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class USBDevice -- USB Device driver

docs/library/machine.USBDevice.rst

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

class USBDevice -- USB Device driver

.. note:: machine.USBDevice is currently only supported for esp32, rp2 and samd ports. Native USB support is also required, and not every board supports native USB.

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 :ref:soft reset <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 :ref:boot.py file on the :ref:device VFS <filesystem>. On each reset this file is executed before the USB subsystem is initialised (and before :ref: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.remote_wakeup(self)

Wake up host if we are in suspend mode and the REMOTE_WAKEUP feature
is enabled by the host. This has to be enabled in the USB attributes,
and on the host. Returns ``True`` if remote wakeup was enabled and
active and the host was woken up.

.. 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