.. currentmodule:: machine
.. _machine.USBDevice:

class USBDevice -- USB Device driver
====================================

.. note:: ``machine.USBDevice`` is currently only supported on the rp2 and samd
          ports.

USBDevice provides a low-level Python API for implementing USB device functions using
Python code.

.. warning:: This low-level API assumes familiarity with the USB standard. There
   are high-level `usb driver modules in micropython-lib`_ which provide a
   simpler interface and more built-in functionality.

Terminology
-----------

- A "Runtime" USB device interface or driver is one which is defined using this
  Python API after MicroPython initially starts up.

- A "Built-in" USB device interface or driver is one that is compiled into the
  MicroPython firmware, and is always available. Examples are USB-CDC (serial
  port) which is usually enabled by default. Built-in USB-MSC (Mass Storage) is an
  option on some ports.

Lifecycle
---------

Managing a runtime USB interface can be tricky, especially if you are communicating
with MicroPython over a built-in USB-CDC serial port that's part of the same USB
device.

- A MicroPython soft reset will always clear all runtime USB interfaces, which
  results in the entire USB device disconnecting from the host. If MicroPython
  is also providing a built-in USB-CDC serial port then this will re-appear
  after the soft reset.

  This means some functions (like ``mpremote run``) that target the USB-CDC
  serial port will immediately fail if a runtime USB interface is active,
  because the port goes away when ``mpremote`` triggers a soft reset. The
  operation should succeed on the second try, as after the soft reset there is
  no more runtime USB interface.

- To configure a runtime USB device on every boot, it's recommended to place the
  configuration code in the ``boot.py`` file on the :ref:`device VFS
  <filesystem>`. On each reset this file is executed before the USB subsystem is
  initialised (and before ``main.py``), so it allows the board to come up with the runtime
  USB device immediately.

- For development or debugging, it may be convenient to connect a hardware
  serial REPL and disable the built-in USB-CDC serial port entirely. Not all ports
  support this (currently only ``rp2``). The custom build should be configured
  with ``#define MICROPY_HW_USB_CDC (0)`` and ``#define
  MICROPY_HW_ENABLE_UART_REPL (1)``.

Constructors
------------

.. class:: USBDevice()

   Construct a USBDevice object.

   .. note:: This object is a singleton, each call to this constructor
             returns the same object reference.

Methods
-------

.. method:: USBDevice.config(desc_dev, desc_cfg, desc_strs=None, open_itf_cb=None, reset_cb=None, control_xfer_cb=None, xfer_cb=None)

    Configures the ``USBDevice`` singleton object with the USB runtime device
    state and callback functions:

    - ``desc_dev`` - A bytes-like object containing
      the new USB device descriptor.

    - ``desc_cfg`` - A bytes-like object containing the
      new USB configuration descriptor.

    - ``desc_strs`` - Optional object holding strings or bytes objects
       containing USB string descriptor values. Can be a list, a dict, or any
       object which supports subscript indexing with integer keys (USB string
       descriptor index).

       Strings are an optional USB feature, and this parameter can be unset
       (default) if no strings are referenced in the device and configuration
       descriptors, or if only built-in strings should be used.

       Apart from index 0, all the string values should be plain ASCII. Index 0
       is the special "languages" USB descriptor, represented as a bytes object
       with a custom format defined in the USB standard. ``None`` can be
       returned at index 0 in order to use a default "English" language
       descriptor.

       To fall back to providing a built-in string value for a given index, a
       subscript lookup can return ``None``, raise ``KeyError``, or raise
       ``IndexError``.

    - ``open_itf_cb`` - This callback is called once for each interface
      or Interface Association Descriptor in response to a Set
      Configuration request from the USB Host (the final stage before
      the USB device is available to the host).

      The callback takes a single argument, which is a memoryview of the
      interface or IAD descriptor that the host is accepting (including
      all associated descriptors). It is a view into the same
      ``desc_cfg`` object that was provided as a separate
      argument to this function. The memoryview is only valid until the
      callback function returns.

    - ``reset_cb`` - This callback is called when the USB host performs
      a bus reset. The callback takes no arguments. Any in-progress
      transfers will never complete. The USB host will most likely
      proceed to re-enumerate the USB device by calling the descriptor
      callbacks and then ``open_itf_cb()``.

    - ``control_xfer_cb`` - This callback is called one or more times
      for each USB control transfer (device Endpoint 0). It takes two
      arguments.

      The first argument is the control transfer stage. It is one of:

      - ``1`` for SETUP stage.
      - ``2`` for DATA stage.
      - ``3`` for ACK stage.

      Second argument is a memoryview to read the USB control request
      data for this stage. The memoryview is only valid until the
      callback function returns. Data in this memoryview will be the same
      across each of the three stages of a single transfer.

      A successful transfer consists of this callback being called in sequence
      for the three stages. Generally speaking, if a device wants to do
      something in response to a control request then it's best to wait until
      the ACK stage to confirm the host controller completed the transfer as
      expected.

      The callback should return one of the following values:

      - ``False`` to stall the endpoint and reject the transfer. It won't
        proceed to any remaining stages.
      - ``True`` to continue the transfer to the next stage.
      - A buffer object can be returned at the SETUP stage when the transfer
        will send or receive additional data. Typically this is the case when
        the ``wLength`` field in the request has a non-zero value. This should
        be a writable buffer for an ``OUT`` direction transfer, or a readable
        buffer with data for an ``IN`` direction transfer.

    - ``xfer_cb`` - This callback is called whenever a non-control
      transfer submitted by calling :func:`USBDevice.submit_xfer` completes.

