MEMORY DEVICES
Week 13
BASICS OF SEMICONDUCTOR MEMORY
Memory is the portion of a system for storing binary data
in large quantities. Semiconductor memories consists of
arrays of storage elements that are generally either latches
or semiconductors
UNITS:
BITS
: The smallest unit of binary data.
BYTE
: Unit of 8 bits or multiplies of 8 bit units .
NIBBLES : A byte split into two 4 bit units
WORD : Complete unit of information.
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Semiconductor memory array
►
Each storage element in a memory can retain either a 1 or 0 and is
called a cell. Memories are made up of arrays of cells as shown in fig
1. .Each block in the memory array represents one storage cell ,and
its location can be identified by specifying a row and a column.
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Memory Address and Capacity
►
ADDRESS: The location of a unit in a memory array .The
address of a byte is specified only by row. The address of
a bit is specified by row and column.
CAPACITY: It is the total number of data units that can
be stored.
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Basic memory operations
►
WRITE: The write operation puts data into a specified
address in the memory
►
READ: The read operation takes data out of a specified
address in the memory
►
ADDRESSING : The addressing operation which is part of
both read and write ,selects the specified memory address.
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System Bus
►
DATA BUS: Data units go into memory during a write operation
and come out of the memory during a read operation on a set of
lines called the data bus. The data bus is bidirectional.
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System Bus
►
ADDRESS BUS: For a write or read operation , an address
is selected by placing a binary code representing the
desired address on a set of lines called address lines.
►
The address code is decoded internally and the appropriate
address is selected . The number of lines in the address
bus depends on the addressing capacity..
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System Bus
8 bit
FFFF
FFFE
Instruction 1
Instruction 2
Instruction 2
Word length
1 word instruction
2 word instruction
Word: no. of bits
micro-P recognizes and
processes at a time ( 4
- 64bit ).
► Instruction:
combination
of
bit
patterns with specific
meaning known to
micro-P.
► Program: Set of all
instructions.
►
64KByte
0001
0000
address
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Illustration of the write operation.
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►
To store a byte of data in memory ,a code held in the
address register is placed on the address bus. Once the
address code is on the bus ,the address decoder decodes
the address and selects the specified location in the
memory.
►
The memory then gets a write command ,and the data
byte held in the data register is placed on the data bus and
stored in the selected memory address, thus completing
the write operation. When a new byte is written into a
memory address, the current data byte stored at that
address is overwritten and destroyed.
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ILLUSTRATION OF THE READ OPERATION
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ILLUSTRATION OF THE READ OPERATION
►
A code held in the address register is placed on the
address bus. Once the address code is on the address bus
,the address decoder decodes the address and selects the
specified location in the memory.
►
The memory then gets a read command and a copy of the
data byte that is stored in the selected memory address is
placed on the data bus and loaded into the data register
,thus completing the read operation. When a byte is read
form a memory address, it also remains stored at that
address and is not destroyed .This is called non destructive
read.
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RAMs AND ROMs
►
RAM(RANDOM ACCESS MEMORY) : A type of memory in
which all addresses are accessible in an equal amount of
time and can be selected in any order for a read or write
operation .All RAM’S have both read and write capability.
►
ROM(READ ONLY MEMORY): A type of memory where data
can be stored permanently or semi permanently .Data can
be read form a ROM ,but there is no write operation as in
RAM. The ROM is a random access memory
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THE RAM FAMILY
►
The two categories of RAM are static RAM (SRAM) and
dynamic RAM(DRAM) .
►
SRAMs use flip flops as storage elements and can therefore
store data indefinitely as long as dc power is applied
►
DRAMs use capacitors as storage elements and can retain
data very long without the capacitors being recharged by a
process called refreshing .
►
Both SRAMs and DRAMs are volatile memories because
they will lose stored data when dc power is removed.
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Figure 12--6
The RAM family.
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SRAM
Static RAM is a type of RAM that holds its data
without external refresh, for as long as power is
supplied to the circuit.
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SRAM ORGANIZATION
►
An asynchronous SRAM is one in which th e operation is
not synchronized with a system clock .The logic symbol for
a 32k x 8 bit memory is shown in fig.
►
In the READ mode, the eight data bits that are stored in a
selected address appear on the data output lines. In the
write mode ,the eight data bits that are applied to the data
input lines are stored at a selected address. The data input
and data output lines (I/O1 through I/O8 ) are the same
lines. During READ ,they act as output lines (I1 THROUGH
I8)
►
And during WRITE they act as input lines (O1 THROUGH
O8)
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The basic organization of 32kx 8 bit SRAM
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SRAM ORGANIZATION
► The
memory cell array is arranged in 256
rows and 128 columns ,each with 8 bits
.There are actually 215 = 32,768 addresses
and each address contains 8 bits.
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DRAM
Dynamic RAM is a type of RAM that only holds its data if it
is continuously accessed by special logic called a refresh
circuit. The difference between SRAM and DRAMs is the
type of memory cell. DRAM memory cell consists of one
transistor and a capacitor and is much simpler than SRAM
cell .
This allows much great densities in DRAM’S and results in
greater bit capacities for a given chip area ,although much
slower access time.
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SRAM v DRAM
► Both
volatile
 Power needed to preserve data
► Dynamic





cell
Simpler to build, smaller
More dense
Less expensive
Needs refresh
Larger memory units
► Static
 Faster
 Cache
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► Cache
Memory:
It is a relatively small ,high speed memory
that stores the most recently used
instructions or data from the large but
slower main memory. It uses DRAM .
