Abstract:
A method for determining a retention time in a solid state device (SSD), comprising the steps of providing a plurality of write operations to a memory, determining a reference voltage for each of the write operations, determining a difference between (i) the reference voltage after each of the write operations and (ii) a target reference voltage and if the difference is above a predetermined value, generating a flag indicating an excessive retention has occurred.

Description:
FIELD OF THE INVENTION 
     The invention relates to memory devices generally and, more particularly, to a method and/or apparatus for implementing a retention detection and/or channel tracking policy in a flash memory based storage system. 
     BACKGROUND 
     Conventional NAND flash memory cells use a floating gate transistor that stores data in a non-volatile fashion by holding a certain amount of electric charge in the floating gate. After a memory cell is programmed, as time goes by, the floating gate gradually loses its charge due to oxide leakage and charge detrapping. Data retention changes the distribution of threshold voltages Vt of the programmed states. Severe data retention changes can lead to read errors. In a flash memory based storage system, such as a solid state drive (SSD), it is important to avoid and/or handle errors caused by retention. Retention is a function of time and temperature. The longer the data stays in the memory without being re-written, the larger the retention effect. The higher the temperature, the larger the retention effect. Retention also becomes more pronounced as flash memory wears. As flash memory technology scales further towards sub-20 nm, and provides an increasing number of bits per cell (MLC, TLC, etc.), controlling retention becomes more challenging. For some applications, such as a SSD drive, retention time on a conventional flash memory can be longer than the maximum retention time specified by memory manufacturers. 
     SUMMARY 
     The invention concerns a method for determining a retention time in a solid state device (SSD), comprising the steps of providing a plurality of write operations to a memory, determining a reference voltage for each of the write operations, determining a difference between (i) the reference voltage after each of the write operations and (ii) a target reference voltage and if the difference is above a predetermined value, generating a flag indicating an excessive retention has occurred. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the invention will be apparent from the following detailed description and the appended claims and drawings in which: 
         FIG. 1  is a diagram of a solid state drive; 
         FIG. 2  is a more detailed diagram of the controller of the solid state drive of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a retention effect of the voltage threshold distribution; 
         FIG. 4  is a diagram of various reference voltages used in a multilevel flash memory; 
         FIG. 5  is a chart illustrating optimal voltage references that are tracked and stored; 
         FIG. 6  is a flow diagram illustrating a policy for tracking a voltage reference after an erase/program operation; and 
         FIG. 7  is a flow diagram illustrating a retention test mode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention include a method and/or apparatus for providing a retention detection and channel tracking policy that may (i) be used in flash memory based storage systems, (ii) provide a signal processing based approach, (iii) be implemented without tracking time or temperature, (iv) counteract a retention effect that is PEC dependent, (v) provide a DSP based approach, (vi) be integrated with existing channel/VREF tracking procedures, (vii) be implemented with a minimal computational overhead, and/or (viii) be implemented with a low storage overhead. 
     Referring to  FIG. 1 , a block diagram of an example apparatus  50  is shown. The apparatus  50  generally comprises a block (or circuit)  60 , a block (or circuit)  70  and a block (or circuit)  80 . The circuit  70  may include a circuit  90 . The circuit  90  may be a memory configured to store computer instructions (or firmware). The instructions, when executed, may perform a number of steps. The firmware  90  may include a tracking module  100  (to be described in more detail in connection with  FIGS. 5 and 6 ). 
     A signal (e.g., REQ) may be generated by the circuit  60 . 
     The signal REQ may be received by the circuit  70 . The signal REQ may be a request signal that may be used to access data from the circuit  80 . A signal (e.g., I/O) may be generated by the circuit  70  to be presented to/from the circuit  80 . The signal REQ may include one or more address bits. A signal (e.g., DATA) may be one or more data portions received by the circuit  80 . 
     The circuit  60  is shown implemented as a host circuit. The circuit  70  reads and writes data to and from the circuit  80 . The circuit  80  is generally implemented as a nonvolatile memory circuit. The circuit  80  may include a number of modules  90   a - 90   n . The modules  90   a - 90   n  may be implemented as NAND flash chips. In some embodiments, the circuit  80  may be a NAND flash device. In other embodiments, the circuit  70  and/or the circuit  80  may be implemented as all or a portion of a solid state drive having one or more nonvolatile devices. The circuit  80  is generally operational to store data in a nonvolatile condition. When data is read from the circuit  80 , the circuit  70  may access a set of data (e.g., multiple bits) identified in the signal REQ. 