      The callback has three arguments:

      1. The Endpoint number for the completed transfer.
      2. Result value: ``True`` if the transfer succeeded, ``False``
         otherwise.
      3. Number of bytes successfully transferred. In the case of a
         "short" transfer, The result is ``True`` and ``xferred_bytes``
         will be smaller than the length of the buffer submitted for the
         transfer.

      .. note:: If a bus reset occurs (see :func:`USBDevice.reset`),
                ``xfer_cb`` is not called for any transfers that have not
                already completed.

.. method:: USBDevice.active(self, [value] /)

    Returns the current active state of this runtime USB device as a
    boolean. The runtime USB device is "active" when it is available to
    interact with the host, it doesn't mean that a USB Host is actually
    present.

    If the optional ``value`` argument is set to a truthy value, then
    the USB device will be activated.

    If the optional ``value`` argument is set to a falsey value, then
    the USB device is deactivated. While the USB device is deactivated,
    it will not be detected by the USB Host.

    To simulate a disconnect and a reconnect of the USB device, call
    ``active(False)`` followed by ``active(True)``. This may be
    necessary if the runtime device configuration has changed, so that
    the host sees the new device.

.. attribute:: USBDevice.builtin_driver

   This attribute holds the current built-in driver configuration, and must be
   set to one of the ``USBDevice.BUILTIN_`` named constants defined on this object.

   By default it holds the value :data:`USBDevice.BUILTIN_NONE`.

   Runtime USB device must be inactive when setting this field. Call the
   :func:`USBDevice.active` function to deactivate before setting if necessary
   (and again to activate after setting).

   If this value is set to any value other than :data:`USBDevice.BUILTIN_NONE` then
   the following restrictions apply to the :func:`USBDevice.config` arguments:

   - ``desc_cfg`` should begin with the built-in USB interface descriptor data
     accessible via :data:`USBDevice.builtin_driver` attribute ``desc_cfg``.
     Descriptors appended after the built-in configuration descriptors should use
     interface, string and endpoint numbers starting from the max built-in values
     defined in :data:`USBDevice.builtin_driver` attributes ``itf_max``, ``str_max`` and
     ``ep_max``.

   - The ``bNumInterfaces`` field in the built-in configuration
     descriptor will also need to be updated if any new interfaces
     are appended to the end of ``desc_cfg``.

   - ``desc_strs`` should either be ``None`` or a list/dictionary where index
     values less than ``USBDevice.builtin_driver.str_max`` are missing or have
     value ``None``. This reserves those string indexes for the built-in
     drivers. Placing a different string at any of these indexes overrides that
     string in the built-in driver.

.. method:: USBDevice.submit_xfer(self, ep, buffer /)

            Submit a USB transfer on endpoint number ``ep``. ``buffer`` must be
            an object implementing the buffer interface, with read access for
            ``IN`` endpoints and write access for ``OUT`` endpoints.

            .. note:: ``ep`` cannot be the control Endpoint number 0. Control
               transfers are built up through successive executions of
               ``control_xfer_cb``, see above.

            Returns ``True`` if successful, ``False`` if the transfer could not
            be queued (as USB device is not configured by host, or because
            another transfer is queued on this endpoint.)

            When the USB host completes the transfer, the ``xfer_cb`` callback
            is called (see above).

            Raises ``OSError`` with reason ``MP_EINVAL`` If the USB device is not
            active.

.. method:: USBDevice.stall(self, ep, [stall] /)

            Calling this function gets or sets the STALL state of a device endpoint.

            ``ep`` is the number of the endpoint.

            If the optional ``stall`` parameter is set, this is a boolean flag
            for the STALL state.

            The return value is the current stall state of the endpoint (before
            any change made by this function).

            An endpoint that is set to STALL may remain stalled until this
            function is called again, or STALL may be cleared automatically by
            the USB host.

            Raises ``OSError`` with reason ``MP_EINVAL`` If the USB device is not
            active.

Constants
---------

.. data:: USBDevice.BUILTIN_NONE
.. data:: USBDevice.BUILTIN_DEFAULT
.. data:: USBDevice.BUILTIN_CDC
.. data:: USBDevice.BUILTIN_MSC
.. data:: USBDevice.BUILTIN_CDC_MSC

          These constant objects hold the built-in descriptor data which is
          compiled into the MicroPython firmware. ``USBDevice.BUILTIN_NONE`` and
          ``USBDevice.BUILTIN_DEFAULT`` are always present. Additional objects may be present
          depending on the firmware build configuration and the actual built-in drivers.

          .. note:: Currently at most one of ``USBDevice.BUILTIN_CDC``,
                    ``USBDevice.BUILTIN_MSC`` and ``USBDevice.BUILTIN_CDC_MSC`` is defined
                    and will be the same object as ``USBDevice.BUILTIN_DEFAULT``.
                    These constants are defined to allow run-time detection of
                    the built-in driver (if any). Support for selecting one of
                    multiple built-in driver configurations may be added in the
                    future.

          These values are assigned to :data:`USBDevice.builtin_driver` to get/set the
          built-in configuration.

          Each object contains the following read-only fields:

          - ``itf_max`` - One more than the highest bInterfaceNumber value used
            in the built-in configuration descriptor.
          - ``ep_max`` - One more than the highest bEndpointAddress value used
            in the built-in configuration descriptor. Does not include any
            ``IN`` flag bit (0x80).
          - ``str_max`` - One more than the highest string descriptor index
            value used by any built-in descriptor.
          - ``desc_dev`` - ``bytes`` object containing the built-in USB device
            descriptor.
          - ``desc_cfg`` - ``bytes`` object containing the complete built-in USB
            configuration descriptor.

.. _usb driver modules in micropython-lib: https://github.com/micropython/micropython-lib/tree/master/micropython/usb#readme
