Library internals

Before using the Multi-Protocol Service Layer (MPSL) library, make sure you are familiar with its design.


The following peripherals are owned by MPSL and must not be accessed directly by the application:

  • RTC0

  • TIMER0



  • TEMP

Limited access to these peripherals is provided through the MPSL Timeslot module and through other MPSL APIs.

Thread and Interrupt Safety

The MPSL library is not reentrant, so for thread-safe operation, some considerations must be made.

Interrupt configuration

MPSL enables interrupts for RTC0, TIMER0, POWER_CLOCK, and low_prio_irq. All other interrupts must be enabled and configured by the application. If the Timeslot API is used for RADIO access, the application is responsible for enabling and disabling the interrupt for RADIO.

Interrupts for RTC0, TIMER0, and RADIO must be configured for priority level 0 (MPSL_HIGH_IRQ_PRIORITY) by the application.

The following interrupts do not have real time requirements:

  • POWER_CLOCK interrupt

    It is up to the application to forward any clock related events to MPSL_IRQ_CLOCK_Handler() in lower priority. Irrelevant events are ignored, so the application is free to forward all events for the POWER_CLOCK interrupt.

  • low_prio_irq interrupt

    Low priority work is signaled by MPSL by pending the IRQ specified in the low_prio_irq argument to mpsl_init(). When this interrupt is triggered, mpsl_low_priority_process() should be called as soon as possible, at least within a couple of ms. The application should configure this interrupt priority lower than c:macro:MPSL_HIGH_IRQ_PRIORITY level (that is, a higher numerical value). The interrupt is enabled with mpsl_init() and disabled with mpsl_uninit() by MPSL.


Interaction of the MPSL library with protocol stacks is designed to run at two interrupt priority levels: one for the high priority handlers, and one for the low priority handler. Interaction of the MPSL library with the application happens in thread context and in the low priority handler.

High priority

The high priority handlers are mostly used for timing-critical operations related to radio or scheduling. Interrupting or delaying these handlers leads to undefined behavior.

Low priority

Low priority is used for background tasks that are not directly tied to the radio or scheduling. These tasks are designed in such a way that they can be interrupted by high priority code. The tasks are however not designed to be interrupted by other low priority tasks. Therefore, make sure that only one MPSL API function is called from the application at any time.

  • All protocol stacks using MPSL must be synchronized (i.e. not called concurrently) to avoid concurrent calls to MPSL functions.

  • Application must only call MPSL APIs from non-preemptible threads, or with interrupts disabled (e.g. during initialization).

  • The mpsl_low_priority_process() function should only be called from thread context, i.e. not directly from the software interrupt handler.

  • Alternatively, you can use synchronization primitives to ensure that no MPSL functions are called at the same time.

Other priorities

MPSL inititialization functions, like mpsl_init() and mpsl_uninit(), are not thread-safe. Do not call them while, for example, a protocol timeslot is in progress. This must be enforced by application and protocol stacks.

MPSL should be initialized before any protocol stack is enabled, and uninitialized after all protocol stacks have been disabled.