Do built in keyboards use polling or interrupts?

Question

I was reading this question which says that USB keyboards use polling to see if keys are being pressed whereas PS/2 keyboards use interrupts.

I was wondering which method built in keyboards on a laptop use?

Answer 1

Most classic laptops (with built-in non-detachable keyboards) use a dedicated keyboard controller (direct scanning of key matrix) that comes out of LPC bridge (sometimes called as "Super-I/O"). The LPC (Low Pin Count) bridge also controls all legacy devices, BIOS chip, timers, RTC etc. The LPC bridge comes out of a dedicated PCI unit, and the LPC bus supports all necessary interrupts.

If you check the PC device manager "by connection", and expand "ACPI xxx-based PC", you will eventually see all PCI devices. The keyboard will show up as PS/2, but this is only logically, there is no actual serial interface. So you can say that the laptop keyboard uses interrupts.

Various keyboardless devices as tablets use the USB interface to communicate with keyboard. Detachable keyboards also use USB. The key scanning electronics is local to the keyboard and processes all key-press and key-release events locally. The bus uses USB Periodic Transactions called "Interrupt Pipe" to get keyboard data, which is formally a form of polling.

Answer 2

Built-in keyboards use the method corresponding to the interface they use and that interface is usually either PS/2 or USB.

In Windows 7, you can check by opening up Device Manager and looking under Keyboards. In my sample size of one, I've got a HID Keyboard Device and a Standard PS/2 Keyboard. The former is an external USB keyboard (it disappears when I unplug it), so the latter must be my built-in keyboard.