FPGA and CPLD Architectures: A Tutorial

new types of sophisticated field-programmable devices (FPDs) has dramatically changed the process of designing digital hardware. Unlike previous generations of hardware technology in which board level designs included large numbers of SSI (small-scale integration) chips containing basic gates, virtually every digital design produced today consists mostly of high-density devices. This is true not only of custom devices such as processors and memory but also of logic circuits such as state machine controllers, counters, registers , and decoders. When such circuits are destined for high-volume systems, designers integrate them into high-density gate arrays. However, the high nonrecurring engineering costs and long manufacturing time of gate arrays make them unsuitable for prototyping or other low-volume scenarios. Therefore, most prototypes and many production designs now use FPDs. The most compelling advantages of FPDs are low startup cost, low financial risk, and, because the end user programs the device, quick manufacturing turnaround and easy design changes. The FPD market has grown over the past decade to the point where there is now a wide assortment of devices to choose from. To choose a product, designers face the daunting task of researching the best uses of the various chips and learning the intricacies of vendor-specific software. Adding to the difficulty is the complexity of the more sophisticated devices. To help sort out the confusion, we provide an overview of the various FPD architectures and discuss the most important commercial products, emphasizing devices with relatively high logic capacity. The first user-programmable chip that could implement logic circuits was the programmable read-only memory (PROM), in which address lines serve as logic circuit inputs and data lines as outputs. Logic functions, however, rarely require more than a few product terms, and a PROM contains a full decoder for its address inputs. PROMs are thus inefficient for realizing logic circuits, so designers rarely use them for that purpose. The first device developed specifically for implementing logic circuits was the field-programmable logic array, or simply PLA for short. A PLA consists of two levels of logic gates: a programmable, wired-AND plane followed by a programmable, wired OR plane. A PLA's structure allows any of its inputs (or their complements) to be ANDed together in the AND plane; each AND plane output can thus correspond to any product term of the inputs. Similarly, users can configure each OR This tutorial surveys commercially available, high-capacity field-programmable devices. The authors describe the three …