COURSE / CODE DIGITAL ELECTRONICS FUNDAMENTAL (ECE422 ... · COURSE / CODE DIGITAL ELECTRONICS FUNDAMENTAL (ECE422) PROGRAMMABLE LOGIC DEVICES (PLDs) A PLD, or programmable logic

Mohd Uzir Kamaluddin / Feb 2020 Page 1

COURSE / CODE DIGITAL ELECTRONICS FUNDAMENTAL (ECE422)

PROGRAMMABLE LOGIC DEVICES (PLDs)

A PLD, or programmable logic device, is an electronic component that is used in order to build

digital circuits that are reprogrammable. A programmable logic device does not have a defined

function once manufactured, unlike a logic gate and has to be programmed before it can be used.

Programmable logic devices (PLD) are designed with

configurable logic and flip-flops linked together with

programmable interconnect.

PLDs provide specific functions, including device-to-device

interfacing, data communication, signal processing, data display,

timing and control operations, and almost every other function a

system must perform. Memory cells control and define the

function that the logic performs and how the various logic

functions are interconnected.

FPGAs are used in a wide variety of applications ranging from

data processing and storage, to instrumentation, telecommunications, and digital signal

processing. CPLDs, by contrast, offer much smaller amounts of logic - up to about 10,000 gates.

But CPLDs offer very predictable timing characteristics and are therefore ideal for critical control

applications.

Some CPLDs require extremely low amounts of power and are very inexpensive, making them

ideal for cost-sensitive, battery-operated, portable applications such as mobile phones and digital

handheld assistants.

Categories

Logic devices can be classified into two broad categories - fixed and programmable.

Fixed Logic Devices

As the name suggests, the circuits in a fixed logic device are permanent, they perform one

function or set of functions - once manufactured, they cannot be changed.

With fixed logic devices, the time required to go from design, to prototypes, to a final

manufacturing run can take from several months to more than a year, depending on the

complexity of the device. And, if the device does not work properly, or if the requirements

change, a new design must be developed.


Programmable Logic Devices

On the other hand, programmable logic devices (PLDs) are

standard, off-the-shelf parts that offer customers a wide range of

logic capacity, features, speed, and voltage characteristics - and

these devices can be changed at any time to perform any number

of functions.

With programmable logic devices, designers use inexpensive

software tools to quickly develop, simulate, and test their

designs. Then, a design can be quickly programmed into a

device, and immediately tested in a live circuit. The PLD that is

used for this prototyping is the exact same PLD that will be used in the final production of a piece

of end equipment, such as a network router, a DSL modem, a DVD player, or an automotive

navigation system. There are no NRE costs and the final design is completed much faster than

that of a custom, fixed logic device.

Another key benefit of using PLDs is that during the design phase customers can change the

circuitry as often as they want until the design operates to their satisfaction. That's because PLDs

are based on re-writeable memory technology - to change the design, simply reprogram the

device. Once the design is final, customers can go into immediate production by simply

programming as many PLDs as they need with the final software design file.

Types

Generally, programmable logic devices can be described as being one of three different types:

Simple programmable logic devices (SPLD)

Complex programmable logic devices (CPLD)

Field programmable logic devices (FPGA)

Architecture

There are several manufacturers with many different families of PLD devices, so there are many

variations in architecture. The two major types of programmable logic devices are:

Field programmable gate arrays (FPGAs)

Complex programmable logic devices (CPLDs)

The distinction between the two is often a little fuzzy, with manufacturers designing new,

improved architectures, and frequently muddying the waters for marketing purposes. Together,

CPLDs and FPGAs are often referred to as high-capacity programmable logic devices (HCPLD).

Simple Programmable Logic Devices (SPLD)

Simple programmable logic devices (SPLD) are the simplest, smallest and least-expensive forms

of programmable logic devices. SPLDs can be used in boards to replace 7400-series TTL

components (AND, OR, and NOT gates).

Simple programmable logic devices typically comprise 4 to 22 fully connected macrocells. These

macrocells are typically comprised of some combinatorial logic (such as AND OR gates) and

a flip-flop. In other words, a small Boolean logic equation can be built within each macrocell. This

equation will combine the state of some number of binary inputs into a binary output and, if

necessary, store that output in the flip-flop until the next clock edge. Of course, the particulars of

the available logic gates and flip-flops are specific to each manufacturer and product family. But

the general idea is always the same.

