Hello all,
             Can anyone please help me to clarify several simple queries
regarding asynchronous serial operation using the built in USART module of
the PIC16C65. Firstly I am totally confused about the operation as described
in the data sheets;  for instance: under asynchronous USART operation (p101)
the data sheet states that the TXIF flag bit is set when the TXREG is empty
(i.e. when the data in the TXREG is moved into the TSR register) but the TXIF
bit in the PIR1 register implies that the TXIF bit is set when the transmit
buffer is full ? - (is the transmit buffer the same as the TXREG ?). The
USART operation described on p101 also states that the TXIF flag bit cannot
be cleared in software, whereas the information given on p41 regarding the
set up of the PIR1 register states that the TXIF bit must be cleared in
software ?. - A similar situation also exists regarding the RCIF bit ?
        Also in the set up of both the RCSTA and TXSTA registers, the data sheet
states that the I/O lines RC7/RX and RC6/TX are configured as USART pins when
the SPEN bit in the RCSTA register is set, and BOTH bit 6 and bit 7 of TRISC
are SET ? - Is this correct, only I thought that with bit 6 being TX (i.e. an
output pin) then TRISC bit 6 should be clear ?
        I have included below, two version of code which I have been struggling
with
for several weeks with no luck so far. Can anyone see anything wrong and
please point me in the right direction ?.

Many thanks:      Lee Hewitt (Manchester ENGLAND)

-----------------------------------------------------------------------

        TITLE           "UART with Interupts"
        SUBTITLE        "Version 1.0"


        LIST P=16c65, R=HEX
        include         "p16c65.inc"

        __config        _CP_OFF & _PWRTE_ON & _WDT_OFF & _XT_OSC


;macro definitions
push    macro
        movwf   w_copy          ;save w reg in Buffer
        swapf   w_copy,f        ;swap it
        swapf   STATUS,w        ;get status
        movwf   s_copy          ;save it
        endm
;
pop     macro
        swapf   s_copy,w        ;restore status
        movwf   STATUS          ;       /
        swapf   w_copy,w        ;restore W reg
        endm


;variables
        cblock  20
        w_copy                  ;saved W register for interrupt handler
        s_copy                  ;saved status register for int handler
        RX_BYTE
        TX_BYTE
        endc


        org     0

        goto    reset


        org     4

        push

        btfsc   PIR1,RCIF
        call    rx_int

        btfsc   PIR1,TXIF
        call    tx_int

        pop

        retfie


rx_int  movf    RCREG,w         ;Load received data into w reg
        movwf   RX_BYTE         ;Store received data in file reg REC_BYTE
        return


tx_int  movf    TX_BYTE,w       ;Load data for transmission
        movwf   TXREG           ;Transmit the data when TXREG is loaded
        return

reset:
        movlw   b'10000000'     ;Disable weak pull-ups on Port B
        option

        movlw   00h
        movwf   PORTA
        movlw   000h
        tris    PORTA

        movlw   00h
        movwf   PORTB
        movlw   00h
        tris    PORTB

        movlw   00h
        movwf   PORTC
        movlw   b'10000000'     ;pin 7, RX=i/p; pin 6, TX=o/p
        tris    PORTC

        movlw   00h
        movwf   PORTD
        movlw   TRISD

        movlw   00h
        movwf   PORTE
        movlw   TRISE

        movlw   b'11000000'     ;Enable GIE and PEIE interupts
        movwf   INTCON

        movlw   b'00110000'
        movwf   PIE1            ;Enable USART RX and TX interupts

        call    init_uart


loop    movlw   A'a'
        movwf   TX_BYTE
        goto    loop            ;Keep sending ASCII 'a'


init_uart
        bsf     STATUS,RP0      ;Select bank 1
        bcf     STATUS,RP1
        movlw   .5
        movwf   SPBRG           ;Select 9600 Baud
        movlw   0xA0
        movwf   TXSTA

        bcf     STATUS,RP0      ;Select page 0
        movlw   0x90
        movwf   RCSTA

        return

        end
-------------------------------------------------------------------

        TITLE           "UART Without Interupts"
        SUBTITLE        "Version 1.1"


        LIST P=16c65, R=HEX
        include         "p16c65.inc"


        __config        _CP_OFF & _PWRTE_ON & _WDT_OFF & _XT_OSC



;variables
        cblock  20
        RX_BYTE
        TX_BYTE
        endc


        org     0

        goto    reset

reset:
        movlw   b'10000000'     ;Disable weak pull-ups on Port B
        option

        movlw   00h
        movwf   PORTA
        movlw   000h
        tris    PORTA

        movlw   00h
        movwf   PORTB
        movlw   00h
        tris    PORTB

        movlw   00h
        movwf   PORTC
        movlw   b'10000000'     ;Pin 7, RX=i/p; pin 6, TX=o/p
        tris    PORTC

        movlw   00h
        movwf   PORTD
        movlw   TRISD

        movlw   00h
        movwf   PORTE
        movlw   TRISE

        clrf    INTCON          ;Disable all interupts

        clrf    PIE1            ;Disable all peripheral interupts
        clrf    PIE2            ;Dissable all CCP interupts


;Initialise USART
        bsf     STATUS,RP0      ;Select bank 1
        bcf     STATUS,RP1
        movlw   .5
        movwf   SPBRG           ;Select 9600 Baud
        movlw   0xA0
        movwf   TXSTA

        bcf     STATUS,RP0      ;Select page 0
        movlw   0x90
        movwf   RCSTA


loop    movlw   A'a'
        movwf   TX_BYTE

;poll for received byte
        btfsc   PIR1,RCIF
        movf    RCREG,w         ;Load received data into w reg
        movwf   RX_BYTE         ;Store received data in file reg REC_BYTE

;poll for transmit byte (TXREG full)
        btfsc   PIR1,TXIF
        movf    TX_BYTE,w       ;Load data for transmission
        movwf   TXREG           ;Transmit the data when TXREG is loaded

        end