Hello World - even better version of serial_assembly.asm

serial_assembly2.asm

;serial io echo program from AVR Programming p.81 by Elliot Williams
;help from T. Margush Some Assembly Required
;compile with gavrasm serial_assembly.asm
;flash with avrdude -c avrisp -p m168 -P /dev/tty.usbmodem1411 -b 19200 -U flash:w:serial_assembly.hex
;
; cpu_f = 1,000,000 baud = 9600 use 2x(USX0 = 1)
;Programmer: Me
;2 mhz clock speed, 9600 baud UBBR = 12
.equ UBBRvalue = 12
.equ UCSR0C = 0xc2  ; SRAM address of UCSR0C
.def temp = r16 
.def count = r17 
.def zL = r30 
.def zH = r31 
.device atmega168
        rjmp    main
.cseg
input_string:
.db "Hello World"
        ;inittialize Stack
main:   ldi     temp,low(RAMEND)
        out     SPL,temp
        ldi     temp,high(RAMEND)
        out     SPH,temp
        ;leds display RXD counter
        ldi     temp,0xff
        out     DDRB,temp       ;ddr output
        ldi     temp,0x00
        out     PORTB,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
;--- set up leds to count input
.def rxCount = r0
        clr     rxCount
        com     rxCount
        ;--- send a message
.def byte_tx = r24
        ldi     zH, high(input_string)
        ldi     zL, low(input_string)
        ldi     count,11                        ; 11 characters in string
for1:   lpm     byte_tx,Z+
        rcall   transmit
        dec     count
        brne    for1
lp:     rcall   receive
        brcc    lp
        dec     rxCount
        out     PORTB,rxCount
        rcall   transmit
        rjmp    lp
;--- subroutines
receive:
        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