Communicating With I2C Devices

Pymata-express has the capability to support one or more i2c devices connected to the i2c bus. The API supports several i2c read methods, and a single i2c write method, described below.

NOTE 1:

If you do not specify a callback for any give read method, to retrieve the latest values, you will need to use the i2c_read_saved_data method. The amount of time it takes an i2c device to reply to an i2c read varies from device to device. Therefore it is simpler to use callbacks.

NOTE 2:

Refer to this example for the i2c read and write methods.

Read Commands

i2c_read

 async def i2c_read(self, address, register, number_of_bytes, callback=None)

    Read the specified number of bytes from the specified register for the i2c device.

    :param address: i2c device address

    :param register: i2c register (or None if no register selection is needed)

    :param number_of_bytes: number of bytes to be read

    :param callback: Optional callback function to report i2c data as a result of read command

    callback returns a data list:

    [pin_type, i2c_device_address, i2c_read_register, data_bytes returned, time_stamp]

    The pin_type for i2c = 6

Example:

See NOTE 2 above.

i2c_read_continuous

  async def i2c_read_continuous(self, address, register, number_of_bytes, callback=None)

    Some i2c devices support a continuous streaming data output. This command enables that mode for the device that supports continuous reads.

    :param address: i2c device address

    :param register: i2c register (or None if no register selection is needed)

    :param number_of_bytes: number of bytes to be read

    :param callback: Optional callback function to report i2c data as a result of read command

    callback returns a data list:

    [pin_type, i2c_device_address, i2c_read_register, data_bytes returned, time_stamp]

    The pin_type for i2c = 6

Example:

See NOTE 2 above.

i2c_read_restart_transmission

  async def i2c_read_restart_transmission(self, address, register, number_of_bytes, callback=None)

    Read the specified number of bytes from the specified register for the i2c device. This restarts the transmission after the read. It is required for some i2c devices such as the MMA8452Q accelerometer.

    :param address: i2c device address

    :param register: i2c register (or None if no register selection is needed)

    :param number_of_bytes: number of bytes to be read

    :param callback: Optional callback function to report i2c data as a result of read command

    callback returns a data list:

    [pin_type, i2c_device_address, i2c_read_register, data_bytes returned, time_stamp]

    The pin_type for i2c = 6

Example:

See NOTE 2 above.

i2c_read_saved_data

 async def i2c_read_saved_data(self, address)

    This method retrieves cached i2c data to support a polling mode.

    :param address: I2C device address

    :returns data: [raw data returned from i2c device, time-stamp]

Example:

See NOTE 2 above.

Write Commmand

i2c_write

 async def i2c_write(self, address, args)

    Write data to an i2c device.

    :param address: i2c device address

    :param args: A variable number of bytes to 
                 be sent to the device passed in as a list

Example:

See NOTE 2 above.