;
; hello interrupts
;
; interrupt driven hello mic

.include "tn45def.inc"
;
; definitions
;
.equ tx_pin = PB2; transmit pin
.equ led1 = PB0;
.equ led2 = PB1;
.equ nloop = 200 ; number of samples between framing

.def status_reg = R16 ; status register storage between interrupts
.def temp = R17 ; temp!
.def temp1 = R18 ; 
.def bit_count = R19 ;
.def buffer = R20 ;
.def reload = R21 ;
.def txbyte = R22;

; baud rate settings
.equ C = 1
.equ R = 1
.equ N = 96

; bit positions in status register
.equ BUSY=7; busy-flag (internal - read only)

;
; code segment
;
.cseg
.org $0000
rjmp reset

.org OVF0addr
rjmp timer0_overflow

.org ACIaddr
reti

;
; UART transmitter
;
uart_init:
    ldi temp, R
    out TCCR0A, temp
    ret

putchar:
    ser bit_count ; Erase bit counter
    mov buffer, txbyte ; Copy transmit data to buffer

    ldi temp, 1<<TOIE0 ; set bit 1 in temp
    out TIFR, temp ; clear T/C0 overflow flag
    out TIMSK, temp ; enable T/C0 overflow interrupt

    ldi reload, (256 - N + (8 / C)) ; set reload-value
    ldi temp, (256 - N + (14 / C)) ; Set timer delay to first bit
    out TCNT0, temp ; set timer reload-value (1 bit)

    cbi PORTB, tx_pin
    ret

timer0_overflow:
    in status_reg, SREG ; store status register
    out TCNT0, reload ; reload timer
    inc bit_count ; increment bit counter

    sbrc bit_count, 3 ; if bit 3 in bit_count (>7) is set
      rjmp timer0_stopb ; jump to stop-bit part

    sbrc buffer, 0 ; if LSB in buffer is 1
      sbi PORTB, tx_pin ; set transmit to 1
    sbrs buffer, 0 ; if LSB in buffer is 0
      cbi PORTB, tx_pin ; set transmit to 0
    lsr buffer ; shift buffer right

    out SREG, status_reg ; restore SREG
    reti

timer0_stopb:
    sbi PORTB, tx_pin ; Generate stop-bit
    sbrs bit_count, 0 ; if bit_count == 8 (stop-bit)
      rjmp timer0_ret ;   jump to exit

timer0_ret:
    out SREG, status_reg ;
    reti

;
; spinner delays
;
eight_bit_delay:
    ldi temp, 255
    ebd_loop:
        dec temp
        brne ebd_loop
    ret

sixteen_bit_delay:
    ldi temp, 255
    sbd_loop:
        ldi temp1, 255
        sbd_loop1:
            dec temp1
            brne sbd_loop1
        dec temp
        brne sbd_loop
    ret

;
; Main program
;
reset:  
    sbi DDRB, tx_pin
    sbi PORTB, tx_pin

    sbi DDRB, led1;
    sbi PORTB, led1;
    sbi DDRB, led2;
    sbi PORTB, led2;

    rcall uart_init
    sei

    loop:
        ldi txbyte, 1
        rcall putchar
        ldi txbyte, 1
        rcall putchar
        ldi txbyte, 2
        rcall putchar
        ldi txbyte, 3
        rcall putchar
        ldi txbyte, 5
        rjmp loop