Atmel Studio Dummy_Handler

Question

Occasionally I will get an unexpected interrupt, and my code will hang inside Dummy_Handler() in exceptions.c of the Atmel Studio Framework (ASF). I am using the ATSAM3X8E microcontroller of the Arduino Due.

void Dummy_Handler(void)
{
    while(1) {
    }
}

Any ideas how to determine which interrupt it was?

Of course I could replace this single handler with unique dummy handlers, one for each exception. (There are about fifty of them.) For example change each line in the same exceptions.c file:

void HardFault_Handler  ( void ) __attribute__ ((weak, alias("Dummy_Handler")));

to this

void HardFault_Handler  ( void ) __attribute__ ((weak, alias("Dummy_HardFault_Handler")));

Etc... Or try to reason how my code could have generated which interrupt. But who has that kind of time?

Answer 1

This MCU has an Interrupt Program Status Register that gives some clue as to source. ASF has wrapped it in a function __get_IPSR() in core_cmFunc.h:

uint32_t phantomISR = 9999;

void Dummy_Handler(void)
{
    while(1) {
        phantomISR = __get_IPSR();
    }
}

Then this global variable can be monitored at runtime. (In my case I paused the assembly code for this loop-of-death and saw the value 3 in the R3 register.) The Atmel MCU doc explains its value:

ISR_NUMBER
This is the number of the current exception:
0 = Thread mode
1 = Reserved
2 = NMI
3 = Hard fault
4 = Memory management fault
5 = Bus fault
6 = Usage fault
7-10 = Reserved
11 = SVCall
12 = Reserved for Debug
13 = Reserved
14 = PendSV
15 = SysTick
16 = IRQ0
45 = IRQ29

Both times this happened to me it was the Hard Fault, a kind of blue-screen-of-death for the Ardunio Due. So I also installed a Hard Fault handler of my own.

ISR(HardFault_Handler)
{
    while (1) {
    }
}

Also, detectable in debug mode by pausing. Of course the sequel is, what causes a Hard Fault? I'm guessing memory wipe or infinite recursion.