T325: TECHNOLOGIES FOR DIGITAL MEDIA Block I part2: Information storage 1 1 Arab Open University – Lebanon – Spring 2011

T325: TECHNOLOGIES FOR DIGITAL MEDIA

Block I part2: Information storage1

1Arab Open University – Lebanon – Spring 2011

Outline

Arab Open University – Lebanon – Spring 2011

2

Introduction Rotating media Optical media. Solid-state memory

Introduction


3

Rotating media (hard disk drives (HDDs), CDs and DVDs), and semiconductor memory (flash technology), has seen remarkable development over recent years.

In this part we will be looking in more detail at:how rotating media and semiconductor

memories work?why it has been possible to make such

remarkable improvements?

Rotating media


4

How did they survive competition with semiconductor memories? On the face of it, it is rather surprising that rotating media such

as magnetic hard disks, CDs and DVDs, with their high precision drive mechanisms, have survived competition from semiconductor memories, which have the clear advantages of simple construction with no moving parts to wear out or get damaged by mechanical shock, together with fast access times.

In fact, magnetic disks have made remarkable leaps in capacity and reductions in physical size as a result of major innovations, and this has enabled them to retain their place as large read/write data stores; while optical media continue to offer a cheap and effective way of distributing data offline and making backups.

Disc or Disk ?


5

Both spellings are common Disk for magnetic media (Hard Disk) Disc for optical media (Blue-ray disc)

Rotating media – Magnetic disks


6

Magnetic media have a ferromagnetic surface on which areas can be magnetised (some metals, such as iron and its alloys, may be magnetised). Information may be encoded as a pattern of

magnetism This pattern can subsequently be detected and the

information retrieved Because the patterns persist until the material is

demagnetised or re-magnetised, the medium is ‘non-volatile’ and does not depend on a continual supply of power, unlike many semiconductor memories

Rotating media – Magnetic Disks


7

In a magnetic disk drive, a write head converts electrical signals to magnetised areas on the disk surface, or changes an already magnetised area.

Conversely, a read head produces an electrical signal in response to a magnetic field.

The two heads are constructed as a single assembly and mounted on an arm which can move radially across the spinning disk, so that any point on the recording surface may be reached.

Data is recorded in concentric tracks, with each track being written or read with the heads at a certain nominal radius

Rotating media – Magnetic disks


8

Rotating media - Electromagnetism


9

Close relationship between electricity and magnetism was not discovered until the 19th century

Any electrical conductor carrying a current has associated with it a magnetic field. For example, the wiring in your house has a magnetic field

around it when current is flowing, though it is rather a weak one The live and neutral conductors carry equal currents in opposite

directions, so their magnetic fields tend to cancel each other out.

If a wire is wound into a coil and a current is passed through it, then the magnetic fields produced by each turn of the coil all add together, making a much stronger field.



10

Hans Christian Oersted (1777--1851) noticed that a compass needle was deflected by a nearby wire carrying an electric current, demonstrating a connection between magnetism and electricity.

Investigating further, he concluded that a magnetic field encircled the current carrying wire.



11

André-Marie Ampère (1775-1836) showed that two current carrying wires attracted or repelled each other (depending on the direction of the currents) due to their magnetic fields. So, magnetic forces could be obtained without any permanent magnets being involved.

Ampère formulated a mathematical law linking electric currents and magnetic fields and showed that, by winding wire into a coil or ‘solenoid’, a strong magnetic field like that of a bar magnet could be obtained.



12

Wrapping wire around a bar magnet did not appear to produce an electric current.

Michael Faraday (1791-1867) demonstrated that electric currents could be produced by a magnetic field as long as the field was varying with time, an effect expressed mathematically in his law of magnetic induction.

Rotating media


13

Essentially, a write head is an electromagnet constructed so as to concentrate the magnetic field over a very small area of the surface of the disk.

If there is sufficient current through the write head, the magnetic field is intense enough to magnetise this area permanently (or until the next write).

By varying the current as the disk rotates under the head, a pattern of magnetism is built up along the track, corresponding to a stream of bits.

Rotating media


14

Is reading procedure the converse of the operation ? It does, but there is a subtle and important difference.

