CMOS DIGITAL VLSI DESIGN

Concept of Memory and its Designing-I

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 Outline

• Introduction-Memory
• Memory Classification
• Memory Architectures and Building Blocks
• The Memory Core
a) Read-only Memories
b) Nonvolatile Read-Write Memories
c) Read-Write Memories (RAM)
• Recapitulation

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 Introduction: Memory
• More than half of the transistors in today’s high performance
microprocessors are devoted to cache memories, and this ratio is
expected to further increase.
• Memory cells are combined into large arrays, which minimizes the
overhead caused by peripheral circuitry and increases the storage
density.
• Reliability and power dissipation are two major concerns of the
semiconductor memory designer.

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 Memory Classification
 Size
• The circuit designer tends to define the size of the memory in terms
of bits that are equivalent to the number of individual cells needed to
store the data.
• The chip designer express the memory size in bytes (group of 8 or 9
bits) or its multiple-kilobytes, megabytes, gigabytes etc.
• The system designer quotes the storage requirements in terms of
words, which represent a basic computational entity.

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 Timing Parameters Cont…
• The time require to retrieve from the memory is called Read-Access
Time, which is the delay between the read request and the moment
the data is available at the output.
• The time elapsed between a write request and the final writing of the
input data into the memory is called Write-Access Time.

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Functions Cont…
• Based on memory functionality it is classified as Read-Only memory
(ROM) and Read-Write Memory (RWM).
• RWM uses active circuitry to store the data, so it belong to the class
of volatile memory, in which data is lost when the supply voltage is
turned off.
• ROM belongs to the category of non-volatile memories.
Disconnection of the supply voltage does not result in the loss of the
stored data.
• The EPROM and E2PROM provides the facilities of both read-write
functionality but comes under the category of non-volatile memories.

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 Access pattern Cont…
• Most of the memories are random-access memories in which
memory location can be read and written in a random order.
• Some memories are restricts the order of access, which results in
either fast access time, smaller area or a memory with a special
functionality. Examples of such memory are FIFO, LIFO etc.

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Memory Architecture and Building Blocks
• To implement an N-word memory where each word is M bits wide,
then the most intuitive approach is to stack the subsequent memory
words in a linear fashion.

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Array Structured Memory Organization

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Hierarchical Memory Architecture

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 The Memory Core
 Read only Memories

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 A 4×4 OR ROM Array

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Non-volatile Read-Write Memories-

Floating-Gate Transistor EPROM Flash-EPROM

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Read Write Memories (RAM)
• Storage in RAM memories is based on either positive feedback or
capacitive charge.
a) Static RAM

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 SRAM Read & Write Operation

Read Operation Write Operation

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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b) Dynamic RAM

Source: J. M. Rabaey, A. Chandrakasan and B. Nikolic, “Digital Integrated Circuit,” PHI Learning Pvt. Ltd., 2011.

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 Recapitulation
• The type of memory unit that us preferable for a given application is
a function of required memory size, the time it takes to access the
stored data, the access pattern, the application and the system
requirements.
• RWM belong to the class of volatile memory while ROM belongs to
the category of non-volatile memories.
• EPROM and E2PROM are the memories which offer both read and
write functionality even that are of non-volatile in nature.
• RAM can be classified as Static RAM and Dynamic RAM based on the
storage concept.
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Thank You

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