How does a USB keyboard work?

About the USB keyboard, that may be true only for the cheapest keyboard you could find, but basically not applicable anymore nowadays. Rather than giving you the details, let me link to this video, which is awesome at explaining it and much more -- the USB vs PS/2 part starts here: [VIDEO]

Among other things, the USB host polls the keyboard at a fixed rate. Fantastic breakdown if you have 30 minutes: [VIDEO]

There’s a great video here that goes through all this - the gist is that USB keyboards specify what polling rate to use, 1000hz for the best ones, maybe 125hz for the cheaper ones. An interesting thing about ps/2 is that even though it doesn’t have to wait for the keyboard to poll it transmits data at much slower speed than USB (khz vs mhz) which somewhat offsets the advantage it gets from being interrupt driven [VIDEO] edit: ah, sorry, I see someone's posted a link to this video already!

Apparently [1] PS/2 limited by a slower data rate than USB 2.0 which cancels out almost all the advantage of its lower initial latency to begin the transmission, making them both about equally capable in actual latency - at least in theory. [1] [VIDEO] Even if this conclusion is wrong, that video is a very pleasant and understandable explanation of the USB protocol.

> Embedded programming? Dealing with signals?! God forbid, man. "How does a USB keyboard work? by Ben Eater: * [VIDEO]

If you're interested, here's a really comprehensive video on how USB keyboards work which may answer your question: [VIDEO]

Ben Eater has some videos showing the USB protocol on an oscilloscope. [VIDEO] [LINK] And some videos on using chips and breadboards to meet protocols- [LINK] And a digital protocol too- [LINK]

Piggy-backing this comment to add some context for the OP video: This "USB device discovery" video is actually the latest in a series of videos where Ben Eater is building a simple 8-bit computer on breadboards. He hooked up a PS/2 keyboard as an input device, and then got sucked into a tangent of investigating and explaining keyboard protocols. - He investigates the PS/2 keyboard protocol: [LINK] - Somebody on Patreon asked about USB keyboards having higher latency than PS/2, so he made a video about the basic USB protocol: [VIDEO] - He made the OP video about USB keyboards being limited to holding six keys at once.