Abstract:
A memory system including a memory and, to perform a writing operation to store user data among a plurality of cells of the memory, a pilot generator module, a multiplexer module, and a write module. The pilot generator module is configured to randomly alternate between selection of a first scheme by which pilot data is to be stored, along with the user data, among the plurality of cells of the memory, and a second scheme by which the pilot data is to be stored, along with the user data, among the plurality of cells. The pilot data comprises a known predetermined sequence. The multiplexer module is configured to combine the pilot data and the user data in accordance with the selection of the first scheme and the second scheme. The write module is configured to write the pilot data and the user data among the plurality of cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/986,872, filed on Nov. 27, 2007 (now U.S. Pat. No. 8,190,961), which claims the benefit of U.S. Provisional Application No. 60/867,492, filed on Nov. 28, 2006. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to non-volatile memory devices and more particularly to using pilot signals in non-volatile memory devices. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Referring now to  FIGS. 1 ,  2 A and  2 B, non-volatile semiconductor memory  10  may include flash memory, static random access memory (SRAM), nitride read only memory (NROM), phase change memory, magnetic RAM, multi-state memory, etc. The non-volatile semiconductor memory  10  may include one or more arrays  16 . The arrays  16  may be arranged as B memory blocks  18 - 1 ,  18 - 2 , . . . , and  18 -B (collectively referred to as blocks  18 ). 
     In  FIG. 2A , each block  18  includes P pages  20 - 1 ,  20 - 2 , . . . , and  20 -P (collectively referred to as pages  20 ). In  FIG. 2B , each page  20  may include a plurality of memory cells that are associated with a data portion  24  and may include other memory cells that are associated with an overhead data portion  26  such as error correcting code (ECC) data or other (O) overhead data. 
     The non-volatile semiconductor memory  10  typically communicates with a memory control module of a host device. Usually, the control module addresses the memory using a hardwired block size. Pages in the block may also have a hardwired physical page size and may therefore be referred to as physical pages. The number of memory cells in the data and overhead portions  24  and  26 , respectively may also be hardwired. 
     For example only, a NAND flash array may include 2048 blocks for a total of 2 Gigabytes (GB) of memory. Each block may include 128 kilobytes (kB) in 64 pages. Each page may include 2112 bytes. Of the 2112 bytes, 2048 bytes may be associated with the data portion and 64 bytes may be associated with the overhead portion. Each memory cell may store a bit. To erase data stored in the array, the memory control module typically requires either an entire block and/or an entire page to be erased. 
     In  FIG. 2C , the memory block  18  includes physical pages  50 - 1 ,  50 - 2 , . . . , and  50 -P (collectively referred to as pages  50 ). Each page  50  includes Y memory cells (memory cells  46 - 1 ,  46 - 2 , . . . , and  46 -Y) for the data portion and Z memory cells (memory cells  46 -(Y+1),  46 (Y+2), . . . , and  46 (Y+Z)) for the overhead portion, where Y and Z are fixed values for a particular memory control module. During a first write operation for a first write data block, the memory control module writes data to pages  50 - 1  and  50 - 2  and part of page  50 - 3 . During a second write operation for a second write data block, data is written to pages  50 - 4  and  50 - 5  and part of page  50 - 6 . The remaining memory cells in pages  50 - 3  and  50 - 6  are unused. 
     A logical page may include formatted start and end-points within one or more physical pages. Logical pages may be used when data to be stored in memory has a length that differs from a length of a physical page. In most cases, the logical page size and physical page size are different. Thus, one logical page may be segmented and saved in different physical pages. 
     Referring now to  FIG. 2D , another memory block  60  is illustrated that includes both physical and logical pages. A memory control module (not shown) may format physical pages  50 - 1  to  50 - 6  to appear as logical pages  51 - 1  to  51 - 3  for read/write operations. In other words, data is written to memory blocks according to the logical pages instead of according to the physical pages. 
     SUMMARY 
     A memory system includes a selector module that selects and switches between one of N sequences of signal levels for pilot data. The N sequences are different, and N is an integer greater than 1. A multiplexer module selectively combines data and the pilot data and outputs a combined signal. A write module writes to memory based on the combined signal. 
