memory devices

MEMORY DEVICESMEMORY DEVICES

Week 13 Week 13

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BASICS OF SEMICONDUCTOR MEMORYBASICS OF SEMICONDUCTOR MEMORY

Memory is the portion of a system for storing Memory is the portion of a system for storing binary data in large quantities. Semiconductor binary data in large quantities. Semiconductor memories consists of arrays of storage elements memories consists of arrays of storage elements that are generally either latches or semiconductorsthat are generally either latches or semiconductors

UNITS:UNITS:

BITS : The smallest unit of binary data.BITS : The smallest unit of binary data.

BYTE : Unit of 8 bits or multiplies of 8 bit units .BYTE : Unit of 8 bits or multiplies of 8 bit units .

NIBBLES : A byte split into two 4 bit units NIBBLES : A byte split into two 4 bit units

WORD : Complete unit of informationWORD : Complete unit of information..

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Semiconductor memory arraySemiconductor memory array► Each storage element in a memory can retain either a 1 or 0 and is 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 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 fig 1. .Each block in the memory array represents one storage cell ,and its location can be identified by specifying a row and a cell ,and its location can be identified by specifying a row and a column.column.

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Memory Address and CapacityMemory Address and Capacity

► ADDRESS: The location of a unit in a memory ADDRESS: The location of a unit in a memory array .The address of a byte is specified only by array .The address of a byte is specified only by row. The address of a bit is specified by row and row. The address of a bit is specified by row and column.column.

CAPACITY: It is the total number of data units that CAPACITY: It is the total number of data units that can be stored.can be stored.

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Basic memory operationsBasic memory operations

► WRITE: The write operation puts data into a WRITE: The write operation puts data into a specified address in the memory specified address in the memory

► READ: The read operation takes data out of a READ: The read operation takes data out of a specified address in the memoryspecified address in the memory

► ADDRESSING : The addressing operation which is ADDRESSING : The addressing operation which is part of both read and write ,selects the specified part of both read and write ,selects the specified memory address.memory address.

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► DATA BUS: Data units go into memory during a write DATA BUS: Data units go into memory during a write operation and come out of the memory during a read operation and come out of the memory during a read operation on a set of lines called the data bus. The data bus is operation on a set of lines called the data bus. The data bus is bidirectional.bidirectional.

System BusSystem Bus

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► ADDRESS BUS: For a write or read operation , an ADDRESS BUS: For a write or read operation , an address is selected by placing a binary code address is selected by placing a binary code representing the desired address on a set of lines representing the desired address on a set of lines called address lines.called address lines.

► The address code is decoded internally and the The address code is decoded internally and the appropriate address is selected . The number of appropriate address is selected . The number of lines in the address bus depends on the addressing lines in the address bus depends on the addressing capacity..capacity..

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0000

0001

FFFF

FFFE

64KByte

8 bit

Instruction 1

Instruction 2

Instruction 2

► WordWord: no. of bits micro-P : no. of bits micro-P recognizes and processes recognizes and processes at a time ( 4 - 64bit ).at a time ( 4 - 64bit ).

► InstructionInstruction: combination : combination of bit patterns with of bit patterns with specific meaning known to specific meaning known to micro-P.micro-P.

► ProgramProgram: Set of all : Set of all instructions.instructions.

Word length

1 word instruction

2 word instruction

address


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Illustration of the write operation.Illustration of the write operation.

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► To store a byte of data in memory ,a code held in To store a byte of data in memory ,a code held in the address register is placed on the address bus. the address register is placed on the address bus. Once the address code is on the bus ,the address Once the address code is on the bus ,the address decoder decodes the address and selects the decoder decodes the address and selects the specified location in the memory. specified location in the memory.

► The memory then gets a write command ,and the The memory then gets a write command ,and the data byte held in the data register is placed on the data byte held in the data register is placed on the data bus and stored in the selected memory data bus and stored in the selected memory address, thus completing the write operation. When address, thus completing the write operation. When a new byte is written into a memory address, the a new byte is written into a memory address, the current data byte stored at that address is current data byte stored at that address is overwritten and destroyed.overwritten and destroyed.

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ILLUSTRATION OF THE READ OPERATIONILLUSTRATION OF THE READ OPERATION

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► A code held in the address register is placed on the A code held in the address register is placed on the address bus. Once the address code is on the address bus. Once the address code is on the address bus ,the address decoder decodes the address bus ,the address decoder decodes the address and selects the specified location in the address and selects the specified location in the memory. memory.

► The memory then gets a read command and a copy The memory then gets a read command and a copy of the data byte that is stored in the selected of the data byte that is stored in the selected memory address is placed on the data bus and memory address is placed on the data bus and loaded into the data register ,thus completing the loaded into the data register ,thus completing the read operation. When a byte is read form a memory read operation. When a byte is read form a memory address, it also remains stored at that address and address, it also remains stored at that address and is not destroyed .This is called non destructive read.is not destroyed .This is called non destructive read.

