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Automotive MCUs get flexible

Posted: 16 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FPGA? MCU? structured ASIC?

The MCU, widely applied in automotive electronics, is heading full-speed into a wall of time and cost.

The primary benefit of using MCUs has been high level system integration combined with relatively low cost. However, there are hidden costs associated with these devices beyond the unit price.

Also, with rapidly changing end-market requirements in today's automotive sector, MCUs often become unavailable. MCUs equipped with specialized features and a fixed number of dedicated interfaces do not fulfill market requirements after a short evaluation period. Consequently, system suppliers are being forced to redesign their hardware and rewrite associated software, and in some cases, even change the processor core.

Feature mix
MCU manufacturers are faced with the challenge affecting the entire market. An MCU is an application-specific product. Thus, for each application, a new device with a different set of features is necessary.

To serve a broader market with a single core architecture, manufacturers offer MCU families with various members providing a mix of interfaces and functions. In most cases, this feature mix does not specifically fit customer requirements. Hence, for large customer opportunities, a variant with a new set of interfaces and functions has to be developed around a specific core architecture.

When MCUs were implemented in older technologies with relatively low manufacturing costs, this strategy was successful. Today, with the latest process technologies used increasingly for higher levels of system integration, developing new MCU variants carries a very significant cost. Because only a few customer opportunities offer the necessary volumes, it no longer makes business sense to produce such specialized devices for the requirements of a single customer.

As a result, new MCU variants are equipped with more and more features to attract whole markets, migrating to standard products rather than application-specific devices. While this makes them powerful, it causes product costs to rise dramatically, making it more difficult to serve cost-sensitive markets such as automotive electronics.

The solution is to change the root cause of the problem!the fixed implementation of functions in silicon. A new design approach is clearly required.

Flexible functions
The way out of this dilemma is the implementation of flexible functions in silicon available with FPGAs. These devices offer a powerful, viable alternative to MCUs because they significantly reduce engineering development time and the cost of multiple silicon iterations.

Unlike MCUs that do not possess required features, FPGAs can be programmed and reprogrammed as needed during the design process, enabling more rapid prototyping and faster time-to-market. The devices can also be upgraded in the field if requirements change!even after the devices are deployed in a product.

Automotive graphics controller applications are a key application where FPGAs are typically preferred over traditional controllers. While low-cost FPGAs for isolated functions (i.e. graphics) are very well accepted in the automotive market, more complex functions would become too expensive in a programmable device because of the huge silicon overhead required for programmability.

However, with a seamless migration path from FPGA to structured ASIC now possible, a flexible microcontroller is cost-effective and may be specified exactly to customer requirements!with the features selected from a large library of predefined and scalable building blocks.

The main differentiator from traditional MCUs is the seamless migration path from the prototype FPGA to the resulting MCU. The CPU and the bus architecture are unique to the flexible microcontroller concept, and can be mapped to the design with the exact functions required for a specific customer application.

Bus architecture
Traditionally, single buses have been used in MCUs, where an arbiter monitored the bus as a resource to be distributed. This created a serious disadvantage as the bus!a central resource for the system!quickly became a bottleneck. Consequently, multilayer buses are used in newer systems, particularly for SoC implementations, where several buses work in parallel. Bus fabric structures in today's FPGAs work on a similar principle. The difference is, while the number of layers present is static in other multilayer buses, the FPGA bus fabric approach makes it possible to freely select the number of layers required.

When considering the challenges of EMC and power dissipation, it sometimes makes sense to have a peripheral module run at a different rate from the rest of the overall system. For instance, when running a memory interface at a higher speed to keep access times at a correspondingly low level, the rest of the system is run at a lower clock rate. Another scenario can be integrating many modules where a relatively low clock rate is fully sufficient.

FPGA-to-ASIC integration allows ramping up controller performance and features (lower axis).

To meet EMC or power dissipation needs, decoupling these elements from a portion of the system that runs at a significantly higher rate can be easily accomplished by high-level system design tools such as SOPC Builder. This tool automatically generates the logic required for synchronizing the different clock-rate domains, with the designer only needing to specify which modules are to run in a given clock-rate domain.

Prototyping logic
As the complexity of an automotive MCU system is much higher than a pure graphics controller, the FPGA will be used as the prototyping logic in most cases. Prototyping with an FPGA dramatically minimizes the development risk as it offers opportunities for comprehensive verification, firmware development and field testing. Also, by using an FPGA for prototyping, designers can run the device in-system to exercise it using a real-world situation, allowing identification of potential design flaws that may not have been detected during simulation.

Software development has become a larger part of the overall development cycle. With the extended time and resources necessary for software development, having a prototype system available can shorten the overall development cycle and uncover bugs. It can also uncover compatibility issues and the need for new hardware functions to support functionality that cannot be addressed or implemented by software.

Field testing uncovers system or device flaws that were not seen in the lab. And in many cases, having a demonstration system is a necessary requirement for the salesperson to secure a customer preorder. Another requirement may be new features and functionalities that were not part of the original specification. Whether for undiscovered flaws or to add new features, prototype FPGAs can be modified quickly without large NRE costs or long manufacturing cycles.

The final element in the flexible microcontroller concept is ASIC development. Once the prototype system has been built and tested, the design is ready for conversion to a structured ASIC. With Altera devices, the design is immediately transferred into a HardCopy Structured ASIC device. With this design flow, there is no need to resynthesize the design or perform an additional verification cycle as these devices use the same building blocks as their FPGA counterparts. The fast turnaround time provided by using this structured ASIC flow allows the designer to sign off on the FPGA logic quickly, resulting in a fast, low-cost conversion.

- Axel Zimermann
Automotive Market Development Manager
Altera Europe

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