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USB Composite library for Roger's Melbourne's STM32F1 core:

Protocols supported

  • standard USB HID, with many built-in profiles, and customizable with more

  • MIDI over USB

  • XBox360 wired/wireless controllers

  • Mass storage

  • USB Audio (unidirectional, but both directions are supported)

Basic concepts

Start with:

#include <USBComposite.h>

The library defines one central singleton object:

extern USBCompositeDevice USBComposite;

This controls USB device identification as well as registers the plugins that are connected to it.

Plugin classes included in the library:

USBMultiXBox360<n> / USBXBox360 / USBXBox360W<n>

NOTE: Only one of USBMultiXBox360 / USBXBox360 / USBXBox360W can be registered at a time: they cannot be composited together.

To use the plugins, you need to create instances of them. NOTE: Only one instance of each plugin class can be created.

If you want to make a simple (non-composite) USB device, you can create an instance of the plugin class and call the begin() method, and it will take care of registering itself with USBComposite and starting up USBComposite. If you want to make a composite USB device, however, you need to control the device with USBComposite:

USBComposite.clear(); // clear any plugins previously registered

Of course, you may need to do some further configuring of the plugins (e.g., if plugin1 is USBHID, then you may want to do USBHID.setReportDescriptor(HID_KEYBOARD)) or of the USBComposite device before the USBComposite.begin() call.

After starting up USBComposite, it's a good idea to wait for it to become ready before sending any data:


Finally, there are a number of classes that implement particular protocols for the USBHID class plugin. These include:


And you can customize with more. Moreover, the USBHID plugin itself allows for compositing multiple HID profiles, e.g., Mouse / Keyboard / three joysticks. Each of these has at least one required parameter, which is an instance of USBHID.

Not all combinations will fit within the constraints of the STM32F1 USB system, and not all combinations will be supported by all operating systems.

I recommend calling USBComposite.setDeviceId(device) with a different device number for each combination of plugins and profiles to prevent problems with cached configurations on the host computer.

Uploading with STM32duino bootloader

Normally, the STM32duino bootloader upload method in the Roger Melbourne STM32F1 core sends a command to reset the board via the USB serial port, and thereby put it in bootloader mode, just prior to uploading. If you have installed a sketch that includes a USB serial port in the composite device, this should still work. But if the sketch you've installed doesn't include a USB serial port, then you need to manually activate the bootloader mode next time you want to upload a sketch.

The bootloader mode is active for a short period after the board powers up or resets. So just initiate the upload in the Arduino IDE as usual, but when "Searching for DFU device [1EAF:0003]" is displayed, hit the reset button (or if the device is USB-powered, keep it unplugged from USB and plug it in when you get this message).

Simple USB device configuration

A simple USB device uses a single plugin. You need to create an instance of the plugin class, call any setup methods for the plugin and the begin() method for the plugin. For instance, to inject keyboard data, you should do:

USBHID HID; // create instance of USBHID plugin
HIDKeyboard Keyboard(HID); // create a profile


and then call Keyboard.print("TextToInject") to inject keyboard data. Some plugin configurations may require further initialization code or further code that needs to be called inside the Arduino loop() function.

See the BootKeyboard, midiout and x360 example code for variants on this procedure.

(Additionally, the USBHID plugin has a convenience begin() method that lets you include an instance of a USBCompositeSerial plugin class, and that creates a composite HID-Serial device.)

However, if you want a USB device using more than one plugin, then you will NOT call the plugin's begin() method.

Note that a single HID plugin can support a device with multiple report profiles including a keyboard, several joysticks, a mouse, etc.:

USBHID HID; // create instance of USBHID plugin
HIDKeyboard Keyboard(HID); // create a profile
HIDJoystick Joystick1(HID); // create a profile
HIDJoystick Joystick2(HID); // create a profile
HIDMouse Mouse(HID); // create a profile


Each of the profiles (e.g., Joystick1) contributes a part of the HID report descriptor to USBHID which automatically stitches them together and assigns report IDs. However, you can also make a single overarching custom HID report descriptor and include it in the HID.begin() call. The softjoystick example does this.

Memory limitations

There are 320 bytes of hardware buffer memory available after endpoint 0 is taken into account. The following are the default buffer memory needs of the current components:

  • USB Serial: 144 bytes

  • USB HID: 64 bytes

  • USB Mass Storage: 128 bytes

  • USB MIDI: 128 bytes

  • XBox360 Controller: 64 bytes

This places a limit on what combinations can be used together. For instance, HID+Mass storage+MIDI should be theoretically OK (320 bytes), but Serial+HID+Mass storage (336 bytes) will fail with default settings (and return false from USBComposite.begin()) due to lack of memory.

However, USB Serial, USB HID and USB MIDI allow you to decrease buffer sizes (and allow for more complex composite devices) by calling:

USBCompositeSerial CompositeSerial;


The maximum and default packet size is 64. Smaller packet sizes have not been thoroughly tested and may slow things down. In particular, for HID you should make sure your packet size is sufficient for your largest HID report. The CompositeSerial device also has a control channel whose 16 byte packet size is not adjustable. Note that for reasons that I do not currently understand, CompositeSerial RX packets must be a power of two in size.

Note also that in the above, RX and TX are from the point of view of the MCU, not the host (i.e., RX corresponds to USB Out and TX to USB In).

Endpoint limitations

There is one bidirectional endpoint 0 that all endpoints share, and the hardware allows for seven more in each direction, but there are some complications in that the same endpoint number when used in different directions must have some of the same parameters. The USBComposite library takes care of these complications when allocating endpoints, but if you have too many plugins, you USBComposite.begin() will return false to indicate that you've used up too many.

This is pretty complicated, but a rule of thumb for having enough endpoints is to make sure that when you add up the following contributions for the plugins you use, your total is at most seven.

  • USB Serial: 2 (= 2 TX, 1 RX)

  • USB HID: 1 (= 1 TX)

  • USB Mass Storage: 1 (= 1 TX, 1 RX)

  • USB MIDI: 1 (= 1 TX, 1 RX)

  • XBox360 Controller: 1 per controller (= 1 TX, 1 RX)

  • USB Audio: 1 (= 1 TX or 1 RX depending on mode)

  • USB Multi Serial: 2 per port (= 2 TX, 1 RX)


USB Composite library for STM32F1 (HID, Serial, MIDI and XBox360 controller)







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