Task State

Let’s fix our interrupt handler so that it doesn’t run continuously. To do that, we need to read the character from the UART, causing the FIFO to empty and the interrupt to stop being asserted.

To do that we need to have access to our UART in our interrupt handler.

✅ Open the same stm32-code/standalone-app/src/bin/standalone-rtic-event.rs file as before.

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 the UART in the state of the usart1_handler 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 USART1 driver from init to the usart1_handler task. The USART1 driver then becomes part of the state of the usart1_handler task and can be persistently accessed throughout calls to usart1_handler() through a mutable reference. The resources of a task are available via the Context argument of the task.

To elaborate more on this move action: we only want to have a single USART1 driver because it represents ownership of some hardware. The consequence of this design is that having ownership of an object like usart::Driver means that the function (or task) has exclusive access, or ownership, over the peripheral. This is the case of the init function: it is given the USART driver as part of the NonSecureBoard object and then transfers ownership of it over to a task, using the resource initialization mechanism.

We have moved the USART1 driver into the task because we want to clear the interrupt by reading a character from the UART. If we miss this step the usart1_handler task (function) will be called again once it returns and then again and again and again (ad infinitum).

✅ Modify the program so that it reads characters from the UART, and logs what it read

[INFO] USART1 IRQ!
[INFO] < 0x31
[INFO] < 0x30
[INFO] USART1 IRQ!
[INFO] < 0x48