To prevent the application from accidentally disrupting the protocol stack in any way, the application sandbox also protects the peripherals used by the SoftDevice.
Protected peripheral registers are readable by the application. An attempt to perform a write to a protected peripheral register will result in a Hard Fault. See Error handling for more details on faults due to unallowed memory access. The peripherals are only protected when the SoftDevice is enabled; otherwise, they are available to the application. See Hardware peripherals with limited availability to the application for an overview of the peripherals with access restrictions due to the SoftDevice.