I would have thought they'd be super cheap anyway, if you're looking at $200 keyboards, spending a dollar or so to get a few muxes doesn't seem like a big deal. If they were truly better, I imagine every keyboard enthusiast would be using them.
Edit: I guess since they have multiple muxes, they change the SEL well in advance before scanning the key. So it just requires "clever" scheduling. Just like a matrix requires "clever" diodes, I really don't buy that muxes are an advantage here.
Disclaimer: somehow graduated with an EE, but am an idiot.
The settling time of the switch is the same, regardless of the scanning technique used.
But with a multiplexer, the output of one switch has no impact on the output of another – you can independently actuate each key. They are all essentially isolated switches with individual pull-up resistors.
The multiplexing can be done in a number of different ways, none of which need to be particularly clever. The simplest scheme is simply to connect each key directly to a GPIO on a large-pinout microcontroller, like a TQFP-144. The fewer the keys, the less GPIO you need.
This is the best way to build a keyboard, since input can be done at the clock speed of the MCU, and debouncing can happen on all pins in parallel simultaneously.
Slightly more complex would be to use a smaller MCU with a multiplexer on each input, like a 4051 or 4067. The switching time is like 20-50ns on these parts, so while it's not quite as fast as a GPIO register read, it's still pretty quick, more than fast enough for keyboards.
If you really wanted to get crazy, you could read each switch with a high speed ADC and use an ML model to debounce the keys. But at that point, you're better off just using optical switches which don't really need debouncing.
> The switching time is like 20-50ns on these parts, so while it's not quite as fast as a GPIO register read, it's still pretty quick, more than fast enough for keyboards.
I guess my point was that if you change which input you're reading from the mux and then immediately sample with the MCU, you'll get unstable data. 50ns is 20MHz, I can easily see an MCU used in a keyboard being fast enough to hit this.
It's obviously easy to work around, but so is ghosting.
The switching time of multiplexers is on the scale of 100 nanoseconds, so it's negligible (assuming there's no extra RC components behind the multiplexer to lengthen its settling time further, a reasonable assumption for pure software denouncing).
Rhythm games have knobs to offset the music, most even have sync wizards (tapping to the beat). If you have 200ms input delay, just have the game offset the music by -200ms. It's really not a problem
I got a cabinet for one of my favorite arcade rhythm games, Pop'n Music, a couple of years ago. It's older Firebeat hardware so it's a non-general purpose system specifically built for arcade music games not running a general purpose OS. I was surprised by how much better it felt than the newer versions of the game which run on Windows XP with inherent extra latency in its audio playback. (Other games by the same developer have taken advantage of lower latency audio apis in newer versions of Windows but iirc Pop'n hasn't gotten off XP yet.)
It's got a 16 mhz mcu in a busy-loop reading two CPU pins.. It should be pretty fast and consistent (but I don't know how much time the actual USB protocol stuff takes, and I suspect that more jitter is introduced in the OS anyway)
http://dusted.dk/pages/osukeys/
Yes, I could have used interrupts, but I didn't.
The exception is Model M keyboards, which actually use a membrane instead of a PCB.
99.9% of mechanical keyboards made in the last decade avoid ghost keys by using diodes, yes, primarily because they have been intended for gaming.
However, diodes are not at all universal in mechanical office keyboards, even if they use discrete switches and a PCB. If you'd look at vintage mechanical keyboards, you'll find that while many have them, many don't. Cherry even still makes some older models with Cherry MX switches and 2-key rollover.
You’re going to have to be more precise than this. Most “mechanical” keyboards from the 80s–90s do not (the Model M is not at all exceptional in this sense). I would guess most of the more recent ones (marketed to ordinary typists) don’t either.
For enthusiast-/gamer-targeted keyboards from the past few years you could be right.
Article was good, worth reading.
I can imagine how this matters to gamers -- just asking about the "normal" use of the keyboard.
That is incorrect. You can register and send "key held" input on the first bounce, immediately, then just ignore the key for the debouncing interval. Probably still want some capacitance on the input to not get triggered by any EMI tho.
That moves the latency to key depress but as there is a very little chance someone presses key for less than 10ms it doesn't matter.
The "interrupt, wait, interrupt, send signal" method might be easier to code I guess, especially if you just have few buttons and use hardware interrupts for them.
Or are switches generating spurious signal spikes even when left untouched? That would explain why they need to delay the keypress…
