 1 is reserved for compass writing one-shot acquisition register
- channel 2 is reserved for als reading data */
#define COMPASS_I2C_SLV_READ		0
#define COMPASS_I2C_SLV_WRITE		1
#define ALS_I2C_SLV					2

也就是此处的index为1，表示通道1，其用于设置指南针（磁力计芯片）的寄存器。

这样，s->secondary_state.slv_reg[index].addr就是s->secondary_state.slv_reg[1].addr，在inv_icm20948_init_secondary函数中赋的值是REG_I2C_SLV1_ADDR。REG_I2C_SLV1_ADDR宏在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Defs.h中，定义如下：

#define REG_I2C_SLV0_ADDR       (BANK_3 | 0x03)
#define REG_I2C_SLV0_REG        (BANK_3 | 0x04)
#define REG_I2C_SLV0_CTRL       (BANK_3 | 0x05)
#define REG_I2C_SLV0_DO         (BANK_3 | 0x06)#define REG_I2C_SLV1_ADDR       (BANK_3 | 0x07)
#define REG_I2C_SLV1_REG        (BANK_3 | 0x08)
#define REG_I2C_SLV1_CTRL       (BANK_3 | 0x09)
#define REG_I2C_SLV1_DO         (BANK_3 | 0x0A)#define REG_I2C_SLV2_ADDR       (BANK_3 | 0x0B)
#define REG_I2C_SLV2_REG        (BANK_3 | 0x0C)
#define REG_I2C_SLV2_CTRL       (BANK_3 | 0x0D)
#define REG_I2C_SLV2_DO         (BANK_3 | 0x0E)#define REG_I2C_SLV3_ADDR       (BANK_3 | 0x0F)
#define REG_I2C_SLV3_REG        (BANK_3 | 0x10)
#define REG_I2C_SLV3_CTRL       (BANK_3 | 0x11)
#define REG_I2C_SLV3_DO         (BANK_3 | 0x12)#define REG_I2C_SLV4_CTRL       (BANK_3 | 0x15)

也就是BANK_3 | 0x07 = (3 << 7) | 0x07。

对应于ICM20948芯片手册中的以下内容：