Now we are ready to take some definitions
“‘Real’-time usually means time as prescribed by external sources” For example the time struck by clock (however fast or late it might be). The timings generated by your requirements. You may like to call someone at mid-night and send him a picture. This external timing requirements imposed by the user is the real-time for the embedded system.
“Embodied phenomena are those that by their very nature occur in real time and real space” In other words, A number of systems coexist to discharge a specific function in real time Thus “A Real Time Embedded System” (RTES) is precisely the union of subsystems to discharge a specific task coherently. Hence forth we call them as RTES. RTES as a generic term may mean a wide variety of systems in the real world. However we will be concerned about them which use programmable devices such as microprocessors or microcontrollers and have specific functions. We shall characterize them as follows.
Characteristics of an Rtes
Here “single-functioned” means specific functions. The RTES is usually meant for very specific functions. Generally a special purpose microprocessor executes a program over and over again for a specific purpose. If the user wants to change the functionality, e.g. changing the mobile phone from conversation to camera mode or calculator mode the program gets flushed out and a new program is loaded which carries out the requisite function. These operations are monitored and controlled by an operating system called as Real Time Operating System (RTOS) which has much simpler complexity but more rigid constraints as compared to the conventional operating systems such as Micro Soft Windows and Unix etc.
The constraints on the design and marketability of RTES are more rigid than their non-realtime non-embedded counter parts. Time-domain constraints are the first thing that is taken care while developing such a system. Size, weight, power consumption and cost4 are the other major factors.
Reactive and Real Time
Many embedded systems must continually react to changes in the system’s environment and must compute certain results in real time without delay. For example, a car’s cruise controller continually monitors and reacts to speed and brake sensors. It must compute acceleration or deceleration amounts repeatedly within a limited time; a delayed computation could result in a failure to maintain control of the car. In contrast a desktop computer system typically focuses on computations, with relatively infrequent (from the computer’s perspective) reactions to input devices. In addition, a delay in those computations, while perhaps inconvenient to the computer user, typically does not result in a system failure.
Version 2 EE IIT, Kharagpur