PS/2 (or AT) keyboard is the way to go. The serial interface is simple, the keyboards are cheap.

PS/2 keyboards have a CLOCK line and a DATA line. Both are outputs from the keyboard. The CLOCK line is normally high, and transitions low on the first pulse of DATA. So in other words it can be used to generate interrupts, and reading DATA can be done in the interrupt handler.

1 start bit, 8 data bits, 1 parity bit, 1 stop bit.

The keyboard needs 5V. Can the GBA deliver this?

Put the GBA in SIO General Purpose mode. Use the SI input for CLOCK. Use any other input for DATA.

Serial cables are easy to find. Just get a multiplayer link cable and cut it in two. Ta-da! Two connectors. I got one from Amazon for $6.

I have two MAX3222 serial interface chips. Got free samples from the company that makes them. Dunno how important this schematic is yet. It might work just to hook the PS/2 keyboard right into the GBA.

MALE	FEMALE

Pin 1 - Keyboard clock
Pin 2 - Data
Pin 3 - Reset
Pin 4 - GND
Pin 5 - +5V

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Pin 1 - Data
Pin 2 - Not used
Pin 3 - GND
Pin 4 - +5V
Pin 5 - Keyboard clock
Pin 6 - Not used