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1、Computer SystemDisc Drive Overview第1页/共85页Todays PC ArchitectureIO BusLogicISA BusOther peripheralsCPUPentium ProMemoryPCI BridgeChipVideo GraphicsAdapter CardInterfaceAdapter CardMonitorIDE or SCSIDisc DriveCableRibbon CablePCI Board EdgeConnectionPCI Board EdgeConnectionPCI BusLocal System BusWire
2、d onMother BoardWired onMother BoardWired onMother Board第2页/共85页FilesCollection of BytesTextDocumentComputer InstructionsPictureetc.Sequence of BlocksStored inFile is referenced by a filename rather than location on disk.Files are managed by the computers operating system.The disk drive has no aware
3、ness of files.第3页/共85页Storing Files on Disc Drive ComputerDisc DriveHere are 3 Block of DataStart Storing in Location 5ControllerInterfaceAdapterFile:Letter.DOCLETTWR.DOCANOTHER.DOC01234567891011121314151617181920DIRECTORYTransfer Ratein Mega Mytes per second(MBps)第4页/共85页How to Access FilesDirector
4、y=A List of Filenames and LacationsFilenameLETTER.DOC PROGRAM.EXE ANOTHER.DOC.Block location on disk5,6*,7*1024,1025*,1026*,1027*12,13*,14*,15*,16*.The operating system in the computer keeps track of the directory*In DOS,the directory keeps track of the location for only the 1st block of each file.T
5、he File Allocation Table,or FAT,keeps track of the location of the other blocks.第5页/共85页HDA ComponentsDISCCIRCULATE FILTERCLAMP RINGOD LIMIT STOPBOTTOM POLEVCMPCCPREAMP CHIPID LIMIT STOPHEADFLEXUREARMPIVOT CARTRIDGEBEARINGTOP VIEWDisc Drive Basics第6页/共85页PCB ComponentsHOST CONTROLLERVCM&SPINDLE CONT
6、ROLLERREAD/WRITE CHANNELMICROCONTROLLERSERVO CONTROLLERSRAMDRAMSHOCK SENSORSPINDLE CONNECTORHDA CONNECTORSHOCK IC第7页/共85页Mass Storage Architecture Using Disc DrivesRead/WriteChannelPositionSystemSPMControlSpindle Spindle MotorMotorVCM (Voice VCM (Voice Coil Motor)Coil Motor)ControllerInterfaceAdapte
7、rMemoryCPUPC-AT System PC-AT System Bus(ISA)Bus(ISA)SCSI Ribbon SCSI Ribbon CableCableEmbebbed on mother board or add-in card第8页/共85页Block DefinitionsREAD/WRITE Detects bits from the signal coming from the CHANNEL head(analog)and converts them into digital bitsPOSITION SYSTEMSeeks to and keeps the h
8、eads positioned over the correct track of data on the disk(E-Block-VCM-Servo)SPM CONTROL Keeps the disk rotating and at the proper speed CONTROLLERRecognizes the digital data coming from the Read Channel and organizes it into blocks of bytes第9页/共85页Using Recording Head To Magnetize A FilmFilm Motion
9、Current第10页/共85页Writing Data On A Magnetic FilmFilm MotionCurrent ReversedTransition Results第11页/共85页TrackTrack=A strip of data written on a magnetic filmEach bits value is sampled at regular interval:1 when magnetic transition presents0when magnetic transition does not presentTrack WidthSampling Pe
10、riod第12页/共85页Write Other Tracks by Moving the HeadFilm Motion第13页/共85页Track DensityTrack WidthTrack PitchTrack Density =Number of tracks that fit in one inch(TPI)第14页/共85页Bit Density(Linear Density)Bit LengthBit Density =Number of bits that fit in one inch of track(BPI)第15页/共85页Arial Density1”1”Area
11、l Density=The amount of data that can be stored in 1 square inchAD =BPI*TPI第16页/共85页Reading Data Back by MR Read HeadRun constant current through MR stripe,Measure the resistance.Magnetic field from filmpicked up by stripeField variation in stripechanges the resistanceMR stands for MagnetoResistance
12、.Film Motion第17页/共85页Problem with MR StripeThe MR stripe detects the field from a transition a long way away.Solutions:Space the transitions far apart Detect several overlapping bits at a time Use shields第18页/共85页Shielded MR HeadShields permit only the MR stripe to only see the media below the gap.第
