Patent Grant
9239751
This disclosure relates to error control in memory systems, and in particular, to managing data that is used for soft information error control decoding.
Semiconductor memory devices, including flash memory devices, typically utilize memory cells to store data as an electrical value, such as an electrical charge or a voltage. A flash memory cell, for example, includes a single transistor with a floating gate that is used to store a charge representative of one or more data values. Increases in storage density have been facilitated in various ways, including increasing the density of memory cells, enabled by reductions in the minimum feature sizes of the semiconductor manufacturing processes used to manufacture flash memory devices, 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 more prone to being stored and/or read erroneously. Error control coding (“ECC”) has been utilized to limit the number of uncorrectable errors that are introduced by pseudo-random fluctuations, defects in the storage medium, operating conditions, device history, and/or write-read circuitry, etc. Error control methods using soft information are particularly promising because soft information decoding may improve the error detection and correction capability of a particular error control code, and thus the capacity of the system. However, the utilization of soft information decoding has a number of previously irresolvable drawbacks. For example, soft information decoding implementations tend to introduce undesirable delays (i.e., latencies), have relatively large semiconductor footprints, and are generally power and memory intensive.
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 a system of soft decision decoding that uses less power and/or less memory. In one aspect, a sequence of data values produced by performing multiple soft reads on a memory cell is represented using a plurality of buffer locations, wherein there are fewer buffer locations than the total number of data values in the sequence. For example, in some implementations N values produced by N soft reads of a flash memory cell are represented using M=┌ log2 (N+1)┐ single-bit buffer locations, where ┌x┐ represents the value of “x” rounded up to the closest integer.
So that the present disclosure can be understood in greater detail, a more particular description may be had by reference to the aspects of various implementations, some of which are illustrated in the appended drawings. The appended drawings, however, merely illustrate the more pertinent aspects of the present disclosure and are therefore not to be considered limiting, for the description may admit to other effective aspects.
In accordance with common practice the various features illustrated in the drawings may not be drawn to scale. So 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.
The various implementations described herein include systems, methods and/or devices that may enhance the performance of error control methods used to improve the reliability of a storage medium, such as a flash memory device. Some implementations generate and utilize soft information for decoding error control encoded data read from a storage medium.
More specifically, some implementations include a method of compressing a sequence of read data values into a bit-tuple of a predefined length to enable soft information decoding systems that use less power, less memory and/or less processing time. In some implementations, a bit-tuple of a predefined length is produced using M single-bit buffer locations, where M corresponds to the predefined length of the bit-tuple (i.e., the size of the compressed information unit). Some implementations utilize a collection of characterization vectors that include soft information values associated with the possible permutations of the bit-tuples and the number of read operations performed to generate an individual bit-tuple. In turn, a sequence of bit-tuples (representative of the compressed read data) is converted to a sequence of soft information values by retrieving a characterization vector, and selecting a respective soft information value from the characterization vector for each bit-tuple in the sequence. In some implementations, the sequence of bit-tuples corresponds to a codeword produced by an error control encoder during a write operation.
In some implementations, a soft information value is selected for a particular bit-tuple based at least in part on the number of read operations performed to generate the particular bit-tuple. In some implementations, a characterization vector is selected from a characterization module based on one or more storage medium characterization parameter values associated with the current state of the storage medium. Each characterization vector corresponds to a different set of storage medium characterization parameter values, which provide an indication of at least one of a physical characteristic associated with at least a portion the storage medium, a mode of operation associated with at least a portion of the storage medium, a history of use associated with at least a portion of the storage medium, a condition characteristic associated with at least a portion of the storage medium, a read type associated with at least a portion of the storage medium, and a location associated with at least a portion of the storage medium.
Numerous details are described herein in order to provide a thorough understanding of the example implementations illustrated in the accompanying drawings. However, the invention may be practiced without many of the specific details and is only limited by the language of the claims. 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.
Computer system 110 is coupled to memory controller 120 through data connections 101. Moreover, those skilled in the art will appreciate from the present disclosure that in various 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. 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. Moreover, those skilled in the art will appreciate from the present disclosure that in various implementations 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 be stored) in secondary storage, such as hard disk drivers. 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 wordlines, where each page or wordline is typically an instance of the smallest individually accessible portion in a block. However, in some implementations (e.g., using some types of flash memory) the minimum unit of a data set is a sector, which is subunit of a page. That is, block includes multiple pages, each page contains a plurality of sectors, and each sector is the minimum unit of data for data writing or reading to the flash memory device.
For example, one block may comprise 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 pseudo-random 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. Those skilled in the art will appreciate from the present disclosure that 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 a processor 122 configured to execute instructions in one or more programs, e.g., in management module 121. However, those skilled in the art will appreciate from the present disclosure that processor 122 may be 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 and output buffers 123, 124, 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 between storage medium I/O 128 and the input and output buffers 123, 124. Error control module 125 is provided to limit the number of uncorrectable errors inadvertently introduced into data. 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 (a codeword) is read from the storage medium, decoder 127 applies a decoding process to the codeword 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 from the present disclosure 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 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 a codeword. The codeword is made available to the storage medium I/O 128, which transfers the codeword to storage medium 130 in a manner dependent on the type of storage medium being utilized. During a read operation for the same data, storage medium I/O 128 accesses the portion of storage medium 130 in which the corresponding codeword is stored to read the codeword and provide the codeword to decoder 127. In various embodiments, the codeword has more than 1000 bits, 2000 bits, 4000 bits, or 8000 bits.