Faraday’s law of magnetic induction means that an unchanging magnetic field does not generate a current; what is needed is a changing field.

As the disk rotates under the head, the magnetic field it encounters does indeed change frequently, with consecutive areas being magnetised in either of two directions.

So, a head of this type does produce a voltage signal, but this signal represents the transitions between magnetic states rather than the states themselves.

This is allowed for in the way that data bits are encoded for writing and subsequently decoded for reading.

Rotating media Giant magneto-resistance (GMR)


15

Discovered by Albert Fert in France and Peter Grϋnberg in Germany, working independently. They jointly received the Nobel Prize in Physics in

October 2007 for their discovery. The effect of magnetoresistance is a change in

the electrical resistance of a conductor when it is placed in a magnetic field. Relates the voltage across a conductor to the current

flowing through it in accordance with Ohm’s law: V = I x R

So R can be measured by applying a known voltage to the conductor and measuring the current through it

Rotating mediaGiant Magnetoresistance


16

Magnetoresistance for a read head mechanism: Changes in resistance are, in principle, rather easier

to detect than the tiny amounts of energy that the magnetic bits of a high density disk can generate by induction.

The power used for the measurement now comes from an external voltage (or current) source.

Rotating mediaGiant magneto resistance


17

Although magnetoresistance was observed by William Thomson (Lord Kelvin) 150 years ago, the change in resistance of below 1% was too small to be practically useful.

The real breakthrough came with the discovery of GMR, when huge resistance changes (e.g. 50%) were reported.

GMR read heads were one of the first products of nanotechnology.

The term nanotechnology covers a wide variety of ideas and techniques, but its defining characteristic is that it deals with objects with dimensions in the order of a nanometre (10 - 9 m, or a millionth of a millimetre).

Rotating media


18

Perpendicular magnetic recording technology Another technique which has been used to

increase a real densities is perpendicular magnetic recording.

In conventional recording, or longitudinal magnetic recording, the magnetic bits may be thought of as tiny bar magnets which point along the track.

In perpendicular recording, the bar magnets are aligned at right angles to the disk and point in and out of the disk surface.

Arab Open University – Lebanon – Spring 2011 19

Rotating media

Rotating media


20

Materials used for the recording surface should have a much higher ‘coercively’.

Coercivity is a measure of how difficult a material is to magnetise or demagnetise.

Optical media


21

While the capacity increases in optical storage have not been as spectacular as those in magnetic disks and semiconductor memory

there have been steady improvements from the audio CD standard agreed back in 1979 (the ‘Red Book’ standard), with a capacity of less than a gigabyte, to newer formats capable of storing high definition movies of many gigabytes.

Optical media


22

A laser beam is focused onto the track and reflected back. The reflected beam is analyzed to detect the pits. Feedback

mechanisms (servomechanisms, or ‘servos’ for short) involving optics, mechanics and electronics ensure that the beam is kept focused on the track and does not drift away from the track.

They compensate effectively for minor warping or eccentricity and for some vibration.

The data surface is at the opposite side of a transparent polycarbonate substrate from the laser reader.

The laser light penetrates the transparent surface before it reaches a point of focus, so it is spread over a relatively wide area and, therefore, is less affected by minor scratches and dust particles.

Optical media


23

For read-only CDs, the pits are actual indentations in the data surface

For rewritable media they are simply areas of different reflectivity from the ‘land’ that surrounds them.

These discs use a material that can exist in one of two physical states or phases: crystalline (where the molecules are arranged in regular patterns) and amorphous (where there is a lack of order). One phase reflects light better than the other.

The phase depends on how the material is heated and cooled, and can be changed from one to the other and back again by heat from the laser.

This is called phase-change recording.

Optical media


24

Optical media


25

Blu-ray disc (BD) and HD DVD use blue lasers with a wavelength of 405 nm. As this is shorter than the wavelength of the red laser light used with earlier formats, the laser beam can be focused to a smaller spot on the data surface.

The pits and track pitch (the spacing between tracks) could be reduced in size, increasing the data capacity achievable.

Data in Blu-ray is recorded in a similar way to previous rewritable optical discs, although the details differ.

