By Manuel Alves, Freescale Semiconductor Corp.



Why should microcontroller (MCU) “bit size” matter to a system designer? After all, developers are looking for an MCU that does the job for a particular application at a reasonable price. Next on the list of decision criteria is the availability of high-quality development tools and software from industry-leading vendors. In the automotive world, this means choosing an MCU that provides the lowest system cost that is supported by standard real-time operating systems (RTOS), communication stacks and development tools from trusted partners.

In many instances, adhering to these criteria leaves automotive system developers with a handful of options, usually a mixed bag of 16-bit and 32-bit MCUs. That’s when the question of the number of bits is worth considering, and not just in the first stage of the screening process. MCU bit size indeed matters when looking at the bigger picture of research and development spending versus production volumes and system scalability requirements (increasing or decreasing) in future evolutions.

Today’s 32-bit MCU architectures provide a number of benefits to developers, beginning with an inherently high performance core, a broad linear address space and native 32-bit arithmetic capabilities. These architectures are very attractive to software designers who enjoy the convenience and headroom of a 32-bit address space at their disposal. All these benefits certainly help with software reuse and software platform design. But as every designer knows, there is no such thing as a “free lunch” in embedded systems development. So what are the drawbacks to software-friendly architectures?

First and foremost, they require more memory space for both program and data storage. Although some 32-bit architectures include RISC-based 16-bit instructions, those instructions require more program memory space than CISC instructions used in a stand-alone 16-bit processor, such as Freescale’s S12 MCU core. Going 32-bit typically results in about a 20 percent to 30 percent code size increase, which requires an increase in flash memory space. Additionally, RAM requirements also increase as every application makes extensive use of 8- and 16-bit data for Boolean and Character (Char) type variables (for eg loop counters).

Some 16-bit architectures, such as Freescale’s S12, support misaligned RAM accesses, a technique that enables the storage of back-to-back 8-bit data in the RAM address space. This technique does not leave any wasted “holes” of unused RAM, which is a common issue for 32-bit architectures that store data aligned with 16-bit incremental address boundaries. While 32-bit architectures may offer exceptional performance headroom, 16-bit architectures offer superior headroom in terms of memory space.

Granted that a 16-bit device might not be the most comfortable MCU choice for software designers who want performance headroom, this potential drawback is offset by the inherent simplicity of 16-bit solutions, i.e., fewer registers to initialize, fewer operating modes to worry about and easier-to-use development tools.

High-volume automotive applications make it worthwhile to spend engineering resources on optimizations to achieve the lowest bill of material (BOM). As a result, 16-bit architectures will remain very strong in this market. Freescale’s S12X architecture with the XGATE co-processor is a excellent example of an optimized 16-bit solution that can be used to significantly reduce the system BOM. Although using an S12X device with XGATE may require more software engineering effort than a “simple” 32-bit architecture, the payback comes in the form of fewer and/or more cost-effective peripheral components. The XGATE co-processor may eliminate the need to add ASIC devices, such as an external display driver or even a secondary microcontroller used as a smart watchdog. The XGATE arguably provides automotive suppliers with an opportunity to differentiate their system designs through software versus using mainstream 32-bit solutions.

What about software ecosystems for 16-bit automotive MCUs? Today’s automotive market is highly specialized due to emerging software standards, such as AUTOSAR, as well as automakers lobbying for their preferred solutions. This situation drives high volumes for third-party vendors, too, which fuels their motivation to support both 16-bit and 32-bit architectures while other markets might not justify such support.

New development methodologies, such as model-based design and AUTOSAR, are pulling performance needs upward and driving applications to migrate from 16-bit to 32-bit architectures.

Similarly, increasing functionality on smaller modules is driving a migration from 8-bit to 16-bit at the bottom end of the automotive market, while the 8-bit market enjoys the benefits of emerging applications combined with electromechanical replacements.

Given these automotive market dynamics, the application mix will continue to evolve for each of the 8/16/32-bit MCU architectures, which progressively consume the smaller bit-size of each MCU neighbor’s segment. This 8-to-32-bit automotive MCU ecosystem is sustainable as long as the food chain keeps being fed by new applications at the bottom-end. And within this ecosystem, huge opportunities remain for 16-bit MCU architectures, which occupy a cost-competitive sweet spot within today’s automotive electronics industry. Shown below is a block diagram of a typical 16-bit automotive MCU.
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Manuel Alves is a product marketing manager for Freescale’s Automotive Microcontroller Operation. He is responsible for driving product strategy and launching and promoting 8-bit, 16-bit and 32-bit MCUs targeting the automotive body electronics market. He holds a masters degree in electrical engineering from the ESINSA (Ecole Superieure d'Ingenieurs de Nice Sophia-Antipolis) and has more than 8 years of experience within the automotive semiconductor industry, including positions in field applications engineering and product marketing in Germany, France and the United States. For details on freescale's automotive offerings, click here www.freescale.com