8-bit microcontrollers: still going . . .

Although high-end processors, especially 32-bit MCUs, tend to be the center of attention these days, 8-bit MCUs are more than holding their own. In 2004, 8-bit microcontrollers are expected to continue to lead all MCUs in revenue and unit shipments, accounting for almost 40 percent of the market's revenue, according to IC Insights. The combination of de facto standard architectures (such as the 8051 and 6811) with synthesizable, configurable implementations that are available as licensable silicon intellectual property has enabled 8-bit CPUs to live on far beyond their initially dreamed-of lifetime.

The 8-bit MCU continues to be the workhorse of the automotive industry. It is valued because of its cost-effective control functions, which enable consumers to enjoy the benefits of smart products in the automobile sector. For example, the BMW 745i luxury sedan contains more than sixty 8-bit MCUs.

If 8-bit processors were still stuck in the semiconductor process technologies in which they were originally designed in the 1980s (running at something like 2 MHz), they would have quickly run out of steam. But the introduction of synthesizable MCUs, specifically designed for reuse with synchronous, flip-flop-based styles and simple clocking, has enabled portability to take advantage of the latest process technologies. As "soft" cores, these processors are also configurable, allowing the designer to optimize the MCU to the target application. Enhanced architectures combined with process portability have improved 8-bit MCUs from 100,000 instructions per second in the 1980s to more than 100 million instructions per seconds today-a thousandfold increase.

Clean, synthesizable cores can also take advantage of the latest EDA tool optimizations, which offer an automatic, 50 percent power reduction; insertion of structures for manufacturing test; and help to automatically ensure signal integrity, manufacturability, reliability and high yield. It is absolutely preferable for designers to use proven IP for a mature architecture rather than try to invent such functionality on their own for the same purpose. This is especially important when designers are struggling to achieve the required productivity and cost-effective design required to build modern systems-on-chip. The International Technology Roadmap for Semiconductors predicts that by 2008, 95 percent of the productivity gain will be through the use of predesigned, preverified, reusable IP blocks.

Designers continue to turn to 8-bit processors because of the architectures' familiarity and stability, as well as the massive design ecosystem that has been built around them. Synthesizable cores that adhere to these standard architectures enable the support of legacy software with little or no modification. That's especially important if, say, the original designer retired 10 years ago.

So, where are these 8-bit MCUs going? For some applications, an 8-bitter is "just enough," providing the necessary processing "oomph" to handle the entire application's processing. This is especially true for applications that don't require a lot of processing power, like VCRs, remote controls, animated toys, toasters, refrigerators and washing machines. For many of those applications, designers can turn to highly cost-effective 0.35- and even 0.5-micron fabs.

Companies with high-volume designs currently implemented with a discrete 8-bit MCU and other logic ICs can dramatically reduce their manufacturing costs, increase reliability and reduce power consumption by merging the entire design into a single SoC. This includes, of course, embedding the 8-bit MCU as well as associated memories. To support this kind of move, the MCU must be 100 percent compatible with the discrete part. Costs are reduced through printed-circuit-board reduction, parts reduction and taking advantage of dense modern semiconductor process technology. As a reference, an embedded 8051 implementation can be done in approximately 12,000 gates, with a silicon cost that's a fraction of a cent.

In applications requiring a 32-bit processor for real data crunching, 8-bit MCUs can be used to offload the comparatively larger and more power-hungry main processor. This allows the 32-bit main processor to be run less often and/ or at lower frequency, because while the CPU is in idle or sleep state, the house-keeping functions can be managed by the 8-bit MCU. In some cases, the 8-bit MCU may do enough processing to allow the selection of a smaller or less-powerful 32-bit processor. In real-time applications, the overall reliability of a system can be increased through the use of such multiple MCUs as an 8-bit device coupled with a 16-, 32- or 64-bitter for intermittent, time-critical, low-processing tasks.

While the 32-bit MCU market will undoubtedly overtake the 8-bit MCU in the near future, the end isn't near for venerable 8-bit MCUs. They have re-invented themselves as synthesizable cores, and designers continue to find creative ways to use them in new applications. Key to their continued survival is designers' ability to quickly embed the 8-bit MCUs in their latest designs.

Meghan Le (meghan@synopsys.com) is product-marketing manager for DesignWare microcontrollers at Synopsys Inc. (Mountain View, Calif.).

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