The ARRL Handbook for Radio Communications; Volume 2: Practical Design & Principles PART 1 by ARRL Inc

Author:ARRL Inc. [Inc., ARRL]
Language: eng
Format: azw3
Publisher: ARRL, the national association for Amateur Radio®
Published: 2018-10-07T16:00:00+00:00

8.5.4 Typical DSP Processors

When selecting a DSP device for a new design, often the available development environment is more important than the characteristics of the device itself. Microchip’s dsPIC family of DSPs was chosen for the examples in this chapter because their integrated development environment is extensive and easy to use and the IDE software is available for free download from their website.5 The processor instruction set is a superset of the PIC24 family of general-purpose microcontrollers, with which many hams are already familiar. The company offers a line of low-cost evaluation boards and starter kits as well as an inexpensive in-circuit debugger, the ICD 3. The free IDE ­software includes a simulator that can run dsPIC software on a PC (at a much slower rate, of course), so that you can experiment with DSP algorithms before buying any hardware.

The Microchip DSP family is limited to 70 million instructions per second. In a system with, say, a 70 kHz sample rate, 1000 instructions per sample are available which should be plenty if the calculations are not too complex. However if the sample rate is 1 MHz, then you get only 70 instructions per sample, which likely would be insufficient.

If more horsepower is required, you’ll need to select a processor from a different manufacturer. Look for one with a well-integrated suite of development software that is powerful and easy to use. Also check out the cost and availability of development hardware such as evaluation kits, programmers and debuggers. Once those requirements are met, then you can move on to selecting a specific device with the performance and features required for your application. It can be helpful at the beginning of a project to first write some of the key software routines and test them on a simulator to estimate execution times, in order to determine how powerful a processor is needed.

When estimating execution time, don’t forget to include the effect of interrupts. Most DSP systems require real-time response and make extensive use of interrupts to ensure that certain events happen at the correct times. Although this is hidden from the programmer’s view when programming in C, the interrupt service routines contain quite a bit of overhead each time they are called (to save the processor state when responding and to recall the state just before returning from the interrupt). Sometimes an interrupt may be called more often than you expect, which can eat up processor cycles and so increase the execution time of other unrelated routines.

In the past, may embedded systems were written in assembly language so save memory and increase processing speed. Many early microprocessors and DSPs did not have enough memory to support a high-level language. Today, most processors have sufficient memory and processing speed to support a C kernel and library without difficulty. For anything but the simplest of programs, it is not only faster and easier to develop software in C but it is easier to support and maintain as well, especially if people other than the original programmer might become involved.

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