The recording surface uses a phase-change material, and the pits are areas of contrasting reflectivity.

Optical media


26

CD and DVD -- essential features The ubiquitous 12 cm diameter silvery discs we use, including DVD in its various formats, are all descendants of the original audio CD developed by Philips and Sony, and the essential principles remain the same. Data is written on a spiral track as a series of ‘pits’ of various lengths

Optical media


27

One important design decision was the thickness of the transparent layer covering the data surface.

In CDs the whole 1.2 mm thickness of the disc was used, while the thickness in DVDs was 0.6 mm.

However, there are optical advantages to be gained by having a much thinner transparent cover layer.

If the disc tilts at all, then a thick layer could deflect and defocus the beam.

Would having a thinner transparent layer make the disc more or less resistant to the effects of fingerprints and scratches? What measures might be taken to reduce their effects?

Optical media


28

Having a thinner transparent layer means that where the beam crosses the surface it covers a smaller area.

So, any scratches, fingerprints or particles of dust would be more likely to block out or distort the beam.

The disc would be less resistant to their effects. At first it was thought that scratching would be such a

problem that the disc would need to be contained in a protective cartridge, like a floppy disk. However, a cheaper and less bulky solution was adopted by giving the disc a very hard coating that would resist scratching.

Another possible measure is to accept a poorer raw error rate performance but use better error detection and correction methods to compensate.

Optical media

Unlike the original CD, where the pits were laid in a spiral pattern on an otherwise flat surface, Blu-ray discs have a physical spiral track on which the pits are recorded.

This track is a spiral groove. However, it would be no good just having a

completely flat recording surface, as the recording beam would have nothing to follow.

The preformed groove is there to guide the beam so that it deposits pits in the correct spiral pattern.

29


Optical media


30

Recall that a CD player, or indeed any optical disc player, uses a servomechanism to stay on track.

This is a form of closed-loop control where the position of the beam on the track is constantly monitored.

This signal is fed back to the motors that position the beam in such a way as to correct the departure.

The alternative, open-loop control, does not use monitoring and feedback, but simply relies on very precise mechanics and favourable operating conditions to keep errors to a minimum.)

Optical media


31

Optical media


32

Optical media


33

As the disc rotates, the servomechanism ensures that the laser follows the groove. This means that the servomechanism will be moving the laser from side to side to follow the wobble.

The rate at which it moves from side to side will be determined by the length of the wobbles (the wobble period – the distance between peaks) and the speed of rotation.

The length of the wobbles is predefined, so the equipment can determine the speed of rotation from the rate at which the laser is moving from side to side.

A feedback loop is used to control the speed of rotation, based on measuring the rate at which the servomechanism is having to move the laser from side to side.

In Blu-ray, the ‘wobbling’ technique is taken further, and is used to address 64 Kbyte blocks of data on the disc. This is done by modulation of the wobble signal.

Optical media


34

Important features of Blu-ray are as follows: Blu-ray data is held as a pattern of pits along a track. The laser beam reading the data is kept on the track and in

focus by a servomechanism. The track also produces a wobble signal. The wobble

information is used in locating blocks of data. When data is recorded it is encoded with an error-

correcting code (Reed--Solomon (or RS) code). This allows a large proportion of the errors to be corrected.

Small dust particles may corrupt only a few bytes, but marks or scratches might extend for some distance along the track and cause errors in a long sequence of data.

Solid-state memory


35

If you have been using portable digital equipment over recent years you can’t fail to have noticed the extraordinary developments in data storage based on flash memory.

Memory keys, memory cards (as used for digital cameras) and the built-in memory of mobile phones are all based on flash memory technology.

Even by the standards of digital technology, the development of flash memory has been impressive, in terms of both the storage capacity available and the cost.

Solid-state memory


36

Flash memory is an example of memory that is described as ‘solid-state’.

This use of the term ‘solid-state’ has an interesting history, and exactly what it implies is a little ambiguous. In science ‘the solid state’ means solid as distinct from

liquid, gas or plasma. This is the sense in which the term ‘solid-state

electronics’ was used, drawing the distinction between the transistor, which is entirely made of material in the solid state, and the thermionic valve, which is based on the behavior of electrons in a vacuum tube.

