Arduinoで文字列を数値に変換 ref: http://qiita.com/yjiro0403/items/02ba1b3214c9dcfc443d

file0.txt

void setup() {
  Serial.begin(9600);
}

void loop() {
  if(Serial.available()){
    byte var = Serial.read();
    int val = var - 0x30;
    if (val >= 0 && val < 9) {
      Serial.println(val);
    }
  }
}

file1.txt

int getNum() {
  byte var = 'b';
  int num=0;

  // 'a' is for debug
  while (var != '\t' && var != '\0' && var != 'a') {
    if(Serial.available()){
        var = Serial.read();
        int i = var - 0x30;
        if (i > 0 && i < 9){
          num = (num*10) + i;
        }
        //Serial.println(num);
      }
    }

  return num;
}