Download - Memory and Programmable Logic Devices
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P.RAJA
Assistant Professor,Department of ECE,
Mailam Engineering College,Mailam 604 304
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A memory unit is a collection of cells capable ofstoring a large quantity of binary information
There are two types of memories that are used indigital systems
Random-Access Memory (RAM)
Read-Only Memory (ROM)
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RAM can perform both write and readoperations.
A RAM loses its stored data when power is turned off,thus it is a volatile memory.
The time it takes to transfer information to or from
any desired random location is always the same,hence the name random-access memory (RAM).
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A memory unit stores binary information in
groups of bits called words.
A group of eight bits is called a byte. A block diagram of the memory unit
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The n data inputlines provide the information tobe storedin memory and the n outputlines specifythe binary datacoming out of the memory.
The kaddress lines specify the particular word chosen.
The two control inputs specify the direction of data
transfer required Read and Write.
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Data inputs
Data outputs
3 t 8c d r
A0
A1
A2
A2 A1 A0 = 000
A2 A1 A0 = 101
Memory size = No. of addresslocation v No. of data lines = 8 v 8
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Memory size = 1024 v 16
= 210 v 16
Each word in memory is assigned an
identification number, called an address
(0 up to 2k-1).
The selection of a specific word inside
the memory is done by applying the k-bit
address to the address lines.
A decoderaccepts this address and opens
the paths needed to select the word
specified.
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Write Operations
Apply the binary address of the desired word to the
address lines.
Apply the data bits that must be stored in memory tothe data input lines.
Activate the write input
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Read Operations
Apply the binary address of the desired word to theaddress lines.
Activate the readinput
Some memory chips have a different configuration of controlinputs; one input selects the unit and the other decides the
operation.
Memory Enable Read / Write Memory Operation
0 X None
1 0 Write to selected word
1 1 Read to selected word
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The operation ofa memory is usually controlled bythe CPU.
The access time of a memory is the time required toselect aword and read it.
The cycle time of a memory is the time required to
complete awrite operation.
The access time and cycle time must be within a timeequal to a fixed number of CPU clockcycles.
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StaticRAM (SRAM): Memory cell is constructedusing Latch ( usingMOS transistor )
DynamicRAM (DRAM) : Stores the binary data inthe formofelectric charges on ca acitors.
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Row select
T3 T4
T1 T2
T5T6
Data
line
Data
line
Write
(W)
Read
(R)
T7 T8
T9T10
Data-in Data-out
VDD
D D
Row select T3 - ON T4 - ON
Column select T7
- ON T8
ON
Write T9
- ON T10 OFF
Read T9
- OFF T10 ON
Column select
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Rowselect
Columnselect
CT
Rowselect
Columnselect
CT
Writeoperation Read
operation
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Sl. No SRAM DRAM
1 It contains less memory cells
er unit area
It contains more memory cells
er unit area
2 It access time is less It access time is greater than
SRAM
3 Refreshing circuitry is not
required
Refreshing circuitry is
required
Cost is more Cost i s less
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A decoder with k inputs and 2k outputs requires 2kAND gates with k inputs per gate.
The number ofgates and the number of inputs perstage can be reduced by employing two decoders ina two-dimensionalselection scheme.
In this scheme, two k/2-input decoders are usedinstead of one k-inputdecoder.
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Aread-only memory (ROM) is a memory in which
permanent binary information is stored.
Block diagramof a ROM
We canperform the read operationonly
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Inputs Outputs
A B CIN S COUT
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
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3 to 8Decoder
A
B
CIN
Outputs
S COUT
0 0
1 0
1 0
0 1
1 0
0 1
0 1
1 1
S COUT
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Mask ROM rogram by manufacturer based on the user
specification ROM Programmable ROM User can program their requirement
One time program is possibleEPROM Erasable PROM Program by user
Reprogram by erase the existing data byapplyingUV ( ultra violet) rays
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PLD Programmable Logic Devices Types
PROM
ProgrammableArray Logic ( PAL).
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ProgrammableArray Logic
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