Here is the code for software 2nd serial port by RB0 interrupt, it works
in the background completely and was tested and used on PIC16F876



/***********************************************************************
****
*               INTERRUPT SERVICE ROUTINES
************************************************************************
***/
interrupt isr()
{
        // TIMER0 ISR
        if(T0IF =3D=3D 1)
        {
            if(startBitFlag =3D=3D 1)            // proceed only after
start bit                                                        // has
been received
            {
                // Read serial data=20
                TMR0 =3D (byte)(255 - bitLength);// re-initialise Timer0
to                                                               //
interrupt middle of next bit
//(bit length is 52uSec for 19200
// Baud rate
                if(bitCounter < 9)  // start bit not counted coz it was
detected
// by the interrupt
                {
                    rxShift =3D (rxShift >> 1) | (RxData << 7);=09
                }
                bitCounter++;
                if(bitCounter =3D=3D 9)
                {
                    bitCounter =3D 0;

                    // a byte has been received in rxShift Process it!
                }
// RB0 INTERRUPT=20
    if(INTF =3D=3D 1)
    if(startBitFlag =3D=3D 0)                           // only do if
start
// bit is 0
    {
        TMR0 =3D (byte)(255 - bitLength * 1.5); // set TMR0 to
// interrupt at middle
// of first bit to read it
      T0IE =3D 1;                                               // Enable Timer1
Interrupt
        INTF =3D 0;
        INTE =3D 0;                                             //
disable further RB0
// interrupts till stop bit occurs
        startBitFlag =3D 1;                     // we are in middle of
receiving data
                                                // so enable Timer1
interrupt
    }
// OTHER INTERRUPTS
=09
    if(RCIF)
    {
//        receiveBuffer =3D RCREG;
//        receivebufferfull =3D 1;
    }
}

-----Original Message-----
From: Hubba [mailto:junk@GRYDHOLT.DK]=20
Sent: Thursday, March 21, 2002 2:22 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]:Dual USART PIC

> > > code available for a software UAR that uses the RB0 interrupt
> >
> > You might want to check out the I/O section at www.piclist.com
> > There are interrupt and interruptless code samples
> >
> > I plan to use RB0 interrupts but can't say how soon it will
> > before I have any presentable code. Still pencil-sketching
>
> Jinx,
> I think he wants to *only* use RB0 ints to receive each "bit".  This
would
> be impossible without using a timer, of some sort, to figure out how
many
> consecutive bits you didn't get an interrupt for.  IIRC, if you
trigger on
> the start bit, you (not specifically you Jinx, but you know what I
mean
> ;-D) wouldn't even be guaranteed an interrupt on the stop bit.  I
don't
see
> any "feasible" way to receive serial data using *only* RB0/INT's.
>
> If your gonna have to use a timer anyway, then you would just use
RB0/INT
to
> get the first bit, and TMR0 ints to trigger interrupts at the
appropriate
> time to receive the data bits and stop bit.

If you want to retain RB0/INT for other stuff, you should be able to use
RA0/T0CKI pin as described in Microchip Application note AN555. Please
note
that I am a newbie, so I have not tested it, nor am I sure that this pin
is
available on all common PICs.

Best regards,

Jacob Grydholt Jensen

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