EM357-RTR datasheet(195/241 Pages) SILABS

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11.3. Non-Maskable Interrupt (NMI)

The non-maskable interrupt (NMI) is a special case. Despite being one of the 10 standard ARM® CortexTM-M3

NVIC interrupts, it is sourced from the Event Manager like a peripheral interrupt. The NMI has two second-level

sources; failure of the 24 MHz crystal and watchdog low water mark.

1. Failure of the 24 MHz crystal: If the EM35x's main clock, SYSCLK, is operating from the 24 MHz crystal

and the crystal fails, the EM35x detects the failure and automatically switches to the internal 12 MHz RC

clock. When this failure detection and switch has occurred, the EM35x triggers the CLK24M_FAIL second-

level interrupt, which then triggers the NMI.

2. Watchdog low water mark: If the EM35x's watchdog is active and the watchdog counter has not been reset

for nominally 1.792 s, the watchdog triggers the WATCHDOG_INT second-level interrupt, which then

triggers the NMI.

11.4. Faults

Four of the exceptions in the NVIC are faults: Hard Fault, Memory Fault, Bus Fault, and Usage Fault. Of these,

three (Hard Fault, Memory Fault, and Usage Fault) are standard ARM® CortexTM-M3 exceptions.

The Bus Fault, though, is derived from EM35x-specific sources. The Bus Fault sources are recorded in the

SCS_AFSR register. Note that it is possible for one access to set multiple SCS_AFSR bits. Also note that MPU

configurations could prevent most of these bus fault accesses from occurring, with the advantage that illegal writes

are made precise faults. The four bus faults are:



WRONGSIZE—Generated by an 8-bit or 16-bit read or write of an APB peripheral register. This fault can

also result from an unaligned 32-bit access.



PROTECTED—Generated by a user mode (unprivileged) write to a system APB or AHB peripheral or

protected RAM (see "5.2.2.3. RAM Memory Protection" on page 33).



RESERVED—Generated by a read or write to an address within an APB peripheral's 4 kB block range, but

the address is above the last physical register in that block range. Also generated by a read or write to an

address above the top of RAM or flash.



MISSED—Generated by a second SCS_AFSR fault. In practice, this bit is not seen since a second fault

also generates a hard fault, and the hard fault preempts the bus fault.

EM357-RTR 数据表(PDF) 195 Page - Silicon Laboratories