# Patent

## Title
Optimal multilevel sensing for reading data from a storage medium

## Abstract

Systems, methods and/or devices that enhance the reliability with which data can be stored in and read from a memory utilize an error indicator, obtained from using one reading threshold voltage for decoding, to adaptively determine the reading threshold voltage(s) used for subsequent decoding attempts. For example, in some implementations, the method includes initiating performance of a first read operation, using a first reading threshold voltage, to obtain a first error indicator, and further includes initiating performance of a second set of additional read operations using two or more second reading threshold voltages, the second reading threshold voltages determined in accordance with the first error indicator, to obtain a second error indicator. In some embodiments, when the first error indicator is greater than a first threshold, a difference between two of the second reading threshold voltages is greater than when the first error indicator is less than a first threshold.

## Background
TECHNICAL FIELD
The disclosed embodiments relate generally to memory systems, and in particular, to improving the error correction capability when using soft information error control decoding.
BACKGROUND
Semiconductor memory devices, including flash memory, typically utilize memory cells to store data as an electrical value, such as an electrical charge or voltage. A flash memory cell, for example, includes a single transistor with a floating gate that is used to store a charge representative of a data value. Increases in storage density have been facilitated in various ways, including increasing the density of memory cells on a chip enabled by manufacturing developments, and transitioning from single-level flash memory cells to multi-level flash memory cells, so that two or more bits can be stored by each flash memory cell.
A drawback of increasing storage density is that the stored data is increasingly prone to being stored and/or read erroneously. An error control coding (ECC) engine is utilized to limit the number of uncorrectable errors that are introduced by electrical fluctuations, defects in the storage medium, operating conditions, device history, and/or write-read circuitry, etc.
SUMMARY
Various implementations of systems, methods and devices within the scope of the appended claims each have several aspects, no single one of which is solely responsible for the attributes described herein. Without limiting the scope of the appended claims, after considering this disclosure, and particularly after considering the section entitled “Detailed Description” one will understand how the aspects of various implementations are used to enable reading from a storage medium in a manner that improves the error correction capability when using soft information error control decoding. In one aspect, an error indicator obtained from using one reading threshold voltage for decoding is used to adaptively determine the reading threshold voltage(s) used for subsequent decoding attempts.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the present disclosure can be understood in greater detail, a more particular description may be had by reference to the features of various implementations, some of which are illustrated in the appended drawings. The appended drawings, however, merely illustrate the more pertinent features of the present disclosure and are therefore not to be considered limiting, for the description may admit to other effective features.
FIG. 1 is a block diagram illustrating an implementation of a data storage system, in accordance with some embodiments.
FIG. 2 is a block diagram illustrating an implementation of a management module, in accordance with some embodiments.
FIG. 3A is a prophetic diagram of voltage distributions that may be found in a single-level flash memory cell (SLC) over time, in accordance with some embodiments.
FIG. 3B is a prophetic diagram of voltage distributions that may be found in a multi-level flash memory cell (MLC) over time, in accordance with some embodiments.
FIG. 3C is a prophetic diagram of voltage distributions that may be found in a single-level flash memory cell (SLC) with a low syndrome weight, showing various reading threshold voltages, in accordance with some embodiments.
FIG. 3D is a prophetic diagram of voltage distributions that may be found in a single-level flash memory cell (SLC) with a high syndrome weight, showing various reading threshold voltages, in accordance with some embodiments.
FIGS. 4A-4B illustrate a flowchart representation of a method for reading from a storage medium, in accordance with some embodiments.
FIG. 5 illustrates a flowchart representation of a method for reading from a storage medium, in accordance with some embodiments.



