I2C(Inter－Integrated Circuit)总线是一种由PHILIPS公司开发的两线式串行总线，用于连接微控制器及其外围设备。I2C总线产生于在80年代，最初为音频和视频设备开发，如今主要在服务器管理中使用，其中包括单个组件状态的通信。例如管理员可对各个组件进行查询，以管理系统的配置或掌握组件的功能状态，如电源和系统风扇。可随时监控内存、硬盘、网络、系统温度等多个参数，增加了系统的安全性，方便了管理。
I2C工作原理I2C总线是由数据线SDA和时钟SCL构成的串行总线，可发送和接收数据。在CPU与被控IC之间、IC与IC之间进行双向传送，最高传送速率100kbps。各种被控制电路均并联在这条总线上，但就像电话机一样只有拨通各自的号码才能工作，所以每个电路和模块都有唯一的地址，在信息的传输过程中，I2C总线上并接的每一模块电路既是主控器（或被控器），又是发送器（或接收器），这取决于它所要完成的功能。CPU发出的控制信号分为地址码和控制量两部分，地址码用来选址，即接通需要控制的电路，确定控制的种类；控制量决定该调整的类别（如对比度、亮度等）及需要调整的量。这样，各控制电路虽然挂在同一条总线上，却彼此独立，互不相关。
1. 查看 I2C 源代码vim 24cxx.h 24cXX.c eeprog.c24cXX.c:
#include24cXX.h#include #include #include #include #include #include #include #include #include #include "24cXX.h" static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command, int size, union i2c_smbus_data *data) { struct i2c_smbus_ioctl_data args; args.read_write = read_write; args.command = command; args.size = size; args.data = data; return ioctl(file,I2C_SMBUS,&args); } static inline __s32 i2c_smbus_write_quick(int file, __u8 value) { return i2c_smbus_access(file,value,0,I2C_SMBUS_QUICK,NULL); } static inline __s32 i2c_smbus_read_byte(int file) { union i2c_smbus_data data; if (i2c_smbus_access(file,I2C_SMBUS_READ,0,I2C_SMBUS_BYTE,&data)) return -1; else return 0x0FF & data.byte; } static inline __s32 i2c_smbus_write_byte(int file, __u8 value) { return i2c_smbus_access(file,I2C_SMBUS_WRITE,value, I2C_SMBUS_BYTE,NULL); } static inline __s32 i2c_smbus_read_byte_data(int file, __u8 command) { union i2c_smbus_data data; if (i2c_smbus_access(file,I2C_SMBUS_READ,command, I2C_SMBUS_BYTE_DATA,&data)) return -1; else return 0x0FF & data.byte; } static inline __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value) { union i2c_smbus_data data; data.byte = value; return i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_BYTE_DATA, &data); } static inline __s32 i2c_smbus_read_word_data(int file, __u8 command) { union i2c_smbus_data data; if (i2c_smbus_access(file,I2C_SMBUS_READ,command, I2C_SMBUS_WORD_DATA,&data)) return -1; else return 0x0FFFF & data.word; } static inline __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value) { union i2c_smbus_data data; data.word = value; return i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_WORD_DATA, &data); } static inline __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value) { union i2c_smbus_data data; data.word = value; if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_PROC_CALL,&data)) return -1; else return 0x0FFFF & data.word; } static inline __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values) { union i2c_smbus_data data; int i; if (i2c_smbus_access(file,I2C_SMBUS_READ,command, I2C_SMBUS_BLOCK_DATA,&data)) return -1; else { for (i = 1; i <= data.block[0]; i++) values[i-1] = data.block[i]; return data.block[0]; } } static inline __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, __u8 *values) { union i2c_smbus_data data; int i; if (length > 32) length = 32; for (i = 1; i <= length; i++) data.block[i] = values[i-1]; data.block[0] = length; return i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_BLOCK_DATA, &data); } static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command, __u8 *values) { union i2c_smbus_data data; int i; if (i2c_smbus_access(file,I2C_SMBUS_READ,command, I2C_SMBUS_I2C_BLOCK_DATA,&data)) return -1; else { for (i = 1; i <= data.block[0]; i++) values[i-1] = data.block[i]; return data.block[0]; } } static inline __s32 i2c_smbus_write_i2c_block_data(int file, __u8 command, __u8 length, __u8 *values) { union i2c_smbus_data data; int i; if (length > 32) length = 32; for (i = 1; i <= length; i++) data.block[i] = values[i-1]; data.block[0] = length; return