Programming Embedded Systems, 2nd Edition

eCos Interrupt Handling

As explained in Chapter 8, it is important to keep interrupt processing down to a minimum so that other (potentially higher-priority) interrupts in the system can be serviced, and so high-priority tasks can run. Thus, programmers typically divide interrupt handling into two categories: a short ISR, and a more leisurely DSR.

The question of how to make the split—what to put in the ISR and what to put in the DSR—depends on the application and the processor. Basically, you should defer whatever you can. OS functions that might block cannot be called from an ISR or DSR. And unlike many other operating systems, eCos in particular does not allow an ISR even to signal a semaphore via a nonblocking call. In eCos, semaphore signaling must be done via a call from the DSR.

In order to get a better understanding of the split interrupt handling scheme, take a look at the following eCos example, which shows the use of an interrupt to handle the timing for the Blinking LED program. In this example, a semaphore is used to signal a task from the interrupt when it is time to toggle the LED.

The initialization sequence between the hardware and software in the cyg_user_start function is important. For example, you wouldn’t want to call timerInit to start the timer interrupt before you have created and installed an interrupt handler for the timer.

First, the LED is initialized. Next, the semaphore, ledToggleSemaphore, which signals the blinkLedTask when it is time to toggle ...