Task State

Now let's say we want to change the previous program to count how many times the USB cable (port J3) has been connected and disconnected.

✅ Open the src/bin/resource.rs file.

Tasks run from start to finish, like functions, in response to events. To preserve some state between the different executions of a task we can add a resource to the task. In RTIC, resources are the mechanism used to share data between different tasks in a memory safe manner but they can also be used to hold task state.

To get the desired behavior we'll want to store some counter in the state of the on_power_event task.

The starter code shows the syntax to declare a resource, the Resources struct, and the syntax to associate a resource to a task, the resources list in the #[task] attribute.

In the starter code a resource is used to move (by value) the POWER peripheral from init to the on_power_event task. The POWER peripheral then becomes part of the state of the on_power_event task and can be persistently accessed throughout calls to on_power_event() through a reference. The resources of a task are available via the Context argument of the task.

To elaborate more on this move action: in the svd2rust API, peripheral types like POWER are singletons (only a single instance of the type can ever exist). The consequence of this design is that holding a peripheral instance, like POWER, by value means that the function (or task) has exclusive access, or ownership, over the peripheral. This is the case of the init function: it owns the POWER peripheral but then transfers ownership over it to a task using the resource initialization mechanism.

We have moved the POWER peripheral into the task because we want to clear the USBDETECTED interrupt flag after it has been set by the hardware. If we miss this step the on_power_event task (function) will be called again once it returns and then again and again and again (ad infinitum).

Also note that in the starter code the idle function has been modified. Pay attention to the logs when you run the starter code.

✅ Modify the program so that it prints the number of times the USB cable has been connected to the DK every time the cable is connected, as shown below.

(..)
INFO:resource -- on_power_event: cable connected 1 time
(..)
INFO:resource -- on_power_event: cable connected 2 times
(..)
INFO:resource -- on_power_event: cable connected 3 times

You can find a solution to this exercise in the resource-solution.rs file.