Programming Embedded Systems, 2nd Edition

Learn How to Communicate

Now that you know the names and addresses of the memory and peripherals attached to the processor, it is time to learn how to communicate with the peripherals. There are two basic communication techniques: polling and interrupts. In either case, the processor usually issues some sort of command to the device by writing—by way of the memory or I/O space—particular data values to particular addresses within the device, and then waits for the device to complete the assigned task. For example, the processor might ask a timer to count down from 1,000 to 0. Once the countdown begins, the processor is interested in just one thing: is the timer finished counting yet?

If polling is used, the processor repeatedly checks to see whether the task has been completed. This is analogous to the small child who repeatedly asks, “Are we there yet?” throughout a long trip. Like the child, the processor spends a large amount of otherwise useful time asking the question and getting a negative response. To implement polling in software, you need only create a loop that reads the status register of the device in question. Here is an example:

    do
    {
        /* Play games, read, listen to music, etc. */
        ...

        /* Poll to see if we're there yet. */
        status = areWeThereYet();

    } while (status == NO);

The second communication technique uses interrupts. An interrupt is an asynchronous electrical signal from a peripheral to the processor. Interrupts can be generated from peripherals external or internal ...