The distinction now is with rotating media; the significance of the term ‘solid’ seems in contrast with the moving mechanical parts of the hard drive.

Solid-state memory


37

Memory basics Our main interest is in memory which stores digital

media files -- audio, and still and moving images. Main memory is comprised of integrated circuits, and

these integrated circuits are of two sorts: ROM and RAM. The processor can both write to and read from the other

type of memory integrated circuit used in main memory, RAM.

The abbreviation RAM stands for random-access memory. This name arises from the fact that a data word in any location in the whole memory can be accessed just as quickly as a data word in any other location.

Solid-state memory


38

An important point about RAM is that it is usually volatile, which means that all the programs and associated data stored in it are lost when the power supply to the RAM is switched off.

This contrasts with ROM and with secondary storage media, both of which are non-volatile.

ROM is used for the programs needed on start-up. It is therefore necessary to ensure that all important

programs and associated data held in RAM are copied to a computer’s secondary memory before the computer is switched off.

Solid-state memory


39

Flash memory is a development from a type of memory known as EPROM.

EPROM is ‘erasable’ PROM, and PROM is ‘programmable’ ROM.

Non-programmable ROM has the data put into it when it is manufactured.

Programmable ROM, by contrast, is manufactured with no data stored in it and can be programmed by the user (the user in this case might be a computer manufacturer).

Special equipment is needed to program PROM, and it cannot be changed once it is programmed (it cannot be erased).

The term ‘programming’ is used, rather than ‘writing’, for putting data into these devices, because it is all done at once ‘up front’, rather than writing individual bits or bytes when needed, although for some devices the distinction is not so clear-cut.

Solid-state memory


40

There is also EEPROM, which is electrically erasable PROM.

As its name suggests, EEPROM is erased with an electrical signal rather than needing UV light.

Whereas erasing EPROM has to be done to the whole of the device (UV light is shone onto the whole of the ‘chip’), EEPROM can be erased one bit or one byte at a time.

A disadvantage of EEPROM compared with EPROM is that EEPROM is more complex and, therefore, costs more and it is not possible to fit as much memory on a single integrated circuit.

Solid-state memory


41

Flash memory is a variation on EEPROM.

The word ‘flash’ is used in reference to the way it is erased, which is done either on the whole device at once or on a block of data in the device and can be done quickly, ‘in a flash’.

Offering the electrical erase capability, traditionally featured by the expensive EEPROM, at cost and density comparable to EPROM, Flash memories not only have taken a big portion of their progenitor’s markets, but in addition they have greatly expanded the fields of application of non-volatile memories. (Source: Bez and Cappelletti, 2005, p. 84)

Solid-state memory


42

The flash memory cell. The memory cell is the part of the memory that can store a single

bit. To do this the cell must be able to exist in each of two states: one

state that represents a data 0 and one state that represents a data 1.

Reading a cell is the process of detecting which of the two states it is in.

Erasing a block of memory sets all the cells in a block to the same known state, that representing a data 1 (see next Figure).

Writing to the cells (programming the memory) is the process of setting some of the cells to the other state, that which represents a data 0.

Since the remaining cells are already in the state representing a data 1 anything to them there is no need to do anything to them.

Solid-state memory


43

The flash memory cell.

Solid-state memory


44

Flash cells use semiconductor technology. They are constructed using the same or similar semiconductor

materials and manufacturing processes to those used in microelectronics such as microprocessors.

The basic building block of microelectronics is the transistor. In flash memory a memory cell uses a single transistor,

whereas several transistors are needed for a single memory cell in some other types of semiconductor memory.

The fact that flash only needs one transistor for a memory cell is one of the reasons that it allows high storage densities.

In the simplest terms, as used in digital electronics, a transistor can be thought of as a controlled switch. Next Figure 2.9

illustrates this.

Solid-state memory


45

Solid-state memory


46

In very broad terms, there are two categories of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs).

The terms I used above, namely source, gate and drain, are the terms used for FETs, which are the transistors that are most widely used in digital circuits.

(In BJTs, the roughly equivalent terms are the emitter, base and collector.)

