Ballistic Chronograph

/*
  Oscillator: 2, 3 on uC
 Input 1 -> Pin8 (5 on uC)    B2
 Input 2 -> Pin7  (6 on uC)   A7
 Taster  ->  Pin6  (7 on uC)  A6

 7Segments Cathodes -> Pin0-2  (12 - 10 on uC)  + Output8 of Shift Register

 888.8
 012 O8

 ShiftRegister -> CL: Latch: Pin3(10) Pin4(9)  DATA: Pin5(8)

 IO:9   -Cathodes = 5 -Inputs = 2


 */
const byte dataPin = 3;
const byte latchPin = 4;
const byte clockPin = 5;

const byte segments[] = {
  252, 144, 122, 218, 150, 206, 238, 152, 254, 222, 2, 108};  //Replace with 7 segment numbers, 255 = '-', 254 = 'C'

void setup()
{
  DDRA = 0b10111111;
  pinMode(7, OUTPUT);    //DECIMAL POINT
  //pinMode(6, INPUT);
  pinMode(8, INPUT);
}

void writeSegment(byte segment, byte value)
{
  byte out = segments[value];
  byte sethigh = false;

  for(byte i = 0; i < 3; i++)
    digitalWrite(i, LOW);

  if(segment == 3)
    bitSet(out, 0);            //replace according to wiring
  else
    sethigh = true;

  shiftOut(dataPin, clockPin, MSBFIRST, out);

  digitalWrite(latchPin, HIGH);
  if(sethigh)digitalWrite(segment, HIGH);
  digitalWrite(latchPin, LOW);
}

void loop()
{
  writeSegment(0, 10);    //show -

  unsigned long bef = 0;
  unsigned long af = 0;

  while(!(PINB & 0b100));      //wait for B2  Input 1
  bef = micros();

  PORTA = 0;

  while(!(PINA & 0b1000000));    //wait for A6 Input 2
  af = micros();

  digitalWrite(8, LOW);


  unsigned long xspeed = (1000000UL / (af - bef));    //10x the actual speed

  if(xspeed < 10000ul)
    digitalWrite(7, HIGH);      //decimal point
  else
    xspeed = xspeed / 10;
  byte digits[4];

  String s = String(xspeed);
  while(s.length() < 4)
    s = "0" + s;

  byte blinkb = 0;
  byte blinkstate = 0;

  if(xspeed > 9999)
    blinkb = true;

    for(int i = 0 ; i < 4; i++)                    //get digits
    {
      digits[i] = s[i] - '0';
    }


  if(!blinkb)
  {
    while(true)                      // show digits
    {
      for(byte i = 0; i < 4; i++)
      {
        writeSegment(i, digits[i]);
        delayMicroseconds(1500);                              //~200 Hz update rate
      }
    }
  }
  else
  {
    while(true)                       // show digits
    {
      for(byte b = 0; b < 30; b++)
      {
        for(byte i = 0; i < 4; i++)
        {
          writeSegment(i, digits[i]);
          delayMicroseconds(1500);                              //~200 Hz update rate
        }
      }
      blinkstate = !blinkstate;
      digitalWrite(7, blinkstate);

    }
  }
}