Selection Criteria for DSP Processor
Choosing the Right DSP Processor
The right DSP processor for a job depends heavily on the application. One processor may perform well for some applications, but be a poor choice for others. With this in mind, one can consider a number of features that vary from one DSP to another in selecting a processor. These features are discussed below.
1. Arithmetic Format
One of the most fundamental characteristics of a programmable digital signal processor is the type of native arithmetic used in the processor. Most DSPs use fixed point arithmetic, while other processors using floating-point arithmetic. Floating-point arithmetic is a more flexible and general mechanism than fixed-point. With floating-point, system designers have access to wider dynamic range (the ratio between the largest and smallest numbers that can be represented). As a result, floating-point DSP processors are generally easier to program than their fixed point cousins, but usually are also more expensive and have higher power consumption. The increased cost and power consumption result from the more complex circuitry required within the floating-point processor, which implies a larger silicon die. The ease-of-use advantage of floating-point processors is due to the fact that in many cases the programmer doesn’t have to be concerned about dynamic range and precision. In contrast, on a fixed-point processor, programmers often must carefully scale signals at various stages of their programs to ensure adequate numeric precision with the limited dynamic range of the fixed-point processor. Most high-volume, embedded applications use fixed point processors because the priority is on low cost and, often, low power. Programmers and algorithm designers determine the dynamic range and precision needs of their application, either analytically or through simulation, and then add scaling operations into the code if necessary.
For applications that have extremely demanding dynamic range and precision requirements, or where ease of development is more important than unit cost, floating-point processors have the advantage. It’s possible to perform general-purpose floating point arithmetic on a fixed-point processor by using software routines that emulate the behavior of a floating point device. However, such software routines are usually very expensive in terms of processor cycles. Consequently, general-purpose floating-point emulation is seldom used. A more efficient technique to boost the numeric range of fixed-point processors is block floating point, wherein a group of numbers with different mantissas but a single, common exponent are processed as a block of data. Block floating-point is usually handled in software, although some processors have hardware features to assist in its implementation.
2. Data Width
All common floating-point DSPs use a 32-bit data word. For fixed-point DSPs, the most common data word size is 16 bits. Motorola’s DSP563xx family uses a 24-bit data word, however, while Zoran’s ZR3800x family uses a 20-bit data word. The size of the data word has a major impact on cost, because it strongly influences the size of the chip and the number of package pins required, as well as the size of external memory devices connected to the DSP. Therefore, designers try to use the chip with the smallest word size that their application can tolerate.
As with the choice between fixed and floating point chips, there is often a trade-off between word size and development complexity. For example, with a 16-bit fixed-point processor, a programmer can perform double-precision 32-bit arithmetic operations by stringing together an appropriate combination of instructions. (Of course, double-precision arithmetic is much slower than single-precision arithmetic.) If the bulk of an application can be handled with single-precision arithmetic, but the application needs more precision for a small section of the code, the selective use of double-precision arithmetic may make sense. If most of the application requires more precision, a processor with a larger data word size is likely to be a better choice.
Note that while most DSP processors use an instruction word size equal to their data word size, not all does. The Analog Devices ADSP-21xx family, for example, uses a 16-bit data word and a 24-bit instruction word.
A key measure of the suitability of a processor for a particular application is its execution speed. There are a number of ways to measure a processor’s speed. Perhaps the most fundamental is the processor’s instruction cycle time: the amount of time required to execute the fastest instruction on the processor. The reciprocal of the instruction cycle time divided by one million and multiplied by the number of instructions executed per cycle is the processor’s peak instruction execution rate in millions of instructions per second, or MIPS.
A problem with comparing instruction execution times is that the amount of work accomplished by a single instruction varies widely from one processor to another. Some of the newest DSP processors use VLIW (very long instruction word) architectures, in which multiple instructions are issued and executed per cycle. These processors typically use very simple instructions that perform much less work than the instructions typical of conventional DSP processors. Hence, comparisons of MIPS ratings between VLIW processors and conventional DSP processors can be particularly misleading, because of fundamental differences in their instruction set styles.
Even when comparing conventional DSP processors, however, MIPS ratings can be deceptive. Although the differences in instruction sets are less dramatic than those seen between conventional DSP processors and VLIW processors, they are still sufficient to make MIPS comparisons inaccurate measures of processor performance. For example, some DSPs feature barrel shifters that allow multi-bit data shifting (used to scale data) in just one instruction, while other DSPs require the data to be shifted with repeated one-bit shift instructions. Similarly, some DSPs allow parallel data moves (the simultaneous loading of operands while executing an instruction) that are unrelated to the ALU instruction being executed, but other DSPs only support parallel moves that are related to the operands of an ALU instruction.
Some newer DSPs allow two MACs to be specified in a single instruction, which makes MIPS-based comparisons even more misleading. One solution to these problems is to decide on a basic operation (instead of an instruction) and use it as a yardstick when comparing processors. A common operation is the MAC operation. Unfortunately, MAC execution times provide little information to differentiate between processors: on many DSPs a MAC operation executes in a single instruction cycle, and on these DSPs the MAC time is equal to the processor’s instruction cycle time.
And, as mentioned above, some DSPs may be able to do considerably more in a single MAC instruction than others. Additionally, MAC times don’t reflect performance on other important types of operations, such as looping, that are present in virtually all applications. A more general approach is to define a set of standard benchmarks and compare their execution speeds on different DSPs. These benchmarks may be simple algorithm “kernel” functions (such as FIR or IIR filters), or they might be entire applications or portions of applications (such as speech coders). Implementing these benchmarks in a consistent fashion across various DSPs and analyzing the results can be difficult.
Two final notes of caution on processor speed: First, be careful when comparing processor speeds quoted in terms of “millions of operations per second” (MOPS) or “millions of floating-point operations per second” (MFLOPS) figures, because different processor vendors have different ideas of what constitutes an “operation.” For example, many floating-point processors are claimed to have a MFLOPS rating of twice their MIPS rating, because they are able to execute a floating-point multiply operation in parallel with a floating-point addition operation. Second, use caution when comparing processor clock rates. A DSP’s input clock may be the same frequency as the processor’s instruction rate, or it may be two to four times higher than the instruction rate, depending on the processor. Additionally, many DSP chips now feature clock doublers or phase-locked loops (PLLs) that allow the use of a lower-frequency external clock to generate the needed high-frequency clock onchip
4. Memory Organization
The organization of a processor’s memory subsystem can have a large impact on its performance. As mentioned earlier, the MAC and other DSP operations are fundamental to many signal processing algorithms. Fast MAC execution requires fetching an instruction word and two data words from memory at an effective rate of once every instruction cycle. There are a variety of ways to achieve this, including multiported memories (to permit multiple memory accesses per instruction cycle), separate instruction and data memories (the “Harvard” architecture and its derivatives), and instruction caches (to allow instructions to be fetched from cache instead of from memory, thus freeing a memory access to be used to fetch data).
Another concern is the size of the supported memory, both on- and off-chip. Most fixed-point DSPs are aimed at the embedded systems market, where memory needs tend to be small. As a result, these processors typically have small-to-medium on-chip memories (between 4K and 64K words), and small external data buses. In addition, most fixed-point DSPs feature address buses of 16 bits or less, limiting the amount of easily-accessible external memory.
Some floating-point chips provide relatively little (or no) on-chip memory, but feature large external data buses. For example, the Texas Instruments TMS320C30 provides 6K words of on-chip memory, one 24-bit external address bus, and one 13-bit external address bus. In contrast, the Analog Devices ADSP-21060 provides 4 Mbits of memory on-chip that can be divided between program and data memory in a variety of ways. As with most DSP features, the best combination of memory organization, size, and number of external buses is heavily application-dependent.
5. Ease of Development
The degree to which ease of system development is a concern depends on the application. Engineers performing research or prototyping will probably require tools that make system development as simple as possible. That said, items to consider when choosing a DSP are software tools (assemblers, linkers, simulators, debuggers, compilers, code libraries, and real-time operating systems), hardware tools (development boards and emulators), and higher-level tools (such as block-diagram based code-generation environments). A fundamental question to ask when choosing a DSP is how the chip will be programmed. Typically, developers choose either assembly language, a high-level language—such as C or Ada—or a combination of both. Surprisingly, a large portion of DSP programming is still done in assembly language. Because DSP applications have voracious number-crunching requirements, programmers are often unable to use compilers, which often generate assembly code that executes slowly. Rather, programmers can be forced to hand-optimize assembly code to lower execution time and code size to acceptable levels. Users of high-level language compilers often find that the compilers work better for floating-point DSPs than for fixed-point DSPs, for several reasons. First, most high-level languages do not have native support for fractional arithmetic. Second, floating-point processors tend to feature more regular, less restrictive instruction sets than smaller, fixed-point processors, and are thus better compiler targets. Third, as mentioned, floating point processors typically support larger memory spaces than fixed-point processors, and are thus better able to accommodate compiler-generated code, which tends to be larger than hand crafted assembly code.