     In some embodiments, the circuit  80  may be implemented as a single-level cell (e.g., SLC) type circuit. An SLC type circuit generally stores a single bit per memory cell (e.g., a logical 0 or 1). In other embodiments, the circuit  80  may be implemented as a multi-level cell (e.g., MLC) type circuit. An MLC type circuit is generally capable of storing multiple (e.g., two) bits per memory cell (e.g., logical 00, 01, 10 or 11). In still other embodiments, the circuit  80  may implement a triple-level cell (e.g., TLC) type circuit. A TLC circuit may be able to store multiple (e.g., three) bits per memory cell (e.g., a logical 000, 001, 010, 011, 100, 101, 110 or 111). 
     Embodiments of the invention provide a method for detecting long retention. A VREF/channel tracking policy tracks pre-defined values of program/erase counts (PEC). The VREF parameter is tracked immediately after a program operation is stored, and used for retention detection. During retention detection, tracking is done and the result of the value of the signal VREF is compared to the stored value of the signal VREF immediately after programming. If the drift of the voltage threshold Vt is larger than a predefined threshold, a long retention is detected. In one example, when a long retention has been detected, a flag may be set. Other types of indications to the controller  90  may be used to indicate that a long retention has been detected. 
     An optimal value for the signal VREF may be determined for a particular read operation. A dedicated procedure may be performed, generally called a retention test. A comparison of the value of the signal VREF may be made to the result to the value found immediately after the last erase/program operation is performed. If the signal VREF drifts down by a large amount, a long retention is detected. The retention detection leverages the results of the existing channel tracking module. An adjustment may be made on how the channel tracking procedure is triggered and/or how the optimal values of the signal VREF are stored. The adjustment may enable effective retention detection. 
     In general, the controller  70  may include an erase/program unit implemented in an R-block configuration. For example, multiple blocks may be read from multiple dies  90   a - 90   n . Channel tracking of the signal VREF and/or the retention detection unit is also included in the R-block configuration. The erase/program unit may be implemented as part of the firmware  90 . The channel tracking module  100  may track a distribution of the voltage threshold Vt for different states. Various channel parameters can be tracked, such as voltage threshold Vt mean, voltage threshold Vt variance, and/or optimal values of the signal VREF. 
     Referring to  FIG. 2 , a more detailed diagram of the controller  70  is shown. The controller  70  generally comprises a block (or circuit)  82 , block (or circuit)  84 , block (or circuit)  86 , block (or circuit)  88 , block (or circuit)  90 , block (or circuit)  92 , and a block (or circuit)  100 . The circuit  82  may be implemented as a host interface. The circuit  84  may be implemented as a buffer. The circuit  86  may be implemented as a decoder, such as an error correction (ECC) decoder. The circuit  88  may be implemented as a processor, such as a backend processor. The circuit  90  may be implemented as a processor, such as a digital signal processor (DSP). The circuit  92  may be implemented as an interface circuit, such as a flash interface circuit. The circuit  100  may be implement as a channel tracking/VREF tracking module. 
     Referring to  FIG. 3 , a diagram illustrating a retention effect of a voltage threshold distribution is shown. The solid lines and the dashed lines represent the distributions of the voltage threshold Vt after and before a retention period. 
     Referring to  FIG. 4 , a diagram showing values of the reference voltages signal VREF used in a MLC flash memory during a read operation is shown. The voltage VREF_B is used for lower page reads. The voltage VREF_A and the voltage VREF_C are used for upper page reads. The tracking module  100  finds the optimal values of the signal VREF to be used in hard-decision reads for the R-block. The particular criterion for determining an optimal value can vary to meet the design criteria of a particular implementation. The most typical criterion is to minimize the raw bit error rate. An example of a tracking process can be found in co-pending patent Ser. No. 13/464,433, the appropriate sections of which are incorporated by reference. 
