If you need to refresh a screen every half second, then, depending on what other code is executed in your loop() function, the use of millis() , as in the Blink Without Delay sketch in your Examples folder, just will not cut it.
You would be better off looking at Timer interrupts. However, the subject requires much more depth to it than can adequately be addressed in a reply to your post, so I suggest that you do a Google search for Using Timer Interrupts with Arduino.
Good Luck with your project.