Published on Apr 02, 2024
A quiet revolution is taking place. Over the past few years, the density of the average programmable logic device has begun to skyrocket. The maximum number of gates in an FPGA is currently around 500,000 and doubling every 18 months.
Meanwhile, the price of these chips is dropping. What all of this means is that the price of an individual NAND or NOR is rapidly approaching zero! And the designers of embedded systems are taking note. Some system designers are buying processor cores and incorporating them into system-on-a-chip designs; others are eliminating the processor and software altogether, choosing an alternative hardware-only design.
As this trend continues, it becomes more difficult to separate hardware from software. After all, both hardware and software designers are now describing logic in high-level terms, albeit in different languages, and downloading the compiled result to a piece of silicon. Surely no one would claim that language choice alone marks a real distinction between the two fields. Turing's notion of machine-level equivalence and the existence of language-to-language translators have long ago taught us all that that kind of reasoning is foolish. There are even now products that allow designers to create their hardware designs in traditional programming languages like C. So language differences alone are not enough of a distinction.
Both hardware and software designs are compiled from a human-readable form into a machine-readable one. And both designs are ultimately loaded into some piece of silicon. Does it matter that one chip is a memory device and the other a piece of programmable logic? If not, how else can we distinguish hardware from software?
Regardless of where the line is drawn, there will continue to be engineers like you and me who cross the boundary in our work. So rather than try to nail down a precise boundary between hardware and software design, we must assume that there will be overlap in the two fields. And we must all learn about new things. Hardware designers must learn how to write better programs, and software developers must learn how to utilize programmable logic.
Many types of programmable logic are available. The current range of offerings includes everything from small devices capable of implementing only a handful of logic equations to huge FPGAs that can hold an entire processor core (plus peripherals!). In addition to this incredible difference in size there is also much variation in architecture. In this section, I'll introduce you to the most common types of programmable logic and highlight the most important features of each type.
At the low end of the spectrum are the original Programmable Logic Devices (PLDs). These were the first chips that could be used to implement a flexible digital logic design in hardware. In other words, you could remove a couple of the 7400-series TTL parts (ANDs, ORs, and NOTs) from your board and replace them with a single PLD. Other names you might encounter for this class of device are Programmable Logic Array (PLA), Programmable Array Logic (PAL), and Generic Array Logic (GAL).
As chip densities increased, it was natural for the PLD manufacturers to evolve their products into larger (logically, but not necessarily physically) parts called Complex Programmable Logic Devices (CPLDs). For most practical purposes, CPLDs can be thought of as multiple PLDs (plus some programmable interconnect) in a single chip. The larger size of a CPLD allows you to implement either more logic equations or a more complicated design. In fact, these chips are large enough to replace dozens of those pesky 7400-series parts.
Figure 1 contains a block diagram of a hypothetical CPLD. Each of the four logic blocks shown there is the equivalent of one PLD. However, in an actual CPLD there may be more (or less) than four logic blocks. I've just drawn it that way for simplicity. Note also that these logic blocks are themselves comprised of macro cells and interconnect wiring, just like an ordinary PLD.
Unlike the programmable interconnect within a PLD, the switch matrix within a CPLD may or may not be fully connected. In other words, some of the theoretically possible connections between logic block outputs and inputs may not actually be supported within a given CPLD. The effect of this is most often to make 100% utilization of the macrocells which is very difficult to achieve. Some hardware designs simply won't fit within a given CPLD, even though there are sufficient logic gates and flip-flops available. Because CPLDs can hold larger designs than PLDs, their potential uses are more varied.
They are still sometimes used for simple applications like address decoding, but more often contain high-performance control-logic or complex finite state machines. At the high-end (in terms of numbers of gates), there is also a lot of overlap in potential applications with FPGAs. Traditionally, CPLDs have been chosen over FPGAs whenever high-performance logic is required. Because of its less flexible internal architecture, the delay through a CPLD (measured in nanoseconds) is more predictable and usually shorter.
Before the advent of programmable logic, custom logic circuits were built at the board level using standard components, or at the gate level in expensive application-specific (custom) integrated circuits. The FPGA is an integrated circuit that contains many (64 to over 10,000) identical logic cells that can be viewed as standard components. Each logic cell can independently take on any one of a limited set of personalities. The individual cells are interconnected by a matrix of wires and programmable switches. A user's design is implemented by specifying the simple logic function for each cell and selectively closing the switches in the interconnect matrix. The array of logic cells and interconnect form a fabric of basic building blocks for logic circuits. Complex designs are created by combining these basic blocks to create the desired circuit.
The logic cell architecture varies between different device families. Generally speaking, each logic cell combines a few binary inputs (typically between 3 and 10) to one or two outputs according to a boolean logic function specified in the user program . In most families, the user also has the option of registering the combinatorial output of the cell, so that clocked logic can be easily implemented. The cell's combinatorial logic may be physically implemented as a small look-up table memory (LUT) or as a set of multiplexers and gates. LUT devices tend to be a bit more flexible and provide more inputs per cell than multiplexer cells at the expense of propagation delay.
Field Programmable means that the FPGA's function is defined by a user's program rather than by the manufacturer of the device. A typical integrated circuit performs a particular function defined at the time of manufacture. In contrast, the FPGA's function is defined by a program written by someone other than the device manufacturer. Depending on the particular device, the program is either 'burned' in permanently or semi-permanently as part of a board assembly process, or is loaded from an external memory each time the device is powered up. This user programmability gives the user access to complex integrated designs without the high engineering costs associated with application specific integrated circuits.
Gate array is an approach to design and manufacture of ASICs. A gate array circuit, also known as an Uncommitted Logic Array (ULA) is a prefabricated circuit with no particular function in which transistors, standard logic gates, and other active devices are placed at regular predefined positions and manufactured on a wafer, usually called master slice. Creation of a circuit with a specified function is accomplished by adding metal interconnects to the chips on the master slice late in the manufacturing process, allowing the function of the chip to be customised as desired.
Gate array master slices are usually prefabricated and stockpiled in large quantities regardless of customer orders. The fabrication according to the individual customer specifications may be finished in a shorter time compared with standard cell or full custom design. The gate array approach reduces the mask costs since fewer custom masks need to be produced. In addition manufacturing test tooling lead time and costs are reduced since the same test fixtures may be used for all gate array products manufactured on the same die size.
Drawbacks are somewhat low density and performance than other approaches to ASIC design. However this style is often a viable approach for low production volumes. Field Programmable Gate Arrays (FPGAs) can be used to implement just about any hardware design. One common use is to prototype a lump of hardware that will eventually find its way into an ASIC. However, there is nothing to say that the FPGA can't remain in the final product.
Whether or not it does will depend on the relative weights of development cost and production cost for a particular project. (It costs significantly more to develop an ASIC, but the cost per chip may be lower in the long run. The cost tradeoff involves expected number of chips to be produced and the expected likelihood of hardware bugs and/or changes. This makes for a rather complicated cost analysis, to say the least.)
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