/*
  SERIAL COMMUNICATION (RS232)
  ============================

  Using 9600 baud, one stop bit, no parity.
  Baudrate 9600 baud => 104.167 microsec. per bit
  TMR0 counts each 8th microsec. => 13.02 steps per bit
*/

#define _4_MHz    /* 4 MHz system clock */

#pragma bit RS232_out @ PORTA.0
#pragma bit RS232_in  @ PORTA.1

#ifdef _4_MHz
 #define   TimeStartbit    4
 #define   BitTimeIncr     13
#endif

char bitCount, limit;
char serialData;

#ifdef _4_MHz
 #ifdef UseTMR0
   #define delayStart   limit = TMR0;
   #define delayOneBit      \
     limit += BitTimeIncr;  \
     while ( limit != TMR0) \
         ;
 #else
   #define delayStart   /* empty */
   #define delayOneBit  {      \
        char ti;               \
        ti = 30;               \
        do ; while( --ti > 0); \
        nop();                 \
     }  /* total: 5 + 30*3 - 1 + 1 + 9 \
           = 104 microsec. at 4 MHz */
 #endif
#endif

void sendData( char dout)
/* sends one byte */
{
    RS232_out = 0;  /* startbit */
    delayStart
    for ( bitCount = 9; ; bitCount--)  {
        delayOneBit
        if ( bitCount == 0)
            break;
        Carry = 1;  /* incl. stopbit */
        dout = rr( dout);
        RS232_out = Carry;
    }
}

#define NText 12
char text( char W)
{
    skip(W);
   #pragma return[NText] = "Hello world!"
}


void main( void)
{
    char i;

    PORTA = 0x03; // xxxx xx11
    TRISA = 0xFE; // xxxx xx10
    OPTION = 2;   // prescaler divide by 8

    for ( i = 0; i < NText; i++)
        sendData( text(i));  /* text string */

  WaitNextIO:
    while ( RS232_in == 1)
        ;

  Start_RS232:
    /* sampling once per bit */
    TMR0 = 0;
    limit = TimeStartbit;
    while ( limit != TMR0)
        ;
    if ( RS232_in == 1)
        goto StartBitError;

    bitCount = 8;
    do  {
        limit += BitTimeIncr;
        while ( limit != TMR0)
            ;
        Carry = RS232_in;
        serialData = rr( serialData);  /* rotate carry */
    } while ( -- bitCount > 0);

    limit += BitTimeIncr;
    while ( limit != TMR0)
        ;
    if ( RS232_in == 0)
        goto StopBitError;

    /* 'serialData' can be processed here */
    /* NOTE: limited time (40 - 50 microsec.) */
    goto WaitNextIO;

  StopBitError:
    /* RS232 stopbit error,
       waiting while line is low */
    while ( RS232_in == 0)
        ;
  StartBitError:
    goto WaitNextIO;
}