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Automotives drive hybrid controllers

Posted: 01 Apr 2005 ?? ?Print Version ?Bookmark and Share

Keywords:mcu? dsp? embedded systems? cpu? 8bit?

There is a rapidly emerging category of electronic controller that combines the most useful control functionality of MCUs with the processing power of DSPs. These hybrid digital signal controllers (dscs) are particularly well-suited for the automotive environment, including real-time control of electromechanical systems and actuators such as motors.

MCUs are optimized to perform an array of logical and diagnostic operations on almost any combination of input data from various sources. DSPs are efficient at repetitive, numerically intensive tasks. Emerging DSCs combine the MCU's control-specific functions with the DSP's fast calculation capability to build a hybrid architecture. This creates a programmable core suitable for many of the engineering designs that traditionally have required an MCU and DSP combo.

The DSC's control functionality is based on its overall instruction set and core architecture. To be effective at control tasks, the instruction set must contain elements like bit manipulation, orthogonality and multiple-addressing modes. Software that is mainly doing control-related activities relies heavily on those types of instruction-set features to achieve high code density and minimize the overall memory footprint. The core architecture itself should allow for a large number of general-purpose register files and have the ability to handle interrupts efficiently. MCU applications tend to have a large number of interrupts because they typically interface to the real world.

But the instruction set and architecture are only part of the puzzle. Integrated elements like memory and peripherals can affect the overall control effectiveness of the device and costs. For example, including sufficient integrated, reliable, non-volatile flash memory minimizes board space and lowers overall system costs.

While the number of MIPS the DSC can deliver is important to consider, designers must avoid the common mistake of looking only at the MIPS. In reality, it is the combination of the MIPS capability and the functionality of the peripherals that is the measure of how well a DSC will perform. Powerful, full-function peripherals can reduce the load on the DSC core and perform certain functions much more efficiently. Therefore, when considering the best device for a given application, it is better to look at the raw MIPS capability with that of the integrated peripherals.

Equally important to the device features is the software development toolset and environment that supports them. MCU designers have come to expect the availability of robust, low-cost, software development tools and environments. When selecting a DSC device, designers should seek out those that offer a low-cost development environment with a broad range of cost/performance alternatives. In many projects, multiple engineers will be working on different pieces. It is important to provide each of those engineers with a development environment that is affordable and flexible. For many of the same reasons, designers should also seek out the availability of a low-cost, highly integrated software development toolset.

The advantages of integrating MCU processing, DSP and a powerful peripheral set into a single DSC device are the same advantages that have been driving other types of integration in recent years: reduced overall component cost, reduced board space, minimized logistical controls and increased reliability. There is also a big benefit in software development. In the past, if both MCU and DSP were needed by an application, two devices would have been required. These two would each have had separate software development environments and instruction sets. With a DSC, there is only one development environment and one instruction set with which engineers must deal. This simplifies the software development task and reduces costs.

Several semiconductor companies are leading the way in offering this integrated DSC functionality to designers. Automotive customers are adopting the devices for real-time use in electromechanical systems that require high-performance control capability and DSP-level signal-processing performance.

Currently, the trend is to migrate toward direct-assist electric motor steering systems from the more conventional hydraulic power-steering systems. As steering systems switch from hydraulic to electric control, fast math capability and MCU functionality are required. The primary processor for "steer-by-wire" systems must provide control capability for handling communications protocols and basic safety diagnostics. The processor must also offer signal-processing power to handle the sophisticated algorithms needed for precise operation of the electric motors.

Expectations are high that today's standard braking system, which uses hydraulic fluid, will be replaced in the near future by all-electrical systems. Though some challenges must still be met to realize that vision, the expected advantages over a conventional hydraulic braking system are such that the motivation to develop brake-by-wire systems is strong.

The advantages of brake-by-wire systems include lighter weight, more precise control and easier integration with other automotive subsystems found with future advanced vehicular dynamic-control systems. Each wheel would likely have a motor-controlled actuator with an associated control circuit. Because of the requirement for accurate motor controls, communication between different nodes and complex fail-safe algorithms, a DSC has been identified as an important part of the system.

- Scott Lynch

Operations Manager, DSP Standard Products Operation

Freescale Semiconductor Inc.




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