ILLUSTRATION OF THE READ OPERATIONILLUSTRATION OF THE READ OPERATION

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RAMs AND ROMsRAMs AND ROMs

► RAM(RANDOM ACCESS MEMORY) : A type of RAM(RANDOM ACCESS MEMORY) : A type of memory in which all addresses are accessible in an memory in which all addresses are accessible in an equal amount of time and can be selected in any equal amount of time and can be selected in any order for a read or write operation .All RAM’S have order for a read or write operation .All RAM’S have both read and write capability.both read and write capability.

► ROM(READ ONLY MEMORY): A type of memory ROM(READ ONLY MEMORY): A type of memory where data can be stored permanently or semi where data can be stored permanently or semi permanently .Data can be read form a ROM ,but permanently .Data can be read form a ROM ,but there is no write operation as in RAM. The ROM is a there is no write operation as in RAM. The ROM is a random access memory random access memory

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THE RAM FAMILYTHE RAM FAMILY

► The two categories of RAM are static RAM (SRAM) and The two categories of RAM are static RAM (SRAM) and dynamic RAM(DRAM) .dynamic RAM(DRAM) .

► SRAMs use flip flops as storage elements and can SRAMs use flip flops as storage elements and can therefore store data indefinitely as long as dc power is therefore store data indefinitely as long as dc power is appliedapplied

► DRAMs use capacitors as storage elements and can DRAMs use capacitors as storage elements and can retain data very long without the capacitors being retain data very long without the capacitors being recharged by a process called refreshing .recharged by a process called refreshing .

► Both SRAMs and DRAMs are volatile memories because Both SRAMs and DRAMs are volatile memories because they will lose stored data when dc power is removed.they will lose stored data when dc power is removed.

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Figure 12--6Figure 12--6 The RAM family. The RAM family.

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SRAM SRAM

Static RAM is a type of RAM that holds its Static RAM is a type of RAM that holds its data without external refresh, for as long data without external refresh, for as long as power is supplied to the circuit. as power is supplied to the circuit.

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SRAM ORGANIZATIONSRAM ORGANIZATION

► An asynchronous SRAM is one in which th e operation An asynchronous SRAM is one in which th e operation is not synchronized with a system clock .The logic is not synchronized with a system clock .The logic symbol for a 32k x 8 bit memory is shown in fig.symbol for a 32k x 8 bit memory is shown in fig.

► In the READ mode, the eight data bitsIn the READ mode, the eight data bits that are stored that are stored in a selected address appear on the data output in a selected address appear on the data output lines. In the write mode ,the eight data bits that are lines. In the write mode ,the eight data bits that are applied to the data input lines are stored at a applied to the data input lines are stored at a selected address. The data input and data output selected address. The data input and data output lines lines (I/O(I/O1 through I/OI/O8 ) are the same lines. During READ ,they act as output lines (I1 THROUGH I8)

► And during WRITE they act as input lines And during WRITE they act as input lines (O1 THROUGH O8)

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The basic organization of 32kx 8 bit The basic organization of 32kx 8 bit SRAMSRAM

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►The memory cell array is arranged in The memory cell array is arranged in 256 rows and 128 columns ,each with 256 rows and 128 columns ,each with 8 bits .There are actually 28 bits .There are actually 215 15 = 32,768 = 32,768 addresses and each address contains addresses and each address contains 8 bits.8 bits.

SRAM ORGANIZATIONSRAM ORGANIZATION

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DRAMDRAM Dynamic RAM is a type of RAM that only holds its Dynamic RAM is a type of RAM that only holds its

data if it is continuously accessed by special logic data if it is continuously accessed by special logic called a refresh circuit. The difference between called a refresh circuit. The difference between SRAM and DRAMs is the type of memory cell. DRAM SRAM and DRAMs is the type of memory cell. DRAM memory cell consists of one transistor and a memory cell consists of one transistor and a capacitor and is much simpler than SRAM cell .capacitor and is much simpler than SRAM cell .

This allows much great densities in DRAM’S and This allows much great densities in DRAM’S and results in greater bit capacities for a given chip results in greater bit capacities for a given chip area ,although much slower access time.area ,although much slower access time.

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SRAM v DRAMSRAM v DRAM

► Both volatileBoth volatile Power needed to preserve dataPower needed to preserve data

► Dynamic cell Dynamic cell Simpler to build, smallerSimpler to build, smaller More denseMore dense Less expensiveLess expensive Needs refreshNeeds refresh Larger memory unitsLarger memory units

► StaticStatic FasterFaster CacheCache

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►Cache Memory: Cache Memory:

It is a relatively small ,high speed It is a relatively small ,high speed memory that stores the most recently memory that stores the most recently used instructions or data from the used instructions or data from the large but slower main memory. It uses large but slower main memory. It uses DRAM .DRAM .

