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 machine.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

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
I2C.deinit()

Turn off the I2C bus.

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.

These methods are available on software I2C only.

I2C.start()

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

I2C.stop()

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

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).

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.

Standard bus operations

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

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.

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.

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.

I2C.writevto(addr, vector, stop=True)

Write the bytes contained in vector to the slave specified by addr. vector should be a tuple or list of objects with the buffer protocol. The addr is sent once and then the bytes from each object in vector are written out sequentially. The objects in vector may be zero bytes in length in which case they don’t contribute to the output.

If a NACK is received following the write of a byte from one of the objects in vector then the remaining bytes, and any remaining objects, 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.

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.

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.

The method returns None.

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.

The method returns None.