     In other features, first and second of the N sequences include first and second respective subsequences. The first subsequence includes a highest one of the signal levels repeated M times, and the second subsequence includes a lowest one of the signal levels repeated M times, where M&gt;1. The first and second of the N sequences include third and fourth respective subsequences. The third and fourth subsequences include S signal levels that are between the highest and lowest ones of the signal levels, where S≧n. The S signal levels are different and are repeated X times, where X is an integer greater than 1. The selector module selects from at least one of alternating between the N sequences and randomly selecting from the N sequences for subsequent write operations to multiple cells of the memory. 
     In other features, the system includes a generator module that generates the pilot data. An encoder module encodes the data before the data is combined with the pilot data, and a decoder module decodes the data after the data is combined with the pilot data. The encoder and decoder modules operate on the data according to at least one of a Reed Solomon scheme, a Bose-Chaudhuri-Hocquenghem (BCH) scheme, a Low Density Parity Check (LDPC) scheme, a Gray code scheme, and a combination of two or more of the BCH scheme, the LDPC scheme, and the Gray code scheme. The memory includes a plurality of pages. The plurality of pages include at least one of logical and physical pages. 
     In other features, a read module provides read-back data and read-back pilot data from the memory. The write module writes the pilot data to pilot cells and the data to data cells of the memory based on a predetermined distribution. The predetermined distribution includes at least one of an even distribution and a random distribution. For the even distribution the pilot cells are spaced evenly apart in the memory. The system further includes a format module that determines the predetermined distribution based on at least one of a size of the physical pages and a size of the logical pages. The memory includes multilevel flash memory 
     In other features, the system includes a demultiplexer module that demultiplexes the read-back pilot data and the read-back data. An estimation module estimates characteristics of pilot cells based on the read-back pilot data. A neighbor-finder module processes the read-back data based on the estimated characteristics. The estimation module estimates the characteristics through a least-mean square (LMS) operation. The system further includes a signal processing module that compensates for variations between written data and the read-back data. 
     In other features, a read module determines which of the N sequences was selected for the write to the memory. The read module determines which of the N sequences was selected for the write to the memory based on detection of at least one of the highest one of the signal levels repeated M times and the lowest one of the signal levels repeated M times. The signal levels include pulse-amplitude modulation (PAM) levels. 
     In other features, a method for operating a memory system includes selecting and switching between one of N sequences of signal levels for pilot data. The N sequences are different. N is an integer greater than 1. The method also includes generating a combined signal based on selectively combining data and the pilot data and writing to memory based on the combined signal. 
     In other features, the method includes generating first and second of the N sequences including first and second respective subsequences. The first subsequence includes a highest one of the signal levels repeated M times, and the second subsequence includes a lowest one of the signal levels repeated M times, where M&gt;1. The first and second of the N sequences include third and fourth respective subsequences. The third and fourth subsequences include S signal levels that are between the highest and lowest ones of the signal levels, where S≧1. The S signal levels are different and are repeated X times, where X is an integer greater than 1. 
     In other features, the method includes selecting at least one of alternating between the N sequences and randomly selecting from the N sequences for subsequent write operations to multiple cells of the memory. The method also includes generating the pilot data. The method also includes encoding the data before the data is combined with the pilot data and decoding the data after the data is combined with the pilot data. The method also includes encoding and decoding the data according to at least one of a Reed Solomon scheme, a Bose-Chaudhuri-Hocquenghem (BCH) scheme, a Low Density Parity Check (LDPC) scheme, a Gray code scheme, and a combination of two or more of the BCH scheme, the LDPC scheme, and the Gray code scheme. 
     In other features, the method includes providing read-back data and read-back pilot data from the memory. The method also includes writing the pilot data to pilot cells and the data to data cells of the memory based on a predetermined distribution. The predetermined distribution includes at least one of an even distribution and a random distribution. The method also includes determining the predetermined distribution based on at least one of a size of a physical page and a size of a logical page of the memory. The memory includes multilevel flash memory. 
     In other features, the method includes demultiplexing the read-back pilot data and the read-back data. The method also includes estimating characteristics of pilot cells based on the read-back pilot data. The method also includes processing the read-back data based on the estimated characteristics. The method also includes estimating the characteristics through a least-mean square (LMS) operation. The method also includes compensating for variations between written data and the read-back data. The method also includes determining which of the N sequences was selected for the write to the memory. 
