Arduino nano crashes - please advise

[jhinshel]

@gcjr

what is purpose of the following?

ISR(WDT_vect, ISR_NAKED)
{
  // Setup a pointer to the program counter. It goes in a register so we
  // don't mess up the stack.
  upStack = (uint8_t*)SP;

  // The stack pointer on the AVR micro points to the next available location
  // so we want to go back one location to get the first byte of the address
  // pushed onto the stack when the interrupt was triggered. There will be
  // PROGRAM_COUNTER_SIZE bytes there.
  ++upStack;

  // Newer versions of GCC don't like when naked functions call regular functions
  // so we use call an assembly gate function instead
  __asm__ __volatile__ (
    "call appMon_asm_gate \n"
  );
}

this code includes the interrupt service routine for the watch dog. It comes from

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