I2C C Library

The Onion I2C Library, libonioni2c is a dynamic C library that provides functions to easily read from and write to devices communicating with the Omega via I2C. The library can be used in C and C++ programs.

Also available is a Python module that implements an I2C object using functions from the C library. The module is called onionI2C and is part of the OmegaExpansion package.

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Linux and I2C

I2C devices are usually controlled by a kernel driver, however, it is also possible to access devices through an adapter in the Linux filesystem. The adapter can be found at /dev/i2c-X, where X is the adapter number which can range from 0 to 255. On the Omega, /dev/i2c-0 is available by default. This allows users to interact with I2C slaves from the Linux environment.

The Onion I2C library uses the /dev/i2c-0 adapter, and implements read and write functions I2C devices.

The C Library

The libonioni2c C library is a series of functions that implement I2C communication through the Linux device interface.

Source Code

The source code can be found in the Onion i2c-exp-driver GitHub Repo.

Programming Flow

Each of the read and write functions have been written to be self-contained, so one function call will complete the desired action.

Using the Library

Header File

To add the Onion I2C Library to your C/C++ program, include the header file in your C code:

#include <onion-i2c.h>

Library for Linker

In your project’s makefile, you will need to add the following dynamic libraries to the linker command:

-loniondebug -lonioni2c 

The dynamic libraries are stored in /usr/lib on the Omega.

Example

An example of how the libonioni2c library is used can be found in the GitHub Repo for the drivers for the PWM, Relay, and OLED Expansions.

Specifically, a variety of functions are used in the PWM Expansion source code.

Functions

Each of the main functions implemented in this library are described below.

Function Flow

All of the functions follow the same general pattern:

• Get a file descriptor to the I2C adapter
• Inform the adapter which device to communicate with
• Perform the function’s main operation (read, write to the I2C adapter)
• Release the I2C adapter device handle
• Return the function status

Return Values

All functions follow the same pattern with return values:

If the function operation is successful, the return value will be EXIT_SUCCESS which is a macro defined as 0 in cstdlib.h.

If the function operation is not successful, the function will return EXIT_FAILURE which is defined as 1.

A few reasons why the function might not be successful:

• The specified device address cannot be found on the I2C bus
• The system I2C interface is currently in use elsewhere

An error message will be printed that will give more information on the reason behind the failure.

Read Functions

Functions that perform reads from devices on the I2C bus

Function: i2c_readByte

The i2c_readByte function will read one byte from a register address on a specified device on the I2C bus.

The function is declared as follows:

int i2c_readByte (int devNum, int devAddr, int addr, int *val);

Arguments

The adapter number on the Omega should always be 0 since it will use /dev/i2c-0 to interface with I2C devices. The I2C device address is unique to each type of device and can be found in the device datasheet. The datasheet will also define register addresses to read specific data.

Finally, the val argument is passed by reference, and after the function runs will contain the single byte that was read.

Examples

To read a byte from the 0x01 register from an I2C device with address of 0x5a (the Servo Expansion):

int 	status, rdByte;
status 	= i2c_write(0, 0x5a, 0x01, &rdByte);

Function: i2c_read

The i2c_read function will read a specified number of bytes from a register address on a device on the I2C bus.

The function is declared as follows:

int i2c_read (int devNum, int devAddr, int addr, uint8_t *buffer, int numBytes);

Arguments

The adapter number on the Omega should always be 0 since it will use /dev/i2c-0 to interface with I2C devices. The I2C device address is unique to each type of device and can be found in the device datasheet. The datasheet will also define register addresses to read specific data.

The numBytes argument specifies how many bytes to read from a specific address on the I2C device. The *buffer argument is a unsigned 8-bit integer pointer that will contain all of the bytes read from the device once the function returns

Note that when using a pointer for the buffer argument, it must be allocated before being passed into the function, and it must be allocated with at least the number specified by numBytes. When using an array for the buffer, it must be declared with at least the number of elements specified by the numBytes argument.

C Examples

To read 4 bytes from address 0x00 on a device with an address of 0x48, using a pointer for the buffer:

int 	status;
uint8_t *buffer = malloc(4 * sizeof *buffer);
status 			= i2c_read(0, 0x48, 0x00, buffer, 4);

To read 2 bytes from address 0x10 on a device with an address of 0x48, using an array for the buffer:

int 	status;
uint8_t	buffer[32];
status 			= i2c_read(0, 0x48, 0x10, buffer, 2);

C++ Examples

To read 2 bytes from address 0x00 on a device with an address of 0x48, using a pointer for the buffer:

int 	status;
uint8_t	*buffer = new uint8_t[16];
status 			= i2c_read(0, 0x48, 0x00, buffer, 2);

Write Functions

Functions that perform writes to devices on the I2C bus.

