tty0tty creates 2 pseudo-ttys that are connected together allowing you to run unit tests which use serial ports (like Circuits.UART) without the need for external hardware of adapters.

defmodule SerialTest do
  use ExUnit.Case, async: true

  test "can open a serial port" do
    {:ok, uart} = Circuits.UART.start_link()

    port_name = "/tmp/dummy1"

    # Open serial port before use
    {:ok, _port_sup} = TTY0TTY.open(port_name)

    assert :ok = Cicuits.UART.open(uart, port_name)
  end
end

Under the hood, TTY0TTY.open/2 opens 2 devices (<port_name>, and <port_name>-twin) and connects their TX <-> RX to emulate the serial connection. This allows you to also verifying reading serial data by sending data to the connected twin port:

defmodule SerialTest do
  use ExUnit.Case, async: true

  test "can read a serial port" do
    {:ok, uart} = Circuits.UART.start_link()

    port_name = "/tmp/dummy1"

    # Open serial port before use
    {:ok, _port_sup} = TTY0TTY.open(port_name)

    assert :ok = Cicuits.UART.open(uart, port_name)

    File.write!([port_name, "-twin"], "howdy!")

    assert_receive {:circuits_uart, ^port_name, "howdy!"}
  end
end