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

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• 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???

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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