Embedded Systems Design Europe - May 2008 - (Page 20)

software Definitions of three types of systems’ hardware layer, interface layer, and core software responsibility. System Hardware layer Interface layer Core software layer Elevator Open/Close door button Floor selection button Emergency stop button Floor indicator Floor position sensor(s) Elevator call button Door blocked sensor Display Wheel speed sensor Brake apply switch Brake pressure valves Keypad Monitor Printer Card reader Envelope retriever Digital I/O Serial communications bus Analog I/O Carry passengers to the requested floor Anti-lock brakes ATM Digital I/O Serial communications bus Analog I/O Digital I/O Serial/Parallel communications Prevent individual wheels from locking up during braking Process users money transactions. Print up receipt of transaction. Take users non-cash contributions. Verify users identity. Table 1 tion forces at the tire-to-road contact patch. The perceived system behavior is added stability, increased steering ability of the vehicle, and in most cases, reduced stopping distance. The hardware layer that interfaces to the core processor consists of four wheel-speed sensors and valves at each of the wheels to regulate the brake pressure. The hardware layer may also contain a communication interface for system diagnostics. The electronic control unit and the microcontroller(s) make up the processing intelligence for the system. Keep in mind that the interface layer is hardware dependent. The interface layer handles the methods for transferring the information necessary to the core software and for converting the core software’s calculated output signals into a usable signal by the various peripherals that drive the loads. TRANSLATION SPECIFICATION For example, an accelerometer sensor signal supplying a voltage that is proportional to an acceleration, given in either meters/sec2 or units of gravity (Gs), would have a speciﬁcation that translates voltage into acceleration. A one-volt signal might be equivalent to the sensor measuring 1G. The interface layer would convert a voltage from the 20 analog signal to a mathematical unit of measurement that best represents the desired signal. Since many systems are designed around a ﬁxed-point central processing unit, the interface layer should also be responsible for taking the “real” units signal and converting it into a scaled integer value that is easily processed by the microcontroller. The same approach can be taken for an output variable calculated by the core software. For this example, let’s assume that the core software calculates a desired current that ﬂows through a solenoid/load that is controlled within some desired tolerance. The output from the core software is a current value that is desired to ﬂow through the solenoid/load. The hardware-software interface circuit may consist of a pulse-width modulated output port from the microcontroller that regulates the current through the load by modulating the on/off time for the solenoid. Therefore, the output from the core software is in units of current (amps), which the interface software takes and converts into the required pulse-width modulated signal, in percent on-time or duty cycle, for achieving that current. Table 1 deﬁnes a few systems’ hardware layer, interface layer, and the core software responsibility. If the software architecture takes on the deﬁned structure that is illustrated in Figure 3, the core software can be designed to be independent of the hardware implementation. It’s still necessary to deﬁne the input/output ranges and resolution requirements for processing the data. This is especially important when designing a system that is base on a ﬁxedpoint processor. When using a ﬂoating-point processor, the interface layer is a simple conversion layer that takes a raw measured signal and converts it to a desired unit of measurement. The objective is to develop the interface layer to translate the physical hardware signals into variables that can be directly used by the core software and vice versa. If the interface layer is designed correctly, it will take on the form of a hardware-software interface speciﬁcation (HSIS). It should be designed for easy modiﬁcation in the case that the systems’ context diagram is altered (for example, if a sensor in the system is changed or the microcontroller is suddenly obsolete). The interface layer is the main component in the software that is modiﬁed so that the core software would require minimal changes, if any. Key to the operation of this soft- MAY 2008 | embedded systems design europe | www.embedded.com/europe http://www.embedded.com/europe

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Embedded Systems Design Europe - May 2008 Contents Microsoft Provides Embedded Roadmap Enea Buys Developers Irish Start-Up Raises Funds for Telecom FPGAs Kontron Promotes COM Express Nano Mentor Nucleus Platform Provides UI for Atmel Small Form Factor Boards Head for the SUMIT Proffibus Advances IO-Link Integration Embedded Developers Cautious on Multicore Auto Cooperation Improves Test Altera Launches DO-254 Partner Network Building an ‘Instant-Up’ Real-Time Operating Systems An Architecture for Reusable Embedded Systems Software Free up Bandwidth in PCI Express Evaluating Software in Medical Devices Circuit Sensitivity in Analog Circuits Choosing Flash Memory New Products Advertising Contacts

Embedded Systems Design Europe - May 2008

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