13、19页/共85页The Voltage Being Picked Up is Not Very HighWall Plug220 VoltsComputer Signals3-5 VoltsFlashlight Battery1.5 VoltsEKG waves on your skin0.01 VoltsTV Signal(picked up by antenna)0.0008 VoltsSignal From Recording Head0.0003 Volts0.0003V0.075VPre-amplify the read signal very close to the head第2
14、0页/共85页Inductive Write MR Read HeadIntegrated Inductive Write MR Read Head第21页/共85页Track WidthReader GapMagnetic SpacingHead WidthTrack width is determined by head width(approximately equal).Bit length is determined by reader gap and spacing from gap to media and many others.What Controls Density?第2
15、2页/共85页The rate at which data is read or written through the headmeasured in Million bits per second(Mbps)As Bit Density Increases,So Does Data Rate!Dont confuse data rate with transfer rate,the rate at whichdata transfers over the interface(in Megabytes per secondor MBps)Film MotionData Rate第23页/共8
16、5页Magnetic Storage On A Disc DriveCircular TracksVoice Coil Motor movesthe head in and outSpindle Motor drives the discat constant RPM第24页/共85页Calculate Data Rate0.9 r Too big to deal with We break each track into chunks called sectors:Most common sector Size =512 Bytes(1024 and 2048 bytes common)Ty
17、pical Sectors Per Track =50 to 256 (determined by bit density)Breaking tracks into sectors used up some space-Formatting Efficiency(5%-15%)第26页/共85页Constant Angular Recording(CAR)RidRodRadiusData RateRidRodRadiusRidRodRadiusVelocityBPILess data第27页/共85页Zone Bit RecordingRidRodRadiusBPIRidRodRadiusRi
18、dRodRadiusVelocityData RateZoneMaximize CapacityZone第28页/共85页Zone Table第29页/共85页Constant Angular Recording CapacityCapacity=number of tracks bits per tracknumber of tracks=TPI (Rod Rid)bits per track=BPI Rid 2RidCaptacity=TPI (Rod Rid)bits per trackConstant Angular Recordingbits per track=constantRi
19、dRodRadiusbits per trackArea第30页/共85页Zoning Max CapacityZoned Recordingbits per track=2r BPIRidRodRadiusbits per trackCapacity Improvement=(Rod Rid)2 Rid 50%for 3.5”FF第31页/共85页Zoning Practical CapacityRidRodRadiusbits per trackCapacity Improvement=(Rod Rid)2 Rid(1-N-1)N=number of zones(4 in this exa
20、mple)4 zones 38%improvement8 zones 44%improvement4 zones 47%improvement4 zones 48%improvement Typical zoned drive has 16 zonesFor 3.5”FF drives,the limit to zonings improvement is about 150%第32页/共85页Magnetization Curve of MediaHHcDHM Squareness:Coercive-Squareness:Remanence:Saturation magnetization:
21、Coercivity:Slope at Coercivity:Magnetic Recording Basics第33页/共85页Longitudinal Recording Write FieldHeadHeadHx=2000OeHx=2200OeLines of constanthorizontal fieldintensityGap180020002200240026002800第34页/共85页The Write BubbleInside write bubbleField Hc of 2000OeStrong enough to magnetize mediaOutside writ
22、e bubbleField Hc of 2000OeStrong enough to magnetize mediaHeadHeadGap200022002400260028001800Media LayerHc=2000Oe第35页/共85页Writing a Transition?Media motionTransition written at the trailingEdge fo the write bubbleThis region is magnetized first to the leftand then again to the right第36页/共85页Writing
23、a Transition200022002400260028001800HM Media motionThe media in this area sees1200 Oe in the new direction,Stays magnetized in the old direction!The media in this area sees2400 Oe in the new direction,Being magnetized in the old direction!HcM=0第37页/共85页Real Transitions are Blurry!2000220024002600280
24、01800HM Media motionIt takes distance on the mediato change the direction of magnetizationThis is called“Transition Length”Transition Length第38页/共85页Transition LengthMHhHcxHMHcxMPrevious state of medium-50%50%Hdtransition length(2a)Horizontal Component of Head FieldDemagnetization Field from the Tra
25、nsition第39页/共85页Demagnetization Field from a TransitionMHdatransition length parameterx+MMHdHdTMrA recorded transition generates demagnetization fieldHd第40页/共85页Williams-Comstock Model of a Recorded TransitionMHdHHcxHMHcDH第41页/共85页Calculating The Transition LengthwhereTransition Length Parameter500
