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Python I2C library supporting multiple driver implementations

Introduction

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 initialized when created, or initialized later on.

This library is designed to support different I2C driver implementations. At present below drivers are supported:

• CH341
• CH347

The interface is similar to that of MicroPython’s machine.I2C

Example usage:

from i2cpy import I2C

i2c = I2C()                      # create I2C peripheral

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

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

If you prefer an “int” interface to the “bytes” interface, you can easily write wrapper functions youself. For example,

# assume you already have a gloal i2c object

def i2c_write(addr: int, memaddr: int, *args):
    i2c.writeto_mem(addr, memaddr, bytes(args))

def i2c_read(addr: int, memaddr: int, nbytes: int) -> list[int]:
    got = i2c.readfrom_mem(addr, memaddr, nbytes)
    return list(got)

Installation

The i2cpy Python module itself can be simply pip installed,

pip intall i2cpy

And for the underlying I2C implementations you still need to install their corresponding drivers.

ch341

The CH341 series chip (like CH341A) is USB bus converter which converts USB to UART, parallel port, and common synchronous serial communication interfaces (I2C, SPI). The chip is manufactured by the company Qinheng Microelectronics.

The “ch341” driver shipped with this library is a Python interface to CH341’s official DLLs.

You need the driver DLL files, which are downloadable from Qinheng’s website.

• Windows: https://www.wch-ic.com/downloads/CH341PAR_EXE.html

  They also have a zipball https://www.wch-ic.com/downloads/CH341PAR_ZIP.html . If you use the zipball on Windows it’s recommended to place the DLL files, CH341DLLA64.DLL and/or CH341DLL.DLL depending on the bitness, under Windows System32/SysWOW64 folder. Or if you place them under a different directory, you can add that directory to PATH environment variable.

• Linux: https://www.wch-ic.com/downloads/CH341PAR_LINUX_ZIP.html

  On Linux you need to build the kernel module from source under the downloaded zipball’s driver sub-directory like below.

  $ cd driver
  $ sudo rmmod ch34x_pis
  $ make && sudo make install

  Also you need to either place the libch347.so file for your platform to system supported path like /usr/local/lib, or you make the so file loadable by adding its directory to LD_LIBRARY_PATH environment variable.

  And because Qinheng sometimes updates their Linux driver and breaks backward compatibility, be careful when you upgrade their Linux CH341PAR driver version, and make sure your effective libch347.so file and your compiled kernel module come from the same driver version.

  Still on Linux there could be more details like, system package linux-headers-$(uname -r) being a prerequisite of making the kernel module, and permissioning of /dev/ch34x_pis*, etc.. Those are beyond scope of this doc though.

• macOS is not supported. See also this ticket.

Example usage:

from i2cpy import I2C

i2c = I2C()                                     # ch341 is the default driver
i2c = I2C(driver="ch341")                       # explicitly specify driver

i2c = I2C(0)                                    # override usb id

i2c = I2C("/dev/ch34x_pis0")                    # override device path on Linux

ch347

The CH347 series chip (like CH347T) is USB bus converter manufactured also by Qinheng Microelectronics.

The “ch347” driver shipped with this library is a Python interface to CH347’s official DLLs.

You need the driver DLL files, which are downloadable from Qinheng’s website. As they are actually the same downloadable files as that of CH341, see ch341 section above for how to install.

Example usage:

from i2cpy import I2C

i2c = I2C(driver="ch347")                       # specify ch347 driver

i2c = I2C(0, driver="ch347")                    # override usb id

i2c = I2C("/dev/ch34x_pis0", driver="ch347")    # override device path on Linux

Class I2C

Constructor

I2C.__init__(id=None, *, driver=None, freq=400000, auto_init=True, **kwargs)

Constructor.

• Parameters:
  • id (Union[str, int, None]) – Identifies a particular I2C peripheral. Allowed values depend on the particular driver implementation.
  • freq (int) – I2C bus baudrate, defaults to 400000
  • driver (Optional[str]) – I2C driver name. It corresponds to the I2C driver sub module name shipped with this library. For example “foo” means module “i2cpy.driver.foo”. If not specified, it looks at environment variable “I2CPY_DRIVER”. And if that’s not defined or is empty, it finally falls back to “ch341”.
  • auto_init (bool) – Call init() on object initialization, defaults to True

General methods

I2C.init()

Initialize the I2C bus.

I2C.deinit()

Close the I2C bus.

I2C.scan(start=8, stop=119)

Scan all I2C addresses between start and stop 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.

Depending on the specific driver and OS platform, scan() may or may not work.

• Parameters:
  • start (int) – start address, defaults to 0x08
  • stop (int) – stop address, defaults to 0x77
• Return type: List[int]
• Returns: a list of addresses that respond to scan

Standard bus operations

I2C.writeto(addr, buf, /)

Write the bytes from buf to the peripheral specified by addr.

• Parameters:
  • addr (int) – I2C peripheral deivce address
  • buf (Buffer) – bytes to write

I2C.readfrom(addr, nbytes, /)

Read nbytes from the peripheral specified by addr.

• Parameters:
  • addr (int) – I2C peripheral device address
  • nbytes (int) – number of bytes to read
• Return type: bytes
• Returns: the bytes read

Memory operations

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

Write buf to the peripheral specified by addr starting from the memory address specified by memaddr.

• Parameters:
  • addr (int) – I2C peripheral device address
  • memaddr (int) – memory address
  • buf (Buffer) – bytes to write
  • addrsize (int) – address size in bits, defaults to 8

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

Read nbytes from the peripheral specified by addr starting from the memory address specified by memaddr.

• Parameters:
  • addr (int) – I2C peripheral device address
  • memaddr (int) – memory address
  • nbytes (int) – number of bytes to read
  • addrsize (int) – address size in bits, defaults to 8
• Return type: bytes
• Returns: the bytes read

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

Read into buf from the peripheral specified by addr starting from the memory address specified by memaddr. The number of bytes read is the length of buf.

• Parameters:
  • addr (int) – I2C peripheral device address
  • memaddr (int) – memory address
  • buf (bytearray) – buffer to store the bytes read
  • addrsize (int) – address size in bits, defaults to 8

Nuitka

If you use Nuitka to package your Python code that uses i2cpy, you need to specify the implicit-imported module, as otherwise Nuitka won’t be able to find by itself. For example you can use a nuitka-package.config.yml file with below content.

- module-name: 'i2cpy'
  implicit-imports:
    - depends:
      - 'i2cpy.driver.ch341'

And run the nuitka command like,

nuitka --standalone --onefile --user-package-configuration-file=nuitka-package.config.yml your_script.py

For other packaging tools like Pyinstaller, etc., the idea is similar.