Inside the PC keyboard’s case is an 8042 microcontroller chip that constantly scans the switches on the keyboard to see if any keys are down. (caveat, at least back in the day it was the 8042, i’m not sure if non-PS/2 KBs still utilize it, I need to verify). This processing goes on in parallel with the normal activities of the PC, hence the keyboard never misses a keystroke because the processor in the PC is busy.
A typical keystroke starts with the user pressing a key on the keyboard. This closes an electrical contact in the switch so the microcontroller and sense that you’ve pressed the switch. Alas, switches (being the mechanical things that they are) do not always close (make contact) so cleanly. Often, the contacts bounce off one another several times before coming to rest making a solid contact.
If the microcontroller chip reads the switch constantly, these bouncing contacts will look like a very quick series of key presses and releases. This could generate multiple keystrokes to the PC, a phenomenon known as keybounce which used to be common to many cheap and old keyboards. But even on the most expensive and newest keyboards, keybounce is a problem if you look at the switch a million times a second; mechanical switches simply cannot settle down that quickly. Therefor, most keyboard scanning algorithms control how often they scan the keyboard. A typical inexpensive key will settle down within five milliseconds, so if the keyboard scanning software only looks at the key every ten milliseconds, or so, the controller will effectively miss the keybounce.
References:
- The PS/2 Keyboard Interface – Adam Chapweske
- Art of Assembly – The PC Keyboard (Chap 20) – Randall Hyde
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