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Software Synthesis for Single-Processor DSP Systems Using Ptolemy

1.0 Introduction

Practical signal processing systems today are rarely implemented without software or firmware, even at the ASIC level. Programmable DSPs, in particular, form the heart of many implementations. An aggressive new implementation technology is to use one or more "DSP cores" together with custom circuitry. DSP cores are programmable architectures sold as silicon macro blocks rather than as separate components. They are used as large macrocells in application-specific ICs. Such ASICs are customized to contain precisely the memory and peripherals required by an application, and can also include arbitrary custom logic, configurable logic, or analog circuitry.

The first major market for DSP cores is digital cellular telephony. DSP vendors have developed specialized versions of their commodity DSPs that support both the GSM standard (for Europe) and the IS-54 standard (for the U.S.). For example, the Ericsson HotLine GH197 is a GSM hand-held telephone that uses an ADSP-2102 from Analog Devices. The Motorola DSP56156 is a DSP with carefully chosen peripherals and memory capacity to support the European GSM standard. The Motorola DSP56166 is a variant capable of implementing the VSELP speech coder in the U.S. and Japanese digital cellular standards.

So far, however, the customized core-based ASICs for this application are being designed by the DSP vendor, and not by the producer of the telephone equipment. This approach is viable because the functionality of the ASIC is specified by an international standard, and the market is expected to be very large. However, more proprietary designs cannot proceed in this manner. The design process will more closely resemble that of board-level products using commodity DSPs. Such designs, of course, are mixed hardware and software designs. Our approach to code generation is carefully architected to support such heterogeneous designs.

Any complete system design methodology, therefore, must include software synthesis for programmable devices. Mainstream design tool vendors for signal processing, such as those provided by Comdisco Systems, Mentor Graphics, and CADIS, have recognized this. They have all recently added software synthesis for DSPs to their tools (see for example [1] and [2]). Looking forward, future tools should also include high-level software synthesis for real-time control as well as coupling to high-level hardware synthesis tools. Since the design styles for these capabilities are likely to be radically different from one another, the ideal methodology must cleanly support heterogeneity. This paper will concentrate on code generation for DSP, but will describe a software architecture capable of adapting to such heterogeneous design problems.

A number of design styles can be used to develop signal processing software. One option, of course, is to rely on traditional high-level languages, notably C or Ada. Unfortunately, for many intensive signal processing applications, compilers for these languages are still unable to achieve the code efficiency demanded by designers. Twelve years after the appearance of programmable DSPs, most designers still prefer to program them in assembly language. The difficulty appears to be both in the languages themselves, which are not sufficiently specific to signal processing and poorly matched to fixed point data types; and in the processor architectures, which include features that compilers cannot easily support such as esoteric addressing modes (for example, bit reversed addressing for FFTs and hardware support for circular buffers). Numeric C [3] offers an interesting alternative by modifying the syntax of C to expose to the compiler much of the information it needs. Silage, an applicative language developed b We are pursuing a third alternative, embodied previously in the Gabriel system [5], and more recently implemented in the Ptolemy system [6]. In this methodology, hand written assembly code segments define functional operators on data streams. Code generation consists of two phases, scheduling and synthesis. In the scheduling phase, the functional operators are possibly partitioned for parallel execution, and for each target processor, a sequence of operator invocation is determined. In the synthesis phase, the hand-written assembly code segments (or alternatively, higher-level language code segments or a mixture of both) are stitched together. This methodology has recently been commercialized in the Comdisco DPC system [1] and will be commercialized in the CADIS Descartes [7] systems. The techniques we describe here are complementary to those in DPC and Descartes, and could, in principle, be used in combination. In particular, we focus on management of data passed between functional blocks when synchronous

1.1 - Overview of Ptolemy
1.2 - Code Generation Domains

Software Synthesis for Single-Processor DSP Systems Using Ptolemy - 04 SEP 94

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