接前一篇文章：ICM20948 DMP代码详解（19）
本回继续对inv_icm20948_read_mems_reg函数的其余内容进行解析。为了便于理解和回顾，再次贴出inv_icm20948_read_mems_reg函数源码，在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Transport.c中，如下：
/**
* @brief Read data from a register on MEMs.
* @param[in] Register address
* @param[in] Length of data
* @param[in] Data to be written
* @return 0 if successful.
*/
int inv_icm20948_read_mems_reg(struct inv_icm20948 * s, uint16_t reg, unsigned int length, unsigned char *data)
{int result = 0;unsigned int bytesRead = 0;unsigned char regOnly = (unsigned char)(reg & 0x7F);unsigned char i, dat[INV_MAX_SERIAL_READ];unsigned char power_state = inv_icm20948_get_chip_power_state(s);if((power_state & CHIP_AWAKE) == 0) // Wake up chip since it is asleepresult = inv_icm20948_set_chip_power_state(s, CHIP_AWAKE, 1);if(check_reg_access_lp_disable(s, reg)) // Check if register needs LP_EN to be disabledresult |= inv_icm20948_set_chip_power_state(s, CHIP_LP_ENABLE, 0); //Disable LP_ENresult |= inv_set_bank(s, reg >> 7);while (bytesRead<length) {int thisLen = min(INV_MAX_SERIAL_READ, length-bytesRead);if(s->base_state.serial_interface == SERIAL_INTERFACE_SPI) {result |= inv_icm20948_read_reg(s, regOnly+bytesRead, &dat[bytesRead], thisLen);} else {result |= inv_icm20948_read_reg(s, regOnly+bytesRead, &data[bytesRead],thisLen);}if (result)return result;bytesRead += thisLen;}if(s->base_state.serial_interface == SERIAL_INTERFACE_SPI) {for (i=0; i< length; i++) {*data= dat[i];data++;}}if(check_reg_access_lp_disable(s, reg)) // Check if register needs LP_EN to be enabled result |= inv_icm20948_set_chip_power_state(s, CHIP_LP_ENABLE, 1); //Enable LP_ENreturn result;
}接下来进入while循环，开始数据的接收。代码片段如下：
while (bytesRead<length) {int thisLen = min(INV_MAX_SERIAL_READ, length-bytesRead);if(s->base_state.serial_interface == SERIAL_INTERFACE_SPI) {result |= inv_icm20948_read_reg(s, regOnly+bytesRead, &dat[bytesRead], thisLen);} else {result |= inv_icm20948_read_reg(s, regOnly+bytesRead, &data[bytesRead],thisLen);}if (result)return result;bytesRead += thisLen;}先讲一下regOnly，它在inv_icm20948_read_mems_reg函数的开头处赋值：
unsigned char regOnly = (unsigned char)(reg & 0x7F);代码中是将寄存器和其所属的bank联合起来进行表示的，bit0~6是寄存器，bit7~8代表其所属的bank。这个regOnly表示的就是不带bank、只带寄存器的值。也就是取下图中红色框中的部分：
回到上边while循环。一开始bytesRead的值为0，length的值为1，因此会进入循环。
thisLen为INV_MAX_SERIAL_READ与length-bytesRead（1-0=1）的较小值。INV_MAX_SERIAL_READ宏在EMD-Core\sources\Invn\Devices\Drivers\ICM20948\Icm20948Transport.h中定义，如下：
/** @brief Max size that can be read across I2C or SPI data lines */
#define INV_MAX_SERIAL_READ 16
/** @brief Max size that can be written across I2C or SPI data lines */
#define INV_MAX_SERIAL_WRITE 16那么thisLen的值这里就是1。
接下来走I2C的分支：
else{result |= inv_icm20948_read_reg(s, regOnly+bytesRead, &data[bytesRead], thisLen);}inv_ivm20948_read_reg函数前边已经讲过多次，在此不赘述。直接看一下传给它的实参：
regOnly+bytesRead就是regOnly；&data[bytesRead]第一次进入循环的时候是&data[0]；thisLen的值是1。这句代码的实际意义就是读取USER_CTRL寄存器，读取1个字节，保存到&data[0]。也就是获取USER_CTRL寄存器的内容。
由于只读取1个字节的内容，因此读一次之后就跳出while循环了。
接下来来到以下代码片段：
if(s->base_state.serial_interface == SERIAL_INTERFACE_SPI) {for (i=0; i< length; i++) {*data= dat[i];data++;}}对于SPI接口来说，还要从dat[]到data[]“倒”一下。笔者不太理解为何要这样做，不过当前是用I2C接口，也不用“深抠”这一块了。
接下来就到最后一段了：
if(check_reg_access_lp_disable(s, reg)) // Check if register needs LP_EN to be enabled result |= inv_icm20948_set_chip_power_state(s, CHIP_LP_ENABLE, 1); //Enable LP_ENcheck_reg_access_lp_disable函数上一回讲过，对于USER_CTRL寄存器，总是返回1，会执行inv_icm20948_set_chip_power_state函数。这一句代码的意思是恢复置位LP_EN，也即恢复低功耗使能状态。
至此，inv_icm20948_read_mems_reg函数就都解析完了。其功能概括如下：
唤醒芯片；退出低功耗模式；设置（选择）bank；读取指定寄存器数据；恢复低功耗模式。
下一回将回到inv_icm20948_initialize_lower_driver函数中，继续往下解析接下来的内容。