     In other features, the method includes determining which of the N sequences was selected for the write to the memory based on detection of at least one of the highest one of the signal levels repeated M times and the lowest one of the signal levels repeated M times. The signal levels include pulse-amplitude modulation (PAM) levels. 
     In other features, a memory system includes selector means for selecting and switching between one of N sequences of signal levels for pilot data. The N sequences are different, and N is an integer greater than 1. The system also includes multiplexer means for selectively combining data and the pilot data and for outputting a combined signal. The system also includes write means for writing to memory means for storing data based on the combined signal. 
     In other features, first and second of the N sequences include first and second respective subsequences. The first subsequence includes a highest one of the signal levels repeated M times, and the second subsequence includes a lowest one of the signal levels repeated M times, where M&gt;1. The first and second of the N sequences include third and fourth respective subsequences. The third and fourth subsequences include S signal levels that are between the highest and lowest ones of the signal levels, where S≧1. The S signal levels are different and are repeated X times, where X is an integer greater than 1. The selector means selects from at least one of alternating between the N sequences and randomly selecting from the N sequences for subsequent write operations to multiple cells of the memory means. 
     In other features, the system includes generator means for generating the pilot data. The system also includes encoder means for encoding the data before the data is combined with the pilot data, and decoder means for decoding the data after the data is combined with the pilot data. The encoder and decoder means operate on the data according to at least one of a Reed Solomon scheme, a Bose-Chaudhuri-Hocquenghem (BCH) scheme, a Low Density Parity Check (LDPC) scheme, a Gray code scheme, and a combination of two or more of the BCH scheme, the LDPC scheme, and the Gray code scheme. The memory means includes a plurality of pages. The plurality of pages include at least one of logical and physical pages. 
     In other features, the system includes read means for providing read-back data and read-back pilot data from the memory means. The system also includes write means for writing the pilot data to pilot cells and the data to data cells of the memory based on a predetermined distribution. The predetermined distribution includes at least one of an even distribution and a random distribution. For the even distribution the pilot cells are spaced evenly apart in the memory means. The system further includes format means for determining the predetermined distribution based on at least one of a size of the physical pages and a size of the logical pages. The memory means includes multilevel flash memory 
     In other features, the system includes demultiplexer means for demultiplexing the read-back pilot data and the read-back data. The system also includes estimation means for estimating characteristics of pilot cells based on the read-back pilot data. The system also includes neighbor-finder means for processing the read-back data based on the estimated characteristics. The estimation means estimates the characteristics through a least-mean square (LMS) operation. The system further includes signal processing means for compensating for variations between written data and the read-back data. 
     In other features, the system includes read means for determining which of the N sequences was selected for the write to the memory means. The read means determines which of the N sequences was selected for the write to the memory based on detection of at least one of the highest one of the signal levels repeated M times and the lowest one of the signal levels repeated M times. The signal levels include pulse-amplitude modulation (PAM) levels. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of memory including blocks according to the prior art; 
         FIG. 2A  illustrates pages within the blocks of memory according to the prior art; 
         FIG. 2B  illustrates memory cells within the pages according to the prior art; 
         FIG. 2C  illustrates memory cells arranged in a memory block according to the prior art; 
         FIG. 2D  illustrates memory cells arranged in a memory block according to the prior art; 
         FIG. 3A  is a functional block diagram of a memory system according to the present disclosure; 
         FIG. 3B  illustrates a memory block including pages with variable density, page length and/or overhead; 
         FIG. 3C  illustrates a page including memory cells associated with a data portion and an overhead portion; 
         FIG. 4  is a functional block diagram of a write path module according to the present disclosure; 
         FIG. 5  illustrates pilot cell positions for 8 pulse-amplitude modulation (PAM); 
         FIG. 6  illustrates pilot cell positions for 12PAM pilots; 
         FIG. 7  illustrates pilot cell positions for 16PAM pilots; 
         FIG. 8  is a functional block diagram of a read path module according to the present disclosure; 
         FIG. 9  illustrates a method for operating the memory system according to the present disclosure; 
         FIG. 10A  is a functional block diagram of a hard disk drive; 
         FIG. 10B  is a functional block diagram of a DVD drive; 
         FIG. 10C  is a functional block diagram of a high definition television; 
         FIG. 10D  is a functional block diagram of a vehicle control system; 
         FIG. 10E  is a functional block diagram of a cellular phone; 
         FIG. 10F  is a functional block diagram of a set top box; and 
         FIG. 10G  is a functional block diagram of a mobile device. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     According to the present disclosure, a known group of memory cells in a nonvolatile memory device such as a flash memory device may be referred to as pilot cells. The use and placement of pilot cells in memory allows the pilot cells to have similar read-and-write cycling as other data cells stored in the memory. Data in pilot cells is therefore subject to similar disturbances as other data stored in the memory. The present disclosure includes a control module that determines positions for pilot cells and patterns for writing to levels within the pilot cells. The control module may minimize the number of pilot cells and may also minimize the number of levels within the pilot cells that are used. 
     Disturbance parameters, such as noise, vary across different logical and physical pages due to read/write cycling, manufacturing variations, and/or operating environment conditions. The control module may write pilot and user data to the memory, read-back the pilots and user data, and compare the read-back pilot data to the written pilot data. The control module may determine disturbance parameters based on the comparison. 
     Referring now to  FIGS. 3A-3C , a memory system  66  for non-volatile semiconductor memory  68 , such as multilevel flash memory, is illustrated. In  FIG. 3A , a host device  70  includes a control module  72 . The control module  72  may communicate with the memory  68  via write and read path modules  73 ,  75 . 
     The control module  72  may vary the number of memory cells per page in non-volatile semiconductor memory  68 . The control module  72  may vary the number of memory cells allocated for the data portion and for the overhead portion for each page. The control module  72  may also determine locations for pilot cells within the non-volatile memory  68 . 
     The non-volatile semiconductor memory  68  may include one or more arrays  78 - 1 ,  78 - 2 , . . . , and  78 -A (collectively array  78 ) of memory cells. The array  78  may be arranged in memory blocks  80 - 1 ,  80 - 2 , . . . , and  80 -X (collectively referred to as blocks  80 ). In  FIG. 3B , each block  80  includes physical pages  82 - 1 ,  82 - 2 , . . . , and  82 -Q (collectively referred to as pages  82 ). In  FIG. 3C , each page  82  includes memory cells that are associated with a data portion  90  and other memory cells that are associated with an overhead portion  92 . 
     Memory cells of the memory  68  may store data in multiple levels. The levels may correspond to a threshold voltage distribution of the memory  68 , and therefore different pulse-amplitude modulation (PAM) signaling operations may be used to write to the different levels. Alternatively, any known scheme for writing to multilevel memory may be used. For PAM signaling operations, binary data sequences may be grouped into, for instance, 2 bits per group, and each group may then be assigned to a threshold voltage level. The control module may program the memory cells according to the assigned threshold voltage levels. 
     For example, a 4 level cell may store 2 bits/cell. The 4 levels may therefore correspond to logical states 00, 01, 10, 11, where each logical state has at least one corresponding voltage threshold. 4PAM signaling may therefore be used to write to the 4 levels at the corresponding voltage thresholds. Further, an 8 level cell may include 3 bits/cell that correspond to logical states 000, 001, . . . , and 111. 8PAM signaling may write to the 8 different levels. Further information relating to writing to multiple memory levels may be found in United States Patent Publication Number 20070171714 entitled Flash Memory With Coding and Signal Processing, which is incorporated herein by reference in its entirety. 
     Referring now to  FIG. 4 , the write path module  73  is illustrated. The write path module  73  may include an error correcting code (ECC) encoder module  93  that encodes an overhead portion of the memory  68 . The ECC encoder module  93  may include a cyclic redundancy (CRC) module  120  that generates CRC bits based on user data. The ECC encoder module  93  may include other encoding modules. For example, a Reed Solomon encoder module  124  of the ECC encoder module  93  may perform Reed Solomon encoding based on CRC module signals. A Bose-Chaudhuri-Hocquenghem (BCH)/Low Density Parity Check (LDPC) encoder module  128  of the ECC encoder module  93  may perform either BCH or LDPC encoding based on Reed Solomon encoder module signals. Various other encoding modules may also be used. 
