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I2C API

The I2C API supports Inter-Integrated Circuit (IIC, I2C), a synchronous serial protocol that allows multiple slave chips to communicate with a master chip. A single I2C bus uses 2 pins, SDA (data) and SCL (clock). Multiple I2C buses may be present on a board, represented as index numbers in this API. Implementations should map the I2C bus to the real pins, using the board documentation.

I2C selects the slave device by addressing: the first byte sent by the master chip contains the 7 bit address of a slave, and one bit for direction (read or write). Slaves acknowledge each byte received from the master by sending 0 on SDA (both SDA and SCL are pulled high in I2C).

If read was requested, the master will emit clock, and the selected slave will provide bits to the master as long as clock is emitted.

If write was requested, the master puts the bit on SDA and sends a clock signal for data available. Therefore it is important to select the right speed supported by the master and slave devices. This API uses a Buffer object for both read and write.

The I2C API object

When requiring "i2c", the following steps are run:

  • If there is no permission for using the functionality, throw SecurityError.
  • If the I2C functionality is not supported on the board, throw "NotSupportedError".
  • Return an object that implements the following method.
Method Description
open() synchronous open

See also the Web IDL definition.

The I2C open(options) method

Configures an I2C bus using data provided by the options argument. It runs the following steps:

  • Let i2c be an I2C](#i2c) object.
  • If options is a dictionary and the options.bus property is a number between 0 and 127, let i2c.bus be options.bus, otherwise select the platform default value, and if that is not available, set the value to 0.
  • If options.speed is not a number, let i2c.speed be 10. Otherwise, if it is in the { 10, 100, 400, 1000, 3400 } set, let i2c.speed take that value, otherwise set i2c.speed to the closest matching value.
  • Request the underlying platform to initialize the I2C bus with i2c.speed on board.
  • In case of failure, throw InvalidAccessError.
  • Return i2c.

The I2C interface

Represents the properties and methods that expose I2C functionality.

Property Type Optional Default value Represents
bus octet yes platform selected I2C bus
speed long yes platform selected I2C bus speed
Method Description
write() write data to a device
read() read data from a device
close() close the I2C bus

The bus property denotes the I2C bus number between 0 and 127.

The speed property can take the following numeric values denoting kilobits per second: 10, 100, 400, 1000, 3400.

The write(device, buffer) method

Writes a Buffer using I2C to slave device. The method runs the following steps:

  • If device is not a number between 0 and 127, throw TypeError and terminate these steps.
  • Request the underlying platform to write the bytes specified by buffer argument to the specified device. If the operation fails, throw SystemError.

The read(device, size) method

Reads maximum size number of bytes from I2C device device and returs a Buffer. The method runs the following steps:

  • If device is not a number between 0 and 127, throw TypeError and terminate these steps.
  • Allocate a Buffer.
  • Request the underlying platform to read size number of bytes from the specified device into buffer. If the operation fails, throw SystemError.
  • Return buffer.

The close() method

Closes the current I2C bus and cancels all pending operations.

Examples

try {
  var i2c = require("i2c").open();  // open the default I2C bus
  console.log("I2C bus " + i2c.bus + " opened with bus speed " + i2c.speed);

  i2c.write(0x02, [1, 2, 3]);

  var buffer = i2c.read(0x03, 3);
  console.log("From I2C device 0x03: " + buffer.toString());

  i2c.close();
} catch (err) {
  console.log("I2C error: " + err.message);
}