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Cache
► Small
amount of fast memory
► Sits between normal main memory and CPU
► May be located on CPU chip or module
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THE ROM FAMILY
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ROM Usage
► Permanent
storage
► Microprogramming
► Library subroutines
► Systems programs (BIOS)
► Function tables
► No need to load from secondary device
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ROM Organization
►
The logic symbol for 256 x 4 ROM is shown below.
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►
►
When any one of 256 binary codes is applied to the
address lines ,four data bits appear on the outputs is the
chip enable inputs are low
The memory cell array is actually a 32 x 32 matrix as
shown below.
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►
A0 through A8 are decoded by the row decoder to select
one of the 32 rows.
►
Three of the 8 address lines A5 through A7 are decoded by
the column decoder to select four of the 32 columns.
When an 8 bit address code is applied ,a 4 bit data word
appears on the data outputs when the chip enable lines
are low.
►
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►
The data stored in ROM are always there, whether the
power is on or not. A ROM can be removed from the PC,
and then replaced, and the data it contains will still be
there.
►
Data stored in these chips is unchangeable, provides a
measure of security against accidental or malicious
changes to its contents. Unlike RAM, which can be changed
as easily as it is read
 We will look at five of them to see how they differ in the
way they are programmed, erased, and reprogrammed
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Mask ROM
►
The mask ROM is usually referred to simply as a
ROM.
►
A regular ROM is constructed from hard-wired
logic, encoded in the silicon itself to perform a specific
function that cannot be changed.
►
They consume very little power and reliable but cannot
reprogram or rewrite.
►
Several types of user programmable ROMs have been
developed to overcome this disadvantage.
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Programmable ROM (PROM)
►
A mask ROM chip is very expensive and time-consuming to
create in small quantities from scratch.
►
Mainly, developers created a type of ROM known as
programmable read-only memory (PROM).
►
This is basically a blank ROM chip that can be written only
once using special equipment called a PROM programmer.
►
PROM chips have a grid of columns and rows just as
ordinary ROMs do.
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Programmable ROM (PROM)
►
The difference is that every intersection of a column and
row in a PROM chip has a fuse connecting them.
►
Since all the cells have a fuse, the initial (blank) state of a
PROM chip is all 1s.
►
The user cans selectively burn/blow any of these fuse links
to produce the desired stored memory data.
►
A charge sent through a column will pass through the fuse
in a cell to a grounded row indicating a value of 1.
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Programmable ROM (PROM)
►
To change the value of a cell to 0, you use a PROM
programmer to send a specific amount of current to the
cell to break the connection between the column and row
by burning out the fuse.
►
This process is known as burning the PROM.
►
Very few bipolar PROMs are still available today.
►
TMS27PC256 is a very popular CMOS PROM with a capacity
of 32K  8.
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Erasable Programmable ROM (EPROM)
►
An EPROM is a ROM that can be erased and
reprogrammed as often as desired. Once programmed.
►
The EPROM is a non-volatile memory that will hold its
stored data indefinitely.
►
A little glass window is provided in the top of the ROM
package.
►
Ultraviolet light of a specific frequency can be shined
through this window for a specified period of time, which
will erase all cells at the same time so that an erased
EPROM stores all 1s and allow it to be reprogrammed
again.
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Erasable Programmable ROM (EPROM)
► EPROMs
are configured using an EPROM
programmer that provides voltage at specified
levels depending on the type of EPROM used.
► Obviously
this is much more useful than a regular
PROM, but it does require the erasing light.
► EPROMs
are available in a wide range of capacities
and access times. The 27C64 is an example of 8K
x 8 CMOS EPROM
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Electrically Erasable Programmable ROM
(EEPROM)
► They
require dedicated equipment and a laborintensive process to remove and reinstall them
each time a change is necessary.
► The
next type of ROM is the EEPROM, which can
be erased under software control.
► This
is the most flexible type of ROM, and is now
commonly used for holding BIOS programs
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Electrically Erasable Programmable ROM
(EEPROM)
► In
EEPROMs the chip does not have to be
removed to be rewritten, the entire chip need not
be fully erased to change a specific portion of it,
and changing the contents does not require
additional dedicated equipment.
►
Instead of using UV light, you can return the
electrons in the cells of an EEPROM to normal with
the localized application of an electric field to each
cell.
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Electrically Erasable Programmable ROM
(EEPROM)
► This
erases the targeted cells of the
EEPROM, which can then be rewritten.
► EEPROMs
are changed 1 byte at a time,
which makes them versatile but slow.
► The
Intel 2864 is an example of EEPROM
with 8K  8 array with 13 address inputs
and eight data I/O pins
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Flash Memory
► Flash
memories are so called because of their
rapid erase and write times.
► EEPROM
chips speed is too slow to use in many
products that required quick changes to the data
stored on the chip.
► So
a new type of EEPROM called Flash memory
that uses in-circuit wiring to erase by applying an
electrical field to the entire chip or to
predetermined sections of the chip called blocks.
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Flash Memory
► Flash
memory works much faster than
traditional EEPROMs because it writes data
in chunks, usually 512 bytes in size, instead
of 1 byte at a time.
► The
28F256A CMOS IC is an example of
flash memory chip, which has a capacity of
32K  8.
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