Instructional Material: Exercise: Timing Considerations

Printer-friendly version

When programming robots (or any other real-time system), it is very important to understand how the speed realm of the robot differs from your own. When we look at our robot program, we see the while loop that we use to perform the read-environment/make-decision/act repeating pattern. We tend to think of this loop as happening slowly or at least as we might think of going through these operations.

The reality is that the robot will execute the while loop in our program thousands of times per second. The robot operates much faster than you think. This can lead to bugs in your programs in some situations.

Consider the example code below. The purpose of the program is to perform two actions controlled by the gamepad buttons. The program will count each time your press button A and on each press of button B, toggle a boolean variable from true to false. Download the program in Android Studio and test it on your robot.

So what happened? When you pressed the A button the immediate result is the program counted hundreds or thousands of button presses. Why? Because while the A button press seemed to you to be quick, on the program's time scale the button was down for many repeats of the while loop, and it counted each loop. Same for the B button. The setting of the boolean value bButton seems erratic and definately not a toggle each time the button is pressed. Again, due to the fact that your idea of a button press and the program's idea of a button press are quite different.

What do we do about this? We have to write code to handle this situation and convert the many button presses seen by the while loop into one logical press and act only when the one logical press happends. Here is the example program modified to handle the timing problems:

Download this program into Android studio and try it out. You should see the button presses now handled correctly.

The use of the variables aButtonPressed and bButtonPressed to track the state of the button is called latching.

 

Material Type: 
Lab Activity
Education Level: 
HW Platform: 
SW Platform: 
Interactivity Style: 
Copyright (c) 2013 by the author. All rights reserved.