Adafruit CircuitPython Register library

This library provides a variety of data descriptor class for Adafruit CircuitPython that makes it really simple to write a device drivers for a I2C and SPI register based devices. Data descriptors act like basic attributes from the outside which makes using them really easy to use.

Creating a driver

Creating a driver with the register library is really easy. First, import the register modules you need from the available modules:

from adafruit_register import i2c_bit
from adafruit_bus_device import i2c_device

Next, define where the bit is located in the device’s memory map:

class HelloWorldDevice:
    """Device with two bits to control when the words 'hello' and 'world' are lit."""

    hello = i2c_bit.RWBit(0x0, 0x0)
    """Bit to indicate if hello is lit."""

    world = i2c_bit.RWBit(0x1, 0x0)
    """Bit to indicate if world is lit."""

Lastly, we need to add an i2c_device member of type I2CDevice that manages sharing the I2C bus for us. Make sure the name is exact, otherwise the registers will not be able to find it. Also, make sure that the i2c device implements the busio.I2C interface.

def __init__(self, i2c, device_address=0x0):
    self.i2c_device = i2c_device.I2CDevice(i2c, device_address)

Thats it! Now we have a class we can use to talk to those registers:

import busio
from board import *

with busio.I2C(SCL, SDA) as i2c:
    device = HelloWorldDevice(i2c)
    device.hello = True
    device.world = True

Adding register types

Adding a new register type is a little more complicated because you need to be careful and minimize the amount of memory the class will take. If you don’t, then a driver with five registers of your type could take up five times more extra memory.

First, determine whether the new register class should go in an existing module or not. When in doubt choose a new module. The more finer grained the modules are, the fewer extra classes a driver needs to load in.

Here is the start of the RWBit class:

class RWBit:
    """
    Single bit register that is readable and writeable.

    Values are `bool`

    :param int register_address: The register address to read the bit from
    :param type bit: The bit index within the byte at ``register_address``
    """
    def __init__(self, register_address, bit):
        self.bit_mask = 1 << bit
        self.buffer = bytearray(2)
        self.buffer[0] = register_address

The first thing done is writing an RST formatted class comment that explains the functionality of the register class and any requirements of the register layout. It also documents the parameters passed into the constructor (__init__) which configure the register location in the device map. It does not include the device address or the i2c object because its shared on the device class instance instead. That way if you have multiple of the same device on the same bus, the register classes will be shared.

In __init__ we only use two member variable because each costs 8 bytes of memory plus the memory for the value. And remember this gets multiplied by the number of registers of this type in a driver! Thats why we pack both the register address and data byte into one bytearray. We could use two byte arrays of size one but each MicroPython object is 16 bytes minimum due to the garbage collector. So, by sharing a byte array we keep it to the 16 byte minimum instead of 32 bytes. Each memoryview also costs 16 bytes minimum so we avoid them too.

Another thing we could do is allocate the bytearray only when we need it. This has the advantage of taking less memory up front but the cost of allocating it every access and risking it failing. If you want to add a version of Foo that lazily allocates the underlying buffer call it FooLazy.

Ok, onward. To make a data descriptor we must implement __get__ and __set__.

def __get__(self, obj, objtype=None):
    with obj.i2c_device:
        obj.i2c_device.write(self.buffer, end=1, stop=False)
        obj.i2c_device.read_into(self.buffer, start=1)
    return bool(self.buffer[1] & self.bit_mask)

def __set__(self, obj, value):
    with obj.i2c_device:
        obj.i2c_device.write(self.buffer, end=1, stop=False)
        obj.i2c_device.read_into(self.buffer, start=1)
        if value:
            self.buffer[1] |= self.bit_mask
        else:
            self.buffer[1] &= ~self.bit_mask
        obj.i2c_device.write(self.buffer)

As you can see, we have two places to get state from. First, self stores the register class members which locate the register within the device memory map. Second, obj is the driver class that uses the register class which must by definition provide a I2CDevice compatible object as i2c_device. This object does two thing for us:

  1. Waits for the bus to free, locks it as we use it and frees it after.
  2. Saves the device address and other settings so we don’t have to.

Note that we take heavy advantage of the start and end parameters to the i2c functions to slice the buffer without actually allocating anything extra. They function just like self.buffer[start:end] without the extra allocation.

Thats it! Now you can use your new register class like the example above. Just remember to keep the number of members to a minimum because the class may be used a bunch of times.

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