1. 基本原理

循环冗余校验码（CRC）的基本原理是：在K位信息码后再拼接R位的校验码，整个编码长度为N位，因此，这种编码又叫（N，K）码。对于一个给定的（N，K）码，可以证明存在一个最高次幂为N-K=R的多项式G(x)。根据G(x)可以生成K位信息的校验码，而G(x)叫做这个CRC码的生成多项式。

2. 计算方法

1. 预置一个值为0xFFFF的16位寄存器，此寄存器为CRC寄存器
2. 把第一个8位二进制数与16位的CRC寄存器相异或，异或的结果存在CRC寄存器中
3. CRC寄存器的内容右移一位，用0填补最高位，并检测移出位是0还是1
4. 如果移出位是0，则重复步骤3
如果移出位是1，则与多项式进行异或
5. 重复步骤3、4，直到右移8位，这样整个8位数据都进行了处理
6. 重复步骤2~5，进行下一个字节的处理
7. 最后得到的CRC寄存器的内容即为CRC校验码

3. c语言代码实现

以SHT30温湿度传感器为例，对CRC校验的过程进行代码实现，其中SHT30 CRC检验的相关信息如下图。从图中可以看出：该传感器采用的是CRC8校验，多项式位x8+x5+x4+1，对应的代码为1 0011 0001
而CRC校验码的位数=多项式的位数-1
多项式位数=最高次幂+1
所以得到CRC多项式校验码是0x31，手册中还给到了一个示例，数据是0xBEEF时，生成的校验码是0x92，

以下是crc8的代码实现，对于crc16，crc32或者其他多项式的校验，只需要更改代码9行和10行的初始值即可。

#include <stdio.h>
typedef unsigned char uint8_t;/** crc8校验函数，data为要校验的数据，len为要校验的数据的字节数*/
uint8_t crc8(const uint8_t *data, int len)
{const uint8_t POLYNOMIAL = 0x31;uint8_t crc = 0xFF;int i, j;for (i=0; i<len; ++i) {crc ^= *data++;for (j=0; j<8; ++j) {crc = ( crc & 0x80 )? (crc << 1) ^ POLYNOMIAL: (crc << 1);}}return crc;
}int main(int argc, const char *argv[])
{unsigned char data1[2] = {0x61, 0x04};unsigned char data2[2] = {0xBE, 0xEF};printf("0x%02X\n", crc8(data1, 2));printf("0x%02X\n", crc8(data2, 2));return 0;
}
/* 输出结果
linux@linux-VirtualBox:~$ ./a.out
0xE4
0x92
*/

