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Digital signal processors (DSP) are special microprocessors designed for digital signal processing, usually in real-time. DSPs can also be used to perform general-purpose computations; however, they are not optimized for this function. Instead, DSPs use an instruction set architecture (ISA) that is optimized for rapid signal processing. Important ISA features include deep pipelining to enhance microprocessor performance and the ability to act as a direct memory access device (DMA) for the host environment. Digital signal processors also use Harvard architecture with separate program and data memory. In addition, DSPs use saturation arithmetic so that overflow operations accumulate at the maximum or minimum values that the register can hold instead of wrapping around. With DSPs, the maximum remains the maximum. By contrast, with many general-purpose CPUs, the sum of the maximum plus one equals the minimum.
Most digital signal processors are fixed-point devices because in real-world signal processing, extra precision is not required and there is a large speed benefit. Floating-point DSPs are common in scientific and other applications that require precision. Digital signal processors feature specialized instructions, modulo-addressing in ring buffers, and bit-reversed addressing mode for Fortran function tree (FFT) cross-referencing. Generally, DSPs are dedicated integrated circuits (ICs); however, DSP functionality can also be realized using field programmable gate array (FPGA) chips. DSPs are used in several classes of computer hardware, including sound cards, modems, telephony boards that handle sound and modem functions, and hardware that handles audio and video compression in real time.
How to Select
Selecting digital signal processors requires an analysis of performance specifications. DSPs operate with variety of supply voltages and include data buses that range from 8-bit to 256-bit devices. DSPs also vary in terms of clock speed, which is typically expressed in megahertz (MHz) and gigahertz (GHz). Often, integrated on-chip phase-locked loops (PLLs) with clock frequency synthesis capabilities are used to design high-speed internal clocks for data sampling in DSP applications. Measurements of DSP processing power include million instructions per second (MIPS) and million multiply / accumulates per second (MMACS). For floating-point devices, an additional measurement is million floating-point operations per second (MFLOPS). For all DSPs, the operating current, operating temperature, and power dissipation are also important specifications.
Features and Options
Digital signal processors are available with multiple DMA channels and a variety of I/O ports and interfaces. Some devices also feature an external memory interface that determines the amount of memory a chip can handle. Parallel interfaces include universal asynchronous receiver transmitter (UART) and universal synchronous asynchronous receiver transmitter (UART) technology. Serial interfaces include peripheral component interconnect (PCI), universal serial bus (USB), enhanced synchronous serial interface (ESSI), and serial communications interface (SCI). The Joint Test Actions Group (JTAG), a standards organization, has developed a test access port (TAP) that allows access to the inner workings of ICs. Inter-IC (I²C) is used to control and monitor applications in communications, computer, and industrial settings.
Common package types for digital signal processors include ball grid array (BGA), quad flat package (QFP), single in-line package (SIP), and dual in-line package (DIP). Many packaging variants are available. In terms of additional features, some DSPs include an internal memory interface, embedded timers, or Flash memory. Other devices include on-chip A/D converters that convert analog inputs into digital signal.