Special Report: Choosing a DSP

Special Report: Choosing a DSP
By Don Morgan, Embedded.com
March 13, 2003 (8:16 p.m. EST)
URL: http://www.eetimes.com/story/OEG20030122S0057

Once upon a time, digital signal processors were used only as coprocessors in signal processing applications and programmed mostly in assembly. Today, DSPs are used in almost every application domain. From the nitty to the gritty, here's what to look for in a DSP

When the industry's first digital signal processor (DSP) was used in the Speak & Spell toy in the late seventies who could have predicted that the chip would proliferate the way it has?

DSPs were designed to be number crunchers, usually sampling an analog signal then processing those samples digitally. At first, they were used in telecommunications and media products, because DSPs could run the intense math needed to convert a continuous analog signal into a discrete digital signal, process the data, and spit out a new analog output signal in little time. Its math abilities soon lead the DSP into other uses, such a s motion control with high-end servo control, radio in the form of "soft" or completely programmable radio, cinema, studio and consumer audio, MP3 players, scientific and engineering instrumentation and test equipment, Internet connectivity with Voice over IP and Video over IP, PDAs, and many more. That's a long way from Speak & Spell.

Digital signal processing

In the analog domain, all signals are continuous. In the digital domain, we deal with finite approximations of the continuous. The term digital signal processing refers to the act of translating analog signals into a digital form and then manipulating the data for whatever purpose we choose. The basic algorithms for accomplishing this translation can be implemented on almost any device, from an 8-bit microcontroller to a PC, but if it needs to be done quickly, we generally turn to the DSP.

Big M AC

The single-cycle, multiply and accumulate (MAC) instruction is an innovation particular to the DSP. The MAC multiplies two numbers (generally a weight coefficient and an input sample) and adds the result to another "saved" value. Performed continuously, this process is called a convolution, which is one of the fundamental elements in the implementation of all types of filters and predictors.

A good DSP will be able to execute at least one MAC in a single instruction cycle, and that instruction cycle should require only one clock cycle. Until recently, non-DSP processors have had no rapid and efficient way to process this sort of math. In recent years, however, DSP has expanded into almost every area of engineering. Now, microcontrollers used for engine control include the MAC instruction, the ordinary PC has DSP extensions for audio and video (Intel's MMX instructions), and there are specialized DSPs with on-board peripherals for everything from motion control to audio to telephonics.

Design considerations

If you're planning a DSP-enabled project and you need to evaluate various processors, here are some important characteristics to take into account.

Word length and accuracy
Accuracy depends on the elements (such as A/D) you use to convert from the analog domain to the digital. Precision is the number of bits you use to represent this conversion.

To maintain the highest degree of accuracy, the DSP you choose should have an accumulator long enough to store the intermediate results of all your calculations before finally storing them in system memory. Each time you store your values to system memory, they must be rounded or truncated to the word length of the memory; this is called quantization. Your results are never any better than your input; numerous quantizations during a calculation can create drift or bias in your result.

Not all applications require accuracy. Some, such a s servo control, only require repeatability. It is important in either case that you have enough bits available to represent your results and intermediate results with enough resolution to prevent limit cycling, which occurs when there isn't enough resolution in a system to represent a result in a feedback loop. (The necessary resolution is less than the least significant bit, so the system toggles back and forth between the higher result and the lower result.)

Processing speed
Very often the efficiency and utility of your product depends on the speed of your processor. To determine a DSP's speed, here are some questions you should ask:

• What is the clock speed?
• How many clocks per instruction cycle?
• How many instructions can be executed in a single instruction cycle?
• Does the device have pipelined branches?
• Can any instruction be made conditional?
• Does it have a barrel shifter?
• What kind of architecture does it use? Von Neumann or Harvard?
• Is there an on-chip cache?