csv文件可以像其他文件一样创建。首先打开要写入的文件：

String header = "";
File file = new File("/storage/emulated/0/CharNotifyData.csv");
if (!file.exists()) {
    file.createNewFile();
    header = "Characteristic;Value\n";
}
FileWriter fw = new FileWriter(file.getAbsoluteFile(),true);
BufferedWriter bw = new BufferedWriter(fw);
bw.write(header);

这样写下你的价值观：

bw.write(characteristicName + ";" + value + "\n");

别忘了关闭文件：

bw.close();

编辑：尝试区分两种可能的状态。一种方式可能是这样的：

private void broadcastUpdate(final BluetoothGattCharacteristic characteristic, byte[] value){
   if(isCharacteristicNotifiable(characteristic)){
       String value_str =  bytesToHex(value);
       if(characteristic.getUuid().toString() == TX_CHARACTERISTIC){
          value_str =  bytestoformat(characteristic.getValue());
       }
       String[] line = value_str.split(" ");
       String[] fullLine = new String[4];
       for(int i = 0; i < 4; ++i){
          if(i < line.length){
             fullLine[i] = line[i];
          }else{
             fullLine[i] = "";
          }
       }
       Log.i("broadcastUpdate",  value_str);
       try {
           CSVWriter writer = new CSVWriter(new FileWriter(csv, true));
           writer.writeNext(line);
           writer.close();

       } catch (FileNotFoundException e) {
           e.printStackTrace();
       } catch (IOException e) {
           e.printStackTrace();
       }
   }
}