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5-3Publication# 90008 Rev. A Amendment /0

Issue Date: June 1993

Basic Design with PLDs

AdvancedMicro

Devices

INTRODUCTIONThe Programmable Array Logic device, commonlyknown as the PAL device, was invented at MonolithicMemories in 1978. The concept for this revolutionarytype of device sprang forth as a simple solution to theshort comings of discrete TTL logic.

The successfully proven PROM technology whichallowed the end user to “write on silicon” provided thetechnological basis which made this kind of device notonly possible, but very popular as well.

The availability of design software made it much easierto design with programmable logic. As designers werefreed from the drudgery of low-level implementationissues, new complex designs were easier to implement,and could be completed more quickly.

This chapter outlines some basic information essentialto those who are unfamiliar with Programmable Logicdevices (PLDs). The information may also be useful tothose who are current users of programmable logic. Thespecific issues which need to be addressed are:

What is a PLD?

What other implementations are possible?

What advantages do PLDs have over otherimplementations?

WHAT IS A PLD?

In general, a programmable logic device is a circuitwhich can be configured by the user to perform a logicfunction. Most “standard” PLDs consist of an AND arrayfollowed by an OR array, either (or both) of which isprogrammable. Inputs are fed into the AND array, whichperforms the desired AND functions and generatesproduct terms. The product terms are then fed into theOR array. In the OR array, the outputs of the variousproduct terms are combined to produce the desiredoutputs.

PAL Devices

The PAL device has a programmable AND arrayfollowed by a fixed OR array (Figure 1). The fact that theAND array is programmable makes it possible for thedevices to have many inputs. The fact that the OR arrayis fixed makes the devices small (which means lessexpensive) and fast.

WHAT OTHER IMPLEMENTATIONS AREPOSSIBLE?

There are essentially four alternatives to programmablelogic:

Discrete Logic

Gate Arrays

Standard Cell Circuits

Full Custom Circuits

Discrete Logic

Discrete logic, or conventional TTL logic, has theadvantage of familiarity; hence its popularity. It is alsoquite inexpensive when only unit cost is considered. Thedrawback is that the implementation of even a simpleportion of a system may require many units of discretelogic. There are “hidden” costs associated with each unitthat goes into a system, which can render the overallsystem more expensive.

Designing with discrete chips can also be very tedious.Each design decision directly affects the layout of theboard. Changes are difficult to make. The design is alsomore difficult to document, making it harder to debugand maintain later. These items all contribute to a long

design cycle when discrete chips are used extensively.

Gate Arrays

Gate arrays have been increasing in popularity. Theattractiveness of this solution lies in the device’sflexibility. By packing the functions into the device, agreat majority of the available silicon is actually used.Since such a device is customized for an application, itwould seem to be the optimum device for thatapplication.

However, one also pays substantial development costs,especially in the case of a design which needs changes

after silicon has already been processed. Even thoughthe unit costs are generally quite low for gate arrays, thevolumes required to make their use worthwhile excludesthem as a solution for many designers. This fact, addedto the long design cycle and high risk involved, make thissolution practical for only a limited number of designers.

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5-4 Basic Design with PLDs

I5 I4 I3 I2 I1 I0

O3 O2 O1 O0

Programmable AND Array

Fixed OR Array

Indicates Programmable Connection

Indicates Fixed Connection90008A-1

Figure 1. PAL Device Array Structure

Standard Cell Circuits

Standard cell circuits are quite similar to gate arrays,their main advantage being that they consist of acollection of different parts of circuits which havealready been debugged. These circuits are thenassembled and collected to perform the desiredfunctions. This can ideally lead to reduced turn aroundfrom conception to implementation, and a much moreefficient circuit.

The drawback is that even though the individualcomponents of the circuit have been laid out, a completelayout must still be performed to arrange the cells.Instead of just customizing the metal interconnections,as is done in a gate array, the circuit must be developedfrom the bottom up. Development costs can be evenhigher than for gate arrays, and despite the standard cell

concept, turn around time often tends to be longer thanplanned. Again, the volume must be sufficiently high towarrant the development costs.

Full Custom Circuits

Full custom designs require that a specific chip bedesigned from scratch to perform the needed functions.The intent is to provide a solution which gives thedesigner exactly what is needed for the application inquestion; no more and no less. Ideally, not a square

micron of silicon is wasted. This normally results in thesmallest piece of silicon possible to fit the needs of thedesign, which in turn reduces the system cost.Understandably, though, development costs and risksfor such a design are extremely high, and volumes mustbe commensurately high in order for such a solution tobe ofvalue.

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5-5Basic Design with PLDs

WHAT ADVANTAGES DO PLDs HAVEOVER OTHER IMPLEMENTATIONS?

