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EET 3350 Digital Systems Design

Textbook: John Wakerly

Chapter 9: 9.1

MemoryRead-Only Memory: ROM, PROM, EPROM

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Memory

Sequential circuits all depend upon the presence of

memory A flip-flop can store one bit of information

A register can store a single word

typically 32 or 64 bits

Memory stores a large number of words

Memory stores this large amounts of data using twoprimary device types

Read Only Memory (ROM, PROM, EPROM, EEPROM)

Random Access Memory (RAM)

Static RAM (SRAM)

Dynamic RAM (DRAM)

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Memory

You can think of memory as being

one big array (list) of data The address serves as an array

index

Each address refers to one word

of data (e.g., 8-bits, 16-bits, etc.)

You can read (or modify) the dataat any given memory address,

just like you can read (or modify)the contents of an array at anygiven index

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Address Data

00000000

00000001

00000002

.

.

.

.

.

.

.

.

.

.

FFFFFFFD

FFFFFFFE

FFFFFFFF

word

0110101100111101

1011111100100100

1001110011110111

0110101111010000

1100101000110001

0000101100001111

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Memory

Memory signals fall into three groups:

Address bus - selects one of many memory locations

Data bus -

Read (ROM/RAM): the selected locations stored data is

put on the data bus

Write (RAM): The data on the data bus is stored into theselected location

Control signals - specifies what the memory is to do

Control signals are usually active low

Most common signals are:

CS: Chip Select; must be active to do anything

OE: Output Enable; active to read data

WR: Write; active to write data

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Memory

Memory is not a single chip (device)

Made up of many identical or similar devices

A specific device (part of memory) is selected by controlsignals and the address lines (bus)

All devices are connected to the same bus, and see the

signals at the same time

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Memory

Memory Connection to CPU

RAM and ROM chips are connected to a CPU throughthe data and address buses

The low-order lines in the address bus select the bytewithin the chips and other lines in the address bus select

a particular chip through its chip select inputs

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Memory

Location - the smallest selectable unit in memory

Has 1 or more data bits per location

All bits in location are read/written together

Cannot manipulate single bits in a location

For k address signals, there are 2k locations in a

memory device

Each location contains an n-bit word

Memory size is specified as

#loc x bits per location

224 x 16 RAM - 224 = 16M words, each 16 bits long

24 address lines, 16 data lines

#bits

The total storage capacity is 224 x 16 = 228 bits

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Memory

Memory sizes are usually specified in numbers of

bytes(1 byte= 8 bits)

The 228-bit memory on the previous page translatesinto:

228

bits / 8 bits per byte = 225

bytes

With the abbreviations below, this is equivalent to 32megabytes

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Prefix Base 2 Base 10

K Kilo 2 = 1,024 10 = 1,000M Mega 2 = 1,048,576 10 = 1,000,000

G Giga 2 = 1,073,741,824 10 = 1,000,000,000

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Memory

Non-volatile

If un-powered, its content is retained

Read-only normal operation cannot change

contents

k-bit ADRS specifies the address orlocation to read from

A Chip Select, CS, enables ordisables the RAM/ROM

An Output Enable, OE, turns on or offtri-state output buffers

Data Out will be the n-bit value storedat ADRS

9

2k x n ROM

ADRS Data

Out

kn

CS

OE

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Memory

Content loading (programming) done many

ways depending on device type ROM: mask programmed, loaded at the factory

hardwired - cant be changed

embedded mass-produced systems

PROM: OTP (One Time Programmable),programmed by user, using an externalprogramming device

EPROM: reusable, erased by UV light,programmed by user, using an externalprogramming device

EEPROM: electrically erasable, clears entireblocks with single operation, programmed in-place (no need to remove from circuit board)

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Read-Only Memories

Definition

ROM consists of an array of semiconductor devicesinterconnected to store an array of memory data.

Data can only be read, it cannot be changed undernormal operating conditions.

Types of ROM

Mask programmable ROM (at the factory)

Field-Programmable ROM (PROM)

UV-Erasable and re-Programmable ROM (EPROM) Electrically-Erasable and re-Programmable ROM

(EEPROM)

Flash

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Read-Only Memories

The logic symbol below is used in circuit diagrams

Focus is on the basic structure of a ROM

A combinational logic circuit

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Logic-in-ROM Example

As we discussed previously, a ROM is simply a

combinational circuit, basically a truth-table lookup Can perform any combinational logic function

Address inputs = function inputs

Data outputs = function outputs

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(address) (data)

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Logic-in-ROM Example

Two alternative implementations for the 3-input, 4-output

logic function 2-to-4 decoder with output polarity control

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4x4 Multiplier Example

ROM implementation of a 4x4 unsigned binary

multiplier Multiplier and multiplicand form the address

Product is pre-programmed into the storage location

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4x4 Multiplier Example

ROM contents for the 4x4 unsigned binary multiplier

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x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 xA xB xC xD xE xF

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Internal ROM Structure

Typical implementation of primitive ROM

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5

active low

data output

Diode means a

1 is stored at

this location

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ROM

Commercial ROM types

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Typical Commercial EEPROMs

Logic symbols for representative EEPROMs

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ROM Control and I/O Signals

n Address lines

An-1 A0

b Data lines

Db-1 D0

Chip Select

One or more

Active low

Output Enable

Active low

Tri-state outputbuffers

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ROM Timing

tAA (access time from address): propagation delay from stable

address inputs to valid data output

tACS ( access time from chip select): propagation delay from timeCS is asserted until the valid data output

tOE (output-enable time): propagation delay from time OE and

CS are asserted until the tri-state drivers have left Hi-Z state

tOZ (output-disable time): propagation delay from time OE andCS are negated until the tri-state drivers have entered Hi-Zstate

tOH (output-hold time): the length of time the outputs remainvalid after a change in the address inputs, or after OE and CSare negated

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ROM Timing

tAA access time from address

tACS access time from chip select

tOE/tOZ output-enable/disable time

tOH output-hold time

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ROM -Advantages and Disadvantages

Ease of speed and design

For moderately complex function a ROM-based circuit isusually faster than a circuit using multiple SSI/MSI devices andPLDs

The program that generates the ROM contents can easily be

structured to handle unusual or undefined cases that wouldrequire additional hardware in any other design. e.g. the adderprogram easily handles out-of-range sums.

A ROMs function is easily modified just by changing the stored

pattern, usually without changing any external connections The prices of ROMs are dropping and densities increasing

making them more economical and expanding the scope with asingle chip

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ROM -Advantages and Disadvantages

May consume more power

For functions with more inputs a ROM based circuit isimpractical because of the limit on ROM sizes that are available