Книга: Real-Time Concepts for Embedded Systems
8.3.3 Typical Uses of Event Registers
8.3.3 Typical Uses of Event Registers
Event registers are typically used for unidirectional activity synchronization. It is unidirectional because the issuer of the receive operation determines when activity synchronization should take place. Pending events in the event register do not change the execution state of the receiving task.
In following the diagram, at the time task 1 sends the event X to task 2, no effect occurs to the execution state of task 2 if task 2 has not yet attempted to receive the event.
No data is associated with an event when events are sent through the event register. Other mechanisms must be used when data needs to be conveyed along with an event. This lack of associated data can sometimes create difficulties because of the noncumulative nature of events in the event register. Therefore, the event register by itself is an inefficient mechanism if used beyond simple activity synchronization.
Another difficulty in using an event register is that it does not have a built-in mechanism for identifying the source of an event if multiple sources are possible. One way to overcome this problem is for a task to divide the event bits in the event register into subsets.
The task can then associate each subset with a known source. In this way, the task can identify the source of an event if each relative bit position of each subset is assigned to the same event type.
In Figure 8.9, an event register is divided into 4-bit groups. Each group is assigned to a source, regardless of whether it is a task or an ISR. Each bit of the group is assigned to an event type.
Figure 8.9: Identifying an event source.
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