In this project, the software is really simple, since we just need a procedure that periodically reads the distance and then turn on and off the LEDs accordingly; however, some issues must be pointed out, especially about how to manage the LEDs and the differences between the two setups of the ultrasonic sensor.
Despite of what was presented in the previous chapter about the GPIO's management, it's important to point out that the Linux kernel has several kinds of devices, each one dedicated to a well-defined usage, and one of these special devices are the led devices, which is a particular type of devices that can be used to manage an LED with different triggers. A trigger is a sort of manager of the LED that can be programmed to work in a specific manner. Ok, it's better doing an example instead of trying to explain it!
First of all, we have to define the led devices by using a dedicated device tree as reported in the chapter_02/BB-LEDS-C2-00A0.dts...