USB: when would you need more than one interface?

[Vu.Andy]

Posted on September 19, 2016 at 22:52

I've seen a lot of examples and almost all of them from STM have only a single interface.

And it seems like most of the time, one interface should be enough. Since I am still learning USB, for those who have experiences with USB, when would there be a need for more than one interface? For example, if I am building a HID Audio device, does it need more than one interface?  Thanks.

[tsuneo]

Posted on September 20, 2016 at 03:08

When a device require two or more USB functions, it is equipped as a composite device.

A couple of popular examples, maybe on your desktop, A) USB Audio + HID consumer device - Audio amplifier/speaker with a volume knob and a mute switch. The status of the volume knob and the mute switch is passed to PC over a HID consumer device. B) HID keyboard + HID consumer/system devices - PC keyboard with audio control / sleep-wakeup keys Windows force keyboard in a single interface. And then, other keys, supported by HID consumer/system devices, should move into another separated interface. C) UVC (USB Video class) + audio - Web camera with microphones D) Vendor (debug) class + MSC (Mass-Storage class) + CDC - ST-Link/V2-1 with mbed support on Nucleo and recent Discovery boards E) Your USB device with DFU (Device Firmware Upgrade) etc. Tsuneo