In accordance with common practice the various features illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. In addition, some of the drawings may not depict all of the components of a given system, method or device. Finally, like reference numerals may be used to denote like features throughout the specification and figures.
DETAILED DESCRIPTION
The various implementations described herein include systems, methods and/or devices used to improve the error correction capability when using soft information error control decoding. Some implementations include systems, methods and/or devices to obtain and utilize error information for determining reading threshold voltages used for reading symbols from a storage medium.
More specifically, some implementations include a method for reading from a storage medium. In some implementations, the method includes initiating performance of a first read operation, using a first reading threshold voltage, to obtain a first error indicator. The method further includes initiating performance of a second set of additional read operations using two or more second reading threshold voltages, the second reading threshold voltages determined in accordance with the first error indicator, to obtain a second error indicator.
In some embodiments, when the first error indicator is greater than a first threshold, a difference between two of the second reading threshold voltages is greater than when the first error indicator is less than a first threshold.
In some embodiments, the method further includes initiating performance of a third set of additional read operations using two or more third reading threshold voltages, the third reading threshold voltages determined in accordance with the first error indicator, the second error indicator, or a combination of the first error indicator and the second error indicator, to obtain a third error indicator.
In some embodiments, the first error indicator is based at least in part on a syndrome weight.
In some embodiments, the first error indicator is based at least in part on a bit error count, and the method further comprises setting a set of default reading threshold voltages to the second reading threshold voltages.
In some embodiments, the second error indicator is based at least in part on a bit error count, and the method further comprises setting a set of default reading threshold voltages to the third reading threshold voltages.
In some embodiments, the two or more second reading threshold voltages are different from the first reading threshold voltage.
In some embodiments, at least one of the two or more second reading threshold voltages is the same as the first reading threshold voltage.
Some implementations include a method for reading from a storage medium. In some implementations, the method includes initiating performance of a first read operation, using a first reading threshold voltage, to obtain a first error indicator. The method further includes initiating performance of a second read operation using a second reading threshold voltage, the second reading threshold voltage determined in accordance with the first indicator, to obtain a second error indicator.
In some embodiments, when the first error indicator is greater than a first threshold, a difference between the first reading threshold voltage and the second reading threshold voltage is greater than when the first error indicator is less than a first threshold.
In some embodiments, the storage medium comprises one or more flash memory devices.
In another aspect, a device operable to read from a storage medium is configured to read from the storage medium in accordance with any of the methods described above. In some embodiments, the device includes a storage medium interface for coupling the device to the storage medium, means for initiating performance of a first read operation, using a first reading threshold voltage, to obtain a first error indicator, and means for initiating performance of a second set of additional read operations, using two or more second reading threshold voltages, the second reading threshold voltages determined in accordance with the first error indicator, to obtain a second error indicator. In some embodiments, the device includes a storage medium interface for coupling the device to the storage medium, means for initiating performance of a first read operation, using a first reading threshold voltage, to obtain a first error indicator, and means for initiating performance of a second read operation using a second reading threshold voltage, the second reading threshold voltage determined in accordance with the first indicator, to obtain a second error indicator.
In yet another aspect, a non-transitory computer readable storage medium stores one or more programs configured for execution by a device coupled to a storage medium, the one or more programs comprising instructions for causing the device and/or storage medium to read from the storage medium in accordance with any of the methods described above.
Numerous details are described herein in order to provide a thorough understanding of the example implementations illustrated in the accompanying drawings. However, some embodiments may be practiced without many of the specific details, and the scope of the claims is only limited by those features and aspects specifically recited in the claims. Furthermore, well-known methods, components, and circuits have not been described in exhaustive detail so as not to unnecessarily obscure more pertinent aspects of the implementations described herein.
FIG. 1 is a diagram of an implementation of a data storage system 100, in accordance with some embodiments. While some example features are illustrated, various other features have not been illustrated for the sake of brevity and so as not to obscure more pertinent aspects of the example implementations disclosed herein. To that end, as a non-limiting example, the data storage system 100 includes a memory controller 120, and a storage medium 130, and is used in conjunction with a computer system 110. In some implementations, storage medium 130 is a single flash memory device while in other implementations storage medium 130 includes a plurality of flash memory devices. In some implementations, storage medium 130 is NAND-type flash memory or NOR-type flash memory. Further, in some implementations memory controller 120 is a solid-state drive (SSD) controller. However, other types of storage media may be included in accordance with aspects of a wide variety of implementations.
Computer system 110 is coupled to memory controller 120 through data connections 101. However, in some implementations computer system 110 includes memory controller 120 as a component and/or a sub-system. Computer system 110 may be any suitable computer device, such as a computer, a laptop computer, a tablet device, a netbook, an internet kiosk, a personal digital assistant, a mobile phone, a smart phone, a gaming device, a computer server, or any other computing device. Computer system 110 is sometimes called a host or host system. In some implementations, computer system 110 includes one or more processors, one or more types of memory, a display and/or other user interface components such as a keyboard, a touch screen display, a mouse, a track-pad, a digital camera and/or any number of supplemental devices to add functionality.
Storage medium 130 is coupled to memory controller 120 through connections 103. Connections 103 are sometimes called data connections, but typically convey commands in addition to data, and optionally convey metadata, error correction information and/or other information in addition to data values to be stored in storage medium 130 and data values read from storage medium 130. In some implementations, however, memory controller 120 and storage medium 130 are included in the same device as components thereof. Storage medium 130 may include any number (i.e., one or more) of memory devices including, without limitation, non-volatile semiconductor memory devices, such as flash memory. For example, flash memory devices can be configured for enterprise storage suitable for applications such as cloud computing, or for caching data stored (or to be stored) in secondary storage, such as hard disk drives. Additionally and/or alternatively, flash memory can also be configured for relatively smaller-scale applications such as personal flash drives or hard-disk replacements for personal, laptop and tablet computers.
Storage medium 130 is divided into a number of addressable and individually selectable blocks, such as selectable portion 131. In some implementations, the individually selectable blocks are the minimum size erasable units in a flash memory device. In other words, each block contains the minimum number of memory cells that can be erased simultaneously. Each block is usually further divided into a plurality of pages and/or word lines, where each page or word line is typically an instance of the smallest individually accessible (readable) portion in a block. In some implementations (e.g., using some types of flash memory), the smallest individually accessible unit of a data set, however, is a sector, which is a subunit of a page. That is, a block includes a plurality of pages, each page contains a plurality of sectors, and each sector is the minimum unit of data for reading data from the flash memory device.
