main.asm

;==============================================================================
; AVR USART Serial Interface - Portfolio Refined Version
;
; Target:      ATxmega128A1U
; Peripheral:  USARTE0
; RX / TX:     PE2 / PE3
; Baud:        115200
; Frame:       8 data bits, no parity, 1 stop bit
; System clk:  32 MHz internal oscillator
;
; Description:
;   Implements bidirectional serial communication between the ATxmega128A1U
;   and a PC terminal. The program prompts the user for text input, stores it
;   in SRAM buffers, echoes it through USART, and displays the captured data.
;
; Portfolio refinements:
;   - Reorganized comments and constants for readability
;   - Added bounded string input to prevent SRAM buffer overflow
;   - Improved backspace handling at the start of an empty buffer
;   - Preserved the original program behavior and register-level USART control
;==============================================================================

.include "ATxmega128A1Udef.inc"

;------------------------------------------------------------------------------
; Constants
;------------------------------------------------------------------------------
.equ ENTER       = 0x0D
.equ BACKSPACE   = 0x08
.equ SPACE       = 0x20

.equ YN_MAX      = 3       ; 4-byte buffer: up to 3 chars + null terminator
.equ UNI_MAX     = 39      ; 40-byte buffer: up to 39 chars + null terminator
.equ CITY_MAX    = 39      ; 40-byte buffer: up to 39 chars + null terminator

;------------------------------------------------------------------------------
; SRAM buffers
;------------------------------------------------------------------------------
.dseg
.org 0x2000

ynBuf:      .byte 4
uniBuf:     .byte 40
cityBuf:    .byte 40

;------------------------------------------------------------------------------
; Program memory / reset vector
;------------------------------------------------------------------------------
.cseg
.org 0x0000
    rjmp START

.org 0x00F6

;==============================================================================
; START
;==============================================================================
START:
    ; Initialize stack pointer
    ldi r16, low(RAMEND)
    out CPU_SPL, r16
    ldi r16, high(RAMEND)
    out CPU_SPH, r16

    ; Configure the CPU for the 32 MHz internal oscillator
    rcall setCPUClkto32Mwith32MIntOsc

    ; Configure USARTE0 for 115200 baud, 8-N-1
    rcall initUSARTE0

;==============================================================================
; MAIN APPLICATION
;==============================================================================
MAIN:
    ; Ask whether the user wants to continue
    ldi ZL, low(msg1 << 1)
    ldi ZH, high(msg1 << 1)
    ldi r16, 1
    rcall putsUSARTE0
    rcall newline

READ_YN:
    ; Read the response into ynBuf
    ldi XL, low(ynBuf)
    ldi XH, high(ynBuf)
    ldi r18, YN_MAX
    rcall getsUSARTE0
    rcall newline

    ; Evaluate the first character
    lds r22, ynBuf

    ; Empty input: ask again
    cpi r22, 0
    breq READ_YN

    ; n / N: stop
    cpi r22, 'n'
    breq NO_MORE
    cpi r22, 'N'
    breq NO_MORE

    ; y / Y: continue
    cpi r22, 'y'
    breq CONTINUE
    cpi r22, 'Y'
    breq CONTINUE

    ; Any other response: ask again
    rjmp READ_YN

CONTINUE:
    ; Ask for university name
    ldi ZL, low(msg2 << 1)
    ldi ZH, high(msg2 << 1)
    ldi r16, 1
    rcall putsUSARTE0

    ldi XL, low(uniBuf)
    ldi XH, high(uniBuf)
    ldi r18, UNI_MAX
    rcall getsUSARTE0
    rcall newline

    ; Display university name
    ldi ZL, low(msg3 << 1)
    ldi ZH, high(msg3 << 1)
    ldi r16, 1
    rcall putsUSARTE0

    clr r16
    ldi ZL, low(uniBuf)
    ldi ZH, high(uniBuf)
    rcall putsUSARTE0
    rcall newline

    ; Ask for city
    ldi ZL, low(msg4 << 1)
    ldi ZH, high(msg4 << 1)
    ldi r16, 1
    rcall putsUSARTE0

    ldi XL, low(cityBuf)
    ldi XH, high(cityBuf)
    ldi r18, CITY_MAX
    rcall getsUSARTE0
    rcall newline

    ; Display city
    ldi ZL, low(msg5 << 1)
    ldi ZH, high(msg5 << 1)
    ldi r16, 1
    rcall putsUSARTE0

    clr r16
    ldi ZL, low(cityBuf)
    ldi ZH, high(cityBuf)
    rcall putsUSARTE0
    rcall newline

    rjmp MAIN

NO_MORE:
    ldi ZL, low(msgNoMore << 1)
    ldi ZH, high(msgNoMore << 1)
    ldi r16, 1
    rcall putsUSARTE0
    rcall newline

STOP:
    rjmp STOP

;==============================================================================
; initUSARTE0
;
; Configures USARTE0 for:
;   - PE2 = RX
;   - PE3 = TX
;   - 115200 baud
;   - 8 data bits, no parity, 1 stop bit
;==============================================================================
initUSARTE0:
    ; PE3 output (TX); remaining PORTE pins, including PE2 RX, remain inputs
    ldi r20, 0x08
    sts PORTE_DIR, r20

