versions/v1_4_0/mynewt-core/docs/os/modules/hal/hal_i2c/hal_i2c.rst

I2C
========

The hardware independent interface to I2C Devices.

Description
~~~~~~~~~~~

An Inter-Integrated Circuit (I²C ] I-squared-C) bus is a multi-master,
multi-save serial interface used to connect components on a circuit
board and often peripherals devices located off the circuit board.

I2C is often though of as a 2-wire protocol because it uses two wires
(SDA, SCL) to send data between devices.

For a detailed description of I2C, see the `I²C wikipedia
page <https://en.wikipedia.org/wiki/I²C>`__

HAL\_I2C Theory Of Operation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An I²C transaction typically involves acquiring the bus, sending and/or
receiving data and release the bus. The bus acquisition portion is
important because the bus is typically multi-master so other devices may
be trying to read/write the same peripheral.

HAL\_I2C implements a master interface to the I²C bus. Typical usage of
the interface would involve the following steps.

Initialize an i2c device with: hal\_i2c\_init()

When you wish to perform an i2c transaction, you call one or both of:
hal\_i2c\_master\_write(); hal\_i2c\_master\_read();

These functions will issue a START condition, followed by the device's
7-bit I2C address, and then send or receive the payload based on the
data provided. This will cause a repeated start on the bus, which is
valid in I2C specification, and the decision to use repeated starts was
made to simplify the I2C HAL. To set the STOP condition at an
appropriate moment, you set the ``last_op`` field to a ``1`` in either
function.

For example, in an I2C memory access you might write a register address
and then read data back via: hal\_i2c\_write(); -- write to a specific
register on the device hal\_i2c\_read(); --- read back data, setting
'last\_op' to '1'

An addition API was added called ``hal_i2c_probe``. This command
combines ``hal_i2c_begin()``, ``hal_i2c_read``, and ``hal_i2c_end()`` to
try to read 0-bytes from a specific bus address. its intended to provide
an easy way to probe the bus for a specific device. NOTE: if the device
is write-only, it will not appear with this command.

A slave API is pending for further release.

HAL\_I2C Data
~~~~~~~~~~~~~

Data to read/write is passed to the hal\_i2c APIs via the

::

    struct hal_i2c_master_data {
        uint8_t  address;   /* destination address */
        uint16_t len;       /* number of bytes to transmit or receive */
        uint8_t *buffer;    /* data buffer for transmit or receive */
    };

``buffer`` is a pointer to the data to send. ``len`` is the number of
bytes to send over the bus. ``address`` is a 7-bit bus address of the
device.

When I²C builds its address, it uses the 7-bit address plus a 1-bit R/W
(read/write) indicator to identify the device and direction of the
transaction. Thus when using this API, you should use a 7-bit address in
the data structure and ensure that address is a value between 0-127.

As an example, consider an I²C device address that looks like this:

+-------+------+------+------+------+------+------+-------+
| B7    | B6   | B5   | B4   | B3   | B2   | B1   | B0    |
+=======+======+======+======+======+======+======+=======+
| 1     | 0    | 0    | 0    | 1    | 1    | 0    | R/W   |
+-------+------+------+------+------+------+------+-------+
| MSB   |      |      |      |      |      |      | LSB   |
+-------+------+------+------+------+------+------+-------+

In the HAL\_I2C API you would communicate with this device with address
``0b1000110``, which is hex 0x46 or decimal 70. The I²C drive would add
the R/W bit and transmit it as hex 0x8C (binary 10001100) or 0x8D
(binary 10001101) depending whether it was a read or write command.


API
~~~~~~~~~~

.. doxygengroup:: HALI2c
    :content-only:
    :members: