/*  Init string for Ublox Neo-7M to produce 1 MHz output  
 *  (c) PA3FYM, V0.1 Feb 2019
 *  
 *  Note: this code is written for an ATTiny13a but of course
 *  also runs on an Arduino (Nano, Pro Mini, Blah etc).
 *    
 *  This code may be used (by you) for non commercial purposes, 
 *  and/or intentions, and/or actions as long as you refer to me.
 *  
 *  For commercial use mail me at my QRZ.com address.
*/

// ATTiny13a settings: 1.2 MHz internal osc, no BOD

// PB0 connected to RX pin of Ublox GPS
// PB3 VCO voltage input (10k in series) V(T)CXO side!
// PB4 LED output (active high), plade 1K in series

#define sbi(x,y) x |= _BV(y)                 // set bit
#define cbi(x,y) x &= ~(_BV(y))              // clear bit
#define is_high(x,y) (x & _BV(y) == _BV(y))  // check if the y'th bit of register 'x' is high
                                                   
#define gps 0   // PB0 is connected to GPS
#define vco 3   // PB3 is VCO read pin
#define led 4   // PB4 lock led
#define tick 94 // bit time in us , tweak to 4800 Hz with loop0 as main loop

uint16_t last = 0;

// 1 MHz          
const byte gps_data[] PROGMEM = {0xB5,0x62,0x06,0x31,0x20,0x00,0x00,0x00,0x00,0x00,0x32,0x00,0x00,0x00,0x40,0x42,
                                 0x0F,0x00,0x80,0x84,0x1E,0x00,0x00,0x00,0x00,0x80,0x00,0x00,0x00,0x00,0x00,0x00,
                                 0x00,0x00,0xEB,0x00,0x00,0x00,0xA7,0x8F};  


// 6MHz
//const byte gps_data[] PROGMEM =  {0xB5,0x62,0x06,0x31,0x20,0x00,0x00,0x01,0x00,0x00,0x32,0x00,0x00,0x00,0x80,0x8D,
//                                  0x5B,0x00,0x80,0x84,0x1E,0x00,0x00,0x00,0x00,0x80,0x00,0x00,0x00,0x00,0x00,0x00,
//                                  0x00,0x00,0xEB,0x08,0x00,0x00,0x87,0x0B};

// 8 MHz ref
//const byte gps_data[] PROGMEM = {0xB5,0x62,0x06,0x31,0x20,0x00,0x00,0x00,0x00,0x00,0x32,0x00,0x00,0x00,0x00,0x12,
//                                 0x7A,0x00,0x00,0x12,0x7A,0x00,0x00,0x00,0x00,0x80,0x00,0x00,0x00,0x80,0x00,0x00,
//                                 0x00,0x00,0xEF,0x00,0x00,0x00,0x90,0x03}; 

void send_byte(uint8_t b) {  // sends a byte from PB0, rate: 9600 baud 8N1

   cbi(PORTB,gps);     // send start bit
   _delay_us(tick);    // 1 tick delay

   for (uint8_t i = 0; i < 8; i++) {   // a byte has 8 bits, LSB first

      if (b & 0x01) sbi(PORTB,gps); else cbi(PORTB,gps);
      _delay_us(tick);
      b >>= 1;    // shift right for next bit
      }

   sbi(PORTB,gps);    // make line high 
   _delay_us(tick);   // stopbit is one (1) tick
   _delay_us(tick);   // no parity is also (1) tick
}

void setup () {   

#ifdef __AVR_ATtiny13__
  analogReference(0);          //this line is essential for the Attiny13 when reading analog input
#endif

// define data direction registers for PORTB (1 = output, 0 = input)
DDRB  |= 0b00000001; // PB0 output
PORTB |= 0b00000001; // PB0 high (for TTL rs232)


for (uint8_t i = 0; i<5; i++) {    // to be safe send data several times
    _delay_ms(2000);
    
    for (uint8_t i = 0; i < sizeof(gps_data); i++) {
      send_byte(pgm_read_byte_near(gps_data+i));  // program Ublox GPS with string
      _delay_us(tick);                            // one (1) tick between characters to prevent framing errors
      }
    }
} // setup

void loop0() {   // calibration loop. Measure frequency on PB0, should be as close to 4800 Hz as possible
  _delay_us(tick);
  PINB = bit(gps);  // toggle PB0
}

void loop() { // main loop
  
  uint16_t sample = 0;
  
  for (uint8_t i = 0; i < 8; i++) {   // take 8 samples
    sample += analogRead(vco);        // read ADC
    _delay_ms(10);                    // some delay
    }
    
  sample /= 8;     // average samples

  if (abs(last - sample) < 4) sbi(PORTB,led); // if VCO voltage is stable enough, there is a lock
  else cbi(PORTB,led);                        // otherwise no lock, switch off LED
  last = sample;                              // update last sample
     
  _delay_ms(500);                              // delay
  
} // loop