<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def halt(self):
 """ halt the core """

        self.notify(Notification(event=Target.EVENT_PRE_HALT, source=self, data=Target.HALT_REASON_USER))
        self.write_memory(CortexM.DHCSR, CortexM.DBGKEY | CortexM.C_DEBUGEN | CortexM.C_HALT)
        self.flush()
        self.notify(Notification(event=Target.EVENT_POST_HALT, source=self, data=Target.HALT_REASON_USER))