For example, one block comprises any number of pages, for example, 64 pages, 128 pages, 256 pages or another suitable number of pages. Blocks are typically grouped into a plurality of zones. Each block zone can be independently managed to some extent, which increases the degree of parallelism for parallel operations and simplifies management of storage medium 130.
As noted above, while data storage densities of non-volatile semiconductor memory devices are generally increasing, a drawback of increasing storage density is that the stored data is more prone to being stored and/or read erroneously. As described in greater detail below, error control coding can be utilized to limit the number of uncorrectable errors that are introduced by electrical fluctuations, defects in the storage medium, operating conditions, device history, write-read circuitry, etc., or a combination of these and various other factors.
In some implementations, memory controller 120 includes a management module 121, an input buffer 123, an output buffer 124, an error control module 125 and a storage medium interface (I/O) 128. Memory controller 120 may include various additional features that have not been illustrated for the sake of brevity and so as not to obscure more pertinent features of the example implementations disclosed herein, and that a different arrangement of features may be possible. Input and output buffers 123,124 provide an interface to computer system 110 through data connections 101. Similarly, storage medium I/O 128 provides an interface to storage medium 130 though connections 103. In some implementations, storage medium I/O 128 includes read and write circuitry, including circuitry capable of providing reading signals to storage medium 130 (e.g., reading threshold voltages for NAND-type flash memory).
In some implementations, management module 121 includes one or more processing units (CPUs, also sometimes called processors) 122 configured to execute instructions in one or more programs (e.g., in management module 121). In some implementations, the one or more CPUs 122 are shared by one or more components within, and in some cases, beyond the function of memory controller 120. Management module 121 is coupled to input buffer 123, output buffer 124 (connection not shown), error control module 125 and storage medium I/O 128 in order to coordinate the operation of these components.
Error control module 125 is coupled to storage medium I/O 128, input buffer 123 and output buffer 124. Error control module 125 is provided to limit the number of uncorrectable errors inadvertently introduced into data. In some embodiments, error control module 125 is executed in software by the one or more CPUs 122 of management module 121, and, in other embodiments, error control module 125 is implemented in whole or in part using special purpose circuitry to perform encoding and decoding functions. To that end, error control module 125 includes an encoder 126 and a decoder 127. Encoder 126 encodes data by applying an error control code to produce a codeword, which is subsequently stored in storage medium 130.
When the encoded data (e.g., one or more codewords) is read from storage medium 130, decoder 127 applies a decoding process to the encoded data to recover the data, and to correct errors in the recovered data within the error correcting capability of the error control code. Those skilled in the art will appreciate that various error control codes have different error detection and correction capacities, and that particular codes are selected for various applications for reasons beyond the scope of this disclosure. As such, an exhaustive review of the various types of error control codes is not provided herein. Moreover, those skilled in the art will appreciate that each type or family of error control codes may have encoding and decoding algorithms that are particular to the type or family of error control codes. On the other hand some algorithms, such as the Viterbi algorithm, may be utilized at least to some extent in the decoding of a number of different types or families of error control codes. As such, for the sake of brevity, an exhaustive description of the various types of encoding and decoding algorithms generally available and known to those skilled in the art is not provided herein.
During a write operation, input buffer 123 receives data to be stored in storage medium 130 from computer system 110. The data held in input buffer 123 is made available to encoder 126, which encodes the data to produce one or more codewords. The one or more codewords are made available to storage medium I/O 128, which transfers the one or more codewords to storage medium 130 in a manner dependent on the type of storage medium being utilized.
A read operation is initiated when computer system (host) 110 sends one or more host read commands on control line 111 to memory controller 120 requesting data from storage medium 130. Memory controller 120 sends one or more read access commands to storage medium 130, via storage medium I/O 128, to obtain raw read data in accordance with memory locations (addresses) specified by the one or more host read commands. Storage medium I/O 128 provides the raw read data (e.g., comprising one or more codewords) to decoder 127. If the decoding is successful (e.g., 405—Yes, FIG. 4A), the decoded data is provided to output buffer 124, where the decoded data is made available to computer system 110 (e.g., step 407, FIG. 4A). In some implementations, if the decoding is not successful (e.g., 405—No, FIG. 4A), memory controller 120 may resort to a number of remedial actions or provide an indication of an irresolvable error condition.
Flash memory devices utilize memory cells to store data as electrical values, such as electrical charges or voltages. Each flash memory cell typically includes a single transistor with a floating gate that is used to store a charge, which modifies the threshold voltage of the transistor (i.e., the voltage needed to turn the transistor on). The magnitude of the charge, and the corresponding threshold voltage the charge creates, is used to represent one or more data values. In some implementations, during a read operation, a reading threshold voltage is applied to the control gate of the transistor and the resulting sensed current or voltage is mapped to a data value.
The terms “cell voltage” and “memory cell voltage,” in the context of flash memory cells, means the threshold voltage of the memory cell, which is the minimum voltage that needs to be applied to the gate of the memory cell's transistor in order for the transistor to conduct current. Similarly, reading threshold voltages (sometimes also called reading signals and reading voltages) applied to a flash memory cells are gate voltages applied to the gates of the flash memory cells to determine whether the memory cells conduct current at that gate voltage. In some implementations, when a flash memory cell's transistor conducts current at a given reading threshold voltage, indicating that the cell voltage is less than the reading threshold voltage, the raw data value for that read operation is a “1” and otherwise the raw data value is a “0.”
FIG. 2 is a block diagram illustrating an exemplary management module 121 in accordance with some embodiments. Management module 121 typically includes one or more processing units (CPUs) 122 for executing modules, programs and/or instructions stored in memory 206 and thereby performing processing operations, memory 206, and one or more communication buses 208 for interconnecting these components. Communication buses 208 optionally include circuitry (sometimes called a chipset) that interconnects and controls communications between system components. Management module 121 is coupled to buffer 123, buffer 124, error control module 125, and storage medium I/O 128 by communication buses 208. Memory 206 includes high-speed random access memory, such as DRAM, SRAM, DDR RAM or other random access solid state memory devices, and may include non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid state storage devices. Memory 206 optionally includes one or more storage devices remotely located from the CPU(s) 122. Memory 206, or alternately the non-volatile memory device(s) within memory 206, comprises a non-transitory computer readable storage medium. In some embodiments, memory 206, or the computer readable storage medium of memory 206 stores the following programs, modules, and data structures, or a subset thereof:

    
    
        a reading threshold voltages table 216 that includes a table of reading threshold voltages used for reading symbols from a storage medium;
        a data read module 218 that is used for reading data from a storage medium;
        a data write module 226 that is used for writing data to a storage medium; and
        a data erase module 228 that is used for erasing data from a storage medium.
    
    


In some embodiments, the data read module 218 optionally includes the following modules or sub-modules, or a subset thereof:

    
    
        a read management module 220 that is used for controlling or managing execution of read commands received from a host device such as computer system 110 (FIG. 1);
        an error control management module 222 that is used to receive error indicators (e.g., syndrome weights) from error control module 125, and optionally to control operation of error control module 125; and
        a reading threshold voltage control module 224 that is used for selecting reading threshold voltages from the reading threshold voltages table 216.
    
    


Each of the above identified elements may be stored in one or more of the previously mentioned memory devices, and corresponds to a set of instructions for performing a function described above. The above identified modules or programs (i.e., sets of instructions) need not be implemented as separate software programs, procedures or modules, and thus various subsets of these modules may be combined or otherwise re-arranged in various embodiments. In some embodiments, memory 206 may store a subset of the modules and data structures identified above. Furthermore, memory 206 may store additional modules and data structures not described above. In some embodiments, the programs, modules, and data structures stored in memory 206, or the computer readable storage medium of memory 206, provide instructions for implementing any of the methods described below with reference to FIGS. 4A-4B and 5.
Although FIG. 2 shows a management module 121, FIG. 2 is intended more as functional description of the various features which may be present in a management module than as a structural schematic of the embodiments described herein. In practice, and as recognized by those of ordinary skill in the art, items shown separately could be combined and some items could be separated.
As discussed below with reference to FIG. 3A, a single-level flash memory cell (SLC) stores one bit (“0” or “1”). Thus, the storage density of a SLC memory device is one bit of information per memory cell. A multi-level flash memory cell (MLC), however, can store two or more bits of information per cell by using different ranges within the total voltage range of the memory cell to represent a multi-bit bit-tuple. In turn, the storage density of a MLC memory device is multiple-bits per cell (e.g., two bits per memory cell).
FIG. 3A is a simplified, prophetic diagram of voltage distributions 300a found in a single-level flash memory cell (SLC) over time, in accordance with some embodiments. The voltage distributions 300a shown in FIG. 3A have been simplified for illustrative purposes. In this example, the SLC's voltage range extends approximately from a voltage, VSS, at a source terminal of an NMOS transistor to a voltage, VDD, at a drain terminal of the NMOS transistor. As such, voltage distributions 300a extend between VSS and VDD.
Sequential voltage ranges 301 and 302 between source voltage VSS and drain voltage VDD are used to represent corresponding bit values “1” and “0,” respectively. Each voltage range 301, 302 has a respective center voltage V1 301b, V0 302b. As described above, in many circumstances the memory cell current sensed in response to an applied reading threshold voltages is indicative of a memory cell voltage different from the respective center voltage V1 301b or V0 302b corresponding to the respective bit value written into the memory cell. Errors in cell voltage, and/or the cell voltage sensed when reading the memory cell, can occur during write operations, read operations, or due to “drift” of the cell voltage between the time data is written to the memory cell and the time a read operation is performed to read the data stored in the memory cell. For ease of discussion, these effects are collectively described as “cell voltage drift.” Each voltage range 301, 302 also has a respective voltage distribution 301a, 302a that may occur as a result of any number of a combination of error-inducing factors, examples of which are identified above.
In some implementations, a reading threshold voltage VR is applied between adjacent center voltages (e.g., applied proximate to the halfway region between adjacent center voltages V1 301b and V0 302b). Optionally, in some implementations, the reading threshold voltage is located between voltage ranges 301 and 302. In some implementations, reading threshold voltage VR is applied in the region proximate to where the voltage distributions 301a and 302a overlap, which is not necessarily proximate to the halfway region between adjacent center voltages V1 301b and V0 302b. 
In order to increase storage density in flash memory, flash memory has developed from single-level (SLC) cell flash memory to multi-level cell (MLC) flash memory so that two or more bits can be stored by each memory cell. As discussed below with reference to FIG. 3B, a MLC flash memory device is used to store multiple bits by using voltage ranges within the total voltage range of the memory cell to represent different bit-tuples. A MLC flash memory device is typically more error-prone than a SLC flash memory device created using the same manufacturing process because the effective voltage difference between the voltages used to store different data values is smaller for a MLC flash memory device. Moreover, due to any number of a combination of factors, such as electrical fluctuations, defects in the storage medium, operating conditions, device history, and/or write-read circuitry, a typical error includes a stored voltage level in a particular MLC being in a voltage range that is adjacent to the voltage range that would otherwise be representative of the correct storage of a particular bit-tuple. As discussed in greater detail below with reference to FIG. 3B, the impact of such errors can be reduced by gray-coding the data, such that adjacent voltage ranges represent single-bit changes between bit-tuples.
FIG. 3B is a simplified, prophetic diagram of voltage distributions 300b that may be found in a multi-level flash memory cell (MLC) over time, in accordance with some embodiments. The voltage distributions 300b shown in FIG. 3B have been simplified for illustrative purposes. The cell voltage of a MLC approximately extends from a voltage, VSS, at the source terminal of a NMOS transistor to a voltage, VDD, at the drain terminal. As such, voltage distributions 300b extend between VSS and VDD.
Sequential voltage ranges 311, 312, 313, 314 between the source voltage VSS and drain voltages VDD are used to represent corresponding bit-tuples “11,” “01,” “00,” “10,” respectively. Each voltage range 311, 312, 313, 314 has a respective center voltage 311b, 312b, 313b, 314b. Each voltage range 311, 312, 313, 314 also has a respective voltage distribution 311a, 312a, 313a, 314a that may occur as a result of any number of a combination of factors, such as electrical fluctuations, defects in the storage medium, operating conditions, device history (e.g., number of program-erase (PE) cycles), and/or imperfect performance or design of write-read circuitry.
Ideally, during a write operation, the charge on the floating gate of the MLC would be set such that the resultant cell voltage is at the center of one of the ranges 311, 312, 313, 314 in order to write the corresponding bit-tuple to the MLC. Specifically, the resultant cell voltage would be set to one of V11 311b, V00 312b, V00 313b and V10 314b in order to write a corresponding one of the bit-tuples “11,” “01,” “00” and “10.” In reality, due to the factors mentioned above, the initial cell voltage may differ from the center voltage for the data written to the MLC.
Reading threshold voltages VRA, VRB and VRC are positioned between adjacent center voltages (e.g., positioned at or near the halfway point between adjacent center voltages) and, thus, define threshold voltages between the voltage ranges 311, 312, 313, 314. During a read operation, one of the reading threshold voltages VRA, VRB and VRC is applied to determine the cell voltage using a comparison process. However, due to the various factors discussed above, the actual cell voltage, and/or the cell voltage received when reading the MLC, may be different from the respective center voltage V11 311b, V01 312b, V00 313b or V10 314b corresponding to the data value written into the cell. For example, the actual cell voltage may be in an altogether different voltage range, strongly indicating that the MLC is storing a different bit-tuple than was written to the MLC. More commonly, the actual cell voltage may be close to one of the read comparison voltages, making it difficult to determine with certainty which of two adjacent bit-tuples is stored by the MLC.