If the decoding is successful, the decoded data is provided to output buffer 124, where the decoded data is made available to computer system 110. In some implementations, if the decoding is not successful, memory controller 120 may resort to a number of remedial actions or provide an indication of an irresolvable error condition.
For many error control codes, the decoding process can be improved by using soft information, which takes into consideration the probabilities of interpreting the output of read operations in more than one way. Hard information decoding generally means that absolute decisions are made as to whether a data value (e.g., data-bit or code-bit) is one symbol or another in a particular symbol alphabet. For example, in a binary system, a particular data value can be either “0” or “1”, even if the raw electrical analog value read from a storage location does not indicate that the electrical value representing the data value is sufficient to decide with certainty that the data value is “0” or “1.” In other words, a hard-decision for a particular data value is based on the most likely symbol corresponding to the analog electrical value read from the storage medium, and the probabilities that alternative decisions may be true are ignored by the hard-decision process. Often the hard-decision is based on the Euclidean distances from the analog read value to electrical level(s) defining the symbols.
By contrast, the use of soft information does not ignore the probabilities that different outcomes may be true in view of what is read from the storage medium. Error control methods using soft information are particularly promising because soft information decoding often improves the error detection and correction capability of a particular error control code, and thus the capacity of the system. However, the utilization of soft information decoding has been limited because of a number of previously irresolvable drawbacks, including increased power consumption, memory usage and added delays for generating and conveying the soft information. One particularly difficult challenge has been managing the delay, memory usage and power consumption previously required to create, store and transfer soft information between functional components. For example, using previous methods, soft information generation required reading a memory cell (sometimes called a memory location) multiple times and combining the read results to generate a statistically accurate soft information value for each bit or symbol stored in the memory cell. The accuracy of the soft information generally improves with the number of read operations performed, until a point of diminishing returns. But each read adds a delay, and the aggregate delay to generate accurate soft information for an entire sequence of data values is undesirable. Each read operation also increases the amount of memory used to store the read data values prior to conversion into soft information values. Each read operation also increases power consumption in at least three ways. First, power is consumed for each additional read operation performed. Second, power is consumed by the storage of the additional raw read data values produced by each additional read. Third, power is consumed when the raw read data values are transmitted from one functional block to another.
As discussed in more detail below, some implementations include systems, methods and/or devices enabled to utilize soft information error correction decoding to support the use of flash memory or other storage mediums. In some implementations delay, power consumption, and/or memory usage are reduced during the generation of soft information from multiple read operations. To that end, raw read data from each memory location is compressed into a corresponding bit-tuple representing the result of a number of sequential read operations. The bit-tuple is then used to retrieve a soft information value (e.g., a log-likelihood ratio, LLR) from a stored characterization vector associated with the current state of the storage medium, as identified by a set of storage medium characterization parameter values. In some implementations, memory usage is reduced because the amount of memory required to store a bit-tuple is smaller than the amount of memory required to store the corresponding sequence of raw read data values obtained (read) from a single memory location. As such, in aggregate, the total amount of memory required to store a group of bit-tuples for one or more codewords is significantly smaller (e.g., in some implementations, more than 50% smaller) than the total amount of memory required to store the corresponding raw read data for the one or more codewords, obtained by multiple read operations. In some implementations, power consumption is reduced because less power is consumed to store a group of bit-tuples as compared to storing the raw read data for one or more codewords obtained by multiple read operations. In some implementations, delay is reduced by looking up a soft information value in a characterization vector based on a bit-tuple rather than computing the soft information value from the raw read data. In some implementations, delay is reduced because less time is needed to transfer a set of bit-tuples between system components (e.g., from storage medium 130 to memory controller 120), as compared to the time needed to transfer the corresponding raw read data values between system components.
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, the reading threshold voltages (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 it is a “0”.
In some implementations, the memory cell voltage of a memory cell is read indirectly, by reading the memory cell using one or more reading threshold voltages. More specifically, each read operation produces a result that indicates whether the cell voltage of the memory cell is greater than or less than the reading threshold voltage used during that read operation. By reading the memory cell using multiple reading threshold voltages, the cell voltage can be determine more precisely than if the memory cell were read using only a single reading threshold voltage. Stated another way, the more read operations that are performed on a memory cell, each using a different reading threshold voltage, the more precisely the cell voltage of the memory cell is known.
It is noted that while the specific methodology for reading raw data from a flash memory device may vary from one storage medium to another, the systems and methodologies described here for representing the resulting raw read data using a smaller number of buffer locations than raw data values are independent of the specific methodology used for reading raw data from the flash memory device(s).
A single-level flash memory cell (SLC) stores one bit of information (“0” or “1”). So the storage density of a SLC memory device is one bit per memory cell. A multi-level flash memory cell (MLC) can store two or more bits 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).