The ‘field effect’ in a field-effect transistor is the effect of an electric field that controls the switch.

Next Figure illustrates the structure of a FET, specifically a type of FET known as a metal-oxide-semiconductor field-effect transistor (MOSFET).

Solid-state memory


47

Solid-state memory


48

The substrate is made of a semiconductor material, such as silicon, as are the source and the drain.

The substrate differs from the source and the drain, however, by having different doping.

Doping of semiconductor material is the addition of small amounts of ‘impurities’ to change subtly the electrical characteristics of the semiconductor.

The substrate will be made into a ‘p-type’ semiconductor and both the source and drain into an ‘n-type’ semiconductor

Solid-state memory


49

in the absence of any electric field across the substrate, little or no current will flow between the source and the drain.

If there is a big enough electrical field across the substrate in the right direction, then an electrical channel is created between the source and the drain through which electricity can flow

Solid-state memory


50

The basic technique of building silicon integrated circuits (a process referred to as ‘fabrication’) consists of manufacturing a thin disc, referred to as a wafer, out of a crystal of pure silicon, then modifying it or adding material to the surface to create the transistors or other components.

For example, adding impurities to the silicon (‘doping’ the silicon) changes its electrical conductivity.

Polycrystalline silicon is used where good electrical conductivity is needed, and silicon oxide is used where a good electrical insulator is needed.

Solid-state memory


51

Solid-state memory


52

The flash cell differs from a basic MOSFET by having two gates. One of these has no electrical connections to it (it is surrounded

by insulators) and is described as a floating gate. The gate above it, which does have a connection, is called the

control gate. Though the floating gate has no electrical connections to it, if it

does in some way get electrical charge on it, then the charge will affect the electrical properties of the transistor.

Specifically, the presence of charge on the floating gate alters the field between the gate and the substrate, which in turn alters the resistance of the channel between the source and the drain.

This, then, is the principle that is used to store a data bit. The cell has two states.

In one (representing a data 1) there is no charge on the floating gate and in the other (representing a data 0) there is charge on the floating gate.

Solid-state memory


53

What has to be done to erase a cell? What has to be done to program a cell?

From earlier, erasing involves setting all cells to the state representing a data 1.

This is the state when there is no charge on the floating gate; so, erasing a cell requires the removal of charge from the floating gate.

In programming, setting a cell to the state representing a data 0 requires getting charge on to the floating gate.

Solid-state memory


54

What is needed, therefore, are mechanisms to add charge to the floating gate and to remove charge from it.

In essence the ‘trick’ is simply to use high voltages and/or high currents to force charge on or off!

Applying a high voltage between the control gate and the substrate forces electrons on or off the floating gate through a process known as Fowler-Nordheim (FN) tunnelling

Alternatively, there are methods which impart high energy to electrons which cause them to ‘jump’ through the insulator. These methods are described as ‘hot electron’ or ‘hot carrier’ methods.

Solid-state memory


55

To read data from a cell, a mechanism is needed to detect whether or not there is charge (electrons) on the floating gate.

To read data from a cell, you apply a voltage to the control gate and see whether it succeeds in allowing current to flow between the source and the drain.

If it does allow current to flow -- if current is detected at the drain -- then the data on the cell is a 1;

if it does not allow current to flow -- if no current is detected at the drain -- then the data on the cell is a 0.

Solid-state memory


56

Varieties of flash Such is the demand for flash memory that

manufacturers produce a large range of products, customised to specific applications.

Devices can be designed to be optimised for different applications through, among other things, the details of the cell design, the configuration of the cells on the chip, and the electronic circuits that accompany the memorycells.

It is impossible -- and would be of limited value -- to discuss all variations, but we will briefly look at some products which will highlight a number of features.

Solid-state memory


57

NAND flash memory The word NAND comes from binary logic, and is an

abbreviation of ‘NOT AND’. It is used in reference to the way in which the flash memory

cells are interconnected (which is similar to the way transistors are connected to make a digital electronic NAND gate).

In my description of the flash cell earlier, I said that there would either be charge on the floating gate or there would be no charge.

