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.

Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.

Join 35,000+ makers on Adafruit’s Discord channels and be part of the community! http://adafru.it/discord

Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Follow Adafruit on Instagram for top secret new products, behinds the scenes and more https://www.instagram.com/adafruit/

CircuitPython – The easiest way to program microcontrollers – CircuitPython.org

Maker Business — Chip inventories rise as demand falls

Wearables — How to attach a battery to your next wearables project

Electronics — Check out this shorthand shortcut

Python for Microcontrollers — Python on Microcontrollers Newsletter: 400 CircuitPython Libraries, 3m Thanks and much more! #CircuitPython #Python @ThePSF @micropython @Raspberry_Pi

Adafruit IoT Monthly — 2022 in Recap!

Microsoft MakeCode — MakeCode Thank You!

EYE on NPI — Maxim’s Himalaya uSLIC Step-Down Power Module #EyeOnNPI @maximintegrated @digikey

New Products – Adafruit Industries – Makers, hackers, artists, designers and engineers! — New Products 1/25/2023 Featuring Adafruit ESP32-S2 Reverse TFT Feather! (Video)

Get the only spam-free daily newsletter about wearables, running a "maker business", electronic tips and more! Subscribe at AdafruitDaily.com !

No Comments

No comments yet.

Sorry, the comment form is closed at this time.