As discussed below with reference to
One strategy used to characterize and/or read a data value stored in a memory location is to read the memory location multiple times, using a plurality of distinct reading threshold voltages (sometimes herein called reading voltages or reading signals) distributed across the transition or overlap between adjacent voltage ranges in the memory cell. For example,
Sequential voltage ranges 201, 202 between the source and drain voltages, VSS and VDD, are used to represent corresponding bit values “1” and “0,” respectively. Each voltage range 201 and 202 has a respective center voltage V1 201b and V0 202b. Each voltage range 201 and 202 also has a respective distribution of voltages 201a and 202a that may occur as a result of any number of a combination of error-inducing factors, examples of which are identified above.
Preferably, during a write operation, to write a respective desired bit value to a SLC flash memory cell the charge on the floating gate of the SLC flash memory cell is set such that the resultant cell voltage (i.e., the cell's resultant threshold voltage) is at the center of one of the ranges 201, 202. In other words, the resultant cell voltage is, ideally, set to one of V1 201b and V0 202b in order to write a respective one of the bit values “1” and “0.” Under typical operating conditions, however, due in part to the error-inducing factors described above, the actual cell voltage may differ from the center voltage for the data written to the SLC flash memory cell.
In some implementations, reading threshold voltages VT(1), VT(2), VT(3), VT(4), VT(5) are applied between adjacent center voltages, e.g., applied proximate to the halfway region between adjacent center voltages V1 201b, and V0 202b. Optionally, in some implementations, some or all of the reading threshold voltages are located between voltage ranges 201, 202. In some implementations, reading threshold voltages VT(1), VT(2), VT(3), VT(4), VT(5) are applied in the region proximate to where the voltage distributions 201a, 202a overlap, which is not necessarily proximate to the halfway region between adjacent center voltages V1 201b, and V0 202b. In some implementations, during a sequence of read operations, reading threshold voltages VT(1), VT(2), VT(3), VT(4), VT(5) are sequentially applied, such that the application of each reading threshold voltage yields a raw read data value associated with each memory cell that is read. Consequently, for reading threshold voltages VT(1), VT(2), VT(3), VT(4), VT(5), five separate raw read data values are obtained for each memory location. Those skilled in the art will appreciate that any number of two or more reading threshold voltages can be used in various implementations, and that the five reading threshold voltages described with reference
More generally, the sequence of raw read data values obtained from a memory cell is a N-bit binary sequence, generated by a sequence of N read operations. As described in more detail below, in some implementations, the reading threshold voltages used in the sequence of N read operations is a monotonically increasing or decreasing sequence of reading threshold voltages. In some implementations, the monotonically increasing or decreasing sequence of reading threshold voltages are applied non-sequentially and the resulting raw data values are re-ordered so that the resulting raw data values are thus arranged as though the reading threshold voltages had been applied sequentially.
As described above, in many circumstances the memory cell current sensed in response to the applied reading threshold voltages VT(1), VT(2), VT(3), VT(4), VT(5) is indicative of a memory cell voltage different from the respective center voltage V1 201b or V0 202b 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.”
Again, as compared to a SLC, a MLC memory device is used to store multiple bits by using four or more distinct voltage ranges within the total voltage range of the memory cell to represent different bit-tuples.
Sequential voltage ranges 211, 212, 213, 214 between the source and drain voltages, VSS and VDD, are used to represent corresponding bit-tuples “11”, “01”, “00”, “10”, respectively. Each voltage range 211, 212, 213, 214 has a respective center voltage 211b, 212b, 213b, 214b. Each voltage range 211, 212, 213, 214 also has a respective distribution of voltages 211a, 212a, 213a, 214a that may occur as a result of any number or combination of the error-inducing factors described above.
As noted above, errors in the cell voltage of a MLC and/or the cell voltage sensed are sometimes caused by the cell voltage drifting to a range corresponding to a different bit-tuple from the correct voltage range of the bit-tuple written to the MLC. One way to reduce the impact of a cell voltage drifting from one voltage range to an adjacent voltage range is to grey-code the bit-tuples. Grey-coding the bit-tuples includes constraining the assignment of bit-tuples to memory cell voltage ranges such that the respective bit-tuple of a particular voltage range is different from the respective bit-tuple of an adjacent voltage range by only one bit. For example, as shown in
Preferably, 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 211, 212, 213, 214 in order to write the corresponding bit-tuple to the MLC. In other words, ideally, the resultant cell voltage would be set to one of V11 211b, V10 212b, V00 213b and V10 214b in order to write a corresponding one of the bit-tuples “11”, “01”, “00” and “10.” More typically, though, the resultant cell voltage is set to a value within a predefined range (e.g., 50, 100 or 200 millivolts) of a respective center voltage V11 211b, V10 212b, V00 213b and V10 214b corresponding to the bit-tuple written to the MLC.
As illustrated in
With further reference to
More generally, the sequence of raw read data values is a N-bit binary sequence, generated by a sequence of N lower page read operations from a single memory location. As described in more detail below, the reading threshold voltages used for a sequence of read operations includes a monotonically increasing or decreasing sequence of reading threshold voltages. In some implementations, the monotonically increasing or decreasing sequence of reading threshold voltages can be applied non-sequentially and the resulting raw lower page data values re-ordered so that the resulting raw lower page data values are thus arranged as though the reading threshold voltages had been applied sequentially.
For an upper page read, the upper page bit of a memory location can be read by applying the reading threshold voltages VTb(1), VTb(2), VTb(3), VTb(4), VTb(5) across the respective transition between voltage distributions 211a and 212a or the respective transition between voltage distributions 213a and 214a. As an example only, as shown in
More generally, a sequence of raw read data values for a single memory location is a N-bit binary sequence. In the example shown in
To that end, as a non-limiting example, data storage system 300, and in particular, memory controller 120 has been further configured to compress read data values from multiple read operations and utilize soft information for error control decoding. More specifically, memory controller 120 further includes read data compression module 321, a compressed read data buffer 323, and soft information generation module 325. As discussed below with reference to
Soft information generation module 325 is coupled to management module 121, error control module 125 and storage medium I/O 128. Management module 121 is connected to provide soft information generation module 325 a control signal on a control line 225. Soft information generation module 325 is connected to provide decoder 127 with at least one sequence of soft information corresponding to the raw read data 328 retrieved by the storage medium 130 (obtained via storage medium I/O 128 in embodiments represented by
Read data compression module 321 is coupled to receive raw read data values 328 from storage medium 130 via storage medium I/O 128. Read data compression module 321 is configured to compress an N-bit sequence of raw read data values associated with a particular memory location into a bit-tuple of a predefined length, M. In other words, in some implementations, read data compression module 321 is included in a device operable to compress a N-bit binary sequence, the N-bit binary sequence having an initial sub-sequence of M bits and a remaining sub-sequence of N−M bits. As such, read data compression module 321 produces a sequence of bit-tuples corresponding to an entire codeword by compressing the raw read data values for a set of memory locations associated with the entire codeword. One or more sequences of bit-tuples produced by read data compression module 321 is stored in compressed read data buffer 323. Compressed read data buffer 323 is accessible by soft information generation module 325 to generate corresponding sequences of soft information values. Soft information generation module 325 generates soft information values by utilizing a pre-generated collection of characterization vectors for various combinations of storage medium characterization parameter values. As described in further detail below with reference to
During a write operation, as described above, input buffer 123 receives data to be stored in storage medium 130 from computer system 110. The data in input buffer 123 is made available to encoder 126, which encodes the data to produce a codeword. The codeword is conveyed to storage medium via storage medium I/O 128, which transfers the codeword to storage medium 130 in a manner dependent on the type of storage medium being utilized. For example, for MLC flash memory, each bit-tuple of the codeword is written into a respective memory cell by changing the charge on the floating gate of the respective memory cell.
A read operation is initiated when computer system 110 sends a read command on control line 111 to memory controller 120 to request read data from storage medium 130. Memory controller 120 sends read operation commands to storage medium 130, via storage medium I/O 128, to obtain raw read data values 328 for each memory location that is read in response to the read command. Raw read data values 328 are sent to and processed by read data compression module 321, the operation of which is described in more detail below with reference to
In some implementations, management module 121 also passes the read request to soft information generation module 325 so that soft information generation module 325 can select reading threshold voltages for use by storage medium 130. In some other implementations, management module 121 selects the reading threshold voltages for use by storage medium 130. In yet other implementations, storage medium I/O 128 selects the reading threshold voltages.
As described below with reference to
Extending the read operation example from above, in some implementations memory controller 120 receives one or more storage medium characterization parameter values, which characterize the current state of storage medium 130, from computer system 110 along with the read request. In some other implementations, storage medium characterization parameter values are stored internally by memory controller 120 and memory controller 120 retrieves the one or more storage medium characterization parameter values in response to receiving the read request from computer system 110. The one or more storage medium characterization parameter values are passed to soft information generation module 325. The one or more storage medium characterization parameter values are used by soft information generation module 325 to select a characterization vector associated with the current state of storage medium 130.
To that end, soft information management controller 410 is connected to receive the read request and one or more storage medium characterization parameters on control and data lines 401 and 402, respectively. Soft information management controller 410 is also connected to provide characterization module 420 a selection control signal on control line 411, and to receive a selected characterization vector on data line 412. In some implementations, soft information management controller 410 is further connected to provide reading threshold voltages to storage medium 130 via data line 414 and storage medium I/O 128, and receive bit-tuples from compressed read data buffer 323 through data line 417. Soft information management controller 410 is also connected to provide sequences of soft information values to decoder 127 (
Characterization module 420 includes a collection of characterization vectors 421-1, 421-2, 421-3, . . . 421-n, that each store characterization data, such as soft information values for bit-tuples and reading threshold voltages, associated with storage medium 130 for one or more storage medium characterization parameter values. In some implementations, the characterization data stored in the characterization vectors 421 is statistically derived. More detailed example implementations of characterization vectors 421 are described in co-pending U.S. patent application Ser. No. 13/602,031, entitled “Statistical Read Comparison Signal Generation for Memory Systems,” which is hereby incorporated by reference in its entirety.
Soft information management controller 410 includes a selection module 431, a read controller 432, and a conversion module 433. The selection module 431 is configured to use the one or more storage medium characterization parameters values to select a characterization vector from the stored collection of characterization vectors 421-1, 421-2, 421-3, . . . 421-n in accordance with the current state of storage medium 130. In some implementations, the selection control signal includes one or more of storage medium characterization parameters values and/or an index key associated with a combination of one or more of storage medium characterization parameters values.
Read controller 432 is configured to read a portion of storage medium 130 via storage medium I/O 128. In some implementations, read controller 432 is configured to provide storage medium 130 with reading threshold voltages that are used to read the same portion of storage medium 130 multiple times.
Conversion module 433 is configured to generate a sequence of soft information values corresponding to the bit-tuples stored in compressed read data buffer 323. The sequence of soft information values is generated, at least in part, by selecting a respective soft information value (e.g., a LLR, a transition probability, etc.) from the selected characterization vector for each bit-tuple. In some implementations, soft information is generated by a device characterization process for memory devices produced by a particular manufacturing process, and stored in a characterization module or look-up table included in soft information generation module 325.
Read data compression module 321 includes a compression module 321a and M single-bit buffer locations 321b. As described below with reference to
Even more generally, read data compression module 321 includes a device operable to compress a sequence of data values. In some implementations the device includes a plurality of buffer locations and a compression module. In some implementations, there are fewer buffer locations than the total number of data values in the sequence. In some implementations, the compression module is configured to: populate each of the plurality of buffer locations by setting each buffer location to a corresponding value from an initial sub-sequence of sequence of data values; and update the collective state of the plurality buffer locations for each data value of a remaining sub-sequence of the sequence of data values in sequential order by determining whether the plurality of buffer locations is in a first state and overwriting the first state of the plurality of buffer locations when a respective remaining data value has a first value.
In some implementations, the compression module 321a is further configured to sequentially evaluate each bit of the remaining sub-sequence of N−M bits by: determining whether the M single-bit buffer locations are in the predefined state; determining whether the bit under evaluation has a read value equal to the first predefined value; and maintaining the predefined state of the M single-bit buffer locations in response to a positive determination that the M single-bit buffer locations are in the predefined state and the bit under evaluation has a read value equal to the first predefined value.
To that end, as represented by block 5-1a, method 500a includes receiving a N-bit sequence, that was generated by a sequence of N read operations from a single memory location. For example, with continued reference to
As represented by block 5-2a, the method includes populating the M single-bit buffer locations with the first M-bits of the N-bit sequence. In some implementations, the M single-bit buffer locations are populated by setting each respective single-bit buffer location of the M single-bit buffer locations to a value corresponding to a read value obtained by a respective read operation of the M initial read operations. In some implementations, the M single-bit buffer locations are populated, in a predefined order, from a constituent least-significant-bit location to a constituent most-significant-bit location of the M single-bit buffer locations.
As represented by block 5-3a, method 500a includes determining whether the M single-bit buffer locations are in a first state (i.e., a corresponding predefined state). In some implementations, the first state corresponds to an all-zero state, such that the data value stored in each of the M single-bit buffer locations is a “0.” If the M single-bit buffer locations are not in the first state (“No” path from block 5-3a), as represented by block 5-4a, method 500a includes maintaining the state of the M single-bit buffer locations irrespective of the remaining N−M bits in the N-bit sequence. On the other hand, if the M single-bit buffer locations are in the first state (“Yes” path from block 5-3a), as represented by block 5-5a, method 500a includes determining if there is at least another unchecked bit in the remaining N−M bits of the N-bit sequence, the first of which represents the current bit under evaluation. If there are no more unchecked bits (“No” path from block 5-5a), the N-bit sequence has been fully compressed and the method ends for the N-bit sequence. On the other hand, if there is another sequential unchecked bit in the N-bit sequence (“Yes” path from block 5-5a), as represented by block 5-6a, method 500a includes determining whether the sequential unchecked bit (i.e., the current bit under evaluation) is different from a first predefined value. In some implementations, the first predefined value is a “0.” In some other implementations, the first predefined value is a “1.” If the current bit under evaluation is equal to the first predefined value (“No” path from block 5-6a), as represented by block 5-7a, method 500a includes maintaining the first state of the M single-bit buffer locations. In other words, for each read operation in the N−M subsequent read operations that produces a read value equal to the first predefined read value, the method includes maintaining the first state of the M single-bit buffer locations. Method 500a then circles back to the portion of the method represented by block 5-5a, with the latest current bit under evaluation becoming a checked (or considered) bit. On the other hand, if the current bit under evaluation differs from the first predefined value (“Yes” path from block 5-6a), as represented by block 5-8a, method 500a includes overwriting the state of the M single-bit buffer locations based on the sequence position of the current sequential unchecked bit. In other words, as represented by block 5-8a, method 500a includes updating the state of the M single-bit buffer locations to a state that indicates which read operation in the sequence of N read operations is the first read operation in said sequence to produce a read value that is different from the first predefined read value. Method 500a then circles back to the portion of the method represented by block 5-3a, with the latest current bit under evaluation becoming a checked (or considered) bit.
