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1、DRAM工作原理,Dynamic Random Access MemoryEach cell is a capacitor+a transistorVery small sizeSRAM uses six transistors per cellDivided into banks,rows&columnsEach bank can be independently controlled,DRAM,Main MemoryEverything that happens in the computer is resident in main memoryCapacity:around 100 Mb
2、yte to 100 Gbyte Random access Typical access time is 10-100 nanosecondsWhy DRAM for Main Memory?Cost effective(small chip area than SRAM)High Speed(than HDD,flash)High Density(Gbyte)Mass Production,Main memory,Notation:K,M,G In standard scientific nomenclature,the metricmodifiers K,M,and G to refer
3、 to factors of 1,000,1,000,000 and 1,000,000,000 respectively.Computer engineers have adopted K as thesymbol for a factor of 1,024(210)K:1,024(210)M:1,048,576(220)G:1,073,741,824(230)DRAM density 256M-bit 512M-bit,DRAM Density,What is a DRAM?DRAM stands for Dynamic Random Access Memory.Random access
4、 refers to the ability to access any of the information within the DRAM in random order.Dynamic refers to temporary or transient data storage.Data stored in dynamic memories naturally decays over time.Therefore,DRAM need periodic refresh operation to prevent data loss.,Memory:DRAM position Semicondu
5、ctor memory device ROM:Non volatile Mask ROM EPROM EEPROM Flash NAND:low speed,high density NOR:high speed,low density RAM:Volatile DRAM:Dynamic Random Access Memory SRAM:Static Random Access Memory Pseudo SRAM,DRAM Trend:Future High Speed-DDR(333MHz500MHz),DDR2(533800Mbps),DDR3(8001600Mbps)-Skew-de
6、lay minimized circuit/logic:post-charge logic,wave-pipelining-New Architecture:multi-bank structure,high speed Interface Low Power-5.5V=3.3V(sdr)=2.5V(ddr)=1.8V(ddr2)=1.5v(ddr3)=1.2v?-Small voltage swing I/O interface:LVTTL to SSTL,open drain-Low Power DRAM(PASR,TCSR,DPD)High Density-Memory density:
7、32MB=64MB=.1GB=2GB=4GB-application expansion:mobile,memory DB for shock(than HDD)-Process shrink:145nm(03)=120nm(04)=100nm=90nm=80nm Other Trends-Cost Effectiveness,Technical Compatibility,Stability,Environment.Reliability,Static RAM,SRAMBasic storage element is a 4 or 6 transistor circuit which wil
8、l hold a 1 or 0 as long as the system continues to receive powerNo need for a periodic refreshing signal or a clockUsed in system cacheFastest memory,but expensive,Dynamic RAM,DRAMDenser type of memoryMade up of one-transistor(1-T)memory cell which consists of a single access transistor and a capaci
9、torCheaper than SRAMUsed in main memoryMore complicated addressing scheme,Refresh in DRAMs,Capacitor leaks over time,the DRAM must be“REFRESHED”.,Capacitance Leakage,SRAM vs.DRAM,DRAM Lead Frame and Wire bonding,DRAM Architecture,SDRAM has the multi bank architecture.Conventional DRAM was product th
10、at have single bank architecture.The bank is independent active.memory array have independent internal data bus that have same width as external data bus.Every bank can be activating with interleaving manner.Another bank can be activated while 1st bank being accessed.(Burst read or write),Multi Bank
11、 Architecture,DRAM Multi Bank Architecture,DRAM Single Bank Architecture,DRAM Block Diagram(1),DRAM Block Diagram(2),DRAM Core Architecture,DRAM Address,DRAM Core Architecture,16bit DRAM Core,DRAM Data Path,DRAM 1T-1C structure,RAS:row address strobeCAS:column address strobeWE:write enableAddress:co
12、de to select memory cell locationDQ(I/O):bidirectional channel to transfer and receive dataDRAM cell:storage element to store binary data bitRefresh:the action to keep data from leakageActive:sense data from DRAM cellPre charge:standby state,DRAM Key word,DRAM cell array consist of so many cells.One
13、 transistor&One capacitorSmall sense amplifierLow input gain from charge sharingCS:Small storage capacitor:25fFCBL:Large parasitic capacitor:over 100fFVc:Storage voltageVCP:half Vc for plate biasVBLP:half Vc for BL pre charge bias(initial bias),DRAM Cell,DRAM Array Overview,Simplified Example,Activa
14、ting a Row,Activating a RowMust be done before a read or writeJust latch the row address and turn on a single wordline,Writing,WritingA row must be activeSelect the column addressDrive the data through the column muxStores the charge on a single capacitor,Reading,ReadingA row must be activeSelect th
15、e column addressThe value in the sense-amplifier is driven back out,The Sense-Amplifier,Sense-AmplifierA pair of cross-coupled invertersBasically an SRAM elementWeaker than the column muxWrite data will“outmuscle”the sense-amplifierKeeps the data at full level,Precharge,PrechargeInactive state(no wo
16、rdlines active)Precharge control line highTies the two sides of the sense-amp togetherThis makes the bitlines stay at VDD/2Only stable as long as the precharge control line is highotherwise this is unstable!No capacitors connected,Activation Revisited,ActivationTurn off the precharge control lineMak
