Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 Usually, i2c devices are controlled by a kernel driver. But it is also 2 possible to access all devices on an adapter from userspace, through 3 the /dev interface. You need to load module i2c-dev for this. 4 5 Each registered i2c adapter gets a number, counting from 0. You can 6 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. 7 I2C device files are character device files with major device number 89 8 and a minor device number corresponding to the number assigned as 9 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., 10 i2c-10, ...). All 256 minor device numbers are reserved for i2c. 11 12 13 C example 14 ========= 15 16 So let's say you want to access an i2c adapter from a C program. The 17 first thing to do is "#include <linux/i2c-dev.h>". Please note that 18 there are two files named "i2c-dev.h" out there, one is distributed 19 with the Linux kernel and is meant to be included from kernel 20 driver code, the other one is distributed with lm_sensors and is 21 meant to be included from user-space programs. You obviously want 22 the second one here. 23 24 Now, you have to decide which adapter you want to access. You should 25 inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned 26 somewhat dynamically, so you can not even assume /dev/i2c-0 is the 27 first adapter. 28 29 Next thing, open the device file, as follows: 30 int file; 31 int adapter_nr = 2; /* probably dynamically determined */ 32 char filename[20]; 33 34 sprintf(filename,"/dev/i2c-%d",adapter_nr); 35 if ((file = open(filename,O_RDWR)) < 0) { 36 /* ERROR HANDLING; you can check errno to see what went wrong */ 37 exit(1); 38 } 39 40 When you have opened the device, you must specify with what device 41 address you want to communicate: 42 int addr = 0x40; /* The I2C address */ 43 if (ioctl(file,I2C_SLAVE,addr) < 0) { 44 /* ERROR HANDLING; you can check errno to see what went wrong */ 45 exit(1); 46 } 47 48 Well, you are all set up now. You can now use SMBus commands or plain 49 I2C to communicate with your device. SMBus commands are preferred if 50 the device supports them. Both are illustrated below. 51 __u8 register = 0x10; /* Device register to access */ 52 __s32 res; 53 char buf[10]; 54 /* Using SMBus commands */ 55 res = i2c_smbus_read_word_data(file,register); 56 if (res < 0) { 57 /* ERROR HANDLING: i2c transaction failed */ 58 } else { 59 /* res contains the read word */ 60 } 61 /* Using I2C Write, equivalent of 62 i2c_smbus_write_word_data(file,register,0x6543) */ 63 buf[0] = register; 64 buf[1] = 0x43; 65 buf[2] = 0x65; 66 if ( write(file,buf,3) != 3) { 67 /* ERROR HANDLING: i2c transaction failed */ 68 } 69 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */ 70 if (read(file,buf,1) != 1) { 71 /* ERROR HANDLING: i2c transaction failed */ 72 } else { 73 /* buf[0] contains the read byte */ 74 } 75 76 IMPORTANT: because of the use of inline functions, you *have* to use 77 '-O' or some variation when you compile your program! 78 79 80 Full interface description 81 ========================== 82 83 The following IOCTLs are defined and fully supported 84 (see also i2c-dev.h): 85 86 ioctl(file,I2C_SLAVE,long addr) 87 Change slave address. The address is passed in the 7 lower bits of the 88 argument (except for 10 bit addresses, passed in the 10 lower bits in this 89 case). 90 91 ioctl(file,I2C_TENBIT,long select) 92 Selects ten bit addresses if select not equals 0, selects normal 7 bit 93 addresses if select equals 0. Default 0. This request is only valid 94 if the adapter has I2C_FUNC_10BIT_ADDR. 95 96 ioctl(file,I2C_PEC,long select) 97 Selects SMBus PEC (packet error checking) generation and verification 98 if select not equals 0, disables if select equals 0. Default 0. 99 Used only for SMBus transactions. This request only has an effect if the 100 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just 101 doesn't have any effect. 102 103 ioctl(file,I2C_FUNCS,unsigned long *funcs) 104 Gets the adapter functionality and puts it in *funcs. 105 106 ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset) 107 108 Do combined read/write transaction without stop in between. 109 Only valid if the adapter has I2C_FUNC_I2C. The argument is 110 a pointer to a 111 112 struct i2c_rdwr_ioctl_data { 113 struct i2c_msg *msgs; /* ptr to array of simple messages */ 114 int nmsgs; /* number of messages to exchange */ 115 } 116 117 The msgs[] themselves contain further pointers into data buffers. 118 The function will write or read data to or from that buffers depending 119 on whether the I2C_M_RD flag is set in a particular message or not. 120 The slave address and whether to use ten bit address mode has to be 121 set in each message, overriding the values set with the above ioctl's. 122 123 124 Other values are NOT supported at this moment, except for I2C_SMBUS, 125 which you should never directly call; instead, use the access functions 126 below. 127 128 You can do plain i2c transactions by using read(2) and write(2) calls. 129 You do not need to pass the address byte; instead, set it through 130 ioctl I2C_SLAVE before you try to access the device. 131 132 You can do SMBus level transactions (see documentation file smbus-protocol 133 for details) through the following functions: 134 __s32 i2c_smbus_write_quick(int file, __u8 value); 135 __s32 i2c_smbus_read_byte(int file); 136 __s32 i2c_smbus_write_byte(int file, __u8 value); 137 __s32 i2c_smbus_read_byte_data(int file, __u8 command); 138 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value); 139 __s32 i2c_smbus_read_word_data(int file, __u8 command); 140 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value); 141 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value); 142 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values); 143 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, 144 __u8 *values); 145 All these transactions return -1 on failure; you can read errno to see 146 what happened. The 'write' transactions return 0 on success; the 147 'read' transactions return the read value, except for read_block, which 148 returns the number of values read. The block buffers need not be longer 149 than 32 bytes. 150 151 The above functions are all macros, that resolve to calls to the 152 i2c_smbus_access function, that on its turn calls a specific ioctl 153 with the data in a specific format. Read the source code if you 154 want to know what happens behind the screens.