More generally, with reference to portions of method 500a represented by blocks 5-3a to 5-8a, method 500a includes updating the state of the M single-bit buffer locations to a state that indicates which read operation in the sequence of N read operations is the first read operation in said sequence to produce a read value (e.g., “1”) that is different from the first predefined read value (e.g., “0”). Even more generally, with reference to portions of method 500a represented by blocks 5-3a to 5-8a, method 500a includes updating the collective state of the plurality buffer locations for each data value in a remaining sub-sequence of the sequence of data values in sequential order by: determining whether the plurality of buffer locations is in a first state; and overwriting the first state of the plurality of buffer locations when a respective remaining data value has a first value. The method further includes maintaining (5-7a) the predefined state of the M single-bit buffer locations for each read operation in the N−M subsequent read operations that produces a read value equal to the first predefined read value.
In some implementations of method 500a, the sequence of operations shown in
In some implementations of method 500a, the sequence of operations shown in
To that end, as represented by block 5-1b, method 500b includes performing a read operation to read a bit value from a memory location. Those skilled in the art will appreciate from the present disclosure that performing a read operation on a memory device, such as a flash memory, typically includes reading data values from a number of memory locations together (e.g., all or a portion of the memory locations included in a block, a wordline a page, or a sector of a page). However, for the sake of clarity, method 500b is described with respect to reading data values from a single memory location. Those skilled in the art will appreciate from the present disclosure that in some implementations method 500b is modified to operate on two or more N-bit binary sequences in parallel and/or serially, where the two or more N-bit binary sequences are generated by a sequence of N read operations from two or more memory locations.
As represented by block 5-2b, method 500b includes determining whether the number of read operations performed so far is less than or equal to M, the number of single-bit buffer locations. If the number of read operations performed is less than or equal to M (“Yes” path from block 5-2b), as represented by block 5-3b, method 500b includes populating a respective one of the M single-bit buffer locations with the bit value read from the memory location. Method 500b then circles back to the portion of the method represented by block 5-1b. In some implementations, with reference to block 5-1b to 5-3b, method 500b includes populating the M single-bit buffer locations in order from a constituent least-significant-bit location to a constituent most-significant-bit location of the M single-bit buffer locations. In some implementations, populating the M single-bit buffer locations occurs on a per read basis, as shown in
If the read bit value is equal to the first predefined value (“No” path from block 5-5b), as represented by block 5-7b, method 500b includes maintaining the first state of the M single-bit buffer locations. In other words, for each read operation greater than M that produces a read value equal to the first predefined read value, the method includes maintaining the first state of the M single-bit buffer locations. Following the portion of the method represented by block 5-7b, method 500b proceeds to the portion of the method represented by block 5-8b, which is discussed in greater detail below.
With continued reference to block 5-5b, on the other hand, if the read bit value differs from the first predefined value (“Yes” path from block 5-5b), as represented by block 5-6b, method 500b includes overwriting the state of the M single-bit buffer locations based on how many read operations have been performed thus far. In other words, as represented by block 5-6b, method 500b includes updating the state of the M single-bit buffer locations to a state that indicates which read operation in the sequence of N read operations is the first read operation in said sequence to produce a read value that is different from the first predefined read value. Subsequently, as represented by block 5-8b, method 500b includes determining whether N read operations have been performed. If N read operations have been performed (“Yes” path from block 5-8b), an N-bit sequence has been read from the memory location and compressed. Accordingly, method 500b ends for the memory location. On the other hand, if fewer than N read operations have been performed (“No” path from block 5-8b), method 500b circles back to the portion of the method represented by block 5-1b.
More generally, with reference to portions of method 500b represented by blocks 5-4b to 5-8b, method 500b includes updating the state of the M single-bit buffer locations to a state that indicates which read operation in the sequence of N read operations is the first read operation in said sequence to produce a read value that is different from the first predefined read value. From another viewpoint, with reference to portions of method 500b represented by blocks 5-3b to 5-8b, method 500b includes updating the collective state of the plurality buffer locations for each data value in a remaining sub-sequence of the sequence of data values in sequential order by: determining whether the plurality of buffer locations is in a first state; and overwriting the first state of the plurality of buffer locations when a respective remaining data value has a first value. The method further includes maintaining the predefined state of the M single-bit buffer locations for each read operation in the N−M subsequent read operations that produces a read value equal to the first predefined read value.
Optionally, in some implementations, a modified version of method 500b includes, upon determining at block 5-4b that the M single-bit buffer locations are not in the first state (“No” path from block 5-4b), aborting the sequence of read operations, because method 500b includes maintaining the state of the M single-bit buffer locations irrespective of any remaining bit values subsequently read from the memory location. In the resulting modified method 500b, the “No” branch from block 5-4b would go directly to method “End” instead of block 5-7b. As a result, fewer read operations are performed to obtain the same compressed representation of the raw data read from the memory location, which reduces latency and reduces power consumption. This improvement is potentially most practical in implementation in which method 500b is performed by logic in storage medium 130.