i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_I2C_BLOCK_DATA, &data); } static inline __s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length, __u8 *values) { union i2c_smbus_data data; int i; if (length > 32) length = 32; for (i = 1; i <= length; i++) data.block[i] = values[i-1]; data.block[0] = length; if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command, I2C_SMBUS_BLOCK_PROC_CALL,&data)) return -1; else { for (i = 1; i <= data.block[0]; i++) values[i-1] = data.block[i]; return data.block[0]; } } static int i2c_write_1b(struct eeprom *e, __u8 buf) { int r; // we must simulate a plain I2C byte write with SMBus functions r = i2c_smbus_write_byte(e->fd, buf); if(r < 0) fprintf(stderr, "Error i2c_write_1b: %sn", strerror(errno)); usleep(10); return r; } static int i2c_write_2b(struct eeprom *e, __u8 buf[2]) { int r; // we must simulate a plain I2C byte write with SMBus functions r = i2c_smbus_write_byte_data(e->fd, buf[0], buf[1]); if(r < 0) fprintf(stderr, "Error i2c_write_2b: %sn", strerror(errno)); usleep(10); return r; } static int i2c_write_3b(struct eeprom *e, __u8 buf[3]) { int r; // we must simulate a plain I2C byte write with SMBus functions // the __u16 data field will be byte swapped by the SMBus protocol r = i2c_smbus_write_word_data(e->fd, buf[0], buf[2] << 8 | buf[1]); if(r < 0) fprintf(stderr, "Error i2c_write_3b: %sn", strerror(errno)); usleep(10); return r; } #define CHECK_I2C_FUNC( var, label ) do { if(0 == (var & label)) { fprintf(stderr, "nError: " #label " function is required. Program halted.nn"); exit(1); } } while(0); int eeprom_open(char *dev_fqn, int addr, int type, struct eeprom* e) { int funcs, fd, r; e->fd = e->addr = 0; e->dev = 0; fd = open(dev_fqn, O_RDWR); if(fd <= 0) { fprintf(stderr, "Error eeprom_open: %sn", strerror(errno)); return -1; } // get funcs list if((r = ioctl(fd, I2C_FUNCS, &funcs) < 0)) { fprintf(stderr, "Error eeprom_open: %sn", strerror(errno)); return -1; } // check for req funcs CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_READ_BYTE ); CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_WRITE_BYTE ); CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_READ_BYTE_DATA ); CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_WRITE_BYTE_DATA ); CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_READ_WORD_DATA ); CHECK_I2C_FUNC( funcs, I2C_FUNC_SMBUS_WRITE_WORD_DATA ); // set working device if( ( r = ioctl(fd, I2C_SLAVE, addr)) < 0) { fprintf(stderr, "Error eeprom_open: %sn", strerror(errno)); return -1; } e->fd = fd; e->addr = addr; e->dev = dev_fqn; e->type = type; return 0; } int eeprom_close(struct eeprom *e) { close(e->fd); e->fd = -1; e->dev = 0; e->type = EEPROM_TYPE_UNKNOWN; return 0; } #if 0 int eeprom_24c32_write_byte(struct eeprom *e, __u16 mem_addr, __u8 data) { __u8 buf[3] = { (mem_addr >> 8) & 0x00ff, mem_addr & 0x00ff, data }; return i2c_write_3b(e, buf); } int eeprom_24c32_read_current_byte(struct eeprom* e) { ioctl(e->fd, BLKFLSBUF); // clear kernel read buffer return i2c_smbus_read_byte(e->fd); } int eeprom_24c32_read_byte(struct eeprom* e, __u16 mem_addr) { int r; ioctl(e->fd, BLKFLSBUF); // clear kernel read buffer __u8 buf[2] = { (mem_addr >> 8) & 0x0ff, mem_addr & 0x0ff }; r = i2c_write_2b(e, buf); if (r < 0) return r; r = i2c_smbus_read_byte(e->fd); return r; } #endif int eeprom_read_current_byte(struct eeprom* e) { ioctl(e->fd, BLKFLSBUF); // clear kernel read buffer return i2c_smbus_read_byte(e->fd); } int eeprom_read_byte(struct eeprom* e, __u16 mem_addr) { int r; ioctl(e->fd, BLKFLSBUF); // clear kernel read buffer if(e->type == EEPROM_TYPE_8BIT_ADDR) { __u8 buf = mem_addr & 0x0ff; r = i2c_write_1b(e, buf); } else