Hi everyone!
I'm trying to send a byte from a 16F84 to a 16F874
connected by two lines. For reasons i won't go into i
have to do the communication protocol in software, so
please don't suggest alternative hardware solutions or
use of usart etc! I'm using two PIC boards to test the
software, but it doesn't work as i thought.
I've pasted the gist of the code i'm using: can
anybody see something wrong?

I am sorry for the length of the e-mail.
Thanks in advance for any help!
Regards,
Theodore


;******* CODE FOR 16F84 -- TRANSMIT
;******* PORT/PIN MAP
;        porta,rx: receive line, connects to porta,tx_m of
16F874 (Master)
;        porta,tx: transmit line, connects to porta,rx_m of
16F874

        bcf     porta,tx        ; send request to transmit (a handshake
of sorts)
                                ; active low: tx line high when idle -- clear for
request
        call    delay           ; delay subroutine
        btfsc   porta,rx        ; check Master ready to receive
        goto    $-1
        movf    portb,w         ; the state of portb is to be
transmitted
        movwf   dataout
        movlw   8               ; 8 bit transmission
        movwf   bitcount
        bsf     porta,tx        ; send start bit (high)
bit_out
        call    delay
        call    delay           ; "baud" rate
        rrf     dataout,f
        btfsc   status,c        ; if c high, send high bit
        bsf     porta,tx
        btfss   status,c        ; if c low, send low
        bcf     porta,tx
        decfsz  bitcount,f
        goto    bit_out

        bcf     porta,tx        ; send stop bit (low)
        return

;******* CODE FOR 16F874 -- RECEIVE
        btfss   porta,rx_m
        goto    $-1
        bcf     porta,tx_m      ; ready to receive
        movlw   8               ;8 bit reception
        movwf   bitcount
        btfss   porta,rx_m      ; wait for start bit, high
        goto    $-1
        call    delay           ; extra delay before starting to receive
byte
bit_in
        call    delay
        call    delay
        btfsc   porta,rx_dir
        bsf     byte_in,7      ; register to receive byte
        btfss   porta,rx_m
        bcf     byte_in,7
        rlf     byte_in,f
        decfsz  bitcount,f
        goto    bit_in

        btfsc   porta,rx_m
        goto    $-1             ; wait for stop bit
        bsf     porta,tx_m      ; acknowledge stop bit, set line high
again (high when idle)
        return




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