     Since the distribution of the threshold voltage Vt changes with a number of factors (e.g., PE cycling, retention, read disturb, etc.), the optimal value for the signal VREF also changes. The tracking module  100  can choose to initiate the VREF tracking procedure whenever necessary and then store the optimal values for the signal VREF. However, for retention test purposes, access to the optimal value of the signal VREF tracked immediately after the program operation may be used. 
     Referring to  FIG. 5 , a table illustrating a format to store the value of the reference voltage parameter VREF is shown. The voltage VREF_CUR is shown representing the value of the signal VREF that is most recent for the tracked R-block. The voltage of a parameter VREF_ORG (e.g., a target reference voltage) is shown tracked immediately after programming. 
     In one example, the parameter VREF_ORG can be one value of the signal VREF (e.g., VREF_B) or, in another example, multiple values (e.g., VREF_B and VREF_C). The parameter VREF_ORG is a reference value used by the retention test to detect a long retention. The parameter VREF_ORG is also a reference value used by the tracking module  100  to determine whether the tracking procedure needs to be triggered. For example, a new erase/program operation may eliminate any retention and/or read disturb effect and/or potentially changes the value of the signal VREF. To trigger tracking after programming each R-block is generally inefficient. The voltage VREF_ORG of the module  100  may be used to reduce the frequency of how often the tracking procedure is invoked. 
     Referring to  FIG. 6 , a flow diagram of a method (or process)  200  illustrating a proposed policy for VREF/channel tracking after programming an R-block is shown. The method  200  generally comprises a step (or state)  202 , a decision step (or state)  204 , a step (or state)  206 , a step (or state)  208 , a step (or state)  210 , a decision step (or state)  212 , a step (or state)  214 , and a step (or state)  216 . The state  202  may be entered after programming an entire R-block. Next, the method  200  moves to the decision state  204 . The decision state  204  sets a number of values. The decision state  204  determines whether the value PEC is equal to an initial value after manufacturing (e.g., 1), or whether the value PEC equals some other predefined values. If not, the method  200  moves to the state  206 . If so, the method  200  moves to the state  208 . In the state  206 , the method  200  retrieves current VREF parameters of an R-block. Next, the method  200  moves to the state  210 . The state  210  retrieves the value VREF_CUR stored in the area of parameters. The state  210  compares the current value to the stored channel parameters. The state  210  compares to the value VREF_ORG. Next, the method  200  moves to the decision state  212 . The decision state  212  compares an absolute value of the value VREF_CUR minus VREF_ORG. If this absolute value is greater than a threshold, then the method  200  moves to the state  208 . If not, the method  200  moves to the state  216 . In the state  208 , the method  200  immediately starts tracking the channel for the R-block. Next, the method  200  moves to the state  214 , that updates the channel parameters. 
     The pre-defined PEC (program/erase count) values used to trigger tracking can be based on absolute numbers, such as 100, 200, 300, . . . , or as a percentage of device life, such as 10%, 20%, 25%, 30%, . . . of the device life. The values can be equally spaced or unequally spaced throughout the life of the device. 
     When the PEC is not at one of a number of pre-defined values, tracking is only triggered when there is a non-trivial difference between the signal VREF_ORG and the signal VREF_CUR. The difference can be measured based on a value of the signal VREF (e.g., VREF_B) or averaged over multiple values of the signal VREF. 
     Referring to  FIG. 7 , a tracking policy (or method)  300  for the signal VREF is shown. The method  300  implements a retention test method/flow for stored parameter VREF_ORG. The method  300  generally comprises a step (or state)  302 , a step (or state)  304 , a step (or state)  306 , a step (or state)  308 , a decision step (or state)  310 , a step (or state)  312 , a step (or state)  314 , and a step (or state)  316 . The step  302  generally starts a retention test on a given R-block. Next, the state  304  applies the tracking process to estimate the optimal value VREF for the R-block. Next, the state  306  sets a value VT_DRIFT as equal to the value VREF_ORG minus the value VREF_CUR_AW. The state  308  obtains the value VREF_ORG from the stored VREF parameters. Next, the decision state  310  determines whether a drift in the voltage threshold Vt is larger than a predefined threshold. If so, the method  300  moves to the state  312 . If not, the method  300  moves to the state  314 . In the state  312 , the method  300  declares a long retention as being detected. In the state  314 , the method  300  declares a long retention as not being detected. The state  316  ends the retention test in a given block. 