Most SPLDs use either fuses or non-volatile memory cells (EPROM, EEPROM, FLASH, and

others) to define the functionality.

http://www.globalspec.com/learnmore/semiconductors_electronics/digital_logic_devices/flip_flops


These devices are also known as:

Programmable array logic (PAL)

Generic array logic (GAL)

Programmable logic arrays (PLA)

Field-programmable logic arrays (FPLA)

Programmable logic devices (PLD)

Advantages

PLDs are often used for address decoding, where they have several clear advantages over the

7400-series TTL parts that they replaced:

1. One chip requires less board area, power, and wiring than several do.

2. The design inside the chip is flexible, so a change in the logic does not require any

rewiring of the board. Rather, simply replacing one PLD with another part that has

been programmed with the new design can alter the decoding logic.

Functions

Programmable Logic Devices (PLDs) are digital devices with configurable logic and flip-flops

linked together with programmable interconnect. Logic devices provide specific functions,

including:

Device-to-device interfacing

Data communication

Signal processing

Data display

Timing

Control operations

Almost every other function a

system must perform

Memory cells control and define the function that the logic performs and how the various logic

functions are interconnected.

Programming Technology

The programming technologies for PLD devices are based on the various types of semiconductor

memory. As new types of memories have been developed, the same technology has been applied

to the creation of new types of PLD devices.

The amount of logic resources available is the major distinguishing feature between SPLDs and

HCPLDs. Today, SPLDs are devices that typically contain the equivalent of 600 or fewer gates,

while HCPLDs have thousands and hundreds of thousands of gates available.

Of the two types of HCPLD devices, FPGAs offer the highest amount of logic density, the most

features, and the highest performance.

Examples of PLD applications

There are many applications that use programmable logic devices including in the field of:

Networking

Aerospace and defense

Automotive

Consumer electronics

Computing

Distributed monetary systems

Communications

Audio

Computing

Avionics

Some small scale applications of PLD

Code converter, for instance from binary to gray code.

BCD to 7 segment converters, supporting A-F letters

Mohd. Uzir Kamaluddin / Feb 2020 Page 4

Quadrature decoders and counters

Parity checkers, checksums and error detection and correction

Different types of counters and registers

Memory and I/O controllers for microprocessors

Lookup tables

Question 1:

a) Explain the term PLD. Name the three fundamental types of PLDs.

b) Explain the term PLA and PAL. Describe their differences.

c) What are the problems using standard ICs in building logic circuits?

d) What are the advantages in using PLDs when building logic circuits?

A portion of a logic diagram

A portion of a logic diagram is shown in figure below. The logic diagram shows all of the logic

resources available in a particular device. In each device, inputs are provided in true and

complement versions, as shown in the figure. These drive what are often called “input lines,” which

are the vertical lines in the logic diagram. These input lines can then be connected to product terms.

The name “product term” is really just a fancy name for an AND gate. However, PLDs provide

very wide gates, which can be cumbersome to draw. To save space, the product terms are drawn as

horizontal lines with a small AND gate symbol at one end to indicate the function being performed.

PAL Device Array structure


Question 2:

a) Draw a six input AND gate and a six input OR gate using the array logic symbol.

b) Draw the block diagram of the PROM, PAL and PLA showing their programmable

connectors and normal connectors.

ROM's and PROGRAMMABLE LOGIC

Read Only Memory (ROM)

A ROM is a combinational component for storing

data. The data might be a truth table or the data

might be the control words for a

microprogrammed CPU. A ROM can be

programmed at the factory or in the field.

The following image shows the generic form of a

ROM:

An n x m ROM can store the truth table for m functions defined on log2 n variables:


Example: Implement the following functions in a ROM:

F0 = A

F1 =

Since a ROM stores the complete truth table of a function

(meaning that a ROM decodes every minterm of a function)

the first step is to express each function as a truth table.

For the discussion that follows it may be helpful to keep in mind the canonical form of the function

also:

F0 =

F1 =

The special notation to show the ROM implementation of a

function is:

Exercise 1: Implement the following functions using PROM

F1(A, B, C)=Σ(0, 1, 2, 5, 7) and F2(A, B, C)=Σ(1, 4, 6)

Exercise 2: Design a PROM circuit that takes a 3-bit input and produces its square as 6-bit output.

The image above shows how a 4x3 ROM can be used to implement the two functions F0 and F1.

(Note, there is room in the ROM for 3 functions of two variables. F2 isn't used.

Example shows a ROM with an unused portion to

demonstrate that a ROM may still be the most

efficient implementation even when large sections of

the ROM go unused.) The diagram can be imagined

as a decoder inside the ROM that decodes the inputs

A B. Just like any decoder, one output line is selected

for every unique set of inputs. If the selected output


line is connected to an OR gate the function

associated with the OR gate will have a value of 1

for the particular set of inputs.