if(e->type == EEPROM_TYPE_16BIT_ADDR) { __u8 buf[2] = { (mem_addr >> 8) & 0x0ff, mem_addr & 0x0ff }; r = i2c_write_2b(e, buf); } else { fprintf(stderr, "ERR: unknown eeprom typen"); return -1; } if (r < 0) return r; r = i2c_smbus_read_byte(e->fd); return r; } int eeprom_write_byte(struct eeprom *e, __u16 mem_addr, __u8 data) { if(e->type == EEPROM_TYPE_8BIT_ADDR) { __u8 buf[2] = { mem_addr & 0x00ff, data }; return i2c_write_2b(e, buf); } else if(e->type == EEPROM_TYPE_16BIT_ADDR) { __u8 buf[3] = { (mem_addr >> 8) & 0x00ff, mem_addr & 0x00ff, data }; return i2c_write_3b(e, buf); } fprintf(stderr, "ERR: unknown eeprom typen"); return -1; }
#ifndef _24CXX_H_ #define _24CXX_H_ #includeeeprog.c#include #define EEPROM_TYPE_UNKNOWN 0 #define EEPROM_TYPE_8BIT_ADDR 1 #define EEPROM_TYPE_16BIT_ADDR 2 struct eeprom { char *dev; // device file i.e. /dev/i2c-N int addr; // i2c address int fd; // file descriptor int type; // eeprom type }; int eeprom_open(char *dev_fqn, int addr, int type, struct eeprom*); int eeprom_close(struct eeprom *e); int eeprom_read_byte(struct eeprom* e, __u16 mem_addr); int eeprom_read_current_byte(struct eeprom *e); int eeprom_write_byte(struct eeprom *e, __u16 mem_addr, __u8 data); #endif
#include2. 编译并运行 3. 思考题 I2C总线的优点是什么？#include #include #include #include #include #include #include #include #include "24cXX.h" #define DEVICE_FILE_STRING "/dev/i2c/0" #define DEVICE_ADDRESS 0x50 #define usage_if(a) do { do_usage_if( a , __LINE__); } while(0); void do_usage_if(int b, int line) { const static char *eeprog_usage = "I2C-24C08(256 bytes) Read/Write Program, ONLY FOR TEST!n" "FriendlyARM Computer Tech. 2009n"; if(!b) return; fprintf(stderr, "%sn[line %d]n", eeprog_usage, line); exit(1); } #define die_if(a, msg) do { do_die_if( a , msg, __LINE__); } while(0); void do_die_if(int b, char* msg, int line) { if(!b) return; fprintf(stderr, "Error at line %d: %sn", line, msg); fprintf(stderr, " sysmsg: %sn", strerror(errno)); exit(1); } static int read_from_eeprom(struct eeprom *e, int addr, int size) { int ch, i; for(i = 0; i < size; ++i, ++addr) { die_if((ch = eeprom_read_byte(e, addr)) < 0, "read error"); if( (i % 16) == 0 ) printf("n %.4x| ", addr); else if( (i % 8) == 0 ) printf(" "); printf("%.2x ", ch); fflush(stdout); } fprintf(stderr, "nn"); } static int write_to_eeprom(struct eeprom *e, int addr) { int i; for(i=0, addr=0; i<256; i++, addr++) { if( (i % 16) == 0 ) printf("n %.4x| ", addr); else if( (i % 8) == 0 ) printf(" "); printf("%.2x ", i); fflush(stdout); die_if(eeprom_write_byte(e, addr, i), "write error"); } fprintf(stderr, "nn"); return 0; } int main(int argc, char** argv) { struct eeprom e; int op; op = 0; usage_if(argc != 2 || argv[1][0] != '-' || argv[1][2] != ' '); op = argv[1][1]; //TODO: 将数字改为自己的学号。 write(STDOUT_FILENO, "APP for 123456789012345 ...n", strlen("APP for 123456789012345 ...n")); fprintf(stderr, "Open %s with 8bit moden", DEVICE_FILE_STRING); die_if(eeprom_open(DEVICE_FILE_STRING, DEVICE_ADDRESS, EEPROM_TYPE_8BIT_ADDR, &e) < 0, "unable to open eeprom device file " "(check that the file exists and that it's readable)"); switch(op) { case 'r': fprintf(stderr, " Reading 256 bytes from 0x0n"); read_from_eeprom(&e, 0, 256); break; case 'w': fprintf(stderr, " Writing 0x00-0xff into 24C08 n"); write_to_eeprom(&e, 0); break; default: usage_if(1); exit(1); } eeprom_close(&e); return 0; }
（1.）I2C总线只需要一根数据线和一根时钟线两根线，总线接口已经集成在芯片内部，优化主板空间和成本。
（2.）无论总线上有多少设备，都只使用两条线，保持低引脚/信号数
（3.）真正的支持多主机设备，但是同一时刻只允许一台主机
（4.）I2C总线具有低功耗、抗干扰强的优点，传输距离长的特点。
（5.）连接到相同总线的IC 数量只受到总线的最大电容400pF 限制
I2C总线的启动信号和结束信号有什么特点？启动信号：SCL为高电平时，SDA由高电平向低电平跳变，开始传送数据。
结束信号：SCL为高电平时，SDA由低电平向高电平跳变，结束传送数据。
参考：i2c总线的启动信号和停止信号-与非网
I2C总线介绍_John.Ma的博客-CSDN博客_i2c总线
