15.3.1 Interrupt Vector Usage

To field an interrupt, a function’s address must be placed at the appropriate address in one of the vector tables, with the function preserving any system resources that it uses. It must return to the foreground task using a RETFIE processor instruction. Interrupt functions may be written in C. When a C function is designated as an interrupt handler, the compiler arranges to preserve all the system resources that the compiler uses, and to return from the function using the appropriate instruction. The compiler can optionally arrange for the interrupt vector table to be populated with the interrupt function’s address.

To arrange for the compiler to fill in the interrupt vector to point to the interrupt function, name the function as denoted in the vector tables (see section 15.3.2 Interrupt Vector Tables. For example, the stack error vector will automatically be filled if the following function is defined:

void __attribute__((interrupt(auto_psv))) _StackError(void);

Note the use of the leading underscore. Similarly, the alternate stack error vector will automatically be filled if the following function is defined:

void __attribute__((interrupt(auto_psv))) _AltStackError(void);

Again, note the use of the leading underscore.

For all interrupt vectors without specific handlers, a default interrupt handler will be installed. The default interrupt handler is supplied by the linker and simply resets the device. An application may also provide a default interrupt handler by declaring an interrupt function with the name _DefaultInterrupt.

The last nine interrupt vectors in each table do not have predefined hardware functions. The vectors for these interrupts may be filled by using the names indicated in the vector tables (15.3.2 Interrupt Vector Tables), or you may use names more appropriate to the application, while still filling the appropriate vector entry by using the irq or altirq parameter of the interrupt attribute. For example, to specify that a function should use primary interrupt vector 52, use the following:

void __attribute__((interrupt(auto_psv, irq(52)))) MyIRQ(void);

Similarly, to specify that a function should use alternate interrupt vector 53, use the following:

void __attribute__((interrupt(auto_psv, altirq(52)))) MyAltIRQ(void);

The irq/altirq number can be one of the interrupt request numbers 45 to 53. If the irq parameter of the interrupt attribute is used, the compiler creates the external symbol name __Interruptn, where n is the vector number. Therefore, the C identifiers _Interrupt45 through _Interrupt53 are reserved by the compiler. In the same way, if the altirq parameter of the interrupt attribute is used, the compiler creates the external symbol name __AltInterruptn, where n is the vector number. Therefore, the C identifiers _AltInterrupt45 through _AltInterrupt53 are reserved by the compiler.