26、Magnetic Spacing3”Media Thickness200 Write Field Gradient Factor(0.75)300 Oe/”Media Coercivity2200 OeRemanence Magnetization7500 GCoercive Squareness80%Typical ValuesFrom Williams-Comstock Model第42页/共85页Writing Shorter Sharper TransitionsMedia motionCloser Head-Media Spacing(HMS)Thinner Media LayerS
27、horter Write Gap LengthTighter Media Switching Field Distribution(all the media switch at the same H)Write FieldGradientMediaSquarenessHigh Write Field Gradient(closer bubbles)200022002400260028001800HMTransition LengthHigh Media Squareness(how steep M-H curve)第43页/共85页Reading with a GMR Read HeadBM
28、MBMMvv第44页/共85页Physical Mechanism of GMR EffectM3dFermi levelM4sConduction bandTwo current modelFor normal GMR materialss-d scattering yields energy loss:significantly contributes to resistivity.The number of available 3d states at Fermi surface is different for different spins 第45页/共85页Physical Mec
29、hanism of GMR EffectLow resistance stateMMMMHigh resistance stateScattering of spin electrons occurs within a mono-layer from the interface.Parallel State:Antiparallel State第46页/共85页GMR Read Head Transfer CurveM2M1M2M1q qNon-magneticconductive layer第47页/共85页Characterizing Magnetically Isolated Pulse
30、sdT2aPW50GWhereTransition ParameterShield-to-Shield SpacingMagnetic SeparationMedia ThicknessFrom Williams-Comstock model第48页/共85页Achieving Desirable Isolated PulsesHigh Peak AmplitudeIncrease flux by increasing Mr(Remanence Magnetization)Increase flux by increasing media thicknessDecrease magnetic
31、spacingLonger read gap lengthNarrow Pulse WidthDecreasing magnetic spacingShorten read gap lengthDecrease media thicknessReduce self-demag by increasing coercivityIncrease write head field gradient in head construction(dont use too much current)readingwritingNeed trade-offs第49页/共85页Recording Channel
32、第50页/共85页Recording ChannelChannel write dataInputuser dataECC encoderChannel encoderEqualizerDetectorECC decoderChannel decoderoutputuser dataAnalog readback signal100101101101011011011011010100101101101011011011011010第51页/共85页Data Writing Processwrite current NRZIclock“Data”magnetic mediumT第52页/共85
33、页Data Reading Process S N S N S N S N S N IVT第53页/共85页The Read/Write ChannelWriteCircuitPreampEncoderDecoderReadChannelData To RecordWrite ClockData Read BackRead Ref.ClockFromConrollerToConrollerHDAPCB20 mA200 Vpp50 mVppTTL,ECLTTL,ECL1011101110111011第54页/共85页Pre-amps Write Circuit:H-Bridge DriverVc
34、cRdampHeadPredriverWrite DataWrite Gate第55页/共85页Pre-amps Read Circuit:Differential Pre-ampVV+-Single-endedDifferentialCommon-mode noiseis rejected!NoiseNoise第56页/共85页The Read Channel S N S N S N S N S N ObjectiveOutput a digital pulse corresponding to the peak of each transition on the mediaMEDIARea
35、dSignalDerivedClockRead ChannelOutputT第57页/共85页Peak DetectorThresholdDetectorDifferentiatorZero CrossingDetectorANDRead-backpulse101BitcellBitcellBitcellDetection Window=TNeed timingRecovery circuit第58页/共85页Timing Recovery:Phase Locked Loop(PLL)PhaseDetectorIntegratorVCOFrom PeakdetectorClockPeak De
36、tectorOutputVCO OutputVCOvery veryearlyVCOveryearlyVCOearlyVCOslightlyearlyVCOOn TimeVCOslightlylateVCOlatePulseMissingVCOOn TimeSlowDownSlowDownSlowDownSlowDownDontChangeSpeedUpSpeedUpDontChangeDontChangePhase DetectorOutput第59页/共85页Inter-Symbol Interference(ISI)Linear Superposition of pulsesReadba
37、ckWaveformWrite Current Pulses interfere with each other when written close together:Amplitudes are reduced and Timing is distortedUser Density(UD)=PW50/TTPW50第60页/共85页Peak Detector Output ExamplesPeak DetectionAnalog Read-back WaveformThreshold Detector OutputDifferentiated DataZero Crossing Detect
