Freq      Received HI Duty
50 Hz     49%
200 Hz    49%
500 Hz    48%
1 kHz     47%
2 kHz     43%
5 kHz     37% 
10 kHz    26% 
12 kHz    20-25% 
16 kHz    bad! 

//=============================================================================
//   SEND_RF_BYTE
//=============================================================================
void send_rf_byte(unsigned char txdat)
{
  //-------------------------------------------------------
  // This is a pulse period encoded system to send a byte to RF module.
  // Bits are sent MSB first. Each byte sends 9 pulses (makes 8 periods).
  // Timing;
  //   HI pulse width; always 80uS
  //   0 bit, LO width; 20uS (total 0 bit pulse period 100uS)
  //   1 bit, LO width; 70uS (total 1 bit pulse period 150uS)
  //   space between bytes, LO width; 170uS (total space period 250uS)
  // (timings tested with 20MHz xtal PIC 18F, no pll)
  //-------------------------------------------------------
  unsigned char tbit;

  // make 250uS start bit first
  Delay_uS(170);      // 170uS LO
  LATC.F6 = 1;
  Delay_uS(80-1);     // 80uS HI
  LATC.F6 = 0;

  // now loop and send 8 bits
  for(tbit=0; tbit<8; tbit++)
  {
    Delay_uS(20-1);             // default 0 bit LO period is 20uS
    if(txdat.F7) Delay_uS(50);  // increase the LO period if is a 1 bit!
    LATC.F6 = 1;
    Delay_uS(80-1);             // 80uS HI pulse
    LATC.F6 = 0;
    txdat <<= 1;                // roll data byte left to get next bit
  }
}
//-----------------------------------------------------------------------------

unsigned char rxdat[10];  // (global var) holds received RF bytes

//=============================================================================
//   RECEIVE_RF_PACKET
//=============================================================================
void receive_rf_packet(void)
{
  //-------------------------------------------------------
  // This function receives an RF packet of bytes in my pulse period
  // encoded format. The packet must have 10 valid contiguous bytes
  // or the function will not exit. There is no timeout feature, but could be added.
  // global variable; unsigned char rxdat[10] holds the 10 byte result.
  // Note! TMR0 is running at 500kHz, so 200uS = 100 TMR0 ticks
  //-------------------------------------------------------
  unsigned char rrp_data;
  unsigned char rrp_period;
  unsigned char rrp_bits;
  unsigned char rrp_bytes;

  rrp_bytes = 0;
  while(rrp_bytes < 10)   // loop until it has received 10 contiguous RF bytes
  {
    //-----------------------------------------
    // wait for a start pulse >200uS
    while(1)
    {
      while(!PORTC.F7) continue;    // wait for input / edge
      while(PORTC.F7) continue;     // wait for input \ edge
      rrp_period = TMR0L;           // grab the pulse period!
      TMR0L = 0;                    // and ready to record next period
      if(rrp_period < 100) rrp_bytes = 0;   // clear bytecount if still receiving noise
      else break;                   // exit if pulse was >200uS
    }

    //-----------------------------------------
    // now we had a start pulse, record 8 bits
    rrp_bits = 8;
    while(rrp_bits)
    {
      while(!PORTC.F7) continue;    // wait for input / edge
      while(PORTC.F7) continue;     // wait for input \ edge
      rrp_period = TMR0L;           // grab the pulse period!
      TMR0L = 0;                    // and ready to record next period

      if(rrp_period >= 100) break;  // if >=200uS, is unexpected start pulse!

      if(rrp_period < 61) rrp_data.F0 = 0;    // 61 = 122uS
      else                rrp_data.F0 = 1;
      rrp_data = (rrp_data << 1);   // save the good bit into rrp_data
      rrp_bits--;                   // and record 1 more good bit done
    }

    //-----------------------------------------
    // gets to here after 8 good bits OR after an error (unexpected start pulse)
    if(rrp_bits)      // if error
    {
      rrp_bytes = 0;  // reset bytes, must run from start of a new packet again!
    }
    else              // else 8 good bits were received
    {
      rxdat[rrp_bytes] = rrp_data;  // so save the received byte into array
      rrp_bytes++;                  // record another good byte was saved
    }
  }
}
//-----------------------------------------------------------------------------