Function: i2c_writeBuffer

The i2c_writeBuffer function will write a specified number of bytes from a previously populated pointer or array to a register address on an I2C device.

The function is declared as follows:

int i2c_writeBuffer	(int devNum, int devAddr, int addr, uint8_t *buffer, int size);

Arguments

The adapter number on the Omega should always be 0 since it will use /dev/i2c-0 to interface with I2C devices. The I2C device address is unique to each type of device and can be found in the device datasheet. The datasheet will also define register addresses to read specific data.

The size argument is the number of bytes to write from the buffer pointer. The *buffer argument is a unsigned 8-bit integer pointer that contains all of the bytes to be written to the specified address on the I2C device. Note that buffer[0] will be written first, and then buffer[1], and so on.

When using a pointer for the buffer argument, it must be allocated and populated before being passed into the function, and it must be allocated with at least the number specified by size. When using an array for the buffer, it must be declared with at least the number of elements specified by the numBytes argument.

Examples

Write 2 bytes to the 0x02 register address on an I2C device with an address of 0x08, using an array for the buffer argument:

int 	status;
uint8_t	buffer[32];

// populate the buffer
buffer[0] 	= 0xde;
buffer[1] 	= 0xad;

status 		= i2c_writeBuffer(0, 0x08, 0x01, buffer, 2);

Write 3 bytes to the 0x05 register address on the same device as above using an array for the buffer argument:

int 	status;
uint8_t	buffer[3] = {0xbe, 0xef, 0x80};

status 		= i2c_writeBuffer(0, 0x08, 0x05, buffer, 3);

Write 4 bytes to the 0x54 register address on an I2C device with an address of 0x30, *using a pointer for the buffer argument:

int 	status;
uint8_t *buffer = malloc(4 * sizeof *buffer);

// populate the buffer
buffer[0] 	= 0xb6;
buffer[1] 	= 0xd1;
buffer[2] 	= 0xe3;
buffer[3] 	= 0xff;

status 		= i2c_writeBuffer(0, 0x30, 0x54, buffer, 4);

Function: i2c_writeBytes

The i2c_writeBytes function will write a specified number of bytes from an integer variable to an address on an I2C device. Sometimes it’s a little quicker to pass in an integer rather than create a buffer like the i2c_writeBuffer function above requires. Note that the Least Significant Byte (LSB) of the integer will be written first and that the maximum number of bytes is 4 (since an int holds 32 bits on the Omega).

The function is declared as follows:

int i2c_writeBytes 	(int devNum, int devAddr, int addr, int val, int numBytes);

Arguments

The adapter number on the Omega should always be 0 since it will use /dev/i2c-0 to interface with I2C devices. The I2C device address is unique to each type of device and can be found in the device datasheet. The datasheet will also define register addresses to read specific data.

The numBytes argument is the number of bytes to write from the integer. The val argument is an integer variable that should be preprogrammed with a value to be written.

Note that the bytes of the integer will be written in the following order, assuming that 4 bytes are being written:

• val[7:0]
• val[15:8]
• val[23:16]
• val[31:14]

Examples

Write a byte to the 0x02 register address on an I2C device with an address of 0x08:

int 	status;
int		val  	= 0x04;

status 		= i2c_writeBytes(0, 0x08, 0x01, val, 1);

Write 2 bytes to the 0x05 register address on the same device as above:

int 	status;
int		val  	= 0x1304;

// write 0x04 and then 0x13 to the 0x01 address
status 		= i2c_writeBytes(0, 0x08, 0x01, val, 2);

Write 3 bytes to the 0x54 register address on an I2C device with an address of 0x30:

int 	status;
int		val  	= 0xfe082324;

// write 0x24, then 0x23, and then 0x08 to the 0x54 address
status 		= i2c_writeBytes(0, 0x30, 0x54, val, 3);

Write 4 bytes to the 0x00 register address on an I2C device with an address of 0x30:

int 	status;
int		val  	= 0x27f8e460;

// write 0x60, then 0xe4, then 0xf8, and then 0x27 to the 0x00 address
status 		= i2c_writeBytes(0, 0x30, 0x00, val, 4);