The following code will read a byte from position 0x10, of the register at 0x3f of the device /dev/i2c-2.
To compile this code, you need the helper library i2c-dev.h which can be found in the download package here: [download id=”3312″]
main.c
//Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface
#include "linux/i2c-dev.h"
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
int main() {
const int adapter_nr = 2;
char filename[20];
snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
const int file = open(filename, O_RDWR);
if (file < 0) {
printf("Oh dear, something went wrong with open()! %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
// The I2C address. Got it from `i2cdetect -y 2;`
const int addr = 0x3f;
if (ioctl(file, I2C_SLAVE, addr) < 0) {
printf("Oh dear, something went wrong with ioctl()! %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
// Device register to access.
const __u8 reg = 0x10;
// Using SMBus commands
const __s32 result = i2c_smbus_read_byte_data(file, reg);
if (result < 0) {
// ERROR HANDLING: i2c transaction failed
printf("Oh dear, something went wrong with i2c_smbus_read_byte_data()>i2c_smbus_access()>ioctl()! %s\n", strerror(errno));
exit(EXIT_FAILURE);
} else {
// res contains the read word
printf("0x%+02x\n", result);
}
close(file);
return(EXIT_SUCCESS);
}
linux/i2c-dev.h
/* i2c-dev.h - i2c-bus driver, char device interface Copyright (C) 1995-97 Simon G. Vogl Copyright (C) 1998-99 Frodo Looijaard <[email protected]> This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. */ #ifndef _LINUX_I2C_DEV_H #define _LINUX_I2C_DEV_H #include <linux/types.h> #include <sys/ioctl.h> #include <stddef.h> /* -- i2c.h -- */ /* * I2C Message - used for pure i2c transaction, also from /dev interface */ struct i2c_msg { __u16 addr; /* slave address */ unsigned short flags; #define I2C_M_TEN 0x10 /* we have a ten bit chip address */ #define I2C_M_RD 0x01 #define I2C_M_NOSTART 0x4000 #define I2C_M_REV_DIR_ADDR 0x2000 #define I2C_M_IGNORE_NAK 0x1000 #define I2C_M_NO_RD_ACK 0x0800 short len; /* msg length */ char *buf; /* pointer to msg data */ }; /* To determine what functionality is present */ #define I2C_FUNC_I2C 0x00000001 #define I2C_FUNC_10BIT_ADDR 0x00000002 #define I2C_FUNC_PROTOCOL_MANGLING 0x00000004 /* I2C_M_{REV_DIR_ADDR,NOSTART,..} */ #define I2C_FUNC_SMBUS_PEC 0x00000008 #define I2C_FUNC_SMBUS_BLOCK_PROC_CALL 0x00008000 /* SMBus 2.0 */ #define I2C_FUNC_SMBUS_QUICK 0x00010000 #define I2C_FUNC_SMBUS_READ_BYTE 0x00020000 #define I2C_FUNC_SMBUS_WRITE_BYTE 0x00040000 #define I2C_FUNC_SMBUS_READ_BYTE_DATA 0x00080000 #define I2C_FUNC_SMBUS_WRITE_BYTE_DATA 0x00100000 #define I2C_FUNC_SMBUS_READ_WORD_DATA 0x00200000 #define I2C_FUNC_SMBUS_WRITE_WORD_DATA 0x00400000 #define I2C_FUNC_SMBUS_PROC_CALL 0x00800000 #define I2C_FUNC_SMBUS_READ_BLOCK_DATA 0x01000000 #define I2C_FUNC_SMBUS_WRITE_BLOCK_DATA 0x02000000 #define I2C_FUNC_SMBUS_READ_I2C_BLOCK 0x04000000 /* I2C-like block xfer */ #define I2C_FUNC_SMBUS_WRITE_I2C_BLOCK 0x08000000 /* w/ 1-byte reg. addr. */ #define I2C_FUNC_SMBUS_BYTE (I2C_FUNC_SMBUS_READ_BYTE | \ I2C_FUNC_SMBUS_WRITE_BYTE) #define I2C_FUNC_SMBUS_BYTE_DATA (I2C_FUNC_SMBUS_READ_BYTE_DATA | \ I2C_FUNC_SMBUS_WRITE_BYTE_DATA) #define I2C_FUNC_SMBUS_WORD_DATA (I2C_FUNC_SMBUS_READ_WORD_DATA | \ I2C_FUNC_SMBUS_WRITE_WORD_DATA) #define I2C_FUNC_SMBUS_BLOCK_DATA (I2C_FUNC_SMBUS_READ_BLOCK_DATA | \ I2C_FUNC_SMBUS_WRITE_BLOCK_DATA) #define