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►Small amount of fast memorySmall amount of fast memory►Sits between normal main memory Sits between normal main memory

and CPUand CPU►May be located on CPU chip or moduleMay be located on CPU chip or module

CacheCache

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THE ROM FAMILYTHE ROM FAMILY

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►Permanent storagePermanent storage►MicroprogrammingMicroprogramming►Library subroutinesLibrary subroutines►Systems programs (BIOS)Systems programs (BIOS)►Function tablesFunction tables►No need to load from secondary No need to load from secondary

devicedevice

ROM UsageROM Usage

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ROM OrganizationROM Organization► The logic symbol for 256 x 4 ROM is shown The logic symbol for 256 x 4 ROM is shown

below.below.

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► When any one of 256 binary codes is applied to the When any one of 256 binary codes is applied to the address lines ,four data bits appear on the outputs address lines ,four data bits appear on the outputs is the chip enable inputs are low is the chip enable inputs are low

► The memory cell array is actually a 32 x 32 matrix The memory cell array is actually a 32 x 32 matrix as shown below. as shown below.

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► AA00 through A through A88 are decoded by the row decoder to are decoded by the row decoder to select one of the 32 rows.select one of the 32 rows.

► Three of the 8 address lines AThree of the 8 address lines A55 through A through A77 are are decoded by the column decoder to select four of decoded by the column decoder to select four of the 32 columns.the 32 columns.

► When an 8 bit address code is applied ,a 4 bit data When an 8 bit address code is applied ,a 4 bit data word appears on the data outputs when the chip word appears on the data outputs when the chip enable lines are low.enable lines are low.

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► The data stored in ROM are always there, whether the The data stored in ROM are always there, whether the power is on or not. A ROM can be removed from the PC, power is on or not. A ROM can be removed from the PC, and then replaced, and the data it contains will still be and then replaced, and the data it contains will still be there. there.

► Data stored in these chips is unchangeable, provides a Data stored in these chips is unchangeable, provides a measure of security against accidental or malicious measure of security against accidental or malicious changes to its contents. Unlike RAM, which can be changes to its contents. Unlike RAM, which can be changed as easily as it is readchanged as easily as it is read

We will look at five of them to see how they differ in We will look at five of them to see how they differ in the way they are programmed, erased, and the way they are programmed, erased, and reprogrammedreprogrammed

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Mask ROMMask ROM► The mask ROM is usually referred to simply as a The mask ROM is usually referred to simply as a ROM. ROM.

► A regular ROM is constructed from hard-wired A regular ROM is constructed from hard-wired logic, encoded in the silicon itself to perform a logic, encoded in the silicon itself to perform a

specific specific function that cannot be changed. function that cannot be changed.

► They consume very little power and reliable but They consume very little power and reliable but cannot reprogram or rewrite. cannot reprogram or rewrite.

► Several types of user programmable ROMs have Several types of user programmable ROMs have been been

developed to overcome this disadvantage.developed to overcome this disadvantage.

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Programmable ROM (PROM)Programmable ROM (PROM)

► A mask ROM chip is very expensive and time-A mask ROM chip is very expensive and time-consuming to create in small quantities from scratch. consuming to create in small quantities from scratch.

► Mainly, developers created a type of ROM known as Mainly, developers created a type of ROM known as programmable read-only memory (PROM). programmable read-only memory (PROM).

► This is basically a blank ROM chip that can be written This is basically a blank ROM chip that can be written only once using special equipment called a PROM only once using special equipment called a PROM programmer.programmer.

► PROM chips have a grid of columns and rows just as PROM chips have a grid of columns and rows just as ordinary ROMs do. ordinary ROMs do.

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► The difference is that every intersection of a column The difference is that every intersection of a column and row in a PROM chip has a fuse connecting them.and row in a PROM chip has a fuse connecting them.

► Since all the cells have a fuse, the initial (blank) state Since all the cells have a fuse, the initial (blank) state of a PROM chip is all 1s. of a PROM chip is all 1s.

► The user cans selectively burn/blow any of these fuse The user cans selectively burn/blow any of these fuse links to produce the desired stored memory data. links to produce the desired stored memory data.

► A charge sent through a column will pass through the A charge sent through a column will pass through the fuse in a cell to a grounded row indicating a value of 1. fuse in a cell to a grounded row indicating a value of 1.

Programmable ROM Programmable ROM (PROM)(PROM)

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► To change the value of a cell to 0, you use a PROM To change the value of a cell to 0, you use a PROM programmer to send a specific amount of current to programmer to send a specific amount of current to the cell to break the connection between the the cell to break the connection between the column and row by burning out thecolumn and row by burning out the fuse. fuse.

