I wrote:

> > Actually, sharing the oscillator doesn't buy you any speed; you
> > can go just as fast with separate clocks as you can with a single
> > clock.

and John Payson <PICLIST@MITVMA.MIT.EDU> replied:

> Sure it buys you speed.  Once you've gotten the two PICs
> synchronized, they can exchange unlimitted amounts of data,
> bidirectionally, at a rate of 1 byte per 17 clocks (if the ports are
> located favorably).  By contrast, if you use non-synchronized clocks
> you couldn't do better than 3 clocks/bit, and even then you'd have
> to resync yourself at most every 30 bits or so.

John:

You can synchronize two PICs to within one instruction-clock cycle of
each other WHETHER OR NOT they share an oscillator.

While a shared oscillator obviates the need for resynchronization, a
two-oscillator system can also do without (frequent) resyncs so long
as the two oscillators are closely matched in frequency.

I assume that your "17-cycle" code (I count 18 cycles, but
that includes the load/store) looks something like this:

    BSF     STATUS,RP0   ;Setup to send data.
    CLRF    TRISB^080H   ;

    ....

    MOVWF   PORTB        ;Send a byte.
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;
    RLF     PORTB        ;

    BSF     TRISB^080H,7 ;Prepare to receive a byte.

    RRF     PORTB        ;Receive a byte and store it in W.
    RRF     PORTB        ;
    RRF     PORTB        ;
    RRF     PORTB        ;
    RRF     PORTB        ;
    RRF     PORTB        ;
    RRF     PORTB        ;
    RRF     PORTB,W      ;

Is that correct?  If so, there's a faster way:  Since the "favorable"
arrangement of the ports requires that the PORTB0-6 pins be unused
outputs (or outputs that are tied to devices that don't care whether
they're madly toggled whenever you receive a byte, and how common is
THAT?), you might as well dedicate the ENTIRE port to data transfer.

If you do that, your "transfer a byte in each direction" code
becomes:

    BSF     STATUS,RP0   ;Setup to send data.
    CLRF    TRISB^080H   ;

    ....

    MOVWF   PORTB        ;Send a byte.
    COMF    TRISB^080H   ;Setup to receive data.
    MOVF    PORTB,W      ;Receive a byte.

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499

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