Hi everyone,
I just came up to a Keyboard interface to my 8051 circuit. At first the idea of the matrix struck me but upon careful observation, I find adding diodes to make the 4 × 4 keyboard's output to 4-bit HEX is much simpler. i.e., you have 15 lines from the keyboard and 4 lines at the output. When ever a key is pressed, it codifies into those four-bit hex code using the diodes. ALso, I NOR'ed the four output ports to generate an Interrupt INT0'. So, when ever I get an Interrupt, I can simply take the nibble of the port (to which the keyboard is connected) and know which key is pressed. Since, the key will be in depressed state, there is no fear that the interrupt would re-generate before the RETI statement. Though I've not exactly tried it (planning to do it in a couple of days), I wish to hear comments from people who already used this scheme and any problems they faced.

Regards,
Harsha