Understanding programmable logic means digesting its alphabet soup

Understanding programmable logic means digesting its alphabet soup

 Updated version of an article originally published in Personal Engineering and Instrumentation News, July 1997, pages 75-78. (download original in Adobat Acrobat format)

If you’re not already using high-density programmable logic, it�s probably time to start. Prices for both devices and tools are dropping to where they�re attractive alternatives. In case you haven�t been watching, a massive battle is taking place for supremacy in the programmable-logic industry. The total market rocketed past the $1 billion mark as industry titans Xilinx, Altera, Actel, Lattice, Vantis and others fight for precious market share. As a result, device prices have dropped dramatically. A 2000-gate device that cost more than $100 in 1986 now sells for less than $5 in volume. Similarly, dramatic increases in density pushed gate counts well past the 50,000 mark.

Likewise, development software no longer costs between $5000 and $10,000. For $500 or less, a designer gets design-entry, implementation and simulation capabilities. Granted, a $500 package won�t provide all the options of a $5000 system, but it does provide sufficient capabilities for moderate-density designs. Further, broad support for programmable logic has emerged from CAE software companies. Even more importantly, high-end CAE suppliers now provide alternatives under Win95 and NT.

Thanks to these advances, programmable logic has become a mainstream technology. Look through electronics magazines, and you can�t miss ads for boards using it–even systems selling for less than $100. Another sign is that intellectual property (IP) companies now target their cores or hard macros for programmable logic as well as gate arrays. For engineering managers, it�s important to note that most recent EE graduates have gained experience with programmable logic as part of their undergraduate curriculum. Most recent graduates have the knowledge and skills to use these devices today.

Using programmable logic

Deciding whether or not to use programmable logic depends on the application. If a design requires 300 to 100,000 gates and operates below roughly 60 MHz, the answer is a resounding yes. Multiple devices conquer higher density designs but at a greater expense, especially for high-volume applications.

Because the logic is on-site pro-grammable, design modifications and updates are far easier than with many other technologies. Most of a design resides on one or several chips, thereby reducing the number of “blue-wire” fixes tacked to the backside of a board. Also, changes cost little or nothing. At worst, you might throw away a few one-time programmable devices. Contrast this low cost of change to a gate array where modifications can cost $10,000 and weeks if not months of delay.

Further, many high-density devices are reprogrammable while in a system. Changes are practically free, and you can even make them when a board is deployed in the field. Some designers use this capability to adapt a board to unique configurations. Instead of shipping four different models for four applications, some companies ship one board that performs multiple tasks. This practice reduces engineering costs and simplifies manufacturing.

Design changes with programmable logic are also quick. An engineer can make a modification, recompile a design and download the change into the device in anywhere from 5 min to a few hours, depending on the device size and technology.

Obviously, programmable logic places other technologies. Because of its nature, designers use it in novel applications to replace or enhance existing processors. In some applications, especially DSP, specific compute- intensive algorithms consume most of a processor�s capability. Some designs implement these algorithms in hardware (with programmable logic) to offload time-critical or repetitive tasks. This specialized hardware frees the processor to perform other useful functions. For example, a 16-tap FIR filter in a field-programmable gate array (FPGA) can outperform a standard DSP chip by a factor of ten or more. In another example, a board full of FPGAs outperformed a Cray supercomputer in a pattern-matching application for the Human Genome Project. Programmable logic is also an enabling technology for a new field called reconfigurable computing in which a designer tailors the processing element to the specific application. This customization provides significantly higher performance than is possible with standard processors.

Understanding the lingo

Like most any technology, programmable logic has its unique jargon. While by no means exhaustive, the following terms provide a quick introduction to important concepts.

FPGA (Field-programmable Gate Array)

A generic description of an FPGA is a programmable device with an internal array of logic blocks, surrounded by a ring of programmable I/O blocks, connected by programmable interconnect (Fig 1). The secret to density and performance in these devices lies in the circuitry of their logic blocks and on the performance and efficiency of their routing architecture.


Figure 1. The FPGA architecture uses many relatively small logic blocks with independent I/O blocks.