As represented by block 6-1, method 600 includes performing a first read operation to obtain a bit value from a memory location. As represented by block 6-2, method 600 includes writing the first read bit value to the least significant bit (LSB) of the two single-bit buffer locations. Often the LSB is the right-most bit location in a buffer or a register. However, those skilled in the art will appreciate from the present disclosure that the opposite (i.e., designating the left-most bit location in a buffer or a register as the LSB) and various other arrangements are possible without departing from the scope of the claims. As represented by block 6-3, method 600 includes performing a second read operation to obtain a bit value from the same memory location as the first read operation. As represented by block 6-4, method 600 includes writing the second read bit value to the most significant bit (MSB) of the two single-bit buffer locations. As represented by block 6-5, method 600 includes performing a third read operation to obtain a bit value from the same memory location as the first and second read operations. As represented by block 6-6, method 600 includes determining whether the two single-bit buffer locations are in an all-zero state (i.e., whether the bit value in each of the two single-bit buffer locations is a “0”). If the two single-bit buffer locations are not in the all-zero state (“No” path from block 6-6), method 600 ends for the 3-bit sequence by maintaining the non-zero state of the two single-bit buffer locations that was produced after the first two read operations. On the other hand, if the two single-bit buffer locations are in the all-zero state (“Yes” path from block 6-6), as represented by block 6-7, method 600 includes determining whether the bit value produced by the third read operation is equal to a first predefined value. In some implementations, the first predefined value is a “0.” In some other implementations, the first predefined value is a “1.” If the third read bit value is equal to the first predefined value (“Yes” path from block 6-7), method 600 ends for the 3-bit sequence by maintaining the non-zero state of the two single-bit buffer locations that was produced after the first two read operations. On the other hand, if the third read bit value differs from the first predefined value (“No” path from block 6-7), as represented by block 6-8, method 600 includes overwriting the state of the two single-bit buffer locations. More specifically, in some implementations, as represented by sub-block 6-8a, overwriting the state of the two single-bit buffer locations includes writing a “1” to the LSB of the two single-bit buffer locations. In some implementations, overwriting the state of the two single-bit buffer locations includes writing a “1” to the MSB of the two single-bit buffer locations (not shown in
In response to a second read operation, as represented by block 6-4 of
In response to a third read operation, as represented by block 6-5 of
As represented by block 8-1, method 800 includes performing a read operation to read a bit value from a memory location. As noted above, those skilled in the art will appreciate from the present disclosure that performing a read operation on a memory device, such as a flash memory, typically includes reading data values from a number of memory locations together (e.g., all or a portion of the memory locations included in a block, a zone, a word-line and/or a page). However, for the sake of clarity, method 800 is described with respect to reading data values from a single memory location. Nevertheless, those skilled in the art will appreciate from the present disclosure that in some implementations method 800 is modified to operate on two or more 7-bit binary sequences in parallel and/or serially, where the two or more 7-bit binary sequences are generated by a sequence of seven read operations from a corresponding two or more memory locations.
As represented by block 8-2, method 800 includes determining whether the number of read operations performed so far is less than or equal to three. If the number of read operations performed thus far is greater than three (“No” path from block 8-2), method 800 proceeds to the portion of the method represented by block 8-8, which is described in greater detail further below. On the other hand, the number of read operations performed so far is less than or equal to three (“Yes” path from block 8-2), as represented by block 8-3, method 800 includes determining whether or not the latest read operation performed is the first read operation on the memory location. If the latest read operation is the first read operation on the memory location (“Yes” path from block 8-3), as represented by block 8-5, method 800 includes writing the raw read data value (i.e., the read bit) to the LSB of the three single-bit buffer locations. On the other hand, if the latest read operation is not the first read operation on the memory location (“No” path from block 8-3), as represented by block 8-4, method 800 includes, determining whether or not the latest read operation performed is the second read operation on the memory location. If the latest read operation is the second read operation on the memory location (“Yes” path from block 8-4), as represented by block 8-6, method 800 includes writing the raw read data value (i.e., the read bit) to the center significant bit (CSB) of the three single-bit buffer locations. On the other hand, if the latest read operation is not the second read operation on the memory location (“No” path from block 8-4), as represented by block 8-7, method 800 includes writing the raw read data value (i.e., the read bit) to the most significant bit (MSB) of the three single-bit buffer locations.
Alternatively, in some implementations, the operation represented by block 8-6 is determined in accordance with the value of the LSB; in particular, if the LSB has been set to a predefined value (e.g., “1”), then the CSB is set to the same predefined value regardless of the value of the second read bit; otherwise, the CSB is set to the value of the second read bit. Similarly, in some implementations, the operation represented by block 8-7 is determined in accordance with the value of the LSB; in particular, if the CSB has been set to a predefined value (e.g., “1”), then the MSB is set to the same predefined value regardless of the value of the second read bit; otherwise, the MSB is set to the value of the third read bit.
