“As a kid, I got the book “Upgrading and Repairing PCs.” (Now in its 22nd edition.) It was the first book to explain to me the PC architecture. I considered, how were there so few pins on an AT-style keyboard connector when there were 101 keys on the keyboard? That is when I first learned about the keyboard matrix.
The keyboard matrix itself did not amaze me, but instead the idea there was an entirely separate 8-bit microcontroller inside of the keyboard. Early keyboards may have used the P8049AH, which, there is still some stock available to purchase. I was fascinated with the idea an entire computer was necessary to run the keyboard, to use my “real” computer. Why did it take something as complicated as a microcontroller?
The key benefit (get it?) of a keyboard matrix is that it reduces the number of pins necessary to capture the input of a large number of the keys. Even though a PC keyboard has 101 keys, it does not mean there is a microcontroller with 101 pins. Nor does it need a cable with over 100 wires.
I will first explain with simple four and nine button examples.”