Note that the image above also shows a new way

to indicate multiple connections with a single

line.

For example:

A single vertical line that intersects 4 horizontal lines represents

potentially 4 different lines or connections. This is a notational

convenience when talking about ROM's, PLA's, and PAL's because it

makes the diagrams much easier to read.

The circles at the intersection of two lines indicates a connection.

Connections are either formed at the factory or in the field. If they

are formed in the field a special programmable ROM is used. One

type of programmable ROM is a ROM that has a fuse at every

connection. Fuses at connections not wanted are burned by running

high current through the fuse. What is left are the connections that define the data within the ROM.

Observations

Not very efficient implementation of sparse functions.

A ROM that implements two functions does not require twice the number of gates as a

ROM that implements one function. (The decoder is shared by every output function.)

Some extra points

ROMs programmed at the factory are called mask ROMS because during fabrication the circuit

patterns are determined by a mask. There are several different types of field programmable ROMS:

PROM (Programmable Read-Only Memory) - This is the type that was discussed above.

Connections are fused and burned in the field with a PROM programmer.

EPROM (Erasable Programmable Read-Only Memory) - This type of ROM can be re-written

by shining an ultraviolet light through a window on the IC.

EEPROM (Electrically Erasable Programmable Read-Only Memory) - Rather than ultraviolet

light an extra high voltage is used to re-write the contents of this type of ROM.

Flash Memory - Instead of requiring extra high voltages flash memory devices work with

regular device voltages.

Programmable Logic

A programmable logic device works like a ROM but is a

more efficient solution for implementing sparse output

functions. (Not all minterms are decoded.)

There are two types of programmable logic devices:

PLA (Programmable Logic Array)

PAL (Programmable Array Logic)

As suggested earlier that a ROM had a decoder inside it

and could be visualized as:

The image above also defines two terms that will be used to distinguish between PLA and PAL

devices:


AND Array - this is the portion of the device that decodes the inputs. The AND array

determines the minterms decoded by the device. A ROM decodes all possible minterms.

OR Array - this is the portion of the device that combines the minterms for the definition of

a function.

PLA

A PLA is a programmable logic device with a programmable AND array and a programmable OR

array.

A PLA with n inputs has fewer than 2n AND gates (otherwise there would be no advantage over a

ROM implementation of the same size). A PLA only needs to have enough AND gates to decode as

many unique terms as there are in the functions it will implement.

Because of the control-ability of the AND array and there is a limit to the number of terms that can

be specified in the AND array, it may be more economical to simplify the function before

implementing it with a PLA. If simplification is done to the function and then intended to

implement with a PAL device, it should also be kept in mind that product terms can be shared

between functions. (Product sharing is when two functions share a product term decoded by the

AND array. For example, in the image below the product term AB is shared between F0 and F1.)

Example: Implement the functions F0 F1 we introduced above using a PLA with 2 inputs, 3 product

terms, and 2 outputs.

The un-programmed PLA from the manufacture looks like:

After programming for the two functions

F0 F1 the state of the PLA is:

Notice that it only need three AND gates because there

are only three unique minterms in the functions F0 and

F1. Also, notice that since that the OR array can be

controlled, it can share with the minterm AB in the

definitions of both functions.

Note, there may be an advantage to simplifying the

functions before implementing. In the example used here

there is no advantage. The simplified form of the

functions F0 and F1 still require 3 unique product terms. Because product terms can be shared

between functions it’s important to look for common product terms when simplifying.


PAL

A PAL is a programmable logic device with a

programmable AND array and a fixed OR array.

A PAL has a fixed OR array. For example, here is what

an un-programmed PAL might look like straight from

the manufacture:

A fixed OR array makes the device less expensive to

manufacture. On the other hand, having a fixed OR

array means you can't share product terms between

functions.

Example: Implement the functions F0 F1 we

introduced above using the PAL given above.

For this implementation it will need to simplify the

functions F0 F1 because the PAL that are given has an

output function that can accommodate only one

product term. The simplified form of the functions

are:

F0 = A

F1 =

After programming for the two functions F0 F1 the

state of the PAL is:

So, in summary:

A PLA device has a programmable AND and programmable OR array

A PAL device has a programmable AND and fixed OR array

(A ROM has a fixed AND and programmable OR array)

When implementing with a ROM there is no advantage to minimizing the functions since the input

is fully decoded. When implementing with a PLA there may be an advantage to minimizing the

expression but also have to keep in mind that product terms can be shared between functions. So,

when minimizing one function, it is needed to consider the form of other functions and watch for

product terms that can be shared. When implementing with a PAL there may also be some

advantages to minimizing the function first. However,

since it can't share product terms with a PAL it did not

have to consider the form of other functions when

minimizing.

