FPGAs ABSTRACT This report provides a survey of architectures of commercially available highcapacity field-programmable logic devices (FPLDs) ex. FPGAs and also the applications of FPGAs. We first define the relevant terminology in the field and then describe the recent evolution of FPLDs. The three main categories of FPLDs are delineated: Simple PLDs (SPLDs), Complex PLDs (CPLDs) and Field-Programmable Gate Arrays (FPGAs). The details of the architectures of the most important commercially available FPGAs are given. 1 FPGAs CHAPTER 1 INTRODUCTION TO HIGH CAPACITY FPLDs
Prompted by the development of new types of sophisticated field-programmable logic devices (FPLDs), the process of designing digital hardware has changed dramatically over the past few years. Unlike previous generations of technology, in which board-level designs included large numbers of SSI chips containing basic gates, virtually every digital design produced today consists mostly of high-density devices. This applies not only to custom devices like processors and memory, but also for logic circuits such as state machine controllers, counters, registers, and decoders.
When such circuits are destined for high-volume systems, they have been integrated into high-density gate arrays. However, gate array NRE costs often are too expensive and gate arrays take too long to manufacture to be viable for prototyping or other low-volume scenarios. For these reasons, most prototypes, and also many production designs are now built using FPLDs. The most compelling advantages of FPLDs are instant manufacturing turnaround, low start-up costs, low financial risk and (since programming is done by the end user) ease of design changes....
The market for FPLDs has grown dramatically over the past decade to the point where there is now a wide assortment of devices to choose from. A designer today faces a daunting task to research the different types of chips, understand what they can best be used for, choose a particular manufacturer’s product, learn the intricacies of vendor-specific software and then design the hardware. Not only not only the sheer number of FPLDs available exacerbates confusion for designers, but also by the complexity of the more sophisticated devices.
The purpose of this paper is to provide an overview of the architecture of the various types of FPLDs. The emphasis is on devices with relatively high logic capacity; all of the most important commercial products are discussed. Before proceeding, we provide definitions of the terminology in this field. This is necessary because the technical jargon has become somewhat inconsistent over the past few years as companies have attempted to compare and contrast their products in literature. 2 FPGAs 1. 1 Definitions of Relevant Terminology The most important terminology used in this report is defined below. Field-Programmable Logic Device (FLPD) — a general term that refers to any type of integrated circuit used for implementing digital hardware, where the chip can be configured by the end user to realize different designs. Programming of such a device often involves placing the chip into a special programming unit, but some chips can also be configured “in-system”. Another name for FPLDs is programmable logic devices (PLDs); although PLDs encompass the same types of chips as FPLDs, we prefer the term FPD because historically the word PLD has referred
to relatively simple types of devices. PLA — a Programmable Logic Array (PLA) is a relatively small FPLD that contains two levels of logic, an AND-plane and an OR-plane, where both levels are programmable (note: although PLA structures are sometimes embedded into full-custom chips, we refer here only to those PLAs that are provided as separate integrated circuits and are userprogrammable). • PAL—a Programmable Array Logic (PAL) is a relatively small FPLD that has a programmable AND-plane followed by a fixed OR-plane. • SPLD — refers to any type of Simple PLD, usually either a PLA or PAL. CPLD — a more Complex PLD that consists of an arrangement of multiple SPLD-like blocks on a single chip. Alternative names (that will not be used in this paper) sometimes adopted for this style of chip are Enhanced PLD (EPLD), Super PAL, Mega PAL, and others. • FPGA — a Field-Programmable Gate Array is an FPLD featuring a general structure that allows very high logic capacity. Whereas CPLDs feature logic resources with a wide number of inputs (AND planes), FPGAs offer more narrow logic resources. FPGAs also offer a higher ratio of flip-flops to logic resources than do CPLDs. FPGAs • HCPLDs — high-capacity PLDs: a single acronym that refers to both CPLDs and FPGAs. This term has been coined in trade literature for providing an easy way to refer to both types of devices. We do not use this term in the report. • Interconnect — the wiring resources in an FPLD. • Programmable Switch — a user-programmable switch that can connect a logic element to an interconnect wire, or one interconnect wire to another. • Logic Block — a relatively small circuit block that is replicated in an array in an FPLD.
When a circuit is implemented in an FPLD, it is first decomposed into smaller subcircuits that can each be mapped into a logic block. The term logic block is mostly used in the context of FPGAs, but it could also refer to a block of circuitry in an CPLD. • Logic Capacity — the amount of digital logic that can be mapped into a single FPLD. This is usually measured in units of “equivalent number of gates in a traditional gate array”. In other words, the capacity of an FPLD is measured by the size of gate array that it is comparable to.
In simpler terms, logic capacity can be thought of as “number of 2input NAND gates”. • Logic Density — the amount of logic per unit area in an FPLD. • Speed-Performance — measures the maximum operable speed of a circuit when implemented in an FPLD. For combinational circuits, it is set by the longest delay through any path, and for sequential circuits it is the maximum clock frequency for which the circuit functions properly. In the remainder of this section, to provide insight into FPLD development the evolution of FPLDs over the past two decades is described.
Additional background information is also included on the semiconductor technologies used in the manufacture of FPLDs. 4 FPGAs • Full Custom / VLSI IC Technology — In this technology, all layers
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