.include "m128def.inc"

.def temp=r16
.def counter=r17
.def resultl=r18
.def resulth=r19

.cseg

.org $0000
    jmp reset

.org $0018
    jmp TIMER1_COMPA_ISR

.org $0100
reset:
    ldi temp, low(RAMEND)
    out SPL, temp
    ldi temp, high(RAMEND)
    out SPH, temp

    clr counter
    clr resultl
    clr resulth

    ; max compare value: 65535
    ; max prescaler: 1024
    ; 65535 / (4MHz / 1024) = 16s

    ser temp
    out OCR1AH, temp
    out OCR1AL, temp

    ldi temp, (1 << OCIE1A)
    out TIMSK, temp

    clr temp
    out TCCR1A, temp
    ldi temp, (1 << WGM12) | (1 << CS12) | (1 << CS10)  ; CTC mode, prescaler = 1024
    out TCCR1B, temp

    ldi temp, (1 << REFS0)  ; AVcc, input: ADC0
    out ADMUX, temp

    ldi temp, 0b11001111  ; enable, start of conversion, single conversion mode, interrupt enable, max prescaler
    out ADCSRA, temp

    set
    sei

loop:
    sbis ADCSRA, ADIF  ; do we have a ready result?
    rjmp loop

    in temp, ADCL
    add resultl, temp
    in temp, ADCH
    adc resulth, temp

    sbi ADCSRA, ADIF  ; manually clear flag (by setting it)
    sbi ADCSRA, ADSC  ; manually start next conversion (single conversion mode)
    brts loop
    rjmp reset

TIMER1_COMPA_ISR:
    cpi counter, 8  ; 8 * 16s = 128s
    brlo next

    ; divide result by 128 (= 7 right rotations) to get average

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    lsr resulth
    ror resultl

    clt
next:
    inc counter
    reti