;
; This simple program uses interrupts and targets the atmega328P. It is
; presented in GNU assembler syntax.  If you want to use the AVR assembler
; (AVRASM) to assemble it, you will need to modify the syntax slightly.
;

;
; Some equates to associate names with numbers
;
.equ _spl,   0x5d
.equ _ddrd,  0x2a
.equ _portd, 0x2b
.equ _mcucr, 0x55
.equ _eimsk, 0x3d
.equ _eicra, 0x69

rjmp entry
nop
rjmp isr_0
nop

.org 0x60
isr_0:
    add r17,r1
    reti

.org 0x100
entry:
    eor r1,r1
    com r1
    neg r1
    ldi r16,0x80
    ldi r17,0x88
    sts _spl,r16 ; init stack ptr

    lds r24,_ddrd ; load DDRD
    andi r24,0xfb  ; clear PD2 pin
    sts _ddrd,r24 ; write back to DDRD

    lds r24,_portd ; load PORTD
    ori r24,0x04    ; enable pullup for PORTD[2]
    sts _portd,r24 ; write back to PORTD

    ; this sets int0 to trigger on rising edge...
    lds r24,_eicra ; load EICRA
    ori r24,0x03    ; enable isc00,isc01
    sts _eicra,r24 ; write back to EICRA

    lds r24,_eimsk ; load EIMSK
    ori r24,0x01    ; enable INT0 in EIMSK
    sts _eimsk,r24 ; write back to EIMSK 

    ;sei ; enable interrupts

sink:
    add r16,r1
    rjmp sink