4. SHT30代码

SHT30传感器芯片手册连接

4.1 drv.c

#include <linux/module.h>
#include <linux/i2c.h>
#include <linux/mod_devicetable.h>
#include <linux/kdev_t.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/slab.h>
#include <linux/err.h>
#include <linux/cdev.h>
#include <linux/delay.h>
#include <linux/device.h>#include <uapi/asm-generic/errno-base.h>#define SHT30_MAJOR 255
#define SHT30_MINOR 0
#define SHT30_NAME "mpu6050_cdev"struct sht30_dev{struct i2c_client *client;struct cdev cdev;
};struct class  *cls = NULL;
struct device *dev = NULL;static int sht30_open(struct inode *inode, struct file *filp)
{struct sht30_dev *sht30 = container_of(inode->i_cdev, struct sht30_dev, cdev);filp->private_data = sht30;printk("%s -- %d.\n", __FUNCTION__, __LINE__);return 0;
}static int sht30_release(struct inode *inode, struct file *filp)
{printk("%s -- %d.\n", __FUNCTION__, __LINE__);return 0;
}static ssize_t sht30_read(struct file *filp, char __user *userbuf, size_t size, loff_t *offset)
{unsigned char buf[6];int ret;struct sht30_dev *sht30 = filp->private_data;//   printk("%s -- %d.\n", __FUNCTION__, __LINE__);ret = i2c_master_recv(sht30->client, buf, 6);if (ret < 0){printk("i2c_master_recv failed.\n");return ret;}printk("0X%02X 0X%02X 0X%02X 0X%02X 0X%02X 0X%02X.\n", buf[0], buf[1], buf[2], buf[3], buf[4], buf[5], buf[6]);ret = copy_to_user(userbuf, buf, sizeof(buf));if (ret < 0){printk("copy_to_user failed.\n");return ret;}return ret;
}static ssize_t sht30_write(struct file *filp, const char __user *userbuf, size_t size, loff_t *offset)
{unsigned char buf[2];int ret;struct sht30_dev *sht30 = filp->private_data;//   printk("%s -- %d.\n", __FUNCTION__, __LINE__);ret = copy_from_user(buf, userbuf, size);if (ret < 0){printk("copy_from_user failed.\n");return ret;}ret = i2c_master_send(sht30->client, buf, size);if (ret < 0){printk("i2c_master_send failed.\n");return ret;}return ret;
}const struct file_operations sht30_ops = {.open = sht30_open,.release = sht30_release,.read = sht30_read,.write = sht30_write,
};static int sht30_probe(struct i2c_client *client, const struct i2c_device_id *id)
{dev_t dev_no;int ret;struct sht30_dev *sht30;printk("%s -- %d.\n", __FUNCTION__, __LINE__);printk("%p.\n", client);dev_no = MKDEV(SHT30_MAJOR, SHT30_MINOR);ret = register_chrdev_region(dev_no, 1, SHT30_NAME);if ( ret ){printk("register_chrdev_region failed.\n");goto reg_err;}sht30 = kzalloc(sizeof(struct sht30_dev), GFP_KERNEL);    /* GFP_KERNEL = 0 */if ( IS_ERR(sht30) )       /* 判断指针是否有误 */{printk("kzalloc failed.\n");ret = -PTR_ERR(sht30);    /* 返回错误码 ,在include/uapi/asm-generic/errno-base.h中定义 */goto kzalloc_err;}i2c_set_clientdata(client, sht30);   /* 把数据保存到 client->dev.driver_data */sht30->client = client;cdev_init(&sht30->cdev, &sht30_ops);ret = cdev_add(&sht30->cdev, dev_no, 1);if ( ret ){printk("cdev_add failed.\n");goto add_err;}cls = class_create(THIS_MODULE, "sht30_cls");if ( IS_ERR(cls) ){printk("class_create failed.\n");ret = PTR_ERR(cls);goto cls_err;}dev = device_create(cls, NULL, dev_no, NULL, "sht30dev%d", 0);if ( IS_ERR(dev) ){printk("device_create failed.\n");ret = PTR_ERR(dev);goto dev_err;}return 0;dev_err:class_destroy(cls);
cls_err:cdev_del(&sht30->cdev);
add_err:kfree(sht30);
kzalloc_err:unregister_chrdev_region(dev_no, 1);
reg_err:return ret;
}static int sht30_remove(struct i2c_client *client)
{dev_t dev;struct sht30_dev *sht30;printk("%s -- %d.\n", __FUNCTION__, __LINE__);printk("%p.\n", client);dev = MKDEV(SHT30_MAJOR, SHT30_MINOR);sht30 = i2c_get_clientdata(client);    /* client->dev.driver_data */device_destroy(cls, dev);class_destroy(cls);cdev_del(&sht30->cdev);kfree(sht30);unregister_chrdev_region(dev, 1);return 0;
}struct i2c_device_id sht30_id[] = {{.name = "sht30"},{}
};struct i2c_driver sht30_driver = {.probe  = sht30_probe,.remove = sht30_remove,.driver = {.name  = "my_i2cdrv",.owner = THIS_MODULE, },.id_table = sht30_id,
};static int __init i2c_init(void)
{return i2c_add_driver(&sht30_driver);
}static void __exit i2c_exit(void)
{i2c_del_driver(&sht30_driver);
}module_init(i2c_init);
module_exit(i2c_exit);
MODULE_LICENSE("GPL");

4.2 test.c

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>const char *sht30_pathname = "/dev/sht30dev0";unsigned char crc8(const unsigned char *data, int len)
{const unsigned char POLYNOMIAL = 0x31;unsigned char crc = 0xFF;int i, j;for (i=0; i<len; ++i) {crc ^= *data++;for (j=0; j<8; ++j) {crc = ( crc & 0x80 )? (crc << 1) ^ POLYNOMIAL: (crc << 1);}}return crc;
}int main()
{int fd, i = 0, flag = 0;unsigned char write_buf[] = {0x2C, 0x06};unsigned char read_buf[6] = {0};fd = open(sht30_pathname, O_RDWR, 0666);if (fd < 0){printf("open failed\n");return -1;}/* init */while (1){do{write(fd, write_buf, sizeof(write_buf));read(fd, read_buf, sizeof(read_buf));         }while ( !(crc8(read_buf, 2)==read_buf[2] && crc8(read_buf+3, 2)==read_buf[5]) );printf("temp = %.3f℃\n", ( 1.0*175*(read_buf[0]*256 + read_buf[1]) / 65535 - 45 ) );printf("humi = %.3f%% \n", ( 1.0*100*(read_buf[3]*256 + read_buf[4]) / 65535 ) );printf("----------------- count = %d\n", ++i);memset(read_buf,0,sizeof(read_buf));if(i == 36000){i = 0;}sleep(1);}close(fd);return 0;
}

4.3 Makefile

KERNELDIR ?= /home/linux/ti-processor-sdk-linux-am335x-evm-05.02.00.10/board-support/linux-4.14.79/
PWD := $(shell pwd)all:make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -C $(KERNELDIR) M=$(PWD) modulesarm-linux-gnueabihf-gcc test.c -o app
install:sudo cp *.ko  app /tftpbootmake ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -C $(KERNELDIR) M=$(PWD) cleanrm app
clean:make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -C $(KERNELDIR) M=$(PWD) cleanrm appobj-m += drv.o

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