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Comment by pedro_caetano | Hacker News Reader

Well the project description seems to hint at to their motivation:

> 1000 Hz polling rate

> No multiplexing, no ghosting

> FPGA-based, VHDL only, no ALU

It looks like a pure HW 'described' keyboard with no running software meaning it is fully parallel (plus some serialization when reaching the USB device/interface).

So arguably on top of true parallelism the only ceiling for the latency of the whole thing will be the clock period configured in the design and the physics and electrical behaviour of the switches themselves + circuitry.

Probably someone who enjoys working close to hardware and wants to optimize performance.

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Most MCU-based keyboards have a 1000Hz polling rate - and some mainstream gamer-focused keyboards are even going for 8000Hz these days.

Getting rid of multiplexing is a result of having a high number of IO pins: an MCU like the STM32F429BE also has enough pin for direct-attach switches. Ghosting hasn't been an issue for ages, even with a traditional keyboard matrix it's just a matter of adding per-key diodes.

In theory it has a sliiightly lower latency than an MCU-based keyboard using the same USB interface, but I doubt the difference is big enough to even be measurable in real-world scenarios. We're talking about, at most, a few thousand cycles of an MCU running at a few hundred megahertz - and that's only relevant when you press the key right before a polling request arrives. That's the difference between an average input latency of 0.500 ms and 0.505 ms. Meanwhile on the output side, your fancy 480Hz monitor is only showing one frame every 2.1 milliseconds...

The optimization here is somewhat theoretically specially when you take into account the USB bottleneck as well as real-world Human reaction times and Perception.

But it appears from the project description that the author's motivation was indeed performance (irrespective of merit). A neat VHDL + HW project nevertheless.

In that light, it's odd that they specify a polling rate at all, since they use direct-attached key pins linked to an 48MHz clock. There is no grid/matrix sensing period, which is usually what is meant by polling rate. And the claimed value of 1kHz is doubly weird since the debouncing logic uses a period of 5ms, which means they can at most mimic a 200Hz polling rate:

  entity Debouncer is
    generic (
      FILTER_DURATION: delay_length := 5 ms
    );

edit: looked up what the typical bounce time is for a keyboard switch, and 5ms seems to be pretty standard. It's also the default that QMK uses. It seems quite excessive to me, I'd have expected bouncing times to be closer to tens of microseconds than multiple milliseconds.

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