Friday, October 17, 2008

Embedded Systems Time constraints

There are two types of time constraints for embedded systems: stringent and mild.
Stringent time constraints require that the system react in a predefined time frame; otherwise, ca tastrophic events happen. Take for instance a factory where workers have to handle materials being cut by large equipment. As a safety precaution, optical detectors are placed around the blades to detect the presence of the specially colored gloves used by the workers. When the system is alerted that a worker’s hand is in danger, it must stop the blades immediately. It can’t wait for some disk I/O operation involving reading data in from a Linux swap device (for example, swapping back in the memory storing safety management task code) or for some running task to relinquish the CPU. This system has stringent time requirements; it is a hard real-time system. If it doesn’t respond, somebody might lose an arm. Device failure modes don’t get much more painful than that.

Streaming audio systems and consumer devices such as MP3 players and cell phones would also qualify as having stringent requirements, because any transient lagging in audio is usually perceived as bothersome by the users, and failure to contact a cellular tower within a certain time will result in an active call being dropped. Yet, these latter systems would mostly qualify as having soft real-time requirements, because the failure of the application to perform in a timely fashion all the time isn’t catastrophic, as it would be for a hard real-time system. In other words, although infrequent failures will be tolerated—a call being dropped once in a while is an annoying frustration users already live with—the system should be designed to have stringent time requirements. Soft real-time requirements are often the target of embedded Linux vendors that don’t want the (potential) liability of guaranteeing hard real-time but are confident in the abilities of their product to provide, for example, reliable cell phone base-band GSM call management capabilities.

Mild time constraints vary a lot in requirements, but they generally apply to systems where timely responsiveness isn’t necessarily critical. If an automated teller takes 10 more seconds to complete a transaction, it’s generally not problematic (of course, at some point, the user is going to give up on the system and assume it’s never going to respond). The same is true for a PDA that takes a certain number of seconds to start an application. The extra time may make the system seem slow, but it won’t affect the end result. Nonetheless, it’s important that the system make the user aware that it is, in fact, doing something with this time and hasn’t gone out for lunch. Nothing is more frustrating than not knowing whether a system is still working or has crashed.

Source of information : OReilly Building Embedded Linux Systems

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