38、or OutputDetected Data第61页/共85页Read-back Waveforms at Different User DensitiesUD=0.75UD=1.5UD=2.0第62页/共85页Asynchronous DetectionPeakDetectionPLLDetector has NO knowledgeof the bit timingPLL knows the bit timingNo communication to Detector第63页/共85页Synchronous DetectionPeakDetectionPLLDetector has kno
39、wledge of when a pulse may occur(bit timing)Can make a here/not-here decisionMakes better decisionsSignal to Noise Ratio(SNR)can be lowerSampled Detector allows for post compensationModel and remove ISI as an error source第64页/共85页Synchronous Channel:Sampled Peak Detection0.0 -0.2 -0.9 0.0 0.9 0.1 -0
40、.1 -0.8 0.7 -0.8 -0.1 0.1 0.9 0 -0.9 -0.2 0.00 0 -1 0 1 0 0 -1 1 -1 0 0 1 0 -1 0 00 0 1 0 1 0 0 1 1 1 0 0 1 0 1 0 050%-50%Detection ThresholdReceived SamplesTargetvaluesDetectedData第65页/共85页Synchronous Channel:Sampled Peak Detection50%-50%Detection Threshold-0.2 -0.4 -0.8 0.0 0.7 0.3 -0.2 -0.7 0.3 -
41、0.7 -0.2 0.3 0.8 0 -0.8 -0.4 -0.2 0 0 -1 0 1 0 0 -1 0 -1 0 0 1 0 -1 0 00 0 1 0 1 0 0 1 0 1 0 0 1 0 1 0 0Received SamplesTargetvaluesDetectedDataMissing 1 transitionOr have one toomany transitions第66页/共85页Sequence DetectionWe know certain sequences shouldnt exist.Make use of the fact!Step 1:Determine
42、 the rule for which sequences existFor sampled peak detection=polarity of pulses must alternateStep 2:Compare the observed samples with the expected samplesfrom all possible sequences.Choose the closest sequence.Closest=sequence with minimum squared errorClosest=most likely sequence Maximum Likeliho
43、od第67页/共85页Step 1:Rule for Possible SequencesPreconditionedThe TrellisEach path through the trellis corresponds to a possible data sequenceEach path through the trellis predicts a possible sequence of samples to observes第68页/共85页Trellis Example第69页/共85页PRMLPartial Response Maximum LikelihoodBinary d
44、ata transmission method used in communications signal processing used to detect data in a noisy environment Originally used with deep space probesClass 4 applied to magnetic recording channels4 in PR4 refers to the class of partial response system used for magnetic recording channelsTwo relatively i
45、ndependent partsPartial Response-Method for equalizing the readback signal to achieve a sampled three level outputMaximum Likelihood-Sequence Detection 第70页/共85页Partial ResponseClass 4 Partial ResponseFilter or equalize until a transition gives the following waveformTarget more than one non-zero sam
46、ple per pulseEach sample only contains part of the pulse(response)2 non-zero samples(call them+1)All other samples=0第71页/共85页PR4 Equalized Isolated and Dibit PulsesIsolated PulseDibit Pulse第72页/共85页Example Class 4 Partial Response Waveform0 0 1 0 0 0 0 0 1 1 0 1 0 1 1 1 0 0 1 0 1 1 1 0 1 1NRZI第73页/共
47、85页PR4 Eye-patternAll waveforms at clock points pass through one of three points corresponding to sample values of-1,0,and 1.Sampling once each bit period results in three level output第74页/共85页Trellis Diagram for Class 4 Partial Response第75页/共85页PR4 State Diagram第76页/共85页Read-back Waveforms at Diffe
48、rent User DensitiesUD=0.75UD=1.5UD=2.0第77页/共85页Magnetic Channel SpectrumPW50/T=2PW50/T=3PW50/T=1PW50/T=1/2PW50/T=1/3At low recording densities the spectral energy is concentrated near one half of the channel clock rate frequencyAt higher recording densities most of the signal spectrum is below half
49、of the channel clock rate frequencyLimit channel bandwidth to 1/2T without losing information第78页/共85页Why go to higher order Partial ResponsesPR4PW50/T=0.5EPR4E2PR4第79页/共85页PRML Read ChannelAGC-Automatic Gain Control maintains required constant signal level(VGA&Gain Control)Low Pass Filter-Coarse eq
50、ualizationFIR-Finite Impulse Response filter for fine equalizationADC-Analog to Digital Converter samples equalizer outputViterbi Detector-Compares and selects maximum likelihood sequence第80页/共85页PRML Write ChannelRandomizer-Reduces probability of occurrence of worst case repeated patterns that are