     The retention test procedure is normally invoked on an R-block level. The tracking method  200  and/or  300  is called to obtain the current optimal value of the signal VREF (e.g., VREF_CUR_NEW) and compared against the original value of the signal VREF (e.g., VREF_ORG). To save the time spent in retention test, the controller  70  can choose to track a particular one of the values VREF (e.g., VREF_B). Storing the parameter VREF_ORG also reduces the frequency of calling tracking procedures. 
     The criterion for determining a long retention can be PEC independent. For example, the controller  70  can choose a “Vt drift threshold” based on various decoder requirements. At the minimum, one VREF_ORG value needs to be stored for each tracking granularity. 
     The terms “may” and “generally” when used herein in conjunction with “is(are)” and verbs are meant to communicate the intention that the description is exemplary and believed to be broad enough to encompass both the specific examples presented in the disclosure as well as alternative examples that could be derived based on the disclosure. The terms “may” and “generally” as used herein should not be construed to necessarily imply the desirability or possibility of omitting a corresponding element. 
     The functions performed by the diagrams of  FIGS. 6 and 7  may be implemented using one or more of a conventional general purpose processor, digital computer, microprocessor, microcontroller, RISC (reduced instruction set computer) processor, CISC (complex instruction set computer) processor, SIMD (single instruction multiple data) processor, signal processor, central processing unit (CPU), arithmetic logic unit (ALU), video digital signal processor (VDSP) and/or similar computational machines, programmed according to the teachings of the specification, as will be apparent to those skilled in the relevant art(s). Appropriate software, firmware, coding, routines, instructions, opcodes, microcode, and/or program modules may readily be prepared by skilled programmers based on the teachings of the disclosure, as will also be apparent to those skilled in the relevant art(s). The software is generally executed from a medium or several media by one or more of the processors of the machine implementation. 
     The invention may also be implemented by the preparation of ASICs (application specific integrated circuits), Platform ASICs, FPGAs (field programmable gate arrays), PLDs (programmable logic devices), CPLDs (complex programmable logic devices), sea-of-gates, RFICs (radio frequency integrated circuits), ASSPs (application specific standard products), one or more monolithic integrated circuits, one or more chips or die arranged as flip-chip modules and/or multi-chip modules or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
     The invention thus may also include a computer product which may be a storage medium or media and/or a transmission medium or media including instructions which may be used to program a machine to perform one or more processes or methods in accordance with the invention. Execution of instructions contained in the computer product by the machine, along with operations of surrounding circuitry, may transform input data into one or more files on the storage medium and/or one or more output signals representative of a physical object or substance, such as an audio and/or visual depiction. The storage medium may include, but is not limited to, any type of disk including floppy disk, hard drive, magnetic disk, optical disk, CD-ROM, DVD and magneto-optical disks and circuits such as ROMs (read-only memories), RAMs (random access memories), EPROMs (erasable programmable ROMs), EEPROMs (electrically erasable programmable ROMs), UVPROM (ultra-violet erasable programmable ROMs), Flash memory, magnetic cards, optical cards, and/or any type of media suitable for storing electronic instructions. 
     The elements of the invention may form part or all of one or more devices, units, components, systems, machines and/or apparatuses. The devices may include, but are not limited to, servers, workstations, storage array controllers, storage systems, personal computers, laptop computers, notebook computers, palm computers, personal digital assistants, portable electronic devices, battery powered devices, set-top boxes, encoders, decoders, transcoders, compressors, decompressors, pre-processors, post-processors, transmitters, receivers, transceivers, cipher circuits, cellular telephones, digital cameras, positioning and/or navigation systems, medical equipment, heads-up displays, wireless devices, audio recording, audio storage and/or audio playback devices, video recording, video storage and/or video playback devices, game platforms, peripherals and/or multi-chip modules. Those skilled in the relevant art(s) would understand that the elements of the invention may be implemented in other types of devices to meet the criteria of a particular application. 
     While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention.