cycles.
Active input or output I/O devices use interrupts to communicate with real-time applications. Every time an active input device needs to send data or notification of an event to a real-time application, the device generates an interrupt. The interrupt triggers an ISR that executes the minimum code needed to handle the input. If a lot of processing is required, the ISR usually hands off the process to an associated task through an inter-task communication mechanism.
Similarly, active output devices also generate interrupts when they need to communicate with applications. However, interrupts from active output devices are generated when they are ready to receive the next piece of data or notification of some event from the application. The interrupts trigger the appropriate ISR that hands off the required processing to an associated task using an inter-task communication mechanism.
The diagram for both an active I/O device acting as an input or an output to an application and for a device generating interrupts in a synchronous or asynchronous manner is similar to the one illustrated in Figure 14.5.
Figure 14.5: General communication mechanisms for active I/O devices.
Some typical tasks that can result from identifying an active I/O device in a real-time application are listed in Table 14.1.
Table 14.1: Common tasks that interface with active I/O devices.
| Task Type | Description |
|---|---|
| Asynchronous Active Device I/O Task | Assigned to active I/O devices that generate aperiodic interrupts or whose operation is asynchronous with respect to other I/O devices. |
| Synchronous Active Device I/O Task | Assigned to active I/O devices that generate periodic interrupts or whose operation is synchronous with respect to other I/O devices. |
| Resource Control Device I/O Task | Assigned for controlling the access to a shared I/O device or a group of devices. |
| Event Dispatch Device I/O Task | Assigned for dispatching events to other tasks from one or more I/O devices. |
Recommendation 1: Assign separate tasks for separate active asynchronous I/O devices. Active I/O devices that interact with real-time applications do so at their own rate. Each hardware device that uses interrupts to communicate with an application and whose operation is asynchronous with respect to other I/O devices should be considered to have their own separate tasks.
Recommendation 2: Combine tasks for I/O devices that generate infrequent interrupts having long deadlines. In the initial design, each active I/O device can have a separate task assigned to handle processing. Sometimes, however, combining the processing of two I/O devices into a single task makes sense. For example, if two I/O devices generate aperiodic or asynchronous interrupts infrequently and have relatively long deadlines, a single task might suffice.
Recommendation 3: Assign separate tasks to devices that have different input and output rates. Generally speaking, a task that handles a device with a high I/O frequency should have a higher task priority than a task that handles a device with a lower frequency. Higher I/O frequency implies shorter, allowable processing time. However, the importance of the I/O operation, and the consequences of delayed I/O, should be taken into account when assigning task priorities with respect to I/O frequency.
Recommendation 4: Assign higher priorities to tasks associated with interrupt-generating devices. A task that needs to interface with a particular I/O device must be set to a high-enough priority level so that the task can keep up with the device. This requirement exists because the task's execution speed is usually constrained by the speed of the interrupts that an associated I/O device generates and not necessarily the processor on which the application is running.
For I/O devices that generate periodic interrupts, the interrupt period dictates how long a task must handle processing. If the period is very short, tasks associated with these devices need to be set at high priorities.
For I/O devices that generate aperiodic interrupts, it can be difficult to predict how long an associated task will have to process the request before the next interrupt comes in. In some cases, interrupts can occur rapidly. In other cases, however, the interrupts can occur with longer time intervals between them. A rule of thumb is that these types of tasks need their priorities set high to ensure that all interrupt requests can be handled, including ones that occur within short time intervals. If an associated task's priority is set too low, the task might not be able to execute fast enough to meet the hardware device's needs.
Recommendation 5: Assign a resource control task for controlling access to I/O devices. Sometimes multiple tasks need to access a single hardware I/O device. In this case, the device can only serve one task at a time; otherwise, data may be lost or corrupted. An efficient approach is to assign a
This resource control task is not limited to working with just one I/O device. In some cases, one resource task can handle multiple requests that might need to be dispatched to one or more I/O devices.
Recommendation 6: Assign an event dispatch task for I/O device requests that need to be handed off to multiple tasks. Events or requests that come from an I/O device can be propagated across multiple tasks. A single task assigned as an event dispatch task can receive all requests from I/O devices and can dispatch them to the appropriate tasks accordingly.
Passive devices are different from active devices because passive devices do not generate interrupts. They sit passively until an application's task requests them to do something meaningful. Whether the request is for an input or an output, an application's task needs to initiate the event or data transfer sequence. The ways that tasks communicate with these devices is either by polling them in a periodic manner or by making a request whenever the task needs to perform input or output.
The diagram either for a passive I/O device acting as an input or an output to an application or for communicating with the application periodically or aperiodically is similar to the one illustrated in Figure 14.6.
Figure 14.6: General communication mechanisms for passive I/O devices.
Some typical tasks that can result from identifying a passive I/O device in a real-time application are listed in Table 14.2.
Table 14.2: Common tasks that interface with passive I/O devices.
| Task Type | Description |
|---|
