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DSCs come to the forefront

Digital signal controllers bring benefits to integration, value and performance.

With microcontrollers available in a seemingly endless range of configurations, you would have thought it likely that designers would be able to find the right one for their application. However, it would appear there's always room for at least one more variant.

Recognising this apparent shortfall, a new class of microcontroller appeared a few years ago, aimed at providing more effective control of devices such as motors. The digital signal controller (DSC) blends microcontroller and dsp functionality to provide, in the suppliers' opinion, the best of those worlds.

Microchip was one of the first entrants in the market with the dsPIC range, which became broadly available in 2004. Since then, a number of companies have thrown their hats into that particular ring, including Texas Instruments, Freescale and NXP.

NXP's entry into the DSC arena comes on the back of a recent announcement by ARM of the Cortex-M4 core, which is targeted specifically at DSC applications. Along with NXP, TI will also be coming to market with a range of Cortex-M4 based devices to complement its C2000 range. And, according to ARM, three more companies will be using the core in forthcoming products.

Bill Hutchings is DSC product marketing manager with Microchip. He said customers need good integration, good value and good performance. "In addition, signal processing is needed for particular market segments, such as digital power conversion and motor control, where fast execution of control loops is needed."

Microchip has expanded its DSC offering with eight additional models in the 16bit dsPIC33F GS series. These parts include on chip peripherals designed specifically for use with digital power supplies. Peripherals include high speed PWMs, a/d converters and comparators. Potential applications include uninterruptable power supplies, intelligent battery chargers, power factor correction and lighting.

Hutchings noted: "Typically, control has been handled via a range of analogue components. Moving to a DSC allows these components to be replaced and all power control to be handled in software."

In Hutchings' opinion, demand for DSCs is being driven by designers needing parts that help them to lower costs. "They can do this by eliminating the analogue controller and components. Full digital control can remove up to two thirds of components. This means a smaller pcb, which cuts overall costs. At the same time, designers can specify a higher frequency and therefore a smaller transformer and the overall BoM goes down."

Shyam Sadasivan is a product manager in ARM's processor division. He said motor control was one target market for Cortex-M4 based products. "Motor control requires significant signal processing. DSCs can bring energy efficiency, hence lower power consumption, but not at the expense of MHz or mW. The Cortex-M4 core allows users to do more with what's available."

Sadasivan noted that DSCs blend processing and dsp functionality. "This combination of features helps designers to address markets whose requirements lie beyond general purpose mcus. But they also take account of the fact that dsps can be hard to program."

In creating the Cortex-M4, ARM has combined typical mcu and dsp features. MCU features include, C programming, interrupt handling and low power, while dsp features include a Harvard architecture, single cycle MACs, a floating point unit and a barrel shifter. But where the Cortex-M4 differs from other devices is its 32bit core.

Sadasivam was keen to highlight energy efficiency. "If you look at running signal processing algorithms on the Cortex-M4, these will require fewer cycles than competitive devices." In fact, ARM claims the Cortex-M4 is twice as efficient on dsp tasks than 16 and 32bit mcus with dsp extensions.

But it's in programming where ARM believes its offering will make a difference. "It's all about making hardware easy to program," Sadasivan continued. "M4 based devices can be programmed in C and at higher levels of abstraction. And the programs are easy to maintain and reuse across the Cortex family."

NXP is one of the first companies to license the Cortex-M4 processor and says it is the first to demonstrate working silicon.

"Microcontrollers based on the Cortex-M4 processor will be an important part of NXP's high performance mixed signal portfolio, providing embedded system designers with an effective way to process data from complex analogue peripherals," said Geoff Lees, general manager of NXP's microcontroller product line. "The dsp extensions of the Cortex-M4 offer significant advantages, for example, offering 5 to 10 times improvement in complex dsp algorithms."

NXP's Cortex-M4 DSC family will be manufactured on an ultra low-leakage 90nm process technology, enabling performance in excess of 150MHz. The devices, which will also offer 'ultra low' power down currents, are designed for applications such as sophisticated motor control, digital power control and embedded audio.

Hutchings noted DSCs are well suited to motor control. "More products are moving to three phase motors and to variable speed. Designers need to control these products more precisely, at lower cost and more reliably.
"DSCs," he continued, "can match the load with an exact speed, increasing power efficiency. And, by moving to better control algorithms, designers can implement schemes such as vector control, which bring less environment noise and smoother operation."

But Hutchings says that, to do all this, a DSC needs to offer special a/d converters, pwms and processing capability. "In the past, moving control into software required an asic or the use of less efficient algorithms in an mcu."

The eight models added to the dsPIC33F GS series offer from 12 to 18 high speed pwms, each with a resolution of 1ns, and one or two 10bit on chip a/d converters, providing 2 to 4Msample/s. The amount of on chip flash ranges from 32 to 64kbyte and packages range from 64 to 100pin. Unlike the Cortex-M4, the dsPIC33F GS range features a 16bit core. "But this outperforms many 32bit micros," Hutchings contended.

Another licensee of the Cortex-M4 is Texas Instruments, which will be adding M4 based parts to its Stellaris range.

Jean-Anne Booth is TI's worldwide director of Stellaris marketing. She said: "The great thing about the ARM core is that companies have different aims, but can use a common architecture."

Stellaris parts are often used for embedded motion control and Booth claimed TI was one of the lead partners for the M4 simply because of the core's single precision floating point capability. "This can make systems work more accurately. Single precision floating point can get a PID loop working without saturation issues. It also supports field oriented control and allows more accurate angles to be measured."

In Booth's opinion, the M4 also opens up the ARM microcontroller world to those who don't program. "For example, users of meta language products such as LabVIEW and Matlab. The challenge for these users has been that it is hard to translate floating point into fixed point instructions. This development gives the ability for the algorithm to perform well in Embedded LabVIEW."

She added that M4 also allows TI to take advantage of C and C++ so its customers can save time and reduce risk. "Only ARM gives this breadth of reach and the benefit is enormous."

Author
Graham Pitcher

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