Keypad example

; For I/O registers located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", 
; "CBI", and "SBI" instructions must be replaced with instructions that allow access to 
; extended I/O. Typically "LDS" and "STS" combined with "SBRS", "SBRC", "SBR", and "CBR".

.include "m2560def.inc"
.def temp =r16
.def row =r17
.def col =r18
.def mask =r19
.def temp2 =r20
.equ PORTLDIR = 0xF0
.equ INITCOLMASK = 0xEF
.equ INITROWMASK = 0x01
.equ ROWMASK = 0x0F

.cseg
jmp RESET

.org 0x72

RESET:
	ldi temp, low(RAMEND)
	out SPL, temp
	ldi temp, high(RAMEND)
	out SPH, temp
	ldi temp, PORTLDIR ; columns are outputs, rows are inputs
	STS DDRL, temp     ; cannot use out
	ser temp
	out DDRC, temp ; Make PORTC all outputs
	out PORTC, temp ; Turn on all the LEDs
	
; main keeps scanning the keypad to find which key is pressed.
main:
	ldi mask, INITCOLMASK ; initial column mask
	clr col ; initial column

colloop:
	STS PORTL, mask ; set column to mask value
	; (sets column 0 off)
	ldi temp, 0xFF ; implement a delay so the
	; hardware can stabilize
	
delay:
	dec temp
	brne delay
	LDS temp, PINL ; read PORTL. Cannot use in 
	andi temp, ROWMASK ; read only the row bits
	cpi temp, 0xF ; check if any rows are grounded
	breq nextcol ; if not go to the next column
	ldi mask, INITROWMASK ; initialise row check
	clr row ; initial row
	
rowloop:      
	mov temp2, temp
	and temp2, mask ; check masked bit
	brne skipconv ; if the result is non-zero,
	; we need to look again
	rcall convert ; if bit is clear, convert the bitcode
	jmp main ; and start again
	
skipconv:
	inc row ; else move to the next row
	lsl mask ; shift the mask to the next bit
	jmp rowloop          

nextcol:     
	cpi col, 3 ; check if we are on the last column
	breq main ; if so, no buttons were pushed,
	; so start again.
	; else shift the column mask:
	sec ; We must set the carry bit
	rol mask ; and then rotate left by a bit,
	; shifting the carry into
	; bit zero. We need this to make
	; sure all the rows have
	; pull-up resistors
	inc col ; increment column value
	jmp colloop ; and check the next column
	; convert function converts the row and column given to a
	; binary number and also outputs the value to PORTC.
	; Inputs come from registers row and col and output is in
	; temp.
	
convert:
	cpi col, 3 ; if column is 3 we have a letter
	breq letters
	cpi row, 3 ; if row is 3 we have a symbol or 0
	breq symbols
	mov temp, row ; otherwise we have a number (1-9)
	lsl temp ; temp = row * 2
	add temp, row ; temp = row * 3
	add temp, col ; add the column address
	; to get the offset from 1
	inc temp ; add 1. Value of switch is
	; row*3 + col + 1.
	jmp convert_end
	
letters:
	ldi temp, 0xA
	add temp, row ; increment from 0xA by the row value
	jmp convert_end
	
symbols:
	cpi col, 0 ; check if we have a star
	breq star
	cpi col, 1 ; or if we have zero
	breq zero
	ldi temp, 0xF ; we'll output 0xF for hash
	jmp convert_end
	
star:
	ldi temp, 0xE ; we'll output 0xE for star
	jmp convert_end
	
zero:
	clr temp ; set to zero
	
convert_end:
	out PORTC, temp ; write value to PORTC
	ret ; return to caller