Because you are using a wrong header file for your application. With this code the SCL bus is always LOW, means the SCL bus is not released even if there is the i2c_readNak() before to return. s32 i2c_smbus_read_byte_data ( const struct i2c_client * client, u8 command); Where, client – Handle to the slave device command – Byte interpreted by slave. Byte being written Description. s32 i2c_smbus_read_byte_data (const struct i2c_client * client, u8 command) ¶ SMBus “read byte” protocol. This executes the SMBus “ write byte ” protocol, returning negative errno else zero on success. The block read function returns the number of bytes read, so I can use this to determine if the read was successful, but the byte and word read functions return the value read. Part Number: AM3351 Tool/software: Linux Hi, the usb hub 2517 supports only smbus block read/write. This executes the SMBus “send byte” protocol, returning negative errno else zero on success. Hello, As I'm rather new to Cmake usage, I don't really know how to solve this problem. But they are a) not system calls, or b) not accessible from your application. When this flag is used, it will perform the operation directly. When the device is unplugged, the registers read 0xff or 0xffff, but those are also valid register values. * i2c_smbus_read_byte_data - SMBus "read byte" protocol: 124 * @client: Handle to slave device: 125 * @command: Byte interpreted by slave: 126 * 127 * This executes the SMBus "read byte" protocol, returning negative errno: 128 * else a data byte received from the device. It can also cause i2cget to return an invalid value. value. u8 value Byte to be sent. So use at your own risk and only if you know what you're doing. If you see an extern on the function i2c_smbus_read_word_data() in your header, it's a header file for your kernel, but not for your application.The Linux kernel has i2c_smbus_read_word_data() and other i2c smbus functions for its internal use. ‘i2c_smbus_read_byte_data’ was not declared in this scope C Linux Ответ This is mainly meant to be used in scripts. When this flag is used, it will perform the operation directly. By default, i2cget will wait for a confirmation from the user before messing with the I2C bus. It can also cause i2cget to return an invalid value. 1) in this way the 3 values read are 0xFF 0xFF 0xFF instead of 0x5D 0x34 and 0x80 (80 should be not correct because the datasheet is old). It is returning negative errno else a data byte received from the device. So use at your own risk and only if you know what you're doing. i am using below application, but write/read not happning. 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