I did not say anything about how much charge there might be on the gate. The simplest flash cells only distinguish between these two options: charge or no charge.

Multi-level flash is more sophisticated, distinguishing between more than two levels.

Solid-state memory


58

This allows more than one bit to be stored in each cell, and hence a higher storage density.

For example, suppose in a binary cell a data 1 corresponds to no charge on the gate and a data 0 corresponds to an amount of charge we will call C.

If the system is able to add charge more precisely and also measure how much is on the gate accurately, it might be possible to work with four levels of charge: 0, C/3, 2C/3, C. These four levels could be used to represent two bits of binary data, defined as, for example: 0 charge = 11 C/3 = 01 2C/3 = 00 C = 10

Memory using this system would be described as ‘2 bits per cell’ memory.

Solid-state memory


59

Solid-state memory


60

How does the random access time of this NAND flash compare with that of a HDD that has a latency time of 4.17 ms?

The average random access time (latency) for the HDD was 4.17 ms, which is a lot longer than the 60 micro second for the flash.

This is a ratio of

Solid-state memory


61

NOR flash memoryThe word NOR comes from binary logic, and

is an abbreviation of ‘NOT OR’. As with NAND in NAND flash, NOR in NOR

flash is used in reference to the way in which the flash memory cells are interconnected within a memory chip (which is similar to the way transistors are connected to make a digital electronic NOR gate), but for our purposes it is simply a label.

Solid-state memory


62

Whereas NAND flash is optimised for bulk storage, similar to the function of rotating media, NOR flash is optimised for storage of computer code.

We are more interested in this course in memory for bulk storage, so we will not consider NOR flash in as much detail as we did NAND flash.

Solid-State memoryActivity 2.9


63

Figure 2.14 is a simplified display of the memory array configuration (it ignores the use of two planes).

Use information from the data sheet in Figure 2.13 to find: n, the number of pages per block m, the total number of blocks.

From these numbers, calculate (giving separate answers for main memory and spare memory in each case): the total number of bits in a page the total number of bits in a block the total number of bits in the whole

memory. (This should, of course, agree with the total memory size of 16 Gbits of main memory and 512 Mbits of spare memory.)

Solid-state memory


64

The layout of memory cells in NAND flash allows memory cells to be packed more closely together than in NOR flash.

According to a paper by Bez and Cappelletti (2005), if the technology node is F, then the space required by a NOR cell is 10F2 and the space required by a NAND cell is 4.5F2.

If the technology node F is 65 nm, use these formulae to calculate:

The cell size of flash NOR and NAND memory for binary cells (1 bit per cell).

The number of bits that can be stored on a chip that is 5mmx5mm.

Solid-state memory


65

(a) F = 65 nm = 65 x10-9 m.NOR cell size is 10F2, which is 10 x(65 x10-9)2m2

= 4.2 x10-14 m2.NAND cell size is 4.5F2, which is 4.5 x(65 ·10-9)2m2

= 1.9 x10-14 m2. (b) 5 mm square is (5 x10-3)2m2 = 2.5 x10-5m2.The number of NOR cells that will fit in that area is(2.5 x10-5)/(4.2 x10-14) = 5.9 x108. That is 560 Mbits (with M in

the sense of 220 = 1048 576).The number of NAND cells that will fit in that area is(2.5 x10-5)/(1.9 x10-14) = 1.3 x109. That is 1.2 Gbits (with G in

the sense of 230 = 1073 741824).

Solid-state memory


66

The NAND device has a much larger capacity than the NOR device (16 Gbits compared with 1 Gbit).

The random access time for the NOR flash is much shorter than for the NAND flash (96 ns compared with 60 ms).

NOR can program individual words, whereas NAND needs to program a page at a time.

NAND and NOR programming times are similar, being of the order of tens of microseconds.

It takes much longer to erase a block in the NOR flash than in the NAND flash (1 s compared with 2.5 ms).

The endurance of the NOR flash product is around 10 times that of the NAND flash (100 000 cycles compared to 10 000 cycles).

Documents

T325: TECHNOLOGIES FOR DIGITAL MEDIA Block I part2: Information storage 1 1 Arab Open University – Lebanon – Spring 2011