Question 1:

Given the following PAL circuit.

i) What is the PAL specifications?

ii) Determine the Boolean expression for f1 and f2.


Question 2:

Given the following PAL circuit.

i) What is the PAL specifications?

ii) Determine the Boolean expressions for F1, F2,

F3 and F4.

Question 3:

A combinational circuit having the following truth table is implemented using (3 x 4 x 2) PLA

where A, B, C are inputs and F1, F2 are outputs.

A B C F1 F2

0 0 0 1 1

0 0 1 1 0

0 1 0 1 0

0 1 1 0 0

1 0 0 1 0

1 0 1 0 1

1 1 0 0 1

1 1 1 0 1

Show how the circuit is implemented.

Question 4:

Implement the following Boolean functions using (4 x 8 x 3) PAL device.

W(A, B, C) =

X(A, B, C) =

Memory

Memory refers to the physical devices used to store sequences of instructions or data on a

temporary or permanent basis for use in a computer or other digital electronic device. Primary

memory is used for the information in physical systems which function at high-speed (i.e. RAM), as

a distinction from secondary memory, which are physical devices for program and data storage

which are slow to access but offer higher memory capacity. Primary memory stored on secondary

memory is called virtual memory.

The term memory, meaning primary memory is often associated with addressable semiconductor

memory, i.e. integrated circuits consisting of silicon-based transistors, used for example as primary

memory but also other purposes in computers and other digital electronic devices. There are two


main types of semiconductor memory: volatile and non-volatile. Examples of non-volatile memory

are flash memory and ROM/PROM/EPROM/EEPROM memory.

Example of volatile memory are primary memory (typically dynamic RAM, DRAM), and fast CPU

cache memory (typically static RAM, SRAM, which is fast but energy-consuming and offer lower

memory capacity per area unit than DRAM).

Most semiconductor memory is organized into memory cells or bi-stable flip-flops, each storing one

bit (0 or 1). Flash memory organization includes both one bit per memory cell and multiple bits per

cell (called MLC, Multiple Level Cell). The memory cells are grouped into words of fixed word

length, for example 1, 2, 4, 8, 16, 32, 64 or 128 bit. Each word can be accessed by a binary address of

N bit, making it possible to store 2N words in the memory. This implies that processor registers

normally are not considered as memory, since they only store one word and do not include an

addressing mechanism.

The term storage is often used to describe secondary memory such as tape, magnetic disks and

optical discs (CD-ROM and DVD-ROM).

Random Access Memory (RAM)

It is a form of computer data storage. A random-access device allows stored data to be accessed

directly in any random order. In contrast, other data storage media such as hard disks, CDs, DVDs

and magnetic tape, as well as early primary memory types such as drum memory, read and write

data only in a predetermined order, consecutively, because of mechanical design limitations.

Therefore the time to access a given data location varies significantly depending on its physical

location. Today, random-access memory takes the form of integrated circuits. Strictly speaking,

modern types of DRAM are not random access, as data is read in bursts, although the name

DRAM/RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still

random access even in a strict sense. RAM is often associated with volatile types of memory (such

as DRAM memory modules), where its stored information is lost if the power is removed. Many

other types of non-volatile memory are RAM as well, including most types of ROM and a type of

flash memory called NOR-Flash.

The three main forms of modern RAM are static RAM (SRAM), dynamic RAM (DRAM) and phase-

change memory (PRAM). In SRAM, a bit of data is stored using the state of a flip-flop. This form of

RAM is more expensive to produce, but is generally faster and requires less power than DRAM

and, in modern computers, is often used as cache memory for the CPU. DRAM stores a bit

of data using a transistor and capacitor pair, which together comprise a memory cell. The capacitor

holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the

control circuitry on the chip read the capacitor’s state of charge or change it. As this form of

memory is less expensive to produce than static RAM, it is the predominant form of computer

memory used in modern computers.

Both static and dynamic RAM is considered volatile, as their state is lost or reset when power is

removed from the system.


Read Only Memory (ROM)

It is a class of storage medium used in computers and other electronic devices. Data stored in ROM

cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to

distribute firmware (software that is very closely tied to specific hardware and unlikely to need

frequent updates). ROM refers only to mask ROM (the oldest type of solid state ROM), which is

fabricated with the desired data permanently stored in it, and thus can never be modified.

