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Changing times: MCU architectures

Posted: 08 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:MCU? microcontroller? architecture 8bit 16bit?

For a system designer looking to choose a microcontroller, the vast array of available MCUs can make the search quite complex. Silicon Labs has announced an 8bit MCU with a core voltage of 0.9V. Texas Instruments makes many low-power claims with the 16bit MSP430. Infineon and Freescale have many automotive MCUs to go along with their extensive nonautomotive MCU lineups. Atmel's AVR and Microchip's PIC families keep adding variations. New 32bit ARM Cortex-M3 devices are constantly being announced, while the 8bit 8051 core still forms the basis of many diverse microcontrollers. And market leader Renesas has something for everyone.

Changing times
To work through the thicket of choices, it's helpful to assess current trends in the industry. How are MCUs changing? What will future MCUs look like? Perhaps most crucial from the designer's point of view are the trends toward lower power and greater integration.

Power is on the minds of the MCU vendors today, and with good reason. Many applications now run on batteries, in situations where the system power consumption is directly related to the run-time, or life of the system. Also, as the world grapples with rising energy prices and global warming, even applications that are plugged into the wall need to reduce the juice.

There are two types of power dissipation figures to watch for: dynamic and static power. Dynamic, or operating, power addresses the power consumed during a typical application. This is where the general efficiencies of the MCU architecture come into play. Static power (also called standby or leakage power) represents the power the device consumes when in standby or other low-power mode. This is critical for many battery-powered applications, especially those waiting for an external event before any processing takes place.

An MCU may have many low-power modes, depending on the application. There are numerous methods that the MCU can use to lower the static power, including low-leakage transistors and turning off the power to various parts of the MCU. Usually, the deeper "asleep" the device is, the longer it takes to wake up. Wakeup time is an important consideration when determining how just low you can go with low-power modes.

Some vendors have pushed down the supply voltage of their processor cores to lower one part of the power equation. This helps, particularly for dynamic operation; but current levels, naturally, still need to be considered.

Increased silicon integration
In this age of systems-on-chip and advances in semiconductor process technology, it is easier to implement more mixed-signal peripherals onto the microcontroller die. Many vendors now offer a tremendous amount of flexibility in the types and sizes of memory, and all manner of peripheral options.

A quick look at two microcontrollers from different eras shows how far the industry has come.

The Intel 8051, which dates from 1980, has 128bytes of RAM, 4Kbyte of ROM, four bidirectional I/O ports, one UART and two 16bit timers. The Freescale MC9S12X, launched in 2007, packs 32Kbyte of RAM, 4Kbyte of E2PROM, 512Kbyte of flash, six UARTs, three SPI interfaces, an eight-channel timer, an eight-channel 10bit ADC, an eight-channel PWM module, five CAN interfaces, two I2C interfaces, a periodic interrupt timer and an X-gate parallel-processing module.

There are many applications that will never need more than 8 bits of processing power. But the 32bit MCU is becoming more common as modern system design demands more processing clout. With shrinking designs and with an extra emphasis on low-power operation and standby, 32bit MCUs are finding their way into more applications.

Another factor in the battle for processing supremacy is the use of multiple processing cores to perform multithreading. Multicore devices are common in PCs, laptops and servers but are also starting to be seen in MCUs for certain processing-intensive applications, such as industrial equipment and car navigation systems.

As flash memory technology has improved, the silicon real estate required for flash has shrunk. Embedded flash has started to replace other programmable ROM as the program storage of choice because of the speed of the program/erase cycle. And more bytes of this memory are being incorporated into microcontrollers.

Also finding its way into MCUs is digital signal processing functionality. Processor cores for control are different from those that perform very complex mathematical functions. Cores that perform both functions are blurring that line.

In some cases, DSP-like mathematical functions are being added to a regular core's instruction set, with the hardware to support it. And the opposite is occurring too, as DSP cores add control-like instructions.

An alternative is to embed both a controller and a DSP core in the same device, creating a hybrid. Whether these devices are considered MCUs with DSP functionality or DSPs with MCU functionality is up to the vendor to decide.

Large semiconductor vendors that produce MCUs are now trumpeting the fact that their applications or systems engineers can help you design your system using their wide variety of parts. Smaller MCU vendors are doing the same thing, only by cooperating with system design houses to create reference designs.

- Steve Bitton is a technology analyst at TechOnline, a division of TechInsights

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