Columnists

The path to performance: Interchangeable 8- and 32-bit microcontrollers

The ultimate goal is to develop 8- and 32-bit MCUs that share the same socket, the same board, and the same tools.

By Mike McCourt, Freescale Semiconductor -- EDN, 2/14/2007

Technology races forward, and it's putting a lot of pressure on embedded-system developers. Competing in markets with increasingly complex applications, developers often face performance shortfalls that cause them to turn to increasingly higher-performance microcontrollers. Developers using 4-bit devices today are looking for cost-effective 8-bit alternatives, and 8-bit users are turning to compatible DSC (digital signal controller) and 32-bit options. There are many reasons for this migration, which is being further enabled by products with increased memory capacities and improved performance at comparable price levels.

The 8-bit market remains vibrant, with many new applications being unearthed daily that take advantage of the price, power, and code-efficiency benefits of 8-bit microcontrollers. But for advanced applications, market pressures are driving up performance needs and memory sizes. For example, household appliances are now using variable-speed motor control, which requires the higher performance and larger memories of 32-bit devices. On-chip flash memory for 8-bit microcontrollers generally maxes out at 128 kbytes, whereas some of the latest 32-bit embedded processors integrate as much as 3 Mbytes of flash, providing more programming headroom to handle sophisticated control algorithms.

Semiconductor manufacturers are trying to help developers transition to 32-bit devices by easing the migration to higher performance. Low-end 32-bit microcontrollers are priced comparably with high-end 8-bit devices, so unit cost is less of a roadblock. The real expense comes in the transition process itself, including making sure that years of previous experience are retained. Development tools play a major role in this investment.

32-bit development tools are inherently more complex and more expensive than 8-bit tools. Chipmakers and their partners have been working to simplify their 32-bit tool suites and reduce the cost for basic evaluation boards to less than $500. However, developers must still address a number of architectural differences between chips. For example:

• Peripherals on 8- and 32-bit devices may provide similar functions, but the exact functionality and user-visibility programming models will likely be different. Developers will need to rewrite and reverify lower-level device drivers that interface to the on-chip peripheral hardware.
• Engineers must address architectural differences, such as interrupt capabilities and default variable sizes and their layout in memory. Also, any timing loops will probably require rewriting.
• No matter how simple and easy-to-use the 32-bit tools are, they often require an extended learning curve for new developers to master.

Performance and memory requirements may force developers to migrate from 8-bit to 32-bit microcontrollers, but the challenges they then face can lead to time-to-market delays and increased development costs. A novel alternative is to redesign a 32-bit microcontroller to look and act more like an 8-bit device. Identical peripherals and pin-outs and a single tool set suitable for both 8- and 32-bit devices can produce a 32-bit device that's a true pin-compatible, drop-in replacement for an 8-bit product.

Many of the challenges facing designers who want to migrate to 32-bit now can be dealt with at the chip-design level. For instance, a typical 8-bit device may have only a single-pin debugging interface, while a 32-bit device typically requires many more pins. Remapping the 32-bit interface to one pin can greatly enhance compatibility and make the developer's job easier.

The ultimate goal is to develop 8- and 32-bit microcontrollers that share the same socket, the same board, and the same tools. You can take the 8-bit part out of your evaluation-board socket, drop in the 32-bit part, recompile the code, and have a functional 32-bit system. This is the direction we are taking at Freescale. While other paths to higher performance exist, our 8- to 32-bit controller continuum shows great promise as the most attractive alternative—one that has never been tried before.

The migration to 32-bit performance is gaining momentum. Regardless of which devices and tools developers choose, the advantage goes to those who spend less time developing and more time selling their new 32-bit applications.

Mike McCourt is vice president and general manager of the Microcontroller Division at Freescale Semiconductor.