P-2 Digital Design & Applications

Semiconductor Memory

(Unit-V)

By: A K Verma

SOS in Electronics & Photonics Pt. Ravishankar Shukla University, Raipur (C.G.)

1 What is Memory? • In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data (e.g. program state information) on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which function at high-speed (i.e. RAM), as a distinction from secondary memory, which are physical devices for program and data storage which are slow to access but offer higher memory capacity. If needed, primary memory can be stored in secondary memory, through a memory management technique called "virtual memory“.[1]

2 History • In the early 1940s, memory technology mostly permitted a capacity of a few bytes. The first electronic programmable digital computer, the ENIAC, using thousands of octal-base radio vacuum tubes, could perform simple calculations involving 20 numbers of ten decimal digits which were held in the vacuum tube accumulators. • The next significant advance in computer memory came with acoustic delay line memory, developed by J. Presper Eckert in the early 1940s. Through the construction of a glass tube filled with mercury and plugged at each end with a quartz crystal, delay lines could store bits of information within the quartz and transfer it through sound waves propagating through mercury. Delay line memory would be limited to a capacity of up to a few hundred thousand bits to remain efficient.

3 • Two alternatives to the delay line, the Williams tube and electron tube, originated in 1946, both using electron beams in glass tubes as means of storage. Using cathode ray tubes, Fred Williams would invent the Williams tube, which would be the first random access computer memory. The Williams tube would prove more capacious than the electron tube (the electron was limited to 256 bits, while the Williams tube could store thousands) and less expensive. The Williams tube would nevertheless prove to be frustratingly sensitive to environmental disturbances. • Efforts began in the late 1940s to find non-volatile memory. Jay Forrester, Jan A. Rajchman and An Wang developed magnetic core memory, which allowed for recall of memory after power loss. Magnetic core memory would become the dominant form of memory until the development of transistor-based memory in the late 1960s. • Developments in technology and economies of scale have made possible so-called Very Large Memory (VLM) computers.[1]

4 Memory

Memory is required to store:

1. data 2. application programs 3. operating system

5 General Concepts • A memory is an array of storage locations m bits – Each with a unique address 0 1 – Like a collection of registers, 2 but with optimized 3 implementation 4 5 • Address is unsigned-binary 6 encoded

– n address bits ⇒ 2n locations 2n–2 n • All locations the same size 2 –1 – 2n × m bit memory

6 Memory Sizes • Use power-of-2 multipliers – Kilo (K): 210 = 1,024 ≈ 103 – Mega (M): 220 = 1,048,576 ≈ 106 – Giga (G): 230 = 1,073,741,824 ≈ 109 • Example – 32K × 32-bit memory – Capacity = 1,025K = 1Mbit – Requires 15 address bits • Size is determined by application requirements

7 Basic Terms

• Memory cell: A memory cell is the smallest amount of information storage, holding either a 1 or 0. Memory cells are often grouped together to form words.

8 Access time (tacc):

• Access time is one of the most important parameters of any memory component and is the time taken to read data from a given memory location, measured from the start of a read cycle. Access time is made up from two parts; the time taken to locate the required memory location and time taken for data to become available from the memory cell (i.e. valid on the data bus) Many semiconductor memories have identical read and write access times

9 Cycle time (trcyc)

• This is the time which must elapse between two successive read or write accesses.

10 Memory Timing: Definitions

Read Cycle

READ

Write Cycle Read Access Read Access

WRITE

Write Access Data Valid

DATA

Data Written Random access • This is when a memory is configured so that the access time of any cell within it is constant and independent of the physical location of the cell. As far as a processor is concerned random access implies the access time to read from any memory location is constant. If a memory is random access for read cycles, it will usually be random access for write cycles also. Unfortunately the term RAM is now commonly used to indicate a memory which is both read and write. This has nothing to do with the property of random access which indicates an identical access time for all memory cells.

12 Classification of Memory

13 Semiconductor Memory Classification

Non-Volatile Read-Write Memory Read-Write Read-Only Memory Memory

Random Non-Random EPROM Mask-Programmed Access Access 2 E PROM Programmable (PROM)

SRAM FIFO FLASH

DRAM LIFO Shift Register CAM

14 15 Random Access Memory (RAM)

 Can be written to or read from.

 Read/Write memory

 Reading from RAM is non-destructive.

 Access time to read from any memory location is the same.

 As compared to serial access memory.

 Volatile

 Information is lost when power is removed.

16 RAM Types

 Static Random Access Memory (SRAM)  Based on the Flip-Flop  Requires a large number of transistors  Fast

 Dynamic Random Access Memory (DRAM)  Uses a single transistor to store charge  Requires very few transistors  Must be periodically refreshed  Slower

17 Basic Cells

• DRAM • SRAM

VDD

WL WL WL

DL DL DL

18 Random Access Memory (RAM)

19 Read Only Memory (ROM)

 Can only be read from.

 Memory is written (or “programmed”) once

 Reading from ROM is non-destructive.

 Access time to read from any memory location is the same.

 As compared to serial access memory.

 Non-Volatile

 Information is retained even after power is removed.

20 ROM Types

 Programmable Read Only Memory (PROM)

 Can be “programmed”

 Erasable PROM (EPROM)

 Can be “programmed” and erased

 Electrically Erasable PROM (EEPROM)

 Can be erased using an electrical signal

 UV Erasable PROM (UVEPROM)

 Can be erased using Ultraviolet light

21 ROM

• Fuse ROM EEPROM

WL WL

Floating Gate

DL DL

22 Read Only Memory (ROM)

23 Memory

24 Organization of Memory

Random Access Memory

25 Random Access Memory

 Address

 Location in memory of the binary information  Must be decoded to select the appropriate location and read/write the associated data  k-bit address → 2k memory locations

 Data

 Binary information of interest  Stored in a specific location in the memory  Typically organized into words  Each word has n bits

26 Random Access Memory

address data

10-bit address 1024 locations

27 Random Access Memory

 Read

 Indicates that the memory is to be read

 Write

 Indicates that the memory is to be written

28 Random Access Memory

29 Random Access Memory

 Rather than use the Read and Write signals, most commercially available RAM chips use Enable and Read/Write'

 Enable

 Used to enable the selected RAM chip  Aka. “chip select”

 Read/Write'

 RAM is read when Read/Write' = 1  RAM is written when Read/Write' = 0

30 Random Access Memory

31 Random Access Memory

Write Cycle

32 Random Access Memory

Read Cycle

33 Random Access Memory

34 Random Access Memory

35 ROM Organization

36 37 38 39 40 41 42 43 CAM Memory Operation

44 45 46 Application of CAM

In Network Routing and Switching Devices.

CPU and Disk Drives Cache Memory.

47 Memory hierarchy Speed Cost Space

Secondary Memory

48 Application of Memory

• In all types of Computers • In Mobiles • In satellite system • And much more…

49 References

[1]http://en.wikipedia.org/wiki/Computer_memory [2] Digital Design By: Moris Mano , PEARSON Education ( Third Edition) [3] Modern Digital Electronics By: RP Jain, TMH Publication ( Third Edition)

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