collapseos/arch/z80/ti84/blk/615

Sending 0xff to the port resets the keyboard, and then we have
to send groups we want to "listen" to, with a 0 in the group
bit. Thus, to know if *any* key is pressed, we send 0xff to
reset the keypad, then 0x00 to select all groups, if the result
isn't 0xff, at least one key is pressed.