Errors in cell voltage, and/or the cell voltage received when reading the MLC, can occur during write operations, read operations, or due to “drift” of the cell voltage between the time data is written to the MLC and the time a read operation is performed to read the data stored in the MLC. For ease of discussion, sometimes errors in cell voltage, and/or the cell voltage received when reading the MLC, are collectively called “cell voltage drift.”
One way to reduce the impact of a cell voltage drifting from one voltage range to an adjacent voltage range is to gray-code the bit-tuples. Gray-coding the bit-tuples includes constraining the assignment of bit-tuples such that a respective bit-tuple of a particular voltage range is different from a respective bit-tuple of an adjacent voltage range by only one bit. For example, as shown in FIG. 3B, the corresponding bit-tuples for adjacent ranges 301 and 302 are respectively “11” and “01,” the corresponding bit-tuples for adjacent ranges 302 and 303 are respectively “01” and “00,” and the corresponding bit-tuples for adjacent ranges 303 and 304 are respectively “00” and “10.” Using gray-coding, if the cell voltage drifts close to a read comparison voltage level, the error is typically limited to a single bit within the 2-bit bit-tuple.
In some implementations, when encoded data is read from a storage medium (e.g., storage medium 130, FIG. 1) using a reading threshold voltage VR, a decoder (e.g., decoder 127, FIG. 1) applies a decoding process to the encoded data to recover the data, and to correct errors in the recovered data within the error correcting capability of the error control code. In some implementations, after zero iterations or after one iteration of the decoding process, the decoder determines an error indicator. In some embodiments, the error indicator is a syndrome weight, which indicates the number of bit errors detected in a given iteration of the decoding process. A syndrome weight of zero indicates decoding was successful and the data has been recovered.
In FIG. 3C, the relatively small overlap 323 of voltage distributions 321a and 322a indicates a smaller area in which errors occur when reading from the memory cell, and thus relates to a low starting syndrome weight. In FIG. 3D, the relatively large overlap 333 of voltage distributions 331a and 332a indicates a larger area in which errors occur when reading from the memory cell, and thus relates to a high starting syndrome weight.
In some implementations, a decoder processes a read operation using a single reading threshold voltage (e.g., “1-Read decoding”). If the decoding fails or the syndrome weight is too high to be corrected by 1-Read decoding, one or more additional decoding attempts are made with one or more additional read operations using one or more additional reading threshold voltages. For example, if 1-Read decoding fails or if the syndrome weight is too high to be corrected by 1-Read decoding, an additional decoding attempt with two reading threshold voltages (e.g., “2-Read decoding”) is made. The error indicator from the 1-Read decoding is used to determine the reading threshold voltages used for the 2-Read decoding. When the error indicator is greater than a threshold, the difference between the two reading threshold voltages used for the 2-Read decoding is greater (e.g., see FIG. 3D, as described below) than when the error indicator is less than a threshold (e.g., see FIG. 3C, as described below). Thus, if the 1-Read decoding produces a high syndrome weight, the two reading threshold voltages used for the 2-Read decoding are widely separated (e.g., in FIG. 3D). In contrast, if the 1-Read decoding produces a low syndrome weight, the two reading threshold voltages used for the 2-Read decoding are narrowly separated (e.g., in FIG. 3C).
Optionally, if 2-Read decoding fails or the syndrome weight is too high to be corrected by 2-Read decoding, one or more additional decoding attempts are made with one or more additional read operations using one or more additional reading threshold voltages. For example, if 2-Read decoding fails or if the syndrome weight is too high to be corrected by 2-Read decoding, an additional decoding attempt with four reading threshold voltages (e.g., “4-Read decoding”) is made. In some embodiments, the error indicator from the 1-Read decoding is used to determine the reading threshold voltages used for the 4-Read decoding. In some embodiments, the error indicator from the 2-Read decoding is used to determine the reading threshold voltages used for the 4-Read decoding. In some embodiments, a combination of the error indicator from the 1-Read decoding and the error indicator from the 2-Read decoding is used to determine the reading threshold voltages used for the 4-Read decoding. When the error indicator is greater than a threshold, the difference between the four reading threshold voltages used for the 4-Read decoding is greater than when the error indicator is less than a threshold. Thus, if the 1-Read decoding, 2-Read decoding, or a combination of the 1-Read decoding and 2-Read decoding produces a high syndrome weight, the four reading threshold voltages used for the 4-Read decoding are widely separated. In contrast, if the 1-Read decoding, 2-Read decoding, or a combination of the 1-Read decoding and 2-Read decoding produces a low syndrome weight, the four reading threshold voltages used for the 4-Read decoding are narrowly separated.
Although examples of 1-Read decoding, 2-Read decoding, and 4-Read decoding are used to describe various embodiments, this scheme is applicable to any combination of N1-Read decoding, Nj-Read decoding, . . . , and Nk-Read decoding, where j is an integer greater than 1 and k is an integer greater than 2. For example, this scheme is applicable to 1-Read decoding, 3-Read decoding, and 5-Read decoding.
In some embodiments, although decoding was successful and the data has been recovered, the decoder determines an error indicator. In some implementations, the error indicator is a bit error count. In some embodiments, the bit error count is called a fail bit count, and indicates the number of bit errors detected in the decoding. After successful decoding, the bit error count is available. In some embodiments, the bit error count is used to adjust and/or update the default reading threshold voltages for subsequent decoding attempts. For example, after a successful 1-Read decoding, the bit error count from the 1-Read decoding is used to adjust and/or update the default reading threshold voltages for subsequent 2-Read decodings.
FIG. 3C is a simplified, prophetic diagram of voltage distributions 300c found in a single-level flash memory cell (SLC) with a low syndrome weight, in accordance with some embodiments. Explanations provided above in connection with FIG. 3A, with respect to voltage distributions 300a, sequential voltage ranges 301 and 302, center voltages V1 301b and V0 302b, voltage distributions 301a and 302a and their overlap, and reading threshold voltage VR are equally applicable to voltage distributions 300c, sequential voltage ranges 321 and 322, center voltages V1 321b and V0 322b, voltage distributions 321a and 322a and their overlap, and reading threshold voltage VR as shown in FIG. 3C, except as described next.
In FIG. 3C, the relatively small overlap 323 of voltage distributions 321a and 322a indicates a smaller area in which errors occur when reading from the memory cell, and thus relates to a low starting syndrome weight. As described above, if a 1-Read decoding produces a low syndrome weight, the two reading threshold voltages used for the 2-Read decoding are narrowly separated. For example, in FIG. 3C, if a 1-Read decoding using reading threshold voltage VR produces a low syndrome weight, the two reading threshold voltages VRL1 and VRL2 used for 2-Read decoding are narrowly separated.
Although examples of 1-Read decoding and 2-Read decoding are used in the example of FIG. 3C, this scheme is applicable to any combinations of N1-Read decoding, Na-Read decoding, . . . , and Nk-Read decoding, where j is an integer greater than 1 and k is an integer greater than 2. For example, this scheme is applicable to 1-Read decoding, 3-Read decoding, and 5-Read decoding. Similarly, although FIG. 3C illustrates an example using a single-level flash memory cell, this scheme is also applicable to a multi-level flash memory cell (e.g., as illustrated in FIG. 3B).
FIG. 3D is a simplified, prophetic diagram of voltage distributions 300d found in a single-level flash memory cell (SLC) with a high syndrome weight, in accordance with some embodiments. Explanations provided above in connection with FIG. 3A, with respect to voltage distributions 300a, sequential voltage ranges 301 and 302, center voltages V1 301b and V0 302b, voltage distributions 301a and 302a and their overlap, and reading threshold voltage VR are equally applicable to voltage distributions 300d, sequential voltage ranges 331 and 332, center voltages V1 331b and V0 332b, voltage distributions 331a and 332a and their overlap, and reading threshold voltage VR as shown in FIG. 3D, except as described next.
In FIG. 3D, the relatively large overlap 333 of voltage distributions 331a and 332a indicates a larger area in which errors occur when reading from the memory cell, and thus relates to a high starting syndrome weight. As described above, if a 1-Read decoding produces a high syndrome weight, the two reading threshold voltages used for the 2-Read decoding are widely separated. For example, in FIG. 3D, if a 1-Read decoding using reading threshold voltage VR produces a high syndrome weight, the two reading threshold voltages VRH1 and VRH2 used for 2-Read decoding are widely separated.
Although examples of 1-Read decoding and 2-Read decoding are used in the example of FIG. 3D, this scheme is applicable to any combinations of N1-Read decoding, Nj-Read decoding, . . . , and Nk-Read decoding, where j is an integer greater than 1 and k is an integer greater than 2. For example, this scheme is applicable to 1-Read decoding, 3-Read decoding, and 5-Read decoding. Similarly, although FIG. 3D illustrates an example using a single-level flash memory cell, this scheme is also applicable to a multi-level flash memory cell (e.g., as illustrated in FIG. 3B).
FIGS. 4A-4B illustrate a flowchart representation of a method 400 for reading from a storage medium. As noted above with respect to FIG. 1, a read operation is typically initiated when a computer system (e.g., computer system 110, FIG. 1, sometimes called a host) sends one or more host read commands on control line 111 to memory controller 120 requesting data from storage medium 130. In response, memory controller 120 sends one or more read access commands to storage medium 130, via storage medium I/O 128, which initiates performance of method 400.
At least in some implementations, method 400 is performed by a memory controller (e.g., memory controller 120, FIG. 1) or one or more components of the memory controller (e.g., management module 121, FIGS. 1 and 2) to read from a storage medium (e.g., storage medium 130, FIG. 1). In some embodiments, method 400 is governed by instructions that are stored in a non-transitory computer readable storage medium and that are executed by one or more processors of a device, such as the one or more processing units (CPUs) 122 of management module 121, shown in FIGS. 1 and 2.
A memory controller initiates (402) performance of a first read operation, using a first reading threshold voltage (e.g., reading threshold voltage VR, FIG. 3C, or reading threshold voltage VR, FIG. 3D), to obtain a first error indicator. In some embodiments, the first read operation is primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the first error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a read operation from storage medium 130 via storage medium I/O 128.
In some embodiments, the first error indicator is (404) based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful (405—Yes) and the data has been recovered (407).
The memory controller initiates (406) performance of a second set of additional read operations using two or more second reading threshold voltages, the second reading threshold voltages determined in accordance with the first error indicator, to obtain a second error indicator. In some embodiments, the second set of additional read operations are primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the second error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a set of read operations using two or more reading threshold voltages (e.g., reading threshold voltages VRL1 and VRL2, FIG. 3C, or reading threshold voltages VRH1 and VRH2, FIG. 3D). In some embodiments, step 406 is performed in response to a determination that the first read operation failed (405—No). In some embodiments, step 406 is performed in response to a determination that the syndrome weight is too high to be corrected by 1-Read decoding, as explained above.
In some embodiments, the second error indicator is based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful and the data has been recovered.
In some embodiments, when the first error indicator is (408) greater than a first threshold, a difference between two of the second reading threshold voltages is greater than when the first error indicator is less than a first threshold. To illustrate, FIG. 3D shows an example of when the error indicator is greater than a first threshold, and thus, the difference between reading threshold voltages VRH1 and VRH2 is greater than the difference between reading threshold voltages VRL1 and VRL2 in FIG. 3C, which shows an example of when the error indicator is less than the first threshold.
In some embodiments, the first error indicator is (410) based at least in part on a bit error count. The bit error count is sometimes called a fail bit count, and indicates the number of bit errors detected in the decoding. After successful decoding, the bit error count is available. For example, after a successful 1-Read decoding, the bit error count from the 1-Read decoding is available. In some embodiments, the memory controller sets (412) a set of default reading threshold voltages to the second reading threshold voltages. Using FIG. 3C as an example, when the error indicator is based on a bit error count determined after successful 1-Read decoding using reading threshold voltage VR, the memory controller sets the default reading threshold voltages for subsequent 2-Read decoding to reading threshold voltages VRL1 and VRL2. As another example, in FIG. 3D, when the error indicator is based on a bit error count determined after successful 1-Read decoding using reading threshold voltage VR, the memory controller sets the default reading threshold voltages for subsequent 2-Read decoding to reading threshold voltages VRH1 and VRH2.
In some embodiments, the two or more second reading threshold voltages are (414) different from the first reading threshold voltage. For example, FIG. 3C shows two second reading threshold voltages VRL1 and VRL2 which are different from the first reading threshold voltage VR. As another example, FIG. 3D shows two second reading threshold voltages VRH1 and VRH2 which are different from the first reading threshold voltage VR.
In some embodiments, at least one of the two or more second reading threshold voltages is (416) the same as the first reading threshold voltage. Using FIG. 3C as an example, in some implementations, the first reading threshold voltage is reading threshold voltage VR and the second reading threshold voltages are reading threshold voltages VRL1 and VR, where at least one of the two second reading threshold voltages is the same as the first reading threshold voltage. Still using FIG. 3C as an example, in some implementations, the first reading threshold voltage is reading threshold voltage VR and the second reading threshold voltages are reading threshold voltages VRL2 and VR, where at least one of the two second reading threshold voltages is the same as the first reading threshold voltage. Referring to FIG. 3D, an additional example of the second reading threshold voltages is reading threshold voltages VRH1 and VR, and another example is reading threshold voltages VRH2 and VR.
In some embodiments, or in some circumstances, the memory controller initiates (418) performance of a third set of additional read operations using two or more third reading threshold voltages, the third reading threshold voltages determined in accordance with the first error indicator, the second error indicator, or a combination of the first error indicator and the second error indicator, to obtain a third error indicator. In some embodiments, the third set of additional read operations are primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the third error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a set of read operations using two or more reading threshold voltages. In some embodiments, step 418 is performed in response to a determination that the second set of read operations failed. In some embodiments, step 418 is performed in response to a determination that the syndrome weight is too high to be corrected by 2-Read decoding, as explained above.
In some embodiments, the third error indicator is based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful and the data has been recovered.
In some embodiments, when the first error indicator, the second error indicator, or a combination of the first and second error indicators is greater than a threshold, a difference between at least two of the third reading threshold voltages is greater than when the first error indicator, the second error indicator, or the combination of the first and second error indicators is less than a threshold.
In some embodiments, the second error indicator is (420) based at least in part on a bit error count. The bit error count is sometimes called a fail bit count, and indicates the number of bit errors detected in the decoding. After successful decoding, the bit error count is available. For example, after a successful 2-Read decoding, the bit error count from the 2-Read decoding is available. In some embodiments, the memory controller sets (422) a set of default reading threshold voltages to the third reading threshold voltages. Using FIG. 3C as an example, when the error indicator is based on a bit error count determined after successful 2-Read decoding using reading threshold voltages VRL1 and VRL2, the memory controller sets the default reading threshold voltages for subsequent 4-Read decoding to four reading threshold voltages (not shown in FIG. 3C). As another example, in FIG. 3D, when the error indicator is based on a bit error count determined after successful 2-Read decoding using reading threshold voltages VRH1 and VRH2, the memory controller sets the default reading threshold voltages for subsequent 4-Read decoding to four reading threshold voltages (not shown in FIG. 3D).
