The VHDL-AMS language is an undiscovered asset for Mil-Aero digital designers – a powerful tool to define and verify safety-critical requirements in a non-digital context.

The functionality and performance of modern military and aerospace systems has become heavily influenced by their electronic content. Consequently, selecting the right electronic components and choosing the optimal design methodology is vital in developing a successful product. The flexibility and capabilities of new digital components is still growing exponentially. The potential of these devices, however, cannot be fully (and safely) utilized without incorporating the latest design and verification methodologies.

The use of digital devices in military and aerospace applications is widespread. The capacity of these devices to implement and integrate both software and digital-hardware functionality—on a single component—is very attractive. Challenges remain, such as ensuring that these devices are compatible with harsh operating environments and are compliant with the exacting reliability requirements of the industry. The biggest challenge in utilizing these devices, however, may be one of methodology.

Design methodologies for mil-aero applications must consider the complexities of mechatronic systems. Many of the applications are a sophisticated combination of feedback control systems, analog and digital circuitry, multi-physics sensors and actuators—all controlling a multi-physics “plant.” A fly-by-wire flight control system (see Figure 1 for an example) is a feedback control system that implements a control algorithm on a digital device (in software running microcontroller or directly in hardware), getting feedback signals through an analogto- digital converter, from a physical sensor that detects the control surface position. The control algorithm periodically produces a digital control signal that is used to drive a hydraulic actuator, through some mixedsignal electronics, to keep a control surface (the “plant”) in the desired position while it is buffeted by the air passing over it. The importance of unambiguous, verifiable system requirements to the success and safety of such a controller cannot be overemphasized.

Digital designers are familiar with HDL-based, requirements-driven design methodologies for electronic subsystems. But how can requirements be expressed for a system that, while it contains digital elements, is fundamentally non digital? Fortunately, an executable HDL exists that extends the capabilities of the digital VHDL language with continuous time, differential and algebraic equations, multi-physics, transfer functions (both s and z domain), energy conserving analog circuit capabilities (like SPICE), statistical distributions for parametric variations, and functions expressed in software C code. This language is the IEEE Std. 1076.1 VHDL-AMS language. VHDL-AMS is the perfect language for providing continuity in design and verification at all levels: functional specifications; architectural partitions; and component implementations (see Figure 2).

The VHDL-AMS language standard was originally initiated by the US department of defense and was completed in 1999. The description of this language sounds ideal, so why aren’t more designers using the language today? Simply put, implementing the standard has been very difficult technically. Now, however, after years of development, several different tool suppliers are providing simulators that can efficiently execute the VHDL-AMS language. The long-awaited promise of this language standard and the resultant methodology is now a reality.

Figure 1: Mechatronic feedback control system

Designers of safety-critical digital systems, such as Magneti Marelli, have confirmed significant benefits by using the VHDL-AMS language. Since VHDL-AMS is a pure superset of the VHDL language, the designer starts with all of the well-known benefits of HDL design and verification. Then, using the extensions provided by VHDL-AMS, the design can be thoroughly analyzed by incorporating the impact of the neighboring engineering disciplines: analog electrical engineering (Kirchoff’s current and voltage laws), ADC, and DSP circuits; control system transfer functions; mechanical engineering (Newton’s and Bernoulli’s laws); and extensibility any other desired engineering or physics discipline.

To be specific, VHDL-AMS allows expression of simultaneous, nonlinear differential and algebraic equations in any model; the model creator need only express the equations and let the simulator solve them in time or frequency domain. Domain knowledge from any engineering discipline can be encapsulated in reusable libraries4 that are accessible by any member of the design team. It is then possible for the digital developer to start with a clear, executable specification that incorporates all of the requirements (including non-digital) and to use the same specification as a virtual verification environment. Since VHDL-AMS supports the concept of component statistical distributions5, it is also practical to verify that the digital design will operate in the context of tolerance and manufacturing variation, which drive the "non-digital" characteristics of mechatronic systems.

Figure 2: Multi-Discipline design and verfication with VHDL-AMS

by Darrell A. Teegarden

May 1, 2008

Darrell A. Teegarden has over twenty years of experience in development of HDL-based models and software
tools. He currently manages the SystemVision™ VHDL-AMS related tool development for the System Level
Engineering division at Mentor Graphics Corporation in Wilsonville, Oregon. Darrell is an IEEE member and
holds a B.S., Chemical Engineering from Oregon State University and an M.S., Electrical Engineering from
Stanford University. He is a co-author of The System Designer's Guide to VHDL-AMS: Analog, Mixed-Signal,
and Mixed-Technology Modeling.

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