AVR assembly program to toggle a speaker from USART serial connection. Based on serialOrgan.c from E.Williams AVR Programming p.100

organAssembly.asm

;Serial Organ program p.100
;compile with gavrasm organAssembly.asm
;flash with avrdude -c avrisp -p m168 -P /dev/tty.usbmodem1411 -b 19200 -U flash:w:organAssembly.hex;
;USART configure: 2x 1Mhz clock speed, 9600 baud UBBR = 12
.equ UBBRvalue = 12
.def temp = r16
.def zL = r30 
.def zH = r31           ;not defined
.def byte_tx = r18          
.def inB = r20
.def wordL = r24
.def wordH = r25 
.def delayL = r26
.def delayH = r27 

.device atmega168
.cseg
;--inits---
        in      temp,DDRD
        ori     temp,(1 << PD6)
        out     DDRD,temp
        ldi     temp,low(RAMEND)
        out     SPL,temp
        ldi     temp,high(RAMEND)
        out     SPH,temp
        rcall   initUSART
lp:
        rcall   receive
        brcc    lp      ;waits until input
        rcall   transmit    
        clc 
        mov     inB,byte_tx
;--- play notes --- 
.def checkN = r19
.def count = r17
        ldi     count,1
        ldi     zH,high(keyTable * 2)
        ldi     zL,low(keyTable * 2)    ;*2 because LSbit of Z is used as High/Low flag
lp2:    lpm     checkN,Z+
        cp      checkN,inB
        breq    play
        mov     byte_tx,checkN
        rcall   transmit
        inc     count
        cpi     count,19
        brsh    lp  
        rjmp    lp2 
play:
.undef checkN
.def temp2 = r19
;.undef count
        ldi     byte_tx,'X'
        rcall   transmit
        ldi     zL,low(noteTable * 2)
        ldi     zH,high(noteTable * 2)
lp3:    lpm     temp,Z+         ;low byte first
        lpm     temp2,Z+
        dec     count
        brne    lp3             ;get right index
        push    temp
        push    temp2
        ldi     temp,20         ;note duration
        push    temp
        rcall   pNote
        rjmp    lp
;
;--- subroutines ---
initUSART:
        ldi     temp,low(UBBRvalue)
        sts     UBRR0L,temp
        ldi     temp,high(UBBRvalue)
        sts     UBRR0H,temp
        lds     temp,UCSR0A
        ori     temp,(1 << U2X0)
        sts     UCSR0A,temp
        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
        ret
receive:        ; needs byte_tx defined by caller
        clc
        lds     temp,UCSR0A
        sbis    temp,RXC0       ;is byte in rx buffer?
        ret                     ;not yet
        lds     byte_tx,UDR0;
        sec                     ;to indicate a byte was received
        ret
transmit:
        lds     temp,UCSR0A
        sbis    temp,UDRE0      ;wait for Tx buffer to be empty
        rjmp    transmit        ;not ready
        sts     UDR0,byte_tx;
        ret
pNote:
.def duration = r19
.def eoReg = r17
        pop     zH
        pop     zL
        pop     duration
        pop     delayH          ;high byte first
        pop     delayL          ;switched order
outer:  mov     wordL,delayL
        mov     wordH,delayH
dlp:    sbiw    wordH:wordL,1
        brne    dlp
        in      temp,PORTD
        ldi     eoReg,(1 << PD6)
        eor     temp,eoReg
        out     PORTD,temp
        dec     duration
        brne    outer
.undef duration
.undef eoReg
        icall                   ;indirect call points to address in Z

noteTable:
.dw 1043, 984, 929, 877, 827, 781, 737, 696, 657, 620, 585, 552, 521, 492, 464, 438, 414, 390
keyTable:
.db "awsedftgyhjikolp;'"

This program uses the stack to call a function and indirect addressing with the Z register. The pitches aren't exactly right but its working. I need to dial in the delay loop to reflect a microsecond.

The hard part of writing this was figuring out the LPM instruction uses the least significant bit in Z as a flag to tell which byte of the word address to use.