/* Tiny GPS Speedometer v3

   David Johnson-Davies - www.technoblogy.com - 9th March 2017
   ATtiny85 @ 8MHz (external crystal; BOD disabled)
   
   CC BY 4.0
   Licensed under a Creative Commons Attribution 4.0 International license: 
   http://creativecommons.org/licenses/by/4.0/
*/

// Constants
const int DataIn = 0;
const int ClockPin = 2;
const int DataOut = 1;

// Display **********************************************

// Seven segment display setup
const int Clear_Display = 0x76;
const int Decimal_Control = 0x77;
const int Cursor_Control = 0x79;
const int Brightness_Control = 0x7A;

void SendByte(char data) {
  shiftOut(DataOut, ClockPin, MSBFIRST, data);
}

// Clear display
void ClearDisplay () {
  SendByte(Clear_Display);
}

// Display a four/five digit decimal number
void Display (int number) {
  boolean dig = false;
  int j;
  SendByte(Cursor_Control); SendByte(0);
  SendByte(Decimal_Control);
  if (number>9999) { j=10000; SendByte(0x04); }
  else { j=1000; SendByte(0x02); }
  for (int d=0; d<4 ; d++) {
    int i = (number/j) % 10;
    if (!i && !dig && j>100) SendByte(' ');
    else { SendByte(i); dig = true; }
    j=j/10;
  }
}

// USI UART **********************************************

unsigned char ReverseByte (unsigned char x) {
  x = ((x >> 1) & 0x55) | ((x << 1) & 0xaa);
  x = ((x >> 2) & 0x33) | ((x << 2) & 0xcc);
  x = ((x >> 4) & 0x0f) | ((x << 4) & 0xf0);
  return x;    
}

// Initialise USI for UART reception.
void InitialiseUSI (void) {  
  DDRB &= ~(1<<DataIn);           // Define DI as input
  USICR = 0;                      // Disable USI.
  GIFR = 1<<PCIF;                 // Clear pin change interrupt flag.
  GIMSK |= 1<<PCIE;               // Enable pin change interrupts
  PCMSK |= 1<<PCINT0;             // Enable pin change on pin 0
}

// Pin change interrupt detects start of UART reseption.
ISR (PCINT0_vect) {
  if (PINB & 1<<DataIn) return;   // Ignore if DI is high
  GIMSK &= ~(1<<PCIE);            // Disable pin change interrupts
  TCCR0A = 2<<WGM00;              // Timer in CTC mode
  TCCR0B = 2<<CS00;               // Set prescaler to /8
  OCR0A = 103;                    // Shift every (103+1)*8 cycles
  TCNT0 = 206;                    // Start counting from (256-52+2)
  // Enable USI OVF interrupt, and select Timer0 compare match as USI Clock source:
  USICR = 1<<USIOIE | 0<<USIWM0 | 1<<USICS0;
  USISR = 1<<USIOIF | 8;          // Clear USI OVF flag, and set counter
}

// USI overflow interrupt indicates we've received a serial byte
ISR (USI_OVF_vect) {
  USICR  =  0;                    // Disable USI         
  GIFR = 1<<PCIF;                 // Clear pin change interrupt flag.
  GIMSK |= 1<<PCIE;               // Enable pin change interrupts again
  char c = USIDR;
  sei();                          // Allow interrupts
  ParseGPS(ReverseByte(c));
}

// GPS Parser **********************************************

// Example: $GPRMC,194509.000,A,4042.6142,N,07400.4168,W,2.03,5.84,160412,,,A*77
char fmt[]="$GPRMC,dddtdd.ddm,A,eeae.eeee,l,eeeae.eeee,o,djdk,djdc,dddy??,,,?*??";

int state = 0;
unsigned int temp;
long ltmp;

// GPS variables
volatile unsigned int Time, Msecs, Knots, Course, Date;
volatile long Lat, Long;
volatile boolean Fix;

void ParseGPS (char c) {
  if (c == '$') { state = 0; temp = 0; }
  char mode = fmt[state++];
  // If received character matches format string, or format is '?' - return
  if ((mode == c) || (mode == '?')) return;
  // d=decimal digit
  char d = c - '0';
  if (mode == 'd') temp = temp*10 + d;
  // e=long decimal digit
  else if (mode == 'e') ltmp = (ltmp<<3) + (ltmp<<1) + d; // ltmp = ltmp*10 + d;
  // a=angular measure
  else if (mode == 'a') ltmp = (ltmp<<2) + (ltmp<<1) + d; // ltmp = ltmp*6 + d;
  // t=Time - hhmm
  else if (mode == 't') { Time = temp*10 + d; temp = 0; }
  // m=Millisecs
  else if (mode == 'm') { Msecs = temp*10 + d; ltmp=0; }
  // l=Latitude - in minutes*1000
  else if (mode == 'l') { if (c == 'N') Lat = ltmp; else Lat = -ltmp; ltmp = 0; }
  // o=Longitude - in minutes*1000
  else if (mode == 'o') { if (c == 'E') Long = ltmp; else Long = -ltmp; temp = 0; }
   // j/k=Speed - in knots*100
  else if (mode == 'j') { if (c != '.') { temp = temp*10 + d; state--; } }
  else if (mode == 'k') { Knots = temp*10 + d; temp = 0; }
  // c=Course (Track) - in degrees*100
  else if (mode == 'c') { Course = temp*10 + d; temp = 0; }
  // y=Date - ddmm
  else if (mode == 'y') { Date = temp*10 + d ; Fix = 1; }
  else state = 0;
}

// Setup **********************************************

void setup (void) {
  pinMode(ClockPin, OUTPUT);
  pinMode(DataOut, OUTPUT);
  digitalWrite(ClockPin, LOW);
  digitalWrite(DataOut, LOW);
  ClearDisplay();
  for (int i=0; i<4; i++) SendByte('-');
  InitialiseUSI();
}

void loop (void) {
  long temp;
  while(!Fix);
  cli(); temp = Knots; sei();
  Fix = 0;
  // Continuously display the speed in mph
  Display(temp*38/33);
}