Setting interrupt priority levels

The mesh stack has different interrupt priority levels defined by its architecture. It includes various interrupt priority level restrictions that are imposed on the application when using the stack to avoid race condition scenarios and ensure that all operations execute in non-overlapping manner.

Table of contents

• Interrupt priority level list and configuration
• Interrupt priority level restrictions
• Changes in application IRQ priority from nRF5 SDK for Mesh v1.0.0
  • Run application and mesh event handling in low priority interrupt
  • Run application and mesh event handling in main loop

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Interrupt priority level list and configuration

The mesh stack runs in the following interrupt priorities:

• IRQ priority 0 (radio interrupt priority) – for the time-critical parts of the stack.
• Same IRQ priority as the user application's low priority code – for the less time-critical parts of the stack.
  • This can be any priority lower than the SoftDevice API call priority, but it must be the same as the SoftDevice event IRQ handler. When using the nRF5 SDK, this will normally be NRF_MESH_IRQ_PRIORITY_LOWEST (APP_IRQ_PRIORITY_LOWEST). However, if you use the nRF5 SDK's app_scheduler module to run the application from the main loop, the low priority parts of the mesh stack should also run from the main loop.

The mesh network must know in which IRQ priority the user application is running. To know this, nrf_mesh_init_params_t (used by nrf_mesh_init) now has an irq_priority field. Normally, it must be set to NRF_MESH_IRQ_PRIORITY_LOWEST, or NRF_MESH_IRQ_PRIORITY_THREAD if running in the main loop. You can also use the APP_IRQ_PRIORITY_* defines in the nRF5 SDK.

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Interrupt priority level restrictions

You must ensure that no mesh API functions are called from an IRQ priority other than the one specified in the configuration. The only exception is calling the initialization-related functions before calling mesh_stack_start(). Breaking this rule can cause unpredictable behavior.

The mesh stack uses the SoftDevice NVIC APIs for accessing NVIC. In particular, the functions sd_nvic_critical_region_enter() and sd_nvic_critical_region_exit() are used to prevent the mesh event handler from being started in some cases. The application must not use these functions in any callback functions provided by the application to the mesh stack.

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Changes in application IRQ priority from nRF5 SDK for Mesh v1.0.0

Starting with the nRF5 SDK for Mesh v1.0.0, all event processing must run in the same IRQ priority.

Depending on your application, you can choose to:

• Run application and mesh event handling in low priority interrupt
• Run application and mesh event handling in main loop

Before the release of the nRF5 SDK for Mesh v1.0.0, applications made using nRF5 SDK for Mesh v0.10.0-Alpha or an older version of the Mesh SDK would normally run low priority event handling in several IRQ priorities:

• Mesh events were processed in the main loop by simply calling nrf_mesh_process():

  while (true)
  {
   (void)nrf_mesh_process();
  }

• SoftDevice events were processed in the SoftDevice event IRQ handler. For most SoftDevice versions, the IRQ handler would be running in APP_IRQ_PRIORITY_LOWEST by default. Bearer events were handled in APP_IRQ_PRIORITY_LOW.

Running application and mesh event handling in low priority interrupt

When running in a low priority interrupt, the application can no longer call nrf_mesh_process(). You must change the main loop. For example:

while (true)
{
 (void)sd_app_evt_wait();
}

Running application and mesh event handling in main loop

You can use the application scheduler from the nRF5 SDK to run the application and the mesh event handling.

To add app_scheduler to the application and initialize it:

1. In the example directory, edit CMakeList.txt.
2. Add the following module to the executable target sources:

   "${SDK_ROOT}/components/libraries/scheduler/app_scheduler.c"

3. Add the target include directory that corresponds to the executable target source added:

   "${SDK_ROOT}/components/libraries/scheduler"

   Note

   If you are using SEGGER Embedded Studio, use one of the following options:

   • Regenerate the SEGGER project.
   • Manually add the source file and the include directory to your existing project.

4. In the example include directory, add the following defines in app_config.h:

   #define APP_SCHEDULER_ENABLED 1
   #define NRF_SDH_DISPATCH_MODEL 1
   #define APP_TIMER_CONFIG_USE_SCHEDULER 1

   This enables the application scheduler and changes the event dispatch model for the SoftDevice handler and the application timer.
5. In the application main(), initialize app_scheduler:

   APP_SCHED_INIT(APP_SCHED_EVENT_SIZE, APP_SCHED_QUEUE_SIZE);

Note

• APP_SCHED_EVENT_SIZE must be greater than or equal to APP_TIMER_SCHED_EVENT_DATA_SIZE, because the app timer also needs to use the scheduler.
• APP_SCHED_QUEUE_SIZE must be sufficiently large. This is required to be able to hold many events (user events, SoftDevice events, multiple events from various user or stack timers, or both) that could be generated between two successive calls to the app_sched_execute() from the main loop. The value of 32 could be reasonable for typical use cases, when there is a minimal or no additional user code within the main loop.

When running from the main loop, nrf_mesh_process() still needs to be called, but as soon as there are no more events to be processed, the CPU can be put to sleep:

while (true)
{
 app_sched_execute();
 bool done = nrf_mesh_process();
 if (done)
 {
 sd_app_evt_wait();
 }
}

If you need to use simple_hal.c module to handle button press actions, use the scheduler to execute the button_event_handler() function from the thread priority. To do this, use an intermediate function as a button event handler, and post the event to the secondary function using the scheduler. For example:

static void button_event_handler_app_sched(void * p_event_data, uint16_t event_size)
{
 uint8_t * button_number = (uint8_t *) p_event_data;
 NRF_MESH_ASSERT(event_size == 1);
 button_event_handler(*button_number);
}

static void button_event_handler_irq(uint32_t button_number)
{
 static uint8_t bt_num;

 bt_num = button_number;
 app_sched_event_put((void *)&bt_num, sizeof(bt_num), button_event_handler_app_sched);
}

...

static void initialize(void)
{
...
#if BUTTON_BOARD
 ERROR_CHECK(hal_buttons_init(button_event_handler_irq));
#endif
...
}