     A pilot generator module  135  of the write path module  73  may include a generator module  136  that selectively generates pilot data, as will be described below. The pilot data generator module  135  may also include a format module  137  that may set cell locations in the memory  68  for the pilot data. The format module  137  may set locations for pilot data at a start, middle, and/or end of a physical page and/or a logical page. The format module  137  may also set locations for pilot data according to a known pattern. The pilot data generator module  135  may also include a selector module  139  that selects one of a plurality of sequences of PAM levels for writing to pilot cells. The selector module may randomly select a sequence or may alternate between sequences. 
     A multiplexer module  141  receives and selectively combines the encoded user data from the ECC encoder module  93  and the pilot data. The combination may be based on the aforementioned sequences and PAM operations. The multiplexer module  141  selectively outputs the combined pilot and user data in a data stream to a write module  143  that writes to the memory  68 . 
     For example only, during write operations, the selector module  139  may select from two or more sequences for a first write operation. The two sequences may be referred to as sequence A and sequence B. The sequences may have predefined lengths or alternatively may have lengths that are based on the number of pilot cells that will be written to for a particular page or block of the memory  68 . The selector module  139  alternates between sequences A and B in subsequent write operations. Write operations may include writing to one or more cells of the memory  68 . 
     In other words, for a first write operation, sequence A may be selected to write multiple cells of the memory  68 . For a second write operation, sequence B may be selected to write to multiple cells (that may or may not be the same cells as those written to using sequence A). The selector module  138  may make subsequent selections of sequences based on complete or partial write operations to groups of memory cells. The selector module  138  need not complete sequence A before selecting sequence B. 
     The multiplexer module  141  combines pilot data and user data (encoded data stream) to be written to the memory  68 . The combination may be based on the selected sequence and may fix positions of the pilot data for each logical page. 
     In flash memory, different cell levels may have correspondingly different voltage characteristics. The first and last cell levels may have substantially different voltage characteristics, whereas intermediate levels may have relatively similar voltage characteristics. For example, an 8 level cell may include levels 0-7. The lowest level (level 0) and highest level (level 7) may have unique characteristics while levels 1-6 may have similar characteristics. 
     The selector module  139  may select sequences that instruct write operations to write to as few levels as possible. The selector module  139  may therefore select a sequences that includes writing to level 0 and level 7 and two of the intermediate levels (for example levels 2 and 5). The read path module  75  may interpolate voltage characteristics of the unselected levels (levels 1, 3, 4, 6) based on levels 2 and 5 because levels 1-6 have similar voltage characteristics. The selector module sequences may therefore select 4 levels that may provide write/read-back characteristics of an entire cell or group of cells regardless of the number of possible levels. However, the present disclosure is not limited to 4 levels, and any or all levels may be used. 
     The following paragraph indicates 4 exemplary levels used for 8PAM signaling, 12PAM signaling, and 16PAM signaling (non-normalized) signaling. For 8PAM, the selector module  139  selects a sequence that includes signal levels 0, 1, 4, and 7. For 12PAM, the selector module  139  selects a sequence that includes signal levels 0, 1, 6 and 11. For 16PAM, the selector module  139  selects a sequence that includes signal levels 0, 1, 8 and 15. For 32PAM, the selector module  139  selects a sequence that includes pilot levels 0, 1, 16 and 31. 
     The selector module  139  may select from exemplary sequences A and B. The read path  75  may determine whether sequence A or B is used based on the levels of the first 3 pilot cells that are detected that are written to. 
     For sequence A, level 0 is written to for the first 3 pilot cells, while for Scheme B, the highest level may be written for the first 3 pilot cells. The highest level may be 7 for 8PAM, 11 for 12PAM and 15 for 16PAM, respectively. Subsequent pilot levels of sequence A are used in a cyclical pattern that includes the four levels selected. Subsequent pilot levels of sequence B may be a cyclical shift of the pilot levels of sequence A. For example, for 8PAM signaling, sequence A: 0 0 0 1 4 7 0 1 4 7 0 1 4 7 . . . , and sequence B: 7 7 7 4 1 0 7 4 1 0 7 4 1 0 . . . . Sequences A and B are not required to be cyclically shifted but may merely differ in other ways. Sequences A and B are selected so that the read module can easily distinguish between them. In the above example, the read module may do a majority of the decoding on the first three cells to decide whether the pilot sequence A or B is used. Therefore, the write module does not need to explicitly tell the read module which sequence is used. 