    ; Baud-rate settings used by the original EE 235 implementation
    ldi r20, 16
    sts USARTE0_BAUDCTRLA, r20

    clr r20
    sts USARTE0_BAUDCTRLB, r20
    sts USARTE0_CTRLA, r20

    ; Enable receiver and transmitter
    ldi r20, 0x18
    sts USARTE0_CTRLB, r20

    ; Asynchronous 8-bit, no parity, 1 stop bit
    ldi r20, 0x03
    sts USARTE0_CTRLC, r20

    ret

;==============================================================================
; putchUSARTE0
;
; Input:
;   r16 = character to transmit
;==============================================================================
putchUSARTE0:
WAIT_TX:
    lds r20, USARTE0_STATUS
    sbrs r20, USART_DREIF_bp
    rjmp WAIT_TX

    sts USARTE0_DATA, r16
    ret

;==============================================================================
; putsUSARTE0
;
; Inputs:
;   r16 = 1 -> string in program memory
;   r16 = 0 -> string in data memory
;   Z        -> null-terminated string
;==============================================================================
putsUSARTE0:
    cpi r16, 1
    breq PSTRX

DSTRX:
    ld r16, Z+
    cpi r16, 0
    breq DONEX
    rcall putchUSARTE0
    rjmp DSTRX

PSTRX:
    lpm r16, Z+
    cpi r16, 0
    breq DONEX
    rcall putchUSARTE0
    rjmp PSTRX

DONEX:
    ret

;==============================================================================
; newline
;
; Sends carriage return + line feed.
;==============================================================================
newline:
    ldi r16, 0x0D
    rcall putchUSARTE0

    ldi r16, 0x0A
    rcall putchUSARTE0
    ret

;==============================================================================
; getchUSARTE0
;
; Output:
;   r22 = received character
;==============================================================================
getchUSARTE0:
WAIT_RX:
    lds r20, USARTE0_STATUS
    sbrs r20, USART_RXCIF_bp
    rjmp WAIT_RX

    lds r22, USARTE0_DATA
    ret

;==============================================================================
; getsUSARTE0
;
; Inputs:
;   X   -> destination buffer
;   r18 -> maximum number of characters, excluding null terminator
;
; Behavior:
;   - Reads until ENTER
;   - Echoes accepted characters
;   - Supports terminal backspace
;   - Prevents writes beyond the destination buffer
;   - Always null-terminates the resulting string
;==============================================================================
getsUSARTE0:
    clr r19                     ; character count = 0

READ_CHAR:
    rcall getchUSARTE0

    ; ENTER terminates the string
    cpi r22, ENTER
    breq INPUT_DONE

    ; Handle backspace separately
    cpi r22, BACKSPACE
    breq HANDLE_BACKSPACE

    ; Ignore new characters after the buffer is full
    cp r19, r18
    brsh READ_CHAR

    ; Store and echo the received character
    st X+, r22
    inc r19

    mov r16, r22
    rcall putchUSARTE0
    rjmp READ_CHAR

HANDLE_BACKSPACE:
    ; Nothing to erase if the buffer is already empty
    tst r19
    breq READ_CHAR

    ; Move the buffer pointer back and reduce the character count
    sbiw XL, 1
    dec r19

    ; Standard terminal erase sequence: BS, space, BS
    ldi r16, BACKSPACE
    rcall putchUSARTE0
    ldi r16, SPACE
    rcall putchUSARTE0
    ldi r16, BACKSPACE
    rcall putchUSARTE0

    rjmp READ_CHAR

INPUT_DONE:
    ; Null-terminate the string
    clr r20
    st X, r20
    ret

;==============================================================================
; setCPUClkto32Mwith32MIntOsc
;
; Enables the ATxmega128A1U 32 MHz internal oscillator and selects it as the
; system clock with no prescaling.
;==============================================================================
setCPUClkto32Mwith32MIntOsc:
    ; Enable the 32 MHz internal oscillator
    lds r16, OSC_CTRL
    ori r16, OSC_RC32MEN_bm
    sts OSC_CTRL, r16

WAIT_32M:
    ; Wait until the oscillator is stable
    lds r16, OSC_STATUS
    sbrs r16, OSC_RC32MRDY_bp
    rjmp WAIT_32M

    ; Protected write: select 32 MHz RC oscillator
    ldi r16, CCP_IOREG_gc
    sts CPU_CCP, r16
    ldi r16, CLK_SCLKSEL_RC32M_gc
    sts CLK_CTRL, r16

    ; Protected write: disable clock prescaling
    ldi r16, CCP_IOREG_gc
    sts CPU_CCP, r16
    clr r16
    sts CLK_PSCTRL, r16

    ret

;==============================================================================
; Messages stored in program memory
;==============================================================================
msg1:
    .db "Do you want to continue? y/n", 0

msg2:
    .db "What is your university? ", 0

msg3:
    .db "My university is ", 0

msg4:
    .db "What is the city at which your university is located? ", 0

msg5:
    .db "My university is located at ", 0

msgNoMore:
    .db "No more", 0