In some embodiments, the two or more third reading threshold voltages are different from the first reading threshold voltage. In some embodiments, the two or more third reading threshold voltages are different from the two or more second reading threshold voltages.
In some embodiments, at least one of the two or more third reading threshold voltages is the same as the first reading threshold voltage. In some embodiments, at least one of the two or more third reading threshold voltages is the same as at least one of the two or more second reading threshold voltages.
FIG. 5 illustrates a flowchart representation of a method 500 for reading from a storage medium. As noted above with respect to FIG. 1, a read operation is typically initiated when a computer system (e.g., computer system 110, FIG. 1, sometimes called a host) sends one or more host read commands on control line 111 to memory controller 120 requesting data from storage medium 130. In response, memory controller 120 sends one or more read access commands to storage medium 130, via storage medium I/O 128, which initiates performance of method 500.
At least in some implementations, method 500 is performed by a memory controller (e.g., memory controller 120, FIG. 1) or a component of the memory controller (e.g., management module 121, FIGS. 1 and 2) to read from a storage medium (e.g., storage medium 130, FIG. 1). In some embodiments, method 500 is governed by instructions that are stored in a non-transitory computer readable storage medium and that are executed by one or more processors of a device, such as the one or more processing units (CPUs) 122 of management module 121, shown in FIGS. 1 and 2.
A memory controller initiates (502) performance of a first read operation, using a first reading threshold voltage (e.g., reading threshold voltage VR, FIG. 3C, or reading threshold voltage VR, FIG. 3D), to obtain a first error indicator. In some embodiments, the first read operation is primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the first error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a read operation from storage medium 130 via storage medium I/O 128.
In some embodiments, the first error indicator is based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful and the data has been recovered.
The memory controller initiates (504) performance of a second read operation using a second reading threshold voltage, the second reading threshold voltage determined in accordance with the first indicator, to obtain a second error indicator. In some embodiments, the second read operation is primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the second error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a second read operation using a second reading threshold voltage (e.g., reading threshold voltage VRL1 or reading threshold voltage VRL2, FIG. 3C, or reading threshold voltage VRH1 or reading threshold voltage VRH2, FIG. 3D). In some embodiments, step 504 is performed in response to a determination that the first read operation failed. In some embodiments, step 504 is performed in response to a determination that the syndrome weight is too high to be corrected by 1-Read decoding, as explained above.
In some embodiments, the second error indicator is based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful and the data has been recovered.
In some embodiments, when the first error indicator is (506) greater than a first threshold, a difference between the first reading threshold voltage and the second reading threshold voltage is greater than when the first error indicator is less than a first threshold. As an example, suppose that reading threshold voltage VR (e.g., in FIGS. 3C and 3D) is the first reading threshold voltage. In the same example, suppose that reading threshold voltage VRH1 is the second reading threshold voltage in FIG. 3D and reading threshold voltage VRL1 is the second reading threshold voltage in FIG. 3C. As explained above, FIG. 3D shows an example of when the error indicator is greater than a first threshold and FIG. 3C shows an example of when the error indicator is less than a first threshold. In this example, the difference between reading threshold voltage VR and reading threshold voltage VRH1 (e.g., in FIG. 3D) is greater than the difference between reading threshold voltage VR and reading threshold voltage VRL1 (e.g., in FIG. 3C).
In some embodiments, the first error indicator is based at least in part on a bit error count. The bit error count is sometimes called a fail bit count, and indicates the number of bit errors detected in the decoding. After successful decoding, the bit error count is available. For example, after a successful 1-Read decoding, the bit error count from the 1-Read decoding is available. In some embodiments, the memory controller sets a default reading threshold voltage to the second reading threshold voltage.
In some embodiments of method 500, or in some circumstances, the memory controller initiates performance of a third set of additional read operations using two or more third reading threshold voltages, the third reading threshold voltages determined in accordance with the first error indicator, the second error indicator, or a combination of the first error indicator and the second error indicator, to obtain a third error indicator. In some embodiments, the third set of additional read operations are primarily performed to obtain data from the storage medium (e.g., storage medium 130, FIG. 1), and secondarily to obtain the third error indicator. FIG. 1, for example, shows management module 121 (a component of memory controller 120) configured to initiate performance of a set of read operations using two or more reading threshold voltages. In some embodiments, the third set of additional read operations is performed in response to a determination that the second read operation failed, or produced a syndrome weight too high to be corrected, as explained above.
In some embodiments, the third error indicator is based at least in part on a syndrome weight. As explained above, a syndrome weight indicates the number of bit errors detected in a given iteration of the decoding process and a syndrome weight of zero indicates decoding was successful and the data has been recovered.
In some embodiments, the two or more third reading threshold voltages are different from the first reading threshold voltage. In some embodiments, the two or more third reading threshold voltages are different from the second reading threshold voltage.
In some embodiments, at least one of the two or more third reading threshold voltages is the same as the first reading threshold voltage. In some embodiments, at least one of the two or more third reading threshold voltages is the same as the second reading threshold voltage.
In some implementations, with respect to any of the methods described above, the storage medium (e.g., storage medium 130, FIG. 1) is a single flash memory device while in other implementations the storage medium includes a plurality of flash memory devices.
In some implementations, with respect to any of the methods described above, a device operable to read from a storage medium includes a storage medium interface (e.g., storage medium I/O 128, FIG. 1) for coupling the device to the storage medium (e.g., storage medium 130, FIG. 1) and one or more modules, including a memory management module (e.g., management module 121, FIGS. 1 and 2) that includes one or more processors (e.g., CPUs 122, FIGS. 1 and 2) and memory (e.g., memory 206, FIG. 2) storing one or more programs configured for execution by the one or more processors, the one or more modules coupled to the storage medium interface (e.g., storage medium I/O 128, FIG. 1) and configured to perform or control performance of any of the methods described above.
It will be understood that, although the terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first contact could be termed a second contact, and, similarly, a second contact could be termed a first contact, which changing the meaning of the description, so long as all occurrences of the “first contact” are renamed consistently and all occurrences of the second contact are renamed consistently. The first contact and the second contact are both contacts, but they are not the same contact.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the claims. As used in the description of the embodiments and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in accordance with a determination” or “in response to detecting,” that a stated condition precedent is true, depending on the context. Similarly, the phrase “if it is determined [that a stated condition precedent is true]” or “if [a stated condition precedent is true]” or “when [a stated condition precedent is true]” may be construed to mean “upon determining” or “in response to determining” or “in accordance with a determination” or “upon detecting” or “in response to detecting” that the stated condition precedent is true, depending on the context.
The foregoing description, for purpose of explanation, has been described with reference to specific implementations. However, the illustrative discussions above are not intended to be exhaustive or to limit the claims to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The implementations were chosen and described in order to best explain principles of operation and practical applications, to thereby enable others skilled in the art.

