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class USBDevice – USB Device driver


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.


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.


  • 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.


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 file on the device VFS. On each reset this file is executed before the USB subsystem is initialised (and before, 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).


class machine.USBDevice

Construct a USBDevice object.


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


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 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.


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


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 USBDevice.BUILTIN_NONE.

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

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

  • desc_cfg should begin with the built-in USB interface descriptor data accessible via 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 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.


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.

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.


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.

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.



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.


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 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.