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2.4. Fault handling

Faults are a subset of exceptions, see Exception model. All faults result in the HardFault exception being taken or cause Lockup if they occur in the NMI or HardFault handler. The faults are:

  • execution of an SVC instruction at a priority equal or higher than SVCall

  • execution of a BKPT instruction without a debugger attached

  • a system-generated bus error on a load or store

  • execution of an instruction from an XN memory address

  • execution of an instruction from a location for which the system generates a bus fault

  • a system-generated bus error on a vector fetch

  • execution of an Undefined instruction

  • execution of an instruction when not in Thumb-State as a result of the T-bit being previously cleared to 0

  • an attempted load or store to an unaligned address

  • if the device implements the MPU, an MPU fault because of a privilege violation or an attempt to access an unmanaged region.


Only Reset and NMI can preempt the fixed priority HardFault handler. A HardFault can preempt any exception other than Reset, NMI, or another HardFault.

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