Despite the simplicity, speed and economies of scale of mask ROM, field-programmability often

make reprogrammable memories more flexible and inexpensive. Other types of non-volatile

memory such as erasable programmable read only memory (EPROM) and electrically erasable

programmable read-only memory (EEPROM or Flash ROM) are sometimes referred to, in an

abbreviated way, as “read-only memory” (ROM); although these types of memory can be erased

and reprogrammed multiple times, writing to this memory takes longer and may require different

procedures than reading the memory. When used in this less precise way, “ROM” indicates a non-

volatile memory which serves functions typically provided by mask ROM, such as storage of

program code and non-volatile data.


Programmable Read-Only Memory (PROM):

It can be written to or programmed via a special device called a PROM programmer. Typically, this

device uses high voltages to permanently destroy or create internal links (fuses or antifuses) within

the chip. Consequently, a PROM can only be programmed once.

Erasable Programmable Read-Only Memory (EPROM):

It can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer), then

rewritten with a process that again needs higher than usual voltage applied. Repeated exposure to

UV light will eventually wear out an EPROM, but the endurance of most EPROM chips exceeds

1000 cycles of erasing and reprogramming. EPROM chip packages can often be identified by the

prominent quartz “window” which allows UV light to enter. After programming, the window is

typically covered with a label to prevent accidental erasure. Some EPROM chips are factory-erased

before they are packaged, and include no window; these are effectively PROM.

Electrically Erasable Programmable Read-Only Memory (EEPROM):

It is based on a similar semiconductor structure to EPROM, but allows its entire contents (or

selected banks) to be electrically erased, then rewritten electrically, so that they need not be

removed from the computer (or camera, MP3 player, etc.). Writing or flashing an EEPROM is much

slower (milliseconds per bit) than reading from a ROM or writing to a RAM (nanoseconds in both

cases).

Electrically alterable read-only memory (EAROM) is a type of EEPROM that can be modified one

bit at a time. Writing is a very slow process and again needs higher voltage (usually around 12 V)

than is used for read access. EAROMs are intended for applications that require infrequent and

only partial rewriting. EAROM may be used as non-volatile storage for critical system setup

information; in many applications, EAROM has been supplanted by CMOS RAM supplied by

mains power and backed-up with a lithium battery.

Flash memory (or simply flash) can be erased and rewritten faster than ordinary EEPROM, and

newer designs feature very high endurance (exceeding 1,000,000 cycles). Modern NAND flash

makes efficient use of silicon chip area, resulting in individual ICs with a capacity as high as 32 GB

as of 2007; this feature, along with its endurance and physical durability, has allowed NAND flash

to replace magnetic in some applications (such as USB flash drives). Flash memory is sometimes

called flash ROM or flash EEPROM when used as a replacement for older ROM types, but not in

applications that take advantage of its ability to be modified quickly and frequently.


Block diagram of Random Access Memory (RAM)

This block diagram introduces the main interface to the RAM chip.

The chip select input CS will enable or disable the RAM chip.

The address input lines will specify the address or location to read from or to write to.

The READ/ selects between reading from or writing to the memory chip.

The data in/out lines is where the data is coming in (for case of writing) or going out (for the case of

reading).

Block diagram of Read Only Memory (ROM)

This block diagram introduces the main interface to the ROM chip.

The chip select input CS’ will enable or disable the ROM chip.

The address input lines will specify the address or location to read from.

The data out lines is where the data is going out when there is reading operation.

Memory ICs

Shown here are the two commonly found memory ICs in the market.


Exercise 1:

What is the meaning of the following terms related to memory and CPU:

a) Memory cell

b) Memory word

c) Capacity

d) Data Bus

e) Address Bus

f) Control Bus

g) R/ line

h) line

i) line

Exercise 2:

a) In a memory IC, what determines the size of the memory?

b) For the given memory size, determine the number of address lines:

i) 1K ii) 2K iii) 4K iv) 8K

c) For the given number of address lines, determine the memory size:

i) 10 ii) 11 iii) 12 iv) 13

Exercise 3:

If a memory IC is specified as (2K x 8), then determine:

a) Draw the block diagram of this memory IC.

b) The number of words that can be stored in this IC

c) The number of bits per word

d) The capacity of the memory IC

Exercise 4:

If a memory IC is specified as (64K x 8), then determine:

a) How many words can be stored on this IC?

b) How many data input and data output lines does it have?

c) How many memory cells does it contain?

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COURSE / CODE DIGITAL ELECTRONICS FUNDAMENTAL (ECE422 ... · COURSE / CODE DIGITAL ELECTRONICS FUNDAMENTAL (ECE422) PROGRAMMABLE LOGIC DEVICES (PLDs) A PLD, or programmable logic