12.2.4.1 Key-Argument
To protect the module functions against runaway code themselves, a key is required as one of the input arguments. The key-argument will make sure that runaway code entering the function without a function call will be rejected before inflicting any damage. The argument is simply set to be the bitwise inverse of the module flag, i.e.
system_peripheral_<lock_state>(SYSTEM_PERIPHERAL_<module>,
~SYSTEM_PERIPHERAL_<module>);
Where the lock state can be either lock or unlock, and module refer to the peripheral that is to be locked/unlocked.