HID r.2.0 released
I pleased to announce that after a long and difficult development period Human Input Device AKA HID class support has been added to USB Host Shield Library r.2.0 and is available on gitHub – I suggest downloading the whole directory, since some modifications has been also made to core files to accommodate a new class. HID devices include popular devices like keyboards, mice, joysticks, game controllers, bar code scanners, RFID and magnetic card readers, digital scales and UPSes, to name a few.
I previously wrote about interfacing to HID devices here, here, and here. The code examples in these articles were written for legacy USB Host Shield library and can’t be compiled with current revision, however, the basic principles are the same – the device is periodically polled by the host and sends back data block called report containing changes in device controls (buttons, switches, jog dials etc.) since the last poll. Even though different devices have different report formats, for a certain device, report format is stored in the device in data structure called report descriptor. Therefore, it is possible to learn about device controls from the device itself by parsing its report descriptor.
There is one special case where report format is known in advance. Almost all HID keyboards and mice support so-called boot protocol intended for communication to very simple systems like PC configuration screen when computer runs from BIOS. Keyboard boot protocol report consists of 8 bytes containing state of modifier keys (CTRL, SHIFT,etc.) in the first byte, second byte being reserved, and up to 6 key scan codes in the rest of the report. Mouse boot protocol report consists of 3 bytes, first of which contains state of left, right and middle buttons and other 2 store X and Y travel since last poll.
In many cases boot protocol capabilities are more than enough for an Arduino project; for this reason, boot protocol class is the first to be released. To demonstrate operations of this class, 2 simple sketches has been developed, one for mouse, another for keyboard.
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