VLIW-based DSP processors, which typically use simple, orthogonal RISC-based instruction sets and have large register files, are somewhat better compiler targets than traditional DSP processors. However, even compilers for VLIW processors tend to generate code that is inefficient in comparison to hand-optimized assembly code. Hence, these processors, too, are often programmed in assembly language—at least to some degree. Whether the processor is programmed in a high-level language or in assembly language, debugging and hardware emulation tools deserve close attention since, sadly, a great deal of time may be spent with them. Almost all manufacturers provide instruction set simulators, which can be a tremendous help in debugging programs before hardware is ready. If a high-level language is used, it is important to evaluate the capabilities of the high-level language debugger: will it run with the simulator and/or the hardware emulator? Is it a separate program from the assembly-level debugger that requires the user to learn another user interface? Most DSP vendors provide hardware emulation tools for use with their processors. Modern processors usually feature on-chip debugging/emulation capabilities, often accessed through a serial interface that conforms to the IEEE 1149.1 JTAG standard for test access ports. This serial interface allows scan-based emulation—programmers can load breakpoints through the interface, and then scan the processor’s internal registers to view and change the contents after the processor reaches a breakpoint.
Scan-based emulation is especially useful because debugging may be accomplished without removing the processor from the target system. Other debugging methods, such as pod-based emulation, require replacing the processor with a special processor emulator pod. Off-the-shelf DSP system development boards are available from a variety of manufacturers, and can be an important resource. Development boards can allow software to run in real-time before the final hardware is ready, and can thus provide an important productivity boost. Additionally, some low-production-volume systems may use development boards in the final product.
6. Multiprocessor Support
Certain computationally intensive applications with high data rates (e.g., radar and sonar) often demand multiple DSP processors. In such cases, ease of processor interconnection (in terms of time to design interprocessor communications circuitry and the cost of linking processors) and interconnection performance (in terms of communications throughput, overhead, and latency) may be important factors. Some DSP families—notably the Analog Devices ADSP-2106x—provide special-purpose hardware to ease multiprocessor system design. ADSP-2106x processors feature bidirectional data and address buses coupled with six bidirectional bus request lines. These allow up to six processors to be connected together via a common external bus with elegant bus arbitration. Moreover, a unique feature of the ADSP-2106x processor connected in this way is that each processor can access the internal memory of any other ADSP-2106x on the shared bus. Six four-bit parallel communication ports round out the ADSP-2106x’s parallel processing features.
7. Power Consumption and Management
DSPs are increasingly being used in portable applications (such as cellular phones and portable audio players) where power consumption is a major concern. As a result, many processor vendors are reducing processor supply voltages and adding power management features to give programmers greater influence over processor power consumption. Power management features available on some DSPs include:
Reduced voltage operation: Many vendors offer low-voltage (3.3-, 2.5-, or 1.8-volt) versions of their DSP processors. These processors consume far less power than five-volt equivalents at the same clock rate.
Sleep or idle modes: Most DSPs feature modes that turn off the processor’s clock to all but certain sections of the processor, reducing power consumption. In some cases, any unmasked interrupt will bring the processor back from sleep mode, while in other cases, only a few designated external interrupt lines will wake the processor. Some processors provide multiple sleep modes with different power savings and wakeup latencies.
Programmable clock dividers: Some DSPs allow the processor’s clock frequency to be varied under software control to use the minimum clock speed required for a particular task.
Peripheral control: Some DSPs allow the programmer to disable peripherals that are not in use. Regardless of power management features, it is often difficult for design engineers to obtain meaningful power consumption figures for DSPs. This is because a DSP’s power consumption may vary by as much as a factor of three depending on the instructions it executes.
Unfortunately, most vendors publish only “typical” or “maximum” power consumption numbers, usually without specifying what constitutes a “typical” program. One exception is Texas Instruments, which provides application notes that detail power consumption vs. instruction type and processor configuration.
Obviously, processor cost is a major concern for products that are to be produced in volume. For such applications, designers try to use the lowest cost DSP that meets the requirements of the application, even though such devices may be considerably less flexible and more difficult to program than costlier processors. Among processor families, the least expensive family members tend to have significantly fewer features, less on-chip memory, and lower performance than the more expensive members.
A key factor in processor pricing is the dependence of price on device packaging. For example, plastic thin quad flat pack (PQFP and TQFP) packages can be significantly less expensive than pin grid array (PGA) packages. Finally, when considering prices, it is important to remember two things. First, processor prices are continually falling. Second, prices are strongly dependent on quantity, and prices for, say, a quantity 100,000 order may be significantly lower than for a quantity 1,000 order.