In this chapter, we introduced you to GPIOs and how they work. You constructed a circuit that uses GPIOs for both input and output, and then you did some basic testing on the circuit to ensure that the circuit was constructed properly and that the kernel is able to interact with the circuit via the filesystem. You also learned about the portions of the PacktHAL init.{ro.hardware}.rc file and BB-PACKTPUB-00A0.dtbo Device Tree overlay that are responsible for configuring GPIOs and making them available for your app's use.

We showed you how to add PacktHAL into a newly created app project and how to build PacktHAL using the Android NDK. Then, you learned how JNI integrates PacktHAL into your Java app via JNI wrapper functions and explored how each GPIO function of PacktHAL is called and used from within an app.

In the next chapter, you will learn how to integrate I2C bus devices into your apps and begin interacting with hardware that is much more sophisticated than the basic on/off logic...