Giao tiếp PIC với PC bằng USART

[rmitoday]

Dear các bro. Sao mình không mở một cái tutorial về giao tiếp PIC với PC bằng UASART nhỉ. Em làm rồi mà lúc chạy lúc không. Chẳng hiểu sao. OSC đã setup đúng, baud rate đã setup đúng.

        list P = 16F877

        include "P16f877.inc"

; ------------------
; CONFIGURATION FUSE
; ------------------
;
        __CONFIG _CP_OFF & _WRT_ENABLE_ON & _XT_OSC & _WDT_ON & _PWRTE_OFF & _BODEN_ON & _LVP_ON & _CPD_OFF & _DEBUG_OFF

; CRYSTAL SPEED = 4000000Hz
;
        CBLOCK 0x20

dataL

        ENDC
;
; -------------
; PROGRAM START
; -------------
;
        org 0x0000              ; startup = 0000h
;
Start   movlw 0x00              ; setup ports
        movwf PORTA
        movlw 0x00
        movwf PORTB
        movlw 0x40
        movwf PORTC
        movlw 0x00
        movwf PORTD
        movlw 0x00
        movwf PORTE
        bsf STATUS,RP0          ; RAM Page 1
        movlw 0x00
        movwf TRISA
        movlw 0x00
        movwf TRISB
        movlw 0x80
        movwf TRISC
        movlw 0xFF
        movwf TRISD
        movlw 0xFF
        movwf TRISE
;
; -------------------------
; SET ANALOG/DIGITAL INPUTS
; -------------------------
;
        movlw 0x06              ; all digital
        movwf ADCON1
;
; ------------------------------------
; SET BAUD RATE TO COMMUNICATE WITH PC
; ------------------------------------
; Boot Baud Rate = 19200, No Parity, 1 Stop Bit
;
        movlw 0x0C              ; 19200 baud
        movwf SPBRG
        movlw b'00100100'       ; brgh = high (2)
        movwf TXSTA             ; enable Async Transmission, set brgh
        movlw b'10010000'       ; enable Async Reception
        bcf STATUS,RP0          ; RAM Page 0
        movwf RCSTA
;
; ------------------------------------
; PROVIDE A SETTLING TIME FOR START UP
; ------------------------------------
;
        clrf dataL
settle  decfsz dataL,F
        goto settle
;
        movf RCREG,W
        movf RCREG,W
        movf RCREG,W            ; flush receive buffer

Loop    call Receive            ; wait for a char
        movwf TXREG             ; echo data back to sender
        goto Loop
;
; ----------------------------
; RECEIVE CHARACTER FROM RS232
; ----------------------------
; This routine does not return until a character is received.
;
Receive btfss PIR1,RCIF         ; (5) check for received data
        goto Receive
;
        movf RCREG,W
        return
;
; ------------------------------------
; WAIT UNTIL RS232 IS FINISHED SENDING
; ------------------------------------
;
TransWt bsf STATUS,RP0
WtHere  btfss TXSTA,TRMT                ; (1) transmission is complete if hi
        goto WtHere
;
        clrf STATUS             ; RAM Page 0
        return
;
;
        end