Memory and Programmable Logic Devices

8/8/2019 Memory and Programmable Logic Devices

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P.RAJA

Assistant Professor,Department of ECE,

Mailam Engineering College,Mailam 604 304


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A memory unit is a collection of cells capable ofstoring a large quantity of binary information

There are two types of memories that are used indigital systems

Random-Access Memory (RAM)

Read-Only Memory (ROM)


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RAM can perform both write and readoperations.

A RAM loses its stored data when power is turned off,thus it is a volatile memory.

The time it takes to transfer information to or from

any desired random location is always the same,hence the name random-access memory (RAM).


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A memory unit stores binary information in

groups of bits called words.

A group of eight bits is called a byte. A block diagram of the memory unit


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The n data inputlines provide the information tobe storedin memory and the n outputlines specifythe binary datacoming out of the memory.

The kaddress lines specify the particular word chosen.

The two control inputs specify the direction of data

transfer required Read and Write.


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Data inputs

Data outputs

3 t 8c d r

A0

A1

A2

A2 A1 A0 = 000

A2 A1 A0 = 101

Memory size = No. of addresslocation v No. of data lines = 8 v 8


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Memory size = 1024 v 16

= 210 v 16

Each word in memory is assigned an

identification number, called an address

(0 up to 2k-1).

The selection of a specific word inside

the memory is done by applying the k-bit

address to the address lines.

A decoderaccepts this address and opens

the paths needed to select the word

specified.


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Write Operations

Apply the binary address of the desired word to the

address lines.

Apply the data bits that must be stored in memory tothe data input lines.

Activate the write input


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Read Operations

Apply the binary address of the desired word to theaddress lines.

Activate the readinput

Some memory chips have a different configuration of controlinputs; one input selects the unit and the other decides the

operation.

Memory Enable Read / Write Memory Operation

0 X None

1 0 Write to selected word

1 1 Read to selected word


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The operation ofa memory is usually controlled bythe CPU.

The access time of a memory is the time required toselect aword and read it.

The cycle time of a memory is the time required to

complete awrite operation.

The access time and cycle time must be within a timeequal to a fixed number of CPU clockcycles.


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StaticRAM (SRAM): Memory cell is constructedusing Latch ( usingMOS transistor )

DynamicRAM (DRAM) : Stores the binary data inthe formofelectric charges on ca acitors.


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Row select

T3 T4

T1 T2

T5T6

Data

line

Data

line

Write

(W)

Read

(R)

T7 T8

T9T10

Data-in Data-out

VDD

D D

Row select T3 - ON T4 - ON

Column select T7

- ON T8

ON

Write T9

- ON T10 OFF

Read T9

- OFF T10 ON

Column select


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Rowselect

Columnselect

CT

Rowselect

Columnselect

CT

Writeoperation Read

operation


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Sl. No SRAM DRAM

1 It contains less memory cells

er unit area

It contains more memory cells

er unit area

2 It access time is less It access time is greater than

SRAM

3 Refreshing circuitry is not

required

Refreshing circuitry is

required

Cost is more Cost i s less


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A decoder with k inputs and 2k outputs requires 2kAND gates with k inputs per gate.

The number ofgates and the number of inputs perstage can be reduced by employing two decoders ina two-dimensionalselection scheme.

In this scheme, two k/2-input decoders are usedinstead of one k-inputdecoder.


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Aread-only memory (ROM) is a memory in which

permanent binary information is stored.

Block diagramof a ROM

We canperform the read operationonly


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Inputs Outputs

A B CIN S COUT

0 0 0 0 0

0 0 1 1 0

0 1 0 1 0

0 1 1 0 1

1 0 0 1 0

1 0 1 0 1

1 1 0 0 1

1 1 1 1 1


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3 to 8Decoder

A

B

CIN

Outputs

S COUT

0 0

1 0

1 0

0 1

1 0

0 1

0 1

1 1

S COUT


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Mask ROM rogram by manufacturer based on the user

specification ROM Programmable ROM User can program their requirement

One time program is possibleEPROM Erasable PROM Program by user

Reprogram by erase the existing data byapplyingUV ( ultra violet) rays


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PLD Programmable Logic Devices Types

PROM

ProgrammableArray Logic ( PAL).


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ProgrammableArray Logic


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Documents

Memory and Programmable Logic Devices