/* IMPORTANT: I don't know if it's necessary, but I put 10k resistors on
 * the transmit and receive pins of the pic, just to be safe, and it works fine.
 */

/*  rs232 interface (serial port communications) without a level converter.
 *  Therefore microcontroller pins are connected DIRECTLY to the serial cable.
 *  Therefore inverted (from standard) logic is used.
 *  
 *  PIC PIN01 PORTA2 - TRANSMIT (receive at PC end of serial cable) white
 *  PIC PIN02 PORTA3 - RECEIVE (transmit at PC end of serial cable) orange
 */

/*  looking head on at the female connector of a serial mouse coard:
 *  i.e. the end that plugs into your pc
 *
 *  5 4 3 2 1      =      blue    null    orange      white    null
 *   9 8 7 6       =          null    null      yellow     null
 *
 *  the following are loose wires on the other end of the coard.
 *  i.e. where it would have been soldered to the mouse circuit board.
 *
 *  5 = blue   = gnd   = GND
 *  3 = orange = PC_td = PIC PIN02 PORTA3
 *  2 = white  = PC_rd = PIC PIN01 PORTA2
 *  7 = yellow = rts   = unused
 */

/* If you're changing the pinout of the code, these vars are the only changes you have to make.
 * Plus, you'll have to change the TRIS in your main program.
 */
static volatile bit RS232_TD @ (unsigned)&PORTA*8+2;
static volatile bit RS232_RD @ (unsigned)&PORTA*8+3;

///////////////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////////////

/**********************************************************************************************
 * Initialize the transmit pin, 
 * by ensuring it is low for an entire message cycle.
 */
void rs232_initialize()
{
  RS232_TD = 0;
  delay_ms(1);
}

/**********************************************************************************************
 * Send a single char out the transmit pin.
 */
void rs232_send_char(unsigned char c) 
{
  c = ~c;

  // start bit

  RS232_TD = 1;
  delay_us(100);

  // 8 data bits

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  RS232_TD = (c & 1);
  c = c >> 1;
  delay_us(98);

  // stop bit

  RS232_TD = 0;
  delay_us(100);
}

/**********************************************************************************************
 * Send a string of chars out the transmit pin.
 */
void rs232_send_string(const char * s)
{
  while(*s)
  { rs232_send_char(*s++); }
}

/**********************************************************************************************
 * Get a single char from the receive pin.
 * Then send that char over the transmit pin.
 */
unsigned char rs232_echo_char() 
{
  unsigned char c = 0;

  // start bit

  while (RS232_RD == 0) { continue; }
  delay_us(125);

  // 8 data bits

  if (RS232_RD == 0) { c = c | 0b00000001; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b00000010; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b00000100; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b00001000; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b00010000; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b00100000; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b01000000; }
  delay_us(97);

  if (RS232_RD == 0) { c = c | 0b10000000; }
  delay_us(97);

  // stop bit

  delay_us(100);

  rs232_send_char(c);

  return c;
}