CircuitsSim

Interact with simulated I2C devices

Using

CircuitsSim requires Elixir Circuits 2.0 libraries.

You may need to add override: true since not many libraries have been updated to allow Circuits 2.0 versions. Circuits 2.0 is mostly backwards compatible, so this should be safe.

CircuitsSim works by providing an alternative backend for interacting with hardware. This is setup via config/config.exs. In your project, you may only want the simulated backends for MIX_ENV=test configurations, so you'll want to add the following appropriately. For trying it out, it's fine to start with adding this to config.exs:

config :circuits_i2c, default_backend: CircuitsSim.I2C.Backend
config :circuits_spi, default_backend: CircuitsSim.SPI.Backend
config :circuits_gpio, default_backend: CircuitsSim.GPIO.Backend

Note that you don't need to replace all backends. If you're only using I2C, there's no need to bring in SPI and GPIO support.

Demo

CircuitsSim takes a configuration for how to set up the simulated I2C buses and devices. Here's an example configuration:

config :circuits_sim,
  config: [
    {CircuitsSim.Device.MCP23008, bus_name: "i2c-0", address: 0x20},
    {CircuitsSim.Device.AT24C02, bus_name: "i2c-0", address: 0x50},
    {CircuitsSim.Device.ADS7138, bus_name: "i2c-1", address: 0x10},
    {CircuitsSim.Device.MCP23008, bus_name: "i2c-1", address: 0x20},
    {CircuitsSim.Device.MCP23008, bus_name: "i2c-1", address: 0x21},
    {CircuitsSim.Device.TM1620, bus_name: "spidev0.0", render: :binary_clock},
    {CircuitsSim.Device.GPIOLED, gpio_spec: 10},
    {CircuitsSim.Device.GPIOButton, gpio_spec: 11}
  ]

This shows two simulated I2C buses, "i2c-0" and "i2c-1", one SPI bus, and two GPIO. The "i2c-0" bus has two devices, an MCP23008 GPIO expander and an AT24C02 EEPROM.

Here's how it looks when you run IEx:

$ iex -S mix

Interactive Elixir (1.14.3) - press Ctrl+C to exit (type h() ENTER for help)
iex> Circuits.I2C.detect_devices
Devices on I2C bus "i2c-1":
 * 16  (0x10)
 * 32  (0x20)
 * 33  (0x21)

Devices on I2C bus "i2c-0":
 * 32  (0x20)
 * 80  (0x50)

5 devices detected on 2 I2C buses

You can then read and write to the I2C devices similar to how you'd interact with them for real. While they're obviously not real and have limitations, they can be super helpful in mocking I2C devices or debugging I2C interactions without hardware in the loop.

Adding an I2C device

Many I2C devices follow a simple pattern of exposing all operations as register reads and writes. If this is the case for your device, create a new module and implement the CircuitsSim.I2C.SimpleI2CDevice protocol. See CircuitsSim.Device.MCP23008 for an example.

If your device has a fancier interface, you'll need to implement the CircuitsSim.I2C.I2CDevice protocol which just passes the reads and writes through and doesn't handle conveniences like auto-incrementing register addresses on multi-byte reads and writes. See CircuitsSim.Device.ADS7138 for an example.

Adding a SPI device

Simulated SPI devices implement the CircuitsSim.SPI.SPIDevice protocols. See CircuitsSim.Device.TM1620 for an example.