Following the respective portions of the method represented by block 8-5, 8-6 and 8-7, method 800 includes looping back to the portion of the method represented by block 8-1.
With reference to block 8-8, method 800 includes determining whether or not the three single-bit buffer locations are in a first state (e.g., “000”). In some implementations, the first state is an all-zero state in which each of the three single-bit buffer locations stores a “0.” However, those skilled in the art will appreciate that the first state may also include any combination of values stored in the three single-bit buffer locations. If the three single-bit buffer locations are not in the first state (“No” path from block 8-8), as represented by block 8-9, method 800 includes maintaining the state of the three single-bit buffer locations, without regard to the value of the bit read during the latest read operation. Subsequently, as represented by block 8-18, method 800 includes determining whether or not the latest read operation performed is the seventh read operation on the memory location. If the latest read operation is the seventh read operation on the memory location (“Yes” path from block 8-18), method 800 ends for the 7-bit sequence by maintaining the state of the three single-bit buffer locations. On the other hand, if the latest read operation is not the seventh read operation on the memory location (“No” path from block 8-18), method 800 loops back to the portion of the method represented by block 8-1.
With further reference to block 8-8, if the three single-bit buffer locations are in the first state (“Yes” path from block 8-8), as represented by block 8-10, method 800 includes determining whether or not the latest read bit value is different from a first predefined read value. In some implementations, as shown in
In response to a first read operation, described above with reference to
In some implementations, once the three single-bit buffer locations have been fully populated, method 800 operates to overwrite the all-zero state when a subsequent read value is different from first predefined value. As such, if after the third read the three single-bit buffer locations are in state 941 (e.g., “111”), the three single-bit buffer locations sequentially progress to states 951, 961, 971 and 981 in response to the subsequent read operations, without regard to the bit values obtained by those subsequent read operations. Similarly, if after the third read the three single-bit buffer locations are in state 942 (e.g., “110”), the three single-bit buffer locations sequentially progress to states 952, 962, 972 and 982 in response to the subsequent read operations, without regard to the bit values obtained by those subsequent read operations. Similarly, if after the third read the three single-bit buffer locations are in state 943 (e.g., “100”), the three single-bit buffer locations sequentially progress to states 953, 963, 973 and 983 in response to the subsequent read operations, without regard to the bit values obtained by those subsequent read operations.
However, if after the third read the three single-bit buffer locations are in state 944 (an all-zero state) after three read operations, in response to the fourth read the three single-bit buffer locations enter state 955 if a “1” is read, and enter state 954 if a “0” is read. If the three single-bit buffer locations enter state 955, the three single-bit buffer locations sequentially enter states 966, 977 and 988 in response to subsequent read operations, without regard to the bit values obtained by those subsequent read operations.
Similarly, if after the fourth read the three single-bit buffer locations are in state 954 (an all-zero state), in response to the fifth read the three single-bit buffer locations enter state 965 if a “1” is read, and enter state 964 if a “0” is read. If the three single-bit buffer locations enter state 965, the three single-bit buffer locations sequentially enter states 976 and 987 in response to subsequent read operations, without regard to the bit values obtained by those subsequent read operations.
Similarly, if after the fifth read the three single-bit buffer locations are in state 964 (an all-zero state), in response to the sixth read the three single-bit buffer locations enter state 975 if a “1” is read, and enter state 974 if a “0” is read. If the three single-bit buffer locations enter state 975, the three single-bit buffer locations enter state 986 after the seventh read, without regard to the bit value obtained by the seventh read. Similarly, if after the sixth read the three single-bit buffer locations are in state 974 (an all-zero state), in response to the seventh read the three single-bit buffer locations enter state 985 if a “1” is read, and enter state 984 if a “0” is read.
To that end, data storage system 1000 includes a memory system 1010 (e.g., storage medium 130 or a system that includes storage medium 130), an error control code (ECC) application specific integrated circuit (ASIC) 1030, and a transmission lines coupling the memory system 1010 and the ECC ASIC 1030. The memory system 1010 includes a memory array 1011 (e.g., one or more flash memory die), a compression module 1012 and a page buffer 1013 coupled in series. The ECC ASIC 1030 includes a decoding ASIC buffer 1031 and a LLR based decoding module 1032 coupled in series and operable to provide decoded data to a data processing system (not shown). In operation, compression module 1012 compresses sequences of raw data read from memory array 1011 (e.g., as described above with reference to
To that end, the primary difference between data storage systems 1000 and 1100 is that compression module 1131 is implemented on ECC ASIC 1130. In operation, raw data that is read from memory array 1111 is stored in page buffer 1113 without first being compressed. The raw data from page buffer 1113 is transmitted across transmission line 1120, where it is received by compression module 1131. Compression module 1131 compresses sequences of raw data and stores compressed data in decoding ASIC buffer 1132. In turn, LLR based decoding module 1133 decodes the compressed data stored in decoding ASIC buffer 1132.
It will also 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 invention 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 the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various implementations with various modifications as are suited to the particular use contemplated.
This application claims priority to U.S. Provisional Patent Application No. 61/746,551, filed Dec. 27, 2012, which is hereby incorporated by reference in its entirety.
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