As user-programmable semicustom circuits, PLDsprovide a valuable compromise which combines manyof the benefits of discrete logic with many of the benefitsof other semicustom circuits. The overall advantagescan be found in several areas:

Ease of design

Performance

Reliability

Cost savings

Ease of Design

The support tools available for use in designing withPLDs greatly simplify the design process by making thelower-level implementation details transparent. In amatter of one or two hours, a first time PLD user canlearn to design with a PAL device, program it, and

implement the design in a system.

The design support tools consist of design software anda programmer. The design software is used ingenerating the design; the programmer is used toconfigure the device. The software provides the linkbetween the higher-level design and the low-levelprogramming details.

All of the available design software packages performessentially the same tasks. The design is specified withrelatively high-level constructs; the software takes thedesign and converts it into a form which the programmeruses to configure the PLD. Most software packages

provide logic simulation, which allows one to debug thedesign before actually programming a device. Thehigh-level design file also serves as documentation ofthe design. This documentation can be even easier tounderstand than traditional schematics.

Many PLD users do not find it necessary to purchase aprogrammer; it is often quite cost effective andconvenient to have either the manufacturer or anoutside distributor do the programming for them. Fordesign and prototyping, though, it is very helpful to havea programmer; this allows one to implement designsimmediately.

The convenience of programmable logic lies in theability to customize a standard, off-the-shelf product.PLDs can be found in stock to suit a wide range of speedand power requirements. The variety of architecturesavailable also allows a choice of the proper functionality

for the application at hand. Thus, a design can beimplemented using a standard device, with the endresult essentially being a custom device. If a designchange is needed, it is a simple matter to edit the originaldesign and then program a new device, or, in the case ofreprogrammable CMOS devices, erase and reprogramthe old device.

Board layout is vastly simplified with the use ofprogrammable logic. PLDs offer great flexibility in thelocation of inputs and outputs on the device. Sincelarger functions are implemented inside the PLD, boardlayout can begin once the inputs and outputs are known.The details of what will actually be inside the PLD can beworked out independently of the layout. In any cases,any needed design changes can be taken care ofentirely within the PLD, and will not affect the PC board.

Performance

Speed is one of the main reasons that designers usePAL devices. The PAL devices can provide equal or

better performance than the fastest discrete logicavailable. Today’s fastest PAL devices are beingdeveloped on the newest technologies to gain everyextra nanosecond of performance.

Performance cannot come strictly at the expense ofpower consumption. Since PLDs can be used to replaceseveral discrete circuits, the power consumption of aPLD may well be less than that of the combined discretedevices. As more PLDs are developed in CMOStechnology, the option for even lower power becomesavailable, including zero standby power devices forsystems which can tolerate only minute standby powerconsumption.

Reliability

Reliability is an area of increasing concern. As systemsget larger and more complex, the increase in the amountof circuitry tends to reduce the reliability of the system;there are “more things to go wrong.” Thus, a solutionwhich inherently reduces the number of chips in thesystem will contribute to higher reliability. Aprogrammable logic approach can provide a morereliable solution due to the smaller number of devicesrequired.

With the reduction in units and board space, PC boards

can be laid out less densely, which greatly improves thereliability of the board itself. This also reduces crosstalkand other potential sources of noise, making theoperation of the system cleaner and more reliable.

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5-7Basic Design with PLDs

Another economic benefit of the use of PLDs is thatwhen one PAL device is used in several differentdesigns, as is often the case, the user has notcommitted that device to any one of the particulardesigns until the device has been programmed. Thismeans that inventory can be stocked for severaldifferent designs in the form of one device. Asrequirements change, the parts can be programmed to

fit the need. And in the case of reprogrammable CMOSdevices, one is not committed even after programming.

One final subtle cost issue is derived from the ease withwhich a competitor can copy a design. PLDs have aunique feature called a security bit, whose purpose is toprotect a design from being copied. By using securedPLDs extensively in a system, one can safely avoidhaving one’s system easily deciphered. The addeddesign security provided by this feature can buy extramarket time, forcing competitors to do their own originaldesign work rather that copying the designs of others.

Summary

Programmable logic provides the means of creatingsemi-custom designs with readily available standardcomponents. There is a wide variety of PLDs; PALdevices are most widely used, and perform well for basiclogic and some sequencing functions.

By assuming some of the attributes of gate arrays,programmable logic provides the cost savings of anyother semicustom device, without the extra engineeringcosts, risks, and design delays. Reliability is alsoenhanced as quality increases and board complexitydecreases.

The design tasks are greatly simplified due to the design

tools which are now available. Design software anddevice programmers allow top-down high-level designswith a minimum of time spent on actual implementationissues. Simulation allows some design debug before adevice is programmed.

For all of these reasons, programmable logic hasbecome, and will continue to be, the designmethodology of choice among digital systemsdesigners.