► This process is known as burning the PROM. This process is known as burning the PROM.

► Very few bipolar PROMs are still available today. Very few bipolar PROMs are still available today.

► TMS27PC256 is a very popular CMOS PROM with a TMS27PC256 is a very popular CMOS PROM with a capacity of 32K capacity of 32K 8. 8.

Programmable ROM Programmable ROM (PROM)(PROM)

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Erasable Programmable ROM (EPROM)Erasable Programmable ROM (EPROM)

► An EPROM is a ROM that can be erased and An EPROM is a ROM that can be erased and reprogrammed as often as desired. Once reprogrammed as often as desired. Once

programmed.programmed.

► The EPROM is a non-volatile memory that will hold its The EPROM is a non-volatile memory that will hold its stored data indefinitely.stored data indefinitely.

► A little glass window is provided in the top of the ROM A little glass window is provided in the top of the ROM package. package.

► Ultraviolet light of a specific frequency can be shined Ultraviolet light of a specific frequency can be shined through this window for a specified period of time, through this window for a specified period of time, which will erase all cells at the same time so that an which will erase all cells at the same time so that an erased EPROM stores all 1s and allow it to be erased EPROM stores all 1s and allow it to be reprogrammed again. reprogrammed again.

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Erasable Programmable ROM (EPROM)Erasable Programmable ROM (EPROM)

► EPROMs are configured using an EPROM EPROMs are configured using an EPROM programmer that provides voltage at specified programmer that provides voltage at specified levels depending on the type of EPROM used. levels depending on the type of EPROM used.

► Obviously this is much more useful than a Obviously this is much more useful than a regular PROM, but it does require the erasing regular PROM, but it does require the erasing light. light.

► EPROMs are available in a wide range of EPROMs are available in a wide range of capacities and access times. The 27C64 is an capacities and access times. The 27C64 is an example of 8K x 8 CMOS EPROMexample of 8K x 8 CMOS EPROM

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Electrically Erasable Programmable ROM Electrically Erasable Programmable ROM (EEPROM)(EEPROM)

► They require dedicated equipment and a labor-They require dedicated equipment and a labor-intensive process to remove and reinstall them intensive process to remove and reinstall them each time a change is necessary. each time a change is necessary.

► The next type of ROM is the EEPROM, which can The next type of ROM is the EEPROM, which can be erased under software control. be erased under software control.

► This is the most flexible type of ROM, and is This is the most flexible type of ROM, and is now commonly used for holding BIOS programsnow commonly used for holding BIOS programs

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► In EEPROMs the chip does not have to be In EEPROMs the chip does not have to be removed to be rewritten, the entire chip removed to be rewritten, the entire chip need not be fully erased to change a need not be fully erased to change a specific portion of it, and changing the specific portion of it, and changing the contents does not require additional contents does not require additional dedicated equipment.dedicated equipment.

► Instead of using UV light, you can return the Instead of using UV light, you can return the electrons in the cells of an EEPROM to electrons in the cells of an EEPROM to normal with the localized application of an normal with the localized application of an electric field to each cell. electric field to each cell.

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►This erases the targeted cells of the This erases the targeted cells of the EEPROM, which can then be rewritten. EEPROM, which can then be rewritten.

►EEPROMs are changed 1 byte at a time, EEPROMs are changed 1 byte at a time, which makes them versatile but slow. which makes them versatile but slow.

►The Intel 2864 is an example of The Intel 2864 is an example of EEPROM with 8K EEPROM with 8K 8 array with 13 8 array with 13 address inputs and eight data I/O pinsaddress inputs and eight data I/O pins

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Flash MemoryFlash Memory

► Flash memories are so called because of Flash memories are so called because of their rapid erase and write times. their rapid erase and write times.

► EEPROM chips speed is too slow to use in EEPROM chips speed is too slow to use in many products that required quick changes many products that required quick changes to the data stored on the chip. to the data stored on the chip.

► So a new type of EEPROM called Flash So a new type of EEPROM called Flash memory that uses in-circuit wiring to erase memory that uses in-circuit wiring to erase by applying an electrical field to the entire by applying an electrical field to the entire chip or to predetermined sections of the chip or to predetermined sections of the chip called blocks. chip called blocks.

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Flash MemoryFlash Memory

►Flash memory works much faster than Flash memory works much faster than traditional EEPROMs because it writes traditional EEPROMs because it writes data in chunks, usually 512 bytes in data in chunks, usually 512 bytes in size, instead of 1 byte at a time. size, instead of 1 byte at a time.

►The 28F256A CMOS IC is an example The 28F256A CMOS IC is an example of flash memory chip, which has a of flash memory chip, which has a capacity of 32K capacity of 32K 8. 8.

memory devices

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