All posts for the month September, 2021

Now that I’m partly back in my office, I had to have two functioning keyboards again. I dusted off the two latest-version modified NMB keyboards, and tweaked the debounce timing on them slightly, and they’re working fine as my daily drivers. The current debounce logic is very rudimentary; between each loop through scanning the keyboard matrix, the firmware delays by 1ms. If there was a change in key state, this is sent over bluetooth, and the firmware delays by DEBOUNCE_DELAY (currently 20ms) before scanning again. If I were super picky, I’d just ignore bounces in the most recently-changed key, but for real-world typing this seems fine. Any time I get spurious key repeats, I can either clean the switch contacts or increase the delay value.

I do need to implement some kind of anti-ghosting mechanism, since the old keyboards have no NKRO diodes or anything; if I’m sloppy while typing, I’ll easily start getting ghost keypresses. My plan for a rudimentary solution for that is that if there’s ever a key-scanning update that results in two keypresses being added in the same millisecond, then ignore that update until the state changes again. Most of the cases where I’m getting ghost keypresses are when I’m pressing three keys in sequence but I’m late in releasing the earlier keys, so I get events for key1, key2, and then key3+ghost_key. (This happens when two of the keys being pressed share a row, and two of the keys being pressed share a column, and the end result basically shorts out another intersection on the key matrix. So if the keys being pressed are at A1, B1, and A2, when the third key is pressed it will also appear that B2 is pressed, because row 2 and column B are connected through the other three keyswitches.) Anyway, since I don’t know which of the two simultaneous keys being detected is real, I can just ignore both of them, and once one of the first two keys is released the situation should be resolved and the next update can go through.

One of the keyboards is the large Windows-key-equipped version, with everything mounted internally. The other one is one of the smaller 101-key models, and the bluetooth board is mounted on top, in a socket, too tall to fit inside the case. I finally got tired of using it with the top part of the case removed, so I just cut a hole out of the top to accommodate it. Much nicer.