example program using pin change interrupts from AVR Programming by Elliot Williams

capSense.asm

;Capacitive sensor demo in avr assembly
;compile with gavrasm capSense.asm
;flash with avrdude -c avrisp -p m168 -P /dev/tty.usbmodem1411 -b 19200 -U flash:w:capSense.hex
;
;
;what I learned from this program:
; always save the status reg when using interrupts
; and use the right jump vector for interrupt calls
; and keep track of registers used in subroutines
;
;1mhz clock speed X2, 9600 baud UBBR = 12
.equ UBBRvalue = 12
.equ UCSR0C = 0xc2      ;SRAM address of UCSR0C
.def temp = r16 
.def temp2 = r17 
.def trans_t = r18 
.def I_temp = r19 
.def xL = r26           ;x reg used as global counter
.def xH = r27 
.def zL = r30 
.def zH = r31 
.device atmega168
;---- global variables ----
.dseg
cc_count: .byte 2               ;remember this is big endian
.cseg
.org 0
        jmp     reset
.org 0x0008                     ;PCINT1 vector
        jmp     c_change        ;pcint1
c_change:
        in      temp,SREG       ;save status register
        adiw    xH:xL,1
        sbi     DDRC,PC1
        ldi     I_temp,4
i_lp:   dec     I_temp
        brne    i_lp            ;8mhz clock for 1us delay
        cbi     DDRC,PC1
        in      I_temp,PCIFR
        sbr     I_temp,(1 << PCIF1)
        out     PCIFR,I_temp
        out     SREG,temp
        reti
.undef I_temp
;----- Main -----
.org    0x0034
reset:
        cli 
        lds     temp,CLKPR      ;clock_prescale_set
        sbr     temp,(1 << CLKPCE)
        sts     CLKPR,temp
        ldi     temp,0x00
        sts     CLKPR,temp      ;set full speed = 8Mhz
;inittialize Stack
        ldi     temp,low(RAMEND)
        out     SPL,temp
        ldi     temp,high(RAMEND)
         ldi     temp,high(RAMEND)
        out     SPH,temp
;initialize USART
        ldi     temp,high(UBBRvalue)    ;baud rate param
        sts     UBRR0H,temp
        ldi     temp,low(UBBRvalue)
        sts     UBRR0L,temp
        lds     temp,UCSR0A
        ori     temp,(1 << U2X0)        ;set use 2x because %error actual baud > .5
        sts     UCSR0A,temp
;--- USART register values
        ldi     temp,(1 << TXEN0) | (1 << RXEN0) ;enable transmit and receive
        sts     UCSR0B,temp
        ldi     temp,(1 << UCSZ01) | (1 << UCSZ00) ;8 data bits, 1 stop bit
        sts     UCSR0C,temp
;--- send a message
        ldi     zH, high(cap_str * 2)
        ldi     zL, low(cap_str * 2)
        rcall   print_s
;--- set ddr ---
        ldi     temp,0xff
        out     DDRB,temp               ;set output on leds
;--- disable all pullups
        in      temp,MCUCR
        sbr     temp,(1 << PUD)
        out     MCUCR,temp
        sbi     PORTC,PC1               ;leave output high on pc1
;--- init pin change interrupt ---
        lds     temp,PCICR
        sbr     temp,(1 << PCIE1)
        sts     PCICR,temp
        lds     temp,PCMSK1
        sbr     temp,(1 << PC1)         ;enable pin-change interrupts for 1 (bank C)
        sts     PCMSK1,temp             ;enable specific interupt for pin PC1
;--- event loop ---
loop:
        ldi     temp,0x00
        ldi     xH,0x00
        ldi     xL,0x00                 ;xReg is counter
        sbi     DDRC,PC1                ;start with cap charged
        sei                             ;set global interupt
.equ d_cnt = 50000                      ;inline delay
        ldi     temp2,1
outer:  ldi     r24,low(d_cnt)
        ldi     r25,high(d_cnt)
delay:  sbiw    r25:r24,1
        nop                             ;this makes four clock cycles = 200,000 us 
        brne    delay
        dec     temp2
        brne    outer
        cli                             ;clear global interrupt
.equ thresh = 12000
;compare threshhold with count / should work with just high bytes
        cpi     xH,high(thresh)
        brlt    led_on
        ldi     temp,0x00
        out     PORTB,temp
        out     PORTB,temp
        rjmp    msg
led_on: ldi     temp,0xff
        out     PORTB,temp
msg:
        mov     r24,xH
        rcall   transmit
        mov     r24,xL
        rcall   transmit
        ldi     r24,0x0d
        rcall   transmit
        ldi     r24,0x0a
        rcall   transmit
        rjmp    loop
;       
;--- Subroutines ---
;print string function
;takes Z loaded with pointer to string with first byte being length
;uses r16,r18,r24
.def byte_tx = r24
.def count = r18
print_s:
        lpm     count,Z+
for1:   lpm     byte_tx,Z+
wait:   lds     temp,UCSR0A
        sbis    temp,UDRE0      ;wait for Tx buffer to be empty
        rjmp    wait            ;not ready
        sts     UDR0,byte_tx;
        dec     count
        brne    for1
        ret
;send one byte
;uses r24 as byte to send
transmit:
        lds     trans_t,UCSR0A
        sbis    trans_t,UDRE0
        rjmp    transmit
        sts     UDR0,r24
        ret
cap_str: .db 22,"==[ Cap sensor ]==",0x0d,0x0a,0x0d,0x0a,0x00