Hacker News new | ask | show | jobs
by awelkie 1758 days ago
Not only that, but the matrix scanning could be done via interrupts instead of via polling as is typically[0] done. An interrupt could fire on a level shift of any of the column GPIOs, then the MCU needs to search the rows to find the selected key. After the interrupt the MCU will need to poll until all the keys are released. So there's another source of latency that could be improved.

I think the only source of polling left is in USB, but I think that's inherent to the USB protocol (someone correct me if I'm wrong here). Without the USB polling I think it would be possible to have key-press-to-USB-packet completely interrupt driven which should make the latency in the keyboard itself negligible.

[0]: I say typically but like you said a lot of implementations are closed source, so who knows. All of the discussions I've seen on matrix scanning use the polling method, and the open source implementations use polling as well (e.g. Keyberon[1]).

[1]: https://github.com/TeXitoi/keyberon-grid/blob/master/src/bin...
2 comments
ComputerGuru 1757 days ago
It’s counter-intuitive (and you probably already know this but for the benefit of others) but depending on frequency and CPU availability, polling can have a lower latency than interrupts. You just have a much smaller window of time to execute your handling code (unless you don’t care).

It’s actually partially why (some?) hard real-time systems eschew interrupts altogether. They introduce a source of non-determinism into the mix as an interrupt handler can stall out non-interrupt performance-sensitive code (or starve it).

link
Ballas 1757 days ago
The few open source implementations I have looked at scan the matrix as fast as the processor allows. So using interrupts can reduce power consumption a lot, but depending on the actual matrix layout, probably not any worthwhile speedup in the nominal case, as you still have to scan the rows after the interrupt.

AFAIR, the USB poll rate is a function of USB rate, and what the endpoint reports/requests. Low-speed, full-speed and high-speed all have different poll rates.

link