Digital signal controllers combine the best of math hardware with versatile MCU I/O ports.

In April 2009, Freescale announced its family of MC56F8006 processors, which according to the company will interest developers of motor-control and power-supply circuits.  Although not mentioned in the news release, that family includes the MC56F8002 DSC chip.  Freescale's press announcement included the following example, "...a washing machine motor digitally controlled by a DSC can enable more efficient agitation cycles that use less water and shorter spin cycles to reduce drying time and energy consumption." Here in the US west, anything that saves water or power gets a gold star.

You might wonder, "What distinguishes a microcontroller from a DSC?"  After all, engineers have used MCUs to control motors and digitize analog-sensor signals for some time.  In my opinion an MCU can easily handle a washing-machine motor-control loop, but when you must implement a finite-impulse response filter (FIR) or a real-time proportional-integral-differential (PID) control loop, for example, speed becomes the deciding factor. That factor often tilts the decision in favor of a DSC.

DSCs "buy" their speed advantage at the cost of adding more on-chip logic that performs math operations in hardware, allows several memory-access operations to run in parallel, and provides CPU capabilities that perform two or three operations simultaneously.  A 16-by-16-bit multiplication in a DSC might use only one clock cycle, but a similar operation in a small MCU could require many byte-wide operations and many clock cycles. Combine those enhanced CPU capabilities with the cornucopia of readily available MCU-type peripherals, such as analog converters, serial ports, and pulse-width modulators, and you have a powerful DSC chip.

The new Freescale DSC family uses a 16-bit 56800E DSC "engine" that provides a dual Harvard architecture.  You'll find Freescale's "56800E Core Reference Manual," (DSP56800ERM), on the company's Web site.  It will help you understand how the DSC family's architecture increases execution speeds. The manual runs over 700 pages, so it's not light reading.

Freescale's competitors Texas Instruments and Microchip Technology also supply a wide range of DSC chips and development tools, and these latter two companies have dominated the DSC market for some time. So, if you think a design could benefit from using a DSC rather than an MCU, you'll find a variety of interesting products from these three vendors.

Keep in mind that not every power-control application needs a DSC.  MCU vendors also offer a wide range of devices that work well in motion- and power-control applications. And in the case of Texas Instruments, Microchip, and Freescale you have a range of both DSC and MCU chips to choose from. Some DSCs and MCUs can share the same development tools, too.

Have you used a DSC?  What tradeoffs did you consider and did you also consider MCUs? I welcome your comments.

Jon Titus
jon.titus@evalkits.com