This group breaks down into a wide variety of subarchitectures, but you�ll find two primary classes of FPGA architectures. First, coarse-grained architectures consist of fairly large logic blocks, often containing two or more lookup tables and two or more flip-flops. In these architectures, a 4-input lookup table (think of it as a 16 x 1 ROM) implements the actual logic.

The other architecture is called fine-grained. These devices hold a large number of relatively simple logic blocks. Each block usually contains a flip-flop and either a 2-input logic function or a 4:1 multiplexer. These devices excel at bit-serial functions such as shift registers, corner benders or serial arithmetic, and they offer benefits for logic-synthesis-generated designs.

Another difference in architectures concerns the underlying process technology. Currently, the highest-density FPGAs use static memory (SRAM) technology, similar to micro-processors. The other common process technology is antifuse, which has benefits for programmable interconnect (see Table 1).


Table 1. FPGA designers can select any combination of programming method and architecture granularity by their chode of programmable logic vendors.
Architecture Static Memory Anti-Fuse Flash
Coarse-grained Altera: (FLEX)
Atmel: (AT40K)
Lucent: (ORCA)
Vantis: (VF1)
Xilinx: (XC3000,XC4000xx,Spartan,XC5200)
QuickLogic .
Fine-grained Actel: (SPGA)
Atmel: (AT6000)
Xilinx: (XC6200)
Actel: (ACT) Gatefield

NOTE: Also see the summary table of programmable devices and manufacturers.

SRAM-based devices are inherently reprogrammable, even in-system. A configuration memory holds the program that defines how each logic block functions, whether I/O blocks serve as inputs and outputs and how blocks connect to each other. An FPGA either self-loads its configuration program, or an external processor downloads the program. When self-loading, an FPGA addresses a standard byte-wide memory much like a processor or uses a special sequential-access serial PROM. When a processor downloads a program, an FPGA appears much like a standard microprocessor peripheral. The configuration time is typically < 200 msec, depending on device size and configuration method.

In contrast, antifuse devices are one-time programmable (OTP). Once programmed, they can�t be modified but do retain their program when power is off. Antifuse devices require a specialized or a high-end programmer, as most programmers don�t support antifuse.

Many course-grained FPGAs also contain system-level features to boost performance or to simplify system design. For instance, dedicated carry logic boosts performance for arithmetic functions and counters. Bidirectional busing mirrors what you�ll find on many boards and systems, allowing easy integration of bused registers and other logic elements. Further, on-chip RAM simplifies register files, FIFOs and small data-buffering applications.

Generally, FPGAs have many more registers and I/O than CPLDs and typically use less power. FPGAs are usually best for datapath-oriented design but don�t have the fast pin-to-pin performance associated with CPLDs.

CPLD (Complex Programmable Logic Device)

In concept, CPLDs consist of multiple PAL-like logic blocks interconnected with a programmable switch matrix (Fig 2). Typically, each logic block contains four to 16 macrocells depending on the vendor and the architecture.


Figure 2. The CPLD (or EPLD) architecture uses a few large, PAL-like blocks with I/O fixed inside of each block.

A macrocell on most modern CPLDs contains a sum-of-products combinatorial logic function and an optional flip-flop. The combinatorial logic function typically supports four to 16 product terms with wide fan-in. In other words, a macrocell function can have many inputs, but the complexity of the logic function is limited. Contrast this structure to an FPGA logic block where complexity is unlimited, but the lookup table has only four inputs.

CPLDs provide a natural migration path for PAL designers seeking higher density. They offer a PAL-like architecture, and generally four or more PALs comfortably fit into a CPLD. Most CPLDs support PAL development languages such as ABEL, CuPL and Palasm.

CPLDs are generally best for control- oriented designs due in part to their fast pin-to-pin performance. The wide fan-in of their macrocells makes them well-suited to complex, high-performance state machines.

