Architecture of FPGAs and CPLDs

Architecture of FPGAs and CPLDs


This article provides a tutorial survey of architectures of commercially available high-capacity field-programmable devices (FPDs). I first define the relevant terminology in the field and then describe the recent evolution of FPDs.

The three main categories of FPDs are delineated:
1.Simple PLDs (SPLDs),
2.Complex PLDs (CPLDs) and
3.Field-Programmable Gate Arrays (FPGAs).

Definitions of Relevant Terminology

The most important terminology is defined below.

• Field-Programmable Device (FPD) — 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 FPDs is programmable logic devices (PLDs); although PLDs encompass the same types of
chips as FPDs, 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 FPD 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 user-programmable).
• PAL* — a Programmable Array Logic (PAL) is a relatively small FPD 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 FPD 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.
• 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.
* PAL is a trademark of Advanced Micro Devices.
• Interconnect — the wiring resources in an FPD.
• 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 FPD. When a
circuit is implemented in an FPD, it is first decomposed into smaller sub-circuits 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 a CPLD.
• Logic Capacity — the amount of digital logic that can be mapped into a single FPD. This is
usually measured in units of “equivalent number of gates in a traditional gate array”. In other
words, the capacity of an FPD 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 2-input NAND gates”.
• Logic Density—the amount of logic per unit area in an FPD.
• Speed-Performance — measures the maximum operable speed of a circuit when implemented
in an FPD. 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 FPD development the evolution of
FPDs over the past two decades is described. Additional background information is also included
on the semiconductor technologies used in the manufacture of FPDs.



Overview of Commercially Available FPDs

This section provides many examples of commercial FPD products. SPLDs are first discussed
briefly, and then details are given for all of the most important CPLDs and FPGAs.

For more details on the commercial products is to contact the manufacturers,
or their distributors, for the latest data sheets.

Most FPD manufacturers now provide their data sheets on the world wide web, and can be located at URL “http://www.companyname.com”.



Commercially Available SPLDs

As the staple for digital hardware designers for the past two decades, SPLDs are very important
devices. SPLDs represent the highest speed-performance FPDs available, and are inexpensive.
However, they are also fairly straight-forward and well understood, so this paper will discuss
them only briefly.

Two of the most popular SPLDs are the PALs produced by Advanced Micro Devices (AMD)
known as the 16R8 and 22V10. Both of these devices are industry standards and are widely second- sourced by various companies. The name “16R8” means that the PAL has a maximum of 16 inputs (there are 8 dedicated inputs and 8 input/outputs), and a maximum of 8 outputs. The “R” refers to the type of outputs provided by the PAL and means that each output is “registered” by a D flip-flop. Similarly, the “22V10” has a maximum of 22 inputs and 10 outputs. Here, the “V” means each output is “versatile” and can be configured in various ways, some configurations registered and some not.

Another widely used and second sourced SPLD is the Altera Classic EP610. This device is
similar in complexity to PALs, but it offers more flexibility in the way that outputs are produced
and has larger AND- and OR- planes. In the EP610, outputs can be registered and the flip-flops
are configurable as any of D, T, JK, or SR.

In addition to the SPLDs mentioned above many other products are available from a wide
array of companies. All SPLDs share common characteristics, like some sort of logic planes
(AND, OR, NOR, or NAND), but each specific product offers unique features that may be particularly attractive for some applications.
A partial list of companies that offer SPLDs includes:
AMD, Altera, ICT, Lattice, Cypress, and Philips-Signetics.

Commercially Available CPLDs

CPLDs consist of multiple SPLD-like blocks on a single chip. However, CPLD products are much more sophisticated than SPLDs, even at the level of their basic SPLD-like blocks. In this section, CPLDs are discussed in detail, first by surveying the available commercial products and then by discussing the types of applications for which CPLDs are best suited.


A partial list of companies that offer CPLDs includes:
Altera FLASHlogic CPLDs, Xilinx XC7000 CPLDs, Cypress FLASH370 CPLDs, Lattice CPLDs,
Advanced Micro Devices (AMD) CPLDs, Altera CPLDs

Commercially Available FPGAs


There are two basic categories of FPGAs on the market today:
1. SRAM-based FPGAs and
2.antifuse-based FPGAs.
In the first category, Xilinx and Altera are the leading manufacturers in terms of number of users, with the major competitor being AT&T. For antifuse-based products, Actel, Quicklogic and Cypress, and Xilinx offer competing products.

A partial list of companies that offer FPGAs includes:

Xilinx SRAM-based FPGAs, Altera FLEX 8000 and FLEX 10000 FPGAs, AT&T ORCA FPGAs,
Actel FPGAs, Quicklogic pASIC FPGAs,

For more details check the links

http://www.cra.org/Activities/craw/dmp/awards/2005/Hu/files/references/Toronto.pdf

http://iroi.seu.edu.cn/prepare/Reference/File/ebook/Book2/CH05/CH05.2.htm

http://www.cse.iitd.ernet.in/~mbala/cs719/lectures/lect32/sld001.htm