I2C_FUNC_SMBUS_I2C_BLOCK (I2C_FUNC_SMBUS_READ_I2C_BLOCK | \ I2C_FUNC_SMBUS_WRITE_I2C_BLOCK) /* Old name, for compatibility */ #define I2C_FUNC_SMBUS_HWPEC_CALC I2C_FUNC_SMBUS_PEC /* * Data for SMBus Messages */ #define I2C_SMBUS_BLOCK_MAX 32 /* As specified in SMBus standard */ #define I2C_SMBUS_I2C_BLOCK_MAX 32 /* Not specified but we use same structure */ union i2c_smbus_data { __u8 byte; __u16 word; __u8 block[I2C_SMBUS_BLOCK_MAX + 2]; /* block[0] is used for length */ /* and one more for PEC */ }; /* smbus_access read or write markers */ #define I2C_SMBUS_READ 1 #define I2C_SMBUS_WRITE 0 /* SMBus transaction types (size parameter in the above functions) Note: these no longer correspond to the (arbitrary) PIIX4 internal codes! */ #define I2C_SMBUS_QUICK 0 #define I2C_SMBUS_BYTE 1 #define I2C_SMBUS_BYTE_DATA 2 #define I2C_SMBUS_WORD_DATA 3 #define I2C_SMBUS_PROC_CALL 4 #define I2C_SMBUS_BLOCK_DATA 5 #define I2C_SMBUS_I2C_BLOCK_BROKEN 6 #define I2C_SMBUS_BLOCK_PROC_CALL 7 /* SMBus 2.0 */ #define I2C_SMBUS_I2C_BLOCK_DATA 8 /* /dev/i2c-X ioctl commands. The ioctl's parameter is always an * unsigned long, except for: * - I2C_FUNCS, takes pointer to an unsigned long * - I2C_RDWR, takes pointer to struct i2c_rdwr_ioctl_data * - I2C_SMBUS, takes pointer to struct i2c_smbus_ioctl_data */ #define I2C_RETRIES 0x0701 /* number of times a device address should be polled when not acknowledging */ #define I2C_TIMEOUT 0x0702 /* set timeout in units of 10 ms */ /* NOTE: Slave address is 7 or 10 bits, but 10-bit addresses * are NOT supported! (due to code brokenness) */ #define I2C_SLAVE 0x0703 /* Use this slave address */ #define I2C_SLAVE_FORCE 0x0706 /* Use this slave address, even if it is already in use by a driver! */ #define I2C_TENBIT 0x0704 /* 0 for 7 bit addrs, != 0 for 10 bit */ #define I2C_FUNCS 0x0705 /* Get the adapter functionality mask */ #define I2C_RDWR 0x0707 /* Combined R/W transfer (one STOP only) */ #define I2C_PEC 0x0708 /* != 0 to use PEC with SMBus */ #define I2C_SMBUS 0x0720 /* SMBus transfer */ /* This is the structure as used in the I2C_SMBUS ioctl call */ struct i2c_smbus_ioctl_data { __u8 read_write; __u8 command; __u32 size; union i2c_smbus_data *data; }; /* This is the structure as used in the I2C_RDWR ioctl call */ struct i2c_rdwr_ioctl_data { struct i2c_msg *msgs; /* pointers to i2c_msgs */ __u32 nmsgs; /* number of i2c_msgs */ }; #define I2C_RDRW_IOCTL_MAX_MSGS 42 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; } /* Returns the number of read bytes */ 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, const __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); } /* Returns the number of read bytes */ /* Until kernel 2.6.22, the length is hardcoded to 32 bytes. If you ask for less than 32 bytes, your code will only work with kernels 2.6.23 and later. */ static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command, __u8 length, __u8 *values) { union i2c_smbus_data data; int i; if (length > 32) length = 32; data.block[0] = length; if (i2c_smbus_access(file,I2C_SMBUS_READ,command, length == 32 ? I2C_SMBUS_I2C_BLOCK_BROKEN : 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, const __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_BROKEN, &data); } /* Returns the number of read bytes */ 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]; } } #endif /* _LINUX_I2C_DEV_H */
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