N-key Rollover (NKRO) with USB Keyboards

You may read that USB keyboards support only 6-key rollover: that is, a maximum of 6 keys can be pressed at once, and further keypresses are not handled. That is true for USB keyboards operating in “boot” mode, but is not an inherent limitation. Normally the operating system reads a detailed description (the “report descriptor”) of a USB keyboard when you plug it in. The report descriptor specifies which keys it supports, what LEDs it has, how many keypresses are reported at once, etc. However, the USB standard also allows you to declare that a keyboard can operate in “boot” mode, which means that the supplied report descriptor is ignored, and a standard report descriptor is assumed instead. This simplifies the BIOS or other boot code. Boot keyboards are indeed limited to 6-key rollover.

One way people get around this limitation is to have their NKRO keyboard present itself as multiple 6KRO keyboards. But this is not necessary. Myths about USB NKRO and how USB HID works explains this in detail:

A common myth about USB keyboards is that they can only support 6-key rollover, and not report more than 6 keys being pressed at a time. In fact, this is wholly untrue; the USB HID device class can support full N-key rollover (NKRO). While keyboards which purport to support USB NKRO are now common, some of them have the ability to “disable” NKRO for compatibility reasons. This is peculiar and unnecessary and USB HID in no way requires it.

A properly implemented USB keyboard with NKRO can provide full NKRO over USB, and not need any option to “turn NKRO on and off”.

Check out the link for more details. If the link has disappeared, it is archived here.


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