     The format module  137  may set locations for the pilot data that do not depend on the PAM for the memory cells. Alternatively, the multiplexer module  141  may insert pilot data into user data as a function of the PAM of the memory cells. For example, there may be 512 cells in for pilot data per sector of size 33 KB. Thus, for each physical page of 2 KB+64B, 32 cells are allocated for pilot data. In the following, the frequency of pilot cells appearing in the flash memory device is computed for 8PAM, 12PAM, 16PAM and 32PAM. 
     For 8PAM signaling, there may be 5632 cells per physical page. Therefore, every 176 cells may contain one pilot cell. Every 176*3=528 bits may contain 3 pilot bits. For 12PAM signaling, there may be 2414 cell-pairs/physical page. Therefore, every 150 cell-pairs may contain one pair of pilot cells. Every 150*7=1050 bits may contain 7 pilot bits. For 16PAM signaling, there may be 4224 cells per page. Every 132 cells may contain one pilot cell. Every 132*4=528 bits may therefore contain 4 pilot bits. For 32PAM signaling, there may be 3380 cells per page. Every 105 cells may contain one pilot cell. Every 105*5=605 bits may therefore contain 5 pilot bits. 
     Referring now to  FIG. 5 , for 8PAM signaling, the levels the pilot data is written to are listed above. More than one sequence may be used to simulate deterioration of memory cells. The sequences, when implemented by the write module  143 , may include writing to the same level in a number (for example the first three) of pilot cells so that the read path module  75  may recognize which pattern is being used. Based on sequence A, the write module  143  writes to the highest levels of the first 3 cells of sequence A, whereas the write module  143  writes to the lowest levels of the first 3 cells of sequence B. The placement of pilot cells corresponding to 8PAM is depicted in  FIG. 5 . In  FIG. 5 , solid cells denote pilot cells, while the blank cells represent data cells. There are 175 data cells between pilot cells. 
     Referring now to  FIG. 6 , for 12PAM, the situation is slightly different, for example, 2 cells store 7 bits total. Hence, the pilot cells (for example, pilot cells  200 ,  201 ) appear in pairs, in contrast to the 8PAM case. Exemplary 12PAM sequences include: sequence A: 0 0 0 1 6 11 0 1 6 11 0 1 6 11 . . . and sequence B: 11 11 11 6 1 0 11 6 1 0 11 6 1 0 . . . The placement of 12PAM pilot data for sequences A and B are illustrated in  FIG. 6 . Similar notations as in  FIG. 5  are used. 
     Referring now to  FIG. 7 , for 16PAM pilot signal levels and their placement are similar to 8PAM, except that the number of data cells between pilot cells is different. Exemplary 16PAM sequence include: sequence A: 0 0 0 1 8 15 0 1 8 15 0 1 8 15 . . . and sequence B: 15 15 15 8 1 0 15 8 1 0 15 8 1 0 . . . . The pilot cell placement is depicted in  FIG. 7 . The pilot cell placement for 32PAM is similar to 8PAM and 16PAM. 
     A logical page size after encoding varies for different code configurations. Here, a logical page is referring to a block of 2 KB user data along with ECC parity data. To simplify the control logic, the format module  137  may arrange pilot cells in fixed locations relative to the logical page. Increasing the number of locations for pilot data may increase the accuracy of disturbance determinations based on pilot signal processing. Further, all logical pages may include locations for pilot data. All logical and/or all physical pages may include a same number of locations for pilot data. The locations may be the same relative to start and end-points of the pages. 
     Referring now to  FIG. 8 , a read path module  75  is illustrated. The read path module  75  includes a read module  203  that reads user data and pilot data, which also may be referred to as pilot signals, from the memory  68 . A demultiplexer module  204  receives and demultiplexer the user data and pilot signals. A data buffer  208  receives the user data, and a pilot buffer  212  receives the pilot signals. An estimation module  216  receives an output of the pilot buffer  212  and may perform a least means squared (LMS) operation on the pilot data. The estimation module  216  estimates voltage distribution parameters for pilot cells based on the levels that were written to for each cell. A neighbor-finder module  219  that may include a signal processing module  220  receives outputs of the estimation module  216  and the data buffer  208 . The functionality of the neighbor-finder module  219  is to find the signal level(s) that are closest to the received signal sample. The neighbor finder output is then passed to ECC decoder module. 
