Lecture Digital logic design - Lecture 30: Read Only Memory (ROM)

The following will be discussed in this chapter: Read-only memory can normally only be read, internal organization similar to SRAM, ROMs are effective at implementing truth tables, multiple single-bit functions embedded in a single ROM, also used in computer systems for initialization, very useful for implementing FSMs.

Lecture 30

Read Only Memory (ROM)

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° Read-only memory can normally only be read

° Internal organization similar to SRAM

° ROMs are effective at implementing truth tables

• Any logic function can be implemented using ROMs

° Multiple single-bit functions embedded in a single ROM

° Also used in computer systems for initialization

• ROM doesn’t lose storage value when power is removed

° Very useful for implementing FSMs

° Data can be read but not changed

• (normal operating conditions)

Data is written to the ROM once, and read from the ROM many times.

A read-only memory (ROM) consists of an array of

semiconductor devices that are interconnected to store a set

of binary data.

Once binary data is stored in the ROM, it can be read out

° ROMs are actually combinational devices, not sequential

ones!

• You can’t store arbitrary data into a ROM, so the same address will always

contain the same data.

• You can think of a ROM as a combinational circuit that takes an address as

input, and produces some data as the output.

° A ROM table is basically just a truth table.

• The table shows what data is stored at each ROM address.

• You can generate that data combinationally, using the address as the input.

° N input bits

° 2 N words by M bits

° Implement M arbitrary functions of N variables

• Example 8 words by 5 bits:

A B C

ROM – Basic Structure

address

data

° ROM = "Read Only Memory"

• values of memory locations are fixed ahead of time

° A ROM can be used to implement a truth table

• if the address is m-bits, we can address 2 m entries in the ROM.

• our outputs are the bits of data that the address points to.

• m is the "height", and n is the "width"

° Suppose there are 10 inputs

10 address lines (i.e., 2 10 = 1024 different addresses)

° Suppose there are 20 outputs

° ROM is 2 10 x 20 = 20K bits (and a rather unusual size)

° Rather wasteful, since lots of storage bits

• For functions, doesn’t take advantage of K-maps, other

minimization

ROM Implementation

Each minterm of each function can be specified

m Outputs Lines

.

Function

Implementation

3 to 8 decoder

Each column is a new function

Note: two outputs unused!

° We can already convert truth tables to circuits

easily, with decoders.

° For example, you can think of this old circuit as a

memory that “stores” the sum and carry outputs from the truth table on the right.

• A blank ROM just provides a decoder and several OR gates.

• The connections between the decoder and the OR gates are

“programmable,” so different functions can be implemented.

° To program a ROM, you just make the desired

connections between the decoder outputs and the

OR gate inputs.

M exa mpl e

° Blue crosses ( X ) indicate connections between

decoder outputs and OR gates Otherwise there is

no connection.

Same Example

° Here is an alternative presentation of the same 8 x 3 ROM,

using “abbreviated” OR gates to make the diagram neater.

° This combinational circuit can be considered a read-only

memory.

• It stores eight words of data, each consisting of three bits.

• The decoder inputs form an address , which refers to one of the eight

available words.

• So every input combination corresponds to an address, which is “read” to

produce a 3-bit data output.

ROMs vs RAMs

° There are some important differences between

ROM and RAM.

• ROMs are “non-volatile”—data is preserved even without power

On the other hand, RAM contents disappear once power is lost.

• ROMs require special (and slower) techniques for writing, so

they’re considered to be “read-only” devices

° Some newer types of ROMs do allow for easier

writing, although the speeds still don’t compare

with regular RAMs.

• MP3 players, digital cameras and other toys use CompactFlash,

Secure Digital, or MemoryStick cards for non-volatile storage.

• Many devices allow you to upgrade programs stored in “flash

ROM.”

ROM Implementation of a Moore

Machine

° ROMs implement combinational logic

° Note that ROMs do not hold state

° How would you determine the maximum clock

frequency of this circuit?

• Look at the FF to FF path (NS to PS)

Present State

Next State

Outputs Inputs

ROM Implementation of a Mealy

Machine

° ROMs implement combinational logic

° Note that ROMs do not hold state

° How would you determine the maximum clock

frequency of this circuit?

• Look at the FF to FF path (NS to PS)

ROM

ROM

Present State

Next State

Outputs Inputs

° ROMs provide stable storage for data

° ROMs have address inputs and data outputs

• ROMs directly implement truth tables

° ROMs can be used effectively in Mealy and Moore machines

to implement combinational logic

° In normal use ROMs are read-only

• They are only read, not written

° ROMs are often used by computers to store critical

information

• Unlike SRAM, they maintain their storage after the power is turned off