Memory

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Memory
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Memory

• Memory Types
• Using memory to implement logic functions

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Memory Types

• Any sequential circuit has memory of a sort,
  since each flip-flop or latch stores one bit of
  information
• The term memory is usually reserved for that part
  of a system where bits of information is stored
  in a structured way. This is usually a
  2-dimensional array in which one row of bits is
  accessed.

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Read Only Memory (ROM)

• ROM is a combinational circuit with n inputs and
  b outputs. The n inputs are the address and
  usually called A0 A1 A2 A3 . The outputs
  are usually called D0 D1 .
• At each of the 2n unique addresses
  permanently stores a b bit
  value

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ROM as a logic element

• ROM is memory but it is also a combination
  element.
• As a combination element it can perform logic
  functions.
• Table 9-1 versus combination logic for
  implementation.

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Table 9-1

• The truth table

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Implementation

• Have Table 9-1
• Work out logic equations and construct circuit
• How many logic gates does it take?
• What is the delay using logic gates?
• What is the delay using ROM?
• Data sheet for ROM show timing

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ROM Programming

• ROM value at each storage location is stored
  when the ROM is manufactured.
• ROM is non-volatile memory
• PROM Programmable Read Only Memory
• Using a programmer the values are written after
  manufacture.
• Types of PROMs
• One time programmable fuseable links
• UV EPROM The E stands for Erasable
• EEPROM Electrically Erasable PROM

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ROMs for combination logic

• Section 9.1.1 ROMs for random combinational
  logic.

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The solution using ROM

• Use an 8 x 4 PROM
• 3 inputs are A0, A1, and POL
• 4 outputs are Y0, Y1, Y2, and Y3
• What is the programming table???

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Another problem

• Build a 4 x 4 binary multiplier
• Combination logic requirements
• Versus a ROM implementation
• Remember our PROM and its timing?

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Can implement in Gates

• Work out logic equations and then how many
  gates/units does it take?
• Time??
• Or could implement in ROM

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Resulting PROM Programming
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Assignment

• Due Monday Nov 3rd
• Go to www.wikipedia.com and look up PROM, EPROM,
  UV EPROM and EEPROM. Write a ½ to 1 page write
  up summarizing the information you find there.
• Go to www.digikey.com and look up the price and
  features for 2 PAL chips, 2 PLA chips, and 2
  FPGAs. Write a ½ to 1 page summary of this.
• Tuesday Nov 4th VOTE!!!!!!!!!
• Due Wednesday Nov 5th
• Problem 9.1 but only do 6-37 and 6-73. Also,
  show the signals on the address inputs and the
  data outputs.

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Solution

• The solution Table 9-2 and Fig 9-4

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