     An ECC decoder module  221  decodes the read-back signals that were partially encoded by the ECC encoder module  93 . The ECC decoder module  221  may include a log-likelihood ratio (LLR) computation module  224 , a LDPC module  236 , a Gray Code decoder module  228 , and a BCH decoder module  240 . The LLR computation module  224  receives an output of the data buffer  208  and an output of the neighbor-finder module  219 . An output of the data buffer  208  may also be fed directly to the Gray code decoder module  228  via a bypass  230  when the memory module directly outputs binary data. A LDPC decoder module  236  may receive LLR output data from the LLR computation module  224 . A BCH decoder  240  may receive binary outputs from the Gray code decoder module  228 . Finally, the output of LDPC decoder or BCH decoder may further be decoded by a Reed-Solomon decoder (not shown) and subsequently checked by a CRC decoder (not shown). 
     The write and read modules  143 ,  203  may employ column and row select modules (not shown) to select memory cells within the memory  68 . During a write operation, the write module  143  selects write target cells. The write target cells may include any number of memory cells, such as a particular cell, a row of cells, a column of cells, a block of cells, a page of cells, etc., and pilot data associated with the cells. Once the write target cells are selected, the write module  143  generates a write signal. 
     During a read operation, the read module  203  selects read target cells, which may include any number of memory cells, such as a particular cell, a row of cells, a column of cells, a block of cells, a page of cells, etc. Once the read target cells are selected, the read module  203  reads the read target cells. 
     The read path module  75  may read back data from the memory  68  in analog or binary form as an analog or binary signal. If the signal is binary, the demultiplexer module  204  demultiplexes the pilot bits from the input user data, and the user data is directly sent to the Gray code decoder module  228  via the bypass  230 . When the signal is analog, the user data and pilot data are processed with adaptive signal processing algorithms as will be described further below. 
     When the signal is analog, the demultiplexer module  204  separates the pilot data signal from the user data signal and stores them in the pilot buffer  212  and the data buffer  208 , respectively. The estimation module  216  estimates mean and standard deviations of threshold distributions of the levels based on the pilot data. The neighbor-finder module  219  processes the user data in the data buffer  208  using the updated parameters from the estimation module  216 . The received user data and/or pilot data may be received in signals that have corresponding signal constellation features based on a PAM signaling operation. The neighbor-finder module  219  finds signal constellation points that are close to the received signal point (in normalized Euclidean distance sense). 
     If data are LDPC coded, the output from the neighbor-finder module  219  in combination with the original user data are used by the LLR computation module  224  to calculate log-likelihood ratios. Otherwise, the Gray code decoder module  228  translates the output from the neighbor-finder module  219  into coded binary bits for the BCH decoder  240 . 
     The estimation module  216  may determine statistical parameters for disturbances of memory cells. The write path module  73  writes pilot data to the locations, and the estimation module  216  reads the data back and compares the read-back data to the pilot data. Disturbance characteristics may vary for each physical block of the memory  68 . The estimation module  216  may determine parameters for disturbances based on differences of expected to actual pilot signals. The estimation module  216  may include an algorithm, such as the least-mean-squares (LMS) algorithm that determines the parameters and may adapt the algorithm for subsequent read/write operations. 
     Referring now to  FIG. 9 , an exemplary flowchart  300  illustrates the operation of the system. Logic starts in step  302  when an encoder  93  encodes user data that is to be stored in memory  68 . In step  304 , the pilots generator module  135  generates pilot data. In step  306 , the pilots generator module  135  determines which cells in the memory  68  will receive the pilot data. In step  308 , the pilots generator module  135  selects a write sequence for the pilot data. The pilots generator module  135  may variably select between two or more sequences. For example, the pilots generator module  135  may randomly select from two or more sequences or may selectively alternate between two or more sequences. 
     In step  310 , a multiplexer module  141  multