FAST Interrupt Service Routine

The above sequence assumes that the ISR signals or sends a message to a task. However, in many cases, the ISR may not need to notify a task and can simply perform all of its work within the ISR (assuming it can be done quickly). In this case, the ISR will appear as shown in Listing 9-2.

Listing - Fast ISRs with µC/OS-III

          MyShortISR:                                                      (1) 
              Save enough registers as needed by the ISR;                  (2) 
              Clear interrupting device;                                   (3) 
              DO NOT re-enable interrupts;                                 (4) 
              Call user ISR;                                               (5) 
              Restore the saved CPU registers;                             (6) 
              Return from interrupt;                                       (7)

(1) As mentioned above, an ISR is typically written in assembly language. MyShortISR corresponds to the name of the handler that will handle the interrupting device.

(2) Here, you save sufficient registers as required to handle the ISR.

(3) The user probably needs to clear the interrupting device to prevent it from generating the same interrupt once the ISR returns.

(4) Do not re-enable interrupts at this point since another interrupt could make µC/OS-III calls, forcing a context switch to a higher-priority task. This means that the above ISR would complete, but at a much later time and, possible cause other complications.

(5) Now you can take care of the interrupting device in assembly language or call a C function, if necessary.

(6) Once finished, simply restore the saved CPU registers.

(7) Perform a return from interrupt to resume the interrupted task.