## Claims

1. A method of reading from a storage medium, the method comprising:
receiving a host read command specifying an address or location in the storage medium from which to read data;
in response to receiving the host read command specifying the address or location in the storage medium from which to read the data:
initiating performance of a first read operation in accordance with the address or location, using a first reading threshold voltage, to obtain a first error indicator value for the first read operation by decoding data obtained from the first read operation;
in accordance with a determination that the first read operation is successful, returning, as a response to the host read command, decoded data produced by said decoding; and
in accordance with a determination that the first read operation is unsuccessful:
determining, in accordance with the first error indicator value for the first read operation, second reading threshold voltages, wherein when the first error indicator value for the first read operation is greater than a first threshold, a difference between two of the second reading threshold voltages is greater than a difference between the two second reading threshold voltages that is determined when the first error indicator value for the first read operation is less than the first threshold; and
initiating performance of a second set of additional read operations in accordance with the address or location using the two second reading threshold voltages to obtain a second error indicator value for the second set of additional read operations by decoding data obtained from the second set of additional read operations using the two second reading threshold voltages.

2. The method of claim 1, further comprising:
initiating performance of a third set of additional read operations using two or more third reading threshold voltages, the third reading threshold voltages determined in accordance with the first error indicator value for the first read operation, the second error indicator value for the second set of additional read operations, or a combination of the first error indicator value and the second error indicator value, to obtain a third error indicator value for the third set of additional read operations.

3. The method of claim 2, wherein the second error indicator value for the second set of additional read operations is based at least in part on a bit error count, the method further comprising setting a set of default reading threshold voltages to the third reading threshold voltages.

4. The method of claim 1, wherein:
the first error indicator value for the first read operation is based at least in part on a syndrome weight for the first read operation; and
the difference between the two second reading threshold voltages is based on a magnitude of the first error indicator value for the first read operation.

5. The method of claim 1, wherein the first error indicator value for the first read operation is based at least in part on a bit error count, the method further comprising setting a set of default reading threshold voltages to the second reading threshold voltages.

6. The method of claim 1, wherein the two second reading threshold voltages are different from the first reading threshold voltage.

7. The method of claim 1, wherein at least one of the two second reading threshold voltages is the same as the first reading threshold voltage.

8. The method of claim 1, wherein the storage medium comprises one or more flash memory devices.

9. A device operable to read from a storage medium, comprising:
a storage medium interface for coupling the device to the storage medium; and
one or more modules, including a memory management module that includes one or more processors and memory storing one or more programs configured for execution by the one or more processors, the one or more modules coupled to the storage medium interface and configured to:
receive a host read command specifying an address or location in the storage medium from which to read data;
in response to receiving the host read command, specify the address or location in the storage medium from which to read the data;
initiate performance of a first read operation in accordance with the address or location, using a first reading threshold voltage, to obtain a first error indicator value for the first read operation by decoding data obtained from the first read operation;
in accordance with a determination that the first read operation is successful, returning, as a response to the host read command, decoded data produced by said decoding; and
in accordance with a determination that the first read operation is unsuccessful:
determine, in accordance with the first error indicator value for the first read operation, second reading threshold voltages, wherein when the first error indicator value for the first read operation is greater than a first threshold, a difference between two of the second reading threshold voltages is greater than a difference between the two second reading threshold voltages that is determined when the first error indicator value for the first read operation is less than the first threshold; and
initiate performance of a second set of additional read operations in accordance with the address or location, using the two second reading threshold voltages to obtain a second error indicator value for the second set of additional read operations by decoding data obtained from the second set of additional read operations using the two second reading threshold voltages.

10. The device of claim 9, wherein the memory management module is further configured to initiate performance of a third set of additional read operations using two or more third reading threshold voltages, the third reading threshold voltages determined in accordance with the first error indicator value for the first read operation, the second error indicator value for the second set of additional read operations, or a combination of the first error indicator value and the second error indicator value, to obtain a third error indicator value for the third set of additional read operations.

11. The device of claim 10, wherein the second error indicator value for the second set of additional read operations is based at least in part on a bit error count, the memory management module further configured to set a set of default reading threshold voltages to the third reading threshold voltages.

12. The device of claim 9, wherein:
the first error indicator value for the first read operation is based at least in part on a syndrome weight; and
the difference between the two second reading threshold voltages is based on a magnitude of the first error indicator value for the first read operation.

13. The device of claim 9, wherein the first error indicator value for the first read operation is based at least in part on a bit error count, the memory management module further configured to set a set of default reading threshold voltages to the second reading threshold voltages.

14. The device of claim 9, wherein the two second reading threshold voltages are different from the first reading threshold voltage.

15. The device of claim 9, wherein at least one of the two second reading threshold voltages is the same as the first reading threshold voltage.

16. A device operable to read from a storage medium, comprising:
a storage medium interface for coupling the device to the storage medium;
means for receiving a host read command specifying an address or location in the storage medium from which to read data;
means for initiating performance of a first read operation in accordance with the address or location, using a first reading threshold voltage, to obtain a first error indicator value for the first read operation by decoding data obtained from the first read operation; and
means for determining, in accordance with a determination that the first read operation is unsuccessful and in accordance with the first error indicator value for the first read operation, second reading threshold voltages, wherein when the first error indicator value for the first read operation is greater than a first threshold, a difference between two of the second reading threshold voltages is greater than a difference between the two second reading threshold voltages that is determined when the first error indicator value for the first read operation is less than the first threshold;
wherein the means for initiating performance of the first read operation is further configured to initiate, in accordance with the determination that the first read operation is unsuccessful, performance of a second set of additional read operations in accordance with the address or location, using the two second reading threshold voltages to obtain a second error indicator value for the second set of additional read operations by decoding data obtained from the second set of additional read operations using the two second reading threshold voltages.