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Taking the 32bit plunge promises payoffs

Posted: 03 May 2004 ?? ?Print Version ?Bookmark and Share

Keywords:microcontroller? mcu? tcp/ip? cpu? protocol?

InStat/MDR expects 32bit MCUs to grow at a compound annual rate of 22.6 percent between 2001 and 2006. Two driving factors are contributing to that growth. One is the introduction of new applications that require higher performance, including gadgets like digital cameras, cellphones and MP3 players. Second, even such familiar applications as TV sets, car stereos and electronic toys are getting so advanced that they increasingly require performance and memory beyond the scope of 8bit MCUs.

Before discussing the migration from 8bits to 32bits, it is important to clarify where the 16bit MCU fits into the picture. Primary research with embedded engineers across many different markets suggests that most engineers who outgrow 8bit MCUs move directly to 32bit controllers. It seems that for most engineers, a 16bit MCU is not a viable upgrade alternative.

This is particularly interesting in comparison with the massive migration from 4bits to 8bits that has taken place over the past decade. Clearly, history isn't repeating itself, since few designs were migrated directly from 4bits to 16bits. One reason for this break from the past is that the ARM7TDMI has established itself as a standard CPU core. Currently, more than 30 IC vendors are shipping ARM7TDMI-based products, and the core has become even more of a de facto standard than the widely licensed 80C51. Such a dominant architecture was never conceived in the 16bit world, which mostly consists of single-source proprietary cores.

Moreover, process geometry shrinks have almost erased the price difference between 16bit and 32bit MCUs. On-chip memory and peripherals take up so much silicon real estate that the cost of the core nearly disappears in the budget. For most applications, upgrading from a 4bit MCU to a 16bit alternative was not even considered, because of big price differences.

Driving factors

So what specific factors are fueling the shift from 8bit to 32bit architectures? One is the growing need for a broader addressing range. Many 8bit architectures today are limited to 64K of addressing range. Some MCU families are unable to address external program memory, thus limiting the address range to whatever memory is implemented on the chip. A few 8bit architectures are able to address up to a few megabytes of off-chip memory, and in some cases, extended address range is achieved by adding to an existing architecture. But often the solutions vary by company and are not code-compatible, and the extended addressing capabilities still lack the efficiency of a 32bit architecture. An 8bit MCU is still only 8bit, and the arithmetic involved in calculating addresses wider than 16bits imposes a heavy load on most 8bit architectures. Many 32bit MCUs feature a synchronous DRAM, which dramatically lowers the cost of larger data memories.

The need for more performance is also driving the move to 32bit controllers. Not only do applications demand more raw CPU power in terms of clock speed, but the proliferation of communications interfaces such as Ethernet and USB in embedded applications increases the need for advanced on-chip peripherals. The availability of 8bit MCUs with on-chip Ethernet MAC is sparse for a good reason: TCP/IP is too complex a protocol to run properly on most 8bit architectures.

In cases where the MCU runs TCP/IP or other protocols in addition to handling application control functions, the need for an RTOS quickly becomes reality. Most 8bit MCUs are not architected for real-time task switching, and few good RTOSes are available. Also, DMA controllers are common on 32bit MCUs and are used to offload the CPU when the controller is receiving or transmitting large amounts of data.

It is also worth noting that there is a growing tendency among customers to avoid single-source architectures. With the exception of the 80C51, single-source architectures dominate the 8bit MCU market. That implies a significant tool investment and a substantial amount of work every time code is ported from one architecture to another. With the ARM7TDMI becoming the de facto standard architecture for the lower end of the 32bit embedded space, embedded engineers have regained the freedom to choose among MCUs from different vendors without losing the investment in code base and development tools.

Dealing with barriers

Nonetheless, migrating from 8bits to 32bits involves far more than just code recompilation and board relayout. Whereas the 32bit MCU opens up a new world in terms of possibilities, it also introduces a new set of real and perceived issues to deal with.

The myth of higher component cost is one that should be addressed. There is a perception that "more bits cost more," but this is not always valid, since many 32bit MCUs are manufactured in a 0.18m process. Also, one has to consider the application's total bill of materials. Often, engineers compare only the price of the MCU instead of the entire system cost--and that prevents many engineers from even contemplating the move.

When investing in 32bit development tools, one can easily spend far more than $4,000. However, since free GNU debuggers and compilers are available and since most 32bit MCUs do not require an emulator for real-time debugging, one can get started with a modest investment in development tools.

In some cases, fear of complexity could be enough to make engineers hesitate to move to 32bit. Fear that the learning curve is too long will be a barrier to many 8bit design engineers. But when the performance requirements reach the physical limits of what an 8bit MCU can do, the design tweaking involved in adding more features becomes extremely resource-demanding. At this point it is always better to make the leap.

- Geir Kjosavik

Triscend Corp.

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