How does a USB keyboard work?

Among other things, the USB host polls the keyboard at a fixed rate. Fantastic breakdown if you have 30 minutes: ▶ this video

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: ▶ this video

Ben Eater has some videos showing the USB protocol on an oscilloscope. ▶ this video link And some videos on using chips and breadboards to meet protocols- link And a digital protocol too- link

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

> Embedded programming? Dealing with signals?! God forbid, man. "How does a USB keyboard work? by Ben Eater: * ▶ this 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 ▶ this 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] ▶ this video Even if this conclusion is wrong, that video is a very pleasant and understandable explanation of the USB protocol.

Serial ports still exist today. Even in desktop computers (though usually in weird underpowered ones designed for industrial applications and such). Even the most GAMING of motherboards have headers ready for one or even more serial ports to mess with. All you need is a cable and a card to slot into a PCIe bracket! USB itself, at least the 1.1 protocol that's still used for some devices today, isn't all that complicated in itself: ▶ this video Many if the complicated parts here, like device identifiers, are the things you'd need to manually configure if you were to use a serial port. Those hundreds of pages are similar to the hundreds of device types, vendors, and models that Windows would list when you clicked "add new hardware" back in Windows 98. When push comes to shove, you're just sending energy pulses down wires, there's just a chip in there that helps collect pulses so you don't need to manage the timing manually.

PS/2 Keyboards are faster than a USB keyboard with slow-speed negotiation with the host computer but for USB keyboards with high speed negotiation, they are at par[1] [1] ▶ this video

I'm sorry but serious question, why would you need a high-speed for a keyboard? A usb packet for a keystroke is roughly 180 bits (I'm estimating from glancing at ▶ this video ). So 1.5 Mbit/s low speed could roughtly handle over 8,000 keystrokes per second. Do you really need more keystrokes or finer resolution than that?

I'd imagine it would be very difficult, and would require a relatively powerful MCU. Ben Eater's made a few videos discussing how PS/2[0] and USB keyboards[1] work. USB's a vastly more complicated protocol compared to PS/2. [0] link [1] ▶ this video

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: ▶ this video - He made the OP video about USB keyboards being limited to holding six keys at once.