.. currentmodule:: machine
.. _machine.I2C:

class I2C -- a two-wire serial protocol
=======================================

I2C is a two-wire protocol for communicating between devices.  At the physical
level it consists of 2 wires: SCL and SDA, the clock and data lines respectively.

I2C objects are created attached to a specific bus.  They can be initialised
when created, or initialised later on.

Printing the I2C object gives you information about its configuration.

Example usage::

    from machine import I2C

    i2c = I2C(freq=400000)          # create I2C peripheral at frequency of 400kHz
                                    # depending on the port, extra parameters may be required
                                    # to select the peripheral and/or pins to use

    i2c.scan()                      # scan for slaves, returning a list of 7-bit addresses

    i2c.writeto(42, b'123')         # write 3 bytes to slave with 7-bit address 42
    i2c.readfrom(42, 4)             # read 4 bytes from slave with 7-bit address 42

    i2c.readfrom_mem(42, 8, 3)      # read 3 bytes from memory of slave 42,
                                    #   starting at memory-address 8 in the slave
    i2c.writeto_mem(42, 2, b'\x10') # write 1 byte to memory of slave 42
                                    #   starting at address 2 in the slave

Constructors
------------

.. class:: I2C(id=-1, \*, scl, sda, freq=400000)

   Construct and return a new I2C object using the following parameters:

      - *id* identifies a particular I2C peripheral.  The default
        value of -1 selects a software implementation of I2C which can
        work (in most cases) with arbitrary pins for SCL and SDA.
        If *id* is -1 then *scl* and *sda* must be specified.  Other
        allowed values for *id* depend on the particular port/board,
        and specifying *scl* and *sda* may or may not be required or
        allowed in this case.
      - *scl* should be a pin object specifying the pin to use for SCL.
      - *sda* should be a pin object specifying the pin to use for SDA.
      - *freq* should be an integer which sets the maximum frequency
        for SCL.

General Methods
---------------

.. method:: I2C.init(scl, sda, \*, freq=400000)

  Initialise the I2C bus with the given arguments:

     - *scl* is a pin object for the SCL line
     - *sda* is a pin object for the SDA line
     - *freq* is the SCL clock rate

.. method:: I2C.deinit()

   Turn off the I2C bus.

   Availability: WiPy.

.. method:: I2C.scan()

   Scan all I2C addresses between 0x08 and 0x77 inclusive and return a list of
   those that respond.  A device responds if it pulls the SDA line low after
   its address (including a write bit) is sent on the bus.

Primitive I2C operations
------------------------

The following methods implement the primitive I2C master bus operations and can
be combined to make any I2C transaction.  They are provided if you need more
control over the bus, otherwise the standard methods (see below) can be used.

.. method:: I2C.start()

   Generate a START condition on the bus (SDA transitions to low while SCL is high).

   Availability: ESP8266.

.. method:: I2C.stop()

   Generate a STOP condition on the bus (SDA transitions to high while SCL is high).

   Availability: ESP8266.

.. method:: I2C.readinto(buf, nack=True)

   Reads bytes from the bus and stores them into *buf*.  The number of bytes
   read is the length of *buf*.  An ACK will be sent on the bus after
   receiving all but the last byte.  After the last byte is received, if *nack*
   is true then a NACK will be sent, otherwise an ACK will be sent (and in this
   case the slave assumes more bytes are going to be read in a later call).

   Availability: ESP8266.

.. method:: I2C.write(buf)

   Write the bytes from *buf* to the bus.  Checks that an ACK is received
   after each byte and stops transmitting the remaining bytes if a NACK is
   received.  The function returns the number of ACKs that were received.

   Availability: ESP8266.

Standard bus operations
-----------------------

The following methods implement the standard I2C master read and write
operations that target a given slave device.

.. method:: I2C.readfrom(addr, nbytes, stop=True)

   Read *nbytes* from the slave specified by *addr*.
   If *stop* is true then a STOP condition is generated at the end of the transfer.
   Returns a `bytes` object with the data read.

.. method:: I2C.readfrom_into(addr, buf, stop=True)

   Read into *buf* from the slave specified by *addr*.
   The number of bytes read will be the length of *buf*.
   If *stop* is true then a STOP condition is generated at the end of the transfer.

   The method returns ``None``.

.. method:: I2C.writeto(addr, buf, stop=True)

   Write the bytes from *buf* to the slave specified by *addr*.  If a
   NACK is received following the write of a byte from *buf* then the
   remaining bytes are not sent.  If *stop* is true then a STOP condition is
   generated at the end of the transfer, even if a NACK is received.
   The function returns the number of ACKs that were received.

Memory operations
-----------------

Some I2C devices act as a memory device (or set of registers) that can be read
from and written to.  In this case there are two addresses associated with an
I2C transaction: the slave address and the memory address.  The following
methods are convenience functions to communicate with such devices.

.. method:: I2C.readfrom_mem(addr, memaddr, nbytes, \*, addrsize=8)

   Read *nbytes* from the slave specified by *addr* starting from the memory
   address specified by *memaddr*.
   The argument *addrsize* specifies the address size in bits.
   Returns a `bytes` object with the data read.

.. method:: I2C.readfrom_mem_into(addr, memaddr, buf, \*, addrsize=8)

   Read into *buf* from the slave specified by *addr* starting from the
   memory address specified by *memaddr*.  The number of bytes read is the
   length of *buf*.
   The argument *addrsize* specifies the address size in bits (on ESP8266
   this argument is not recognised and the address size is always 8 bits).

   The method returns ``None``.

.. method:: I2C.writeto_mem(addr, memaddr, buf, \*, addrsize=8)

   Write *buf* to the slave specified by *addr* starting from the
   memory address specified by *memaddr*.
   The argument *addrsize* specifies the address size in bits (on ESP8266
   this argument is not recognised and the address size is always 8 bits).

   The method returns ``None``.