17、es the sense-amp unstableit wants to go to either 0 or 1 instead of staying at VDD/2A very very very short time later,turn on the wordline of the row to be activated.Couples the capacitor onto the bitlinesThis“tips”the bitlines to hold the stored value.The sense-amp amplifies the capacitor back to f
18、ull value.(hence the name!),DRAM Refresh,Because the stored memory value is stored on a capacitor(that has resistive leakage),the memory is constantly“forgetting”its contents.Eventually,the charge on the capacitor wont be enough to tip the sense-amp in the right direction.But,activating a row restor
19、es the cells on that row to their full value.There is an explicit refresh command that just activates and immediately deactivates a row.The DRAM has an internal counter that contains the next row to be refreshed and increments every time a refresh command is issued.,DRAM Refresh,Data Retention Time
20、DRAM Cell consists of capacitance which has leakage as time Retention time is period for maintaining its data especially 1 data Usually,DRAM Cell refresh period is 64msRefresh Timing tREF:Real cell retention time(Device characteristic),ex)90ms(Hot)tRFC:Refresh command operating time,ex)75nsRefresh S
21、pec.Burst Refresh:64ms Distribute refresh-128Mb device(12 Row address):64ms/4K=15.6us-256Mb device(13 Row address):64ms/8K=7.8us,AUTO Refresh,When this command is input from the IDLE state,the synchronous DRAM starts autorefresh operation.During the auto-refresh operation,refresh address and bank se
22、lect address are generated inside the Synchronous DRAM.For every auto-refresh cycle,the internal address counter is updated.Accordingly,8192times are required to refresh the entire memory.Before executing the auto-refresh command,all the bank must be IDLE state.In addition,since the Precharge for al
23、l bank is automatically performed after auto-refresh,no Precharge command is required after auto-refresh.,Self Refresh,Self-Refresh EntrySELF:When this command is input during the IDLE state,the Synchronous DRAM starts self-refresh operation.After the execution of this command,selfrefresh continues
24、while CKE is Low.Since self-refresh is performed internally and automatically,external refresh operations are unnecessary.Self-Refresh ExitSELFX:When this command is executed during self-refresh mode,the Sync DRAM can exit from self-refresh mode.After exiting from self-refresh mode,the Sync DRAM ent
25、ers the IDLE state.,no Precharge command is required after auto-refresh.,Mode Register,Special command to initialize the DRAMBurst lengthInterleavingCAS Latency(read command to read data in clocks)For DDR,DLL reset is also here,MRS Block Diagram,Mode Register,Because the stored memory value is store
26、d on a,Extended Mode Register,Special command to initialize DDR DRAMDDR onlydont use for SDRDLL EnableDrive Strength,DRAM Interface,Command SignalsCAS#,RAS#,WE#,CS#CS#+CAS#=ReadCS#+WE#+CAS#=WriteCS#+RAS#+CAS#=RefreshCS#+RAS#=ActivateCS#+WE#=Burst StopCS#+WE#+RAS#=PrechargeCS#+WE#+CAS#+RAS#=MRS or EM
27、RSAll others:NOPOther signals:CLK,DATA,DQS,DRAM Interface,All signals go from the host to the memory except DQS and data which are bi-directional.,Read Cycle,Typical Read CycleBurst Length 4CAS Latency=3,Write Cycle,Typical Write CycleBurst Length 4Write latency is always zero,Data Clocking,CLK is a
28、lways driven by the hostDQS is driven by whoever is driving the dataNV chip drives on write cyclesMemory chip drives on read cyclesThis scheme is called“source-synchronous clocking”Eliminates a lot of the timing headaches from SDRAdds margin,Latencies,All kindsActivate to PrechargeLast write data to
29、 prechargeActivate to ReadActivate to WriteRefresh cycle timeRefresh intervalMinimum row active timeYadda yadda yaddaControlled by PFB_TIMING0,PFB_TIMING1,PFB_TIMING2,Write Cycle,DLLs,A DLL is a Delay-Locked LoopNo transistor can switch in zero time,so there will be a delay between clock and DQS on
30、readsBut,it would make it easier if DQS was always in phase with clock.DLL-off clock-DQS delay not in the specVaries between memory vendorsRe-creates a delayed version of its input clockKeeps DQS on reads aligned with clocksIts an analog circuit and is sensitive to noiseCan lose lock on the input cl
31、ock if the signal is not clean or the DLL power supply is noisy.,DLLs,DLL onDLL off,tAA,tAC,tOH tRCD,tRP Set-up/Hold time Vih,Vil Voh,Vol Ioh,Iol,Timing Parameters,SDRAM Timing Diagram,tAA,tAC,tOH(SDRAM),Setup/hold time,Timing for latching data in Input buffer CLK rising edge is strobe for data(SDRAM)DQS rising&falling edge is strobe for data(DDR SDRAM)During Setup&time,there is no abnormal signal allowed,VIH/VIL,VOH/VOL,IOH/IOL,DC Spec,Thanks!,
