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.

Example:

from pyb import I2C

i2c = I2C(2)                         # create on bus 2
i2c = I2C(2, I2C.MASTER)             # create and init as a master
i2c.init(I2C.MASTER, baudrate=20000) # init as a master
i2c.init(I2C.SLAVE, addr=0x42)       # init as a slave with given address
i2c.deinit()                         # turn off the peripheral

Printing the i2c object gives you information about its configuration.

The basic methods are send and recv:

i2c.send('abc')      # send 3 bytes
i2c.send(0x42)       # send a single byte, given by the number
data = i2c.recv(3)   # receive 3 bytes

To receive inplace, first create a bytearray:

data = bytearray(3)  # create a buffer
i2c.recv(data)       # receive 3 bytes, writing them into data

You can specify a timeout (in ms):

i2c.send(b'123', timeout=2000)   # timeout after 2 seconds

A master must specify the recipient’s address:

i2c.init(I2C.MASTER)
i2c.send('123', 0x42)        # send 3 bytes to slave with address 0x42
i2c.send(b'456', addr=0x42)  # keyword for address

Master also has other methods:

i2c.is_ready(0x42)           # check if slave 0x42 is ready
i2c.scan()                   # scan for slaves on the bus, returning
                             #   a list of valid addresses
i2c.mem_read(3, 0x42, 2)     # read 3 bytes from memory of slave 0x42,
                             #   starting at address 2 in the slave
i2c.mem_write('abc', 0x42, 2, timeout=1000) # write 'abc' (3 bytes) to memory of slave 0x42
                                            # starting at address 2 in the slave, timeout after 1 second

Constructors

class pyb.I2C(bus, ...)

Construct an I2C object on the given bus. bus can be 2 or 4. With no additional parameters, the I2C object is created but not initialised (it has the settings from the last initialisation of the bus, if any). If extra arguments are given, the bus is initialised. See init for parameters of initialisation.

The physical pins of the I2C busses on the OpenMV Cam M4 are:

  • I2C(2) is on the Y position: (SCL, SDA) = (P4, P5) = (PB10, PB11)

The physical pins of the I2C busses on the OpenMV Cam M7 are:

  • I2C(2) is on the Y position: (SCL, SDA) = (P4, P5) = (PB10, PB11)
  • I2C(4) is on the Y position: (SCL, SDA) = (P7, P8) = (PD12, PD13)

Methods

I2C.deinit()

Turn off the I2C bus.

I2C.init(mode, *, addr=0x12, baudrate=400000, gencall=False, dma=False)

Initialise the I2C bus with the given parameters:

  • mode must be either I2C.MASTER or I2C.SLAVE
  • addr is the 7-bit address (only sensible for a slave)
  • baudrate is the SCL clock rate (only sensible for a master)
  • gencall is whether to support general call mode
  • dma is whether to allow the use of DMA for the I2C transfers (note that DMA transfers have more precise timing but currently do not handle bus errors properly)
I2C.is_ready(addr)

Check if an I2C device responds to the given address. Only valid when in master mode.

I2C.mem_read(data, addr, memaddr, *, timeout=5000, addr_size=8)

Read from the memory of an I2C device:

  • data can be an integer (number of bytes to read) or a buffer to read into
  • addr is the I2C device address
  • memaddr is the memory location within the I2C device
  • timeout is the timeout in milliseconds to wait for the read
  • addr_size selects width of memaddr: 8 or 16 bits

Returns the read data. This is only valid in master mode.

I2C.mem_write(data, addr, memaddr, *, timeout=5000, addr_size=8)

Write to the memory of an I2C device:

  • data can be an integer or a buffer to write from
  • addr is the I2C device address
  • memaddr is the memory location within the I2C device
  • timeout is the timeout in milliseconds to wait for the write
  • addr_size selects width of memaddr: 8 or 16 bits

Returns None. This is only valid in master mode.

I2C.recv(recv, addr=0x00, *, timeout=5000)

Receive data on the bus:

  • recv can be an integer, which is the number of bytes to receive, or a mutable buffer, which will be filled with received bytes
  • addr is the address to receive from (only required in master mode)
  • timeout is the timeout in milliseconds to wait for the receive

Return value: if recv is an integer then a new buffer of the bytes received, otherwise the same buffer that was passed in to recv.

I2C.send(send, addr=0x00, *, timeout=5000)

Send data on the bus:

  • send is the data to send (an integer to send, or a buffer object)
  • addr is the address to send to (only required in master mode)
  • timeout is the timeout in milliseconds to wait for the send

Return value: None.

I2C.scan()

Scan all I2C addresses from 0x01 to 0x7f and return a list of those that respond. Only valid when in master mode.

Constants

I2C.MASTER

for initialising the bus to master mode

I2C.SLAVE

for initialising the bus to slave mode