ECE 301 – Digital Electronics

Memory

(Lecture #22)

Random Access Memory

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Random Access Memory

Static Random Access Memory (SRAM)

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Random Access Memory

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Address Decoding

Random Access Memory

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RAM: Address Decoding

k-bit address Decoder requires 2k AND gates

Each AND gate has k inputs For large k this becomes prohibitive.

Use 2-dimensional decoding Two decoders Each decoder requires 2(k/2) AND gates

Each AND gate has k/2 inputs Far less combinational logic

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RAM: Address Decoding

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RAM: Address Decoding

The size of a chip package is often dictated by the number of input and output signals.

For large memories, the number of address lines often becomes prohibitive.

Use address multiplexing The same address lines are used both for

the row address and the column address Use time multiplexing to first latch the row

address and then latch the column address

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RAM: Address Decoding

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Random Access Memory

Building a Bigger Memory System

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RAM Systems

Often RAM chips are smaller than the required memory size.

What if you need a wider memory? Larger word size

What if you need a deeper memory? Greater number of memory locations

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RAM Systems

Exercise:

Design a 32K x 32 memory usingRAM chips that are 32K x 8.

How many address bits are required?How many data bits are required?

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Exercise: 32K x 32 RAM

Addr Addr Addr Addr15

A14

- A0

D31

- D24

8 8 8 8

D23

- D16

D15

- D8

D7 - D

0

Data Data Data Data

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RAM Systems

Exercise:

Design a 128K x 8 memory usingRAM chips that are 32K x 8.

How many address bits are required?How many address bits are connected to the RAM chips?

What are the remaining address bits connected to?

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Example: 128K x 8 RAM

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Read Only Memory

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Read Only Memory ROM store “permanent” binary information

One-time programmable memory Multiple-time programmable memory

Address and Data k address bits n data bits

2k x n ROM includes k-to-2k decoder n 2k-input OR gates

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Read Only Memory

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Read Only Memory

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Read Only Memory

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Read Only Memory

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Read Only Memory

EEPROM (E2PROM) Electrically Erasable Programmable ROM

Flash ROM Similar to E2PROM Has additional circuitry to selectively erase

and program the memory in-circuit Does not require a special programmer

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Programmable Logic Devices

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Programmable Logic Devices

Programmable Logic Arrays (PLA) Programmable Array Logic (PAL) Simple Programmable Logic Device (SPLD) Complex Programmable Logic Device (CPLD) Field Programmable Gate Array (FPGA)

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“That's All Folks!”

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Acknowledgments

The slides used in this lecture were taken, with permission, from those provided by Pearson Prentice Hall for

Digital Design (4th Edition).

They are the property of and are copyrighted by Pearson Education.