ReadOnly Memory ROM

1
Read-Only Memory (ROM)

• Yahya E. Osais
• COE 200
• Term 031

2
Outline

• Overview
• High-level diagram of ROM
• Function implementation with ROM
• ROM internal structure
• Implementation of synchronous sequential circuits
  using ROMs
• Moore
• Mealy
• Summary

3
Overview

• ROM can only be read.
• Any logic function can be implemented using ROMs.
• ROM does not lose storage value when power is
  removed. Thus, it is used in computer systems for
  initialization.
• Once programmed, data in a ROM cannot be changed.

4
High-Level Diagram of ROM

• No. of entries / words 2n n Inputs.
• Word Size M
• Implements M arbitrary functions of n variables.
• Example
• 8 words by 5 bits.

5
Function Implementation with ROM

• Each minterm of each function can be specified

6
Function Implementation with ROM (contd)

• Suppose there are 10 inputs 10 address lines
  (i.e., 210 1024 different addresses)
• Suppose there are 20 outputs
• ROM is 210 x 20 20K bits

7
ROM Internal Structure
8
ROM Internal StructureAlternate View
9
ROM Implementation of a Moore Machine

• ROMs implement combinational logic.
• ROMs do NOT hold state.
• How would you determine the maximum clock
  frequency of this circuit?
• Look at the F/F to F/F path (NS to PS).

10
ROM Implementation of a Mealy Machine

• ROMs implement combinational logic.
• ROMs do NOT hold state.
• How would you determine the maximum clock
  frequency of this circuit?
• Look at the F/F to F/F path (NS to PS).

11
Summary

• 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
• They maintain their storage after the power is
  turned off