One major variation among CPLD architectures concerns how they deal with insufficient product terms. In some architectures, when the number of product terms a design requires exceeds the number available in the macrocell, design software borrows terms from an adjoining macrocell. When borrowing terms, the adjoining macrocell might not be useful any longer in some architectures, while in others, the macrocell retains some basic functionality. In addition, bor-rowed product terms usually increase propagation delay.

Another difference deals with the number of connections within the switch matrix. A switch matrix supporting all possible connections is fully populated, while a partially populated switch supports most connections. The number of connections within the matrix determines how easily a design fits in a given device. With a fully populated switch matrix, a design routes even with a majority of the device resources used and with fixed I/O pin assignment. Generally, delays within a fully populated switch matrix are fixed and predictable.

A device with a partially populated switch matrix might present problems to tools when routing complex designs. Also, it might be difficult to make design changes in these devices without altering the pinout. Routing to a fixed pinout is important because it�s easier to change the internals of a PLD than to relay out a circuit board. Though a partially populated switch matrix is less expensive to manufacture, it might be more difficult to use. The delays within a partially populated matrix can�t be fixed and aren�t easy to predict.

CPLDs are based on one of three process technologies

  • EPROM,
  • EEPROM, or
  • FLASH.

EPROM-based CPLDs are usually one-time programmable (OTP) unless they come in a UV-erasable windowed package. As is the case with antifuse FPGAs, you must use a high-end programmer to program CPLDs.

EEPROM and FLASH processes are erasable technologies. However, not all EEPROM- and FLASH-based devices are programmable while soldered on a board. In-system programmability (ISP) requires special on-chip programming logic, and not all CPLDs come with it, even when built with EEPROM and FLASH technologies. You can erase and program those lacking that circuitry in a device programmer. Those with the circuitry either use a vendor-proprietary interface or a JTAG (IEEE 1149.1) interface for programming.

EPLD (Erasable Programmable Logic Device)

See CPLD. Some companies prefer the term EPLD, others use CPLD. Otherwise they�re essentially the same.


Fitting is a term usually associated with CPLDs. It includes the process of optimizing and grouping logic elements for a design to best fit available resources as well as allocating logic to specific macrocells and interconnecting signals through the switch matrix.

ISP (In-System Programmable)

ISP is generally associated with CPLD devices based on EEPROM or FLASH technology that you can program in the system. However, this term also applies to all FPGAs based on static memory (SRAM) technology, which is inherently in-system programmable.

Logic Block/Cell

A logic block or cell is the basic build-ing block. In an FPGA, the logic block typically contains two or more LUTs and two or more flip-flops. The combination of a LUT and a flip-flop is sometimes called a logic cell. In a CPLD, the logic block typically contains four to 16 macrocells.

LUT (Lookup Table)

A lookup table is a common logic element in most coarse-grained FPGAs. Essentially, it�s equivalent to a small ROM. For example, a 4-input LUT is equivalent to a 16×1 ROM, with the four logic inputs becoming the ROM�s address lines. The ROM�s contents are the logical function of those four inputs. Regardless of complexity, a logic function with four inputs fits into a 4-input LUT and has a fixed delay. For example, a LUT-based implementation of an inverter and a 4-input XOR function have the same delay.

OTP (One-time Programmable)

This term applies to any device you can program only once. They include EPROM-based devices in windowless packages and all antifuse-based devices.


In partitioning or mapping, design software reduces, optimizes and groups logical elements within a design to best fit into the logic blocks or resources available on the programmable- logic device.


A term usually associated with FPGAs, placement is the process of searching for the best location to place a logic block. For FPGAs, it directly affects the performance of the end design and the amount of interconnect resources required.


The term routing appears as either a noun or verb. As a noun, it refers to the interconnect resources available on a device. These resources are simi-lar to the traces on a circuit board and connect logic blocks and I/O blocks to form a user design. As a verb, routing refers to the process of interconnecting logic and I/O blocks. This process is generally compute-intensive, especially for large FPGA devices.

Additional terms

The preceding list is by no means exhaustive. Each vendor has its own list of trademarked names for these same features. A more extensive glossary of terms, including vendor-specific jargon, is available on the web at http://www.reconfig.com/gloss.htm.