[TI-H] Re: PIC16F84 Interfacing

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[TI-H] Re: PIC16F84 Interfacing



On Thu, 09 Aug 2001 20:39:48 -0300 Carlos Becker <cjb@softhome.net>
writes:
> When i connect everything, before executing the program, everything 
> slows 
> down like if the pic was sending data.
>   I think this is an important factor to consider. I don't know what 
> I am 
> doing wrong but I'm sure it's in the RecvByte routine, see below:

Are you setting both lines to be inputs before calling RecvByte?
Could there be something wrong with the cable/calc?

It sounds like at least one line is being pulled low and the calculator
is trying to recieve data.
Hmm: Are you connecting before or after applying power is applied to the
pic?  How does your routine deal with the initial noise generated by
connecting/disconnecting the cable?

I've actually been able to recieve from a TI86 with *no* reisistors at
all, maybe this code could help?
Unfortunately, I no longer know how I set up for it, but I think you need
to set PORTA & TRISA to LINK_MASK (both bits as inputs, both high). 
(actually, I'm not completely sure this is the version that works, but
it's all I have now...)

Something else you might want to try (if it fits with your application)
is to switch to using two of bits 4-7 of portb.  PORTB has internal
pull-up resistors (if you clear /RBPU) and interrupt-on-change with those
four bits.

LINK_PORT       equ PORTA
LINK_TRIS       equ TRISA
RED             equ 2
WHITE           equ 3
LINK_MASK       equ B'01100'

link_bits_left  equ H'10'
link_byte       equ H'11'


link_get_byte:
        movlw   LINK_TRIS
        movwf   FSR
        movlw   8
        movwf   link_bits_left
link_get_loop:
        call    link_get_bit
        iorwf   link_bits_left,w
        btfss   STATUS,Z
        goto    link_get_loop
        retlw   0

link_get_bit:
        movlw   LINK_MASK
        andwf   LINK_PORT,w
        btfsc   STATUS,Z
        goto    link_error
        btfss   LINK_PORT,RED
        goto    link_get_0
        btfss   LINK_PORT,WHITE
        goto    link_get_1
        retlw   0

link_error:                     ; wait for end of error
        movlw   LINK_MASK
        andwf   LINK_PORT,w
        btfsc   STATUS,Z
        goto    link_error
        movlw   8               ; restart this byte
        movwf   link_bits_left
        retlw   1               ; signal an error

link_get_0:
        bcf     LINK_PORT,WHITE ; send other line low
        bcf     INDF,WHITE
        rlf     link_byte,f
        bcf     link_byte,0     ; record the bit
link_g0w:
        btfss   LINK_PORT,RED   ; wait for return to high
        goto    link_g0w
        bsf     INDF,WHITE      ; return other line to high (float)
        retlw   0

link_get_1:
        bcf     LINK_PORT,RED   ; other line low
        bcf     INDF,RED
        rlf     link_byte,f
        bsf     link_byte,0     ; record bit
link_g1w:
        btfss   LINK_PORT,WHITE ; wait for return to high
        goto    link_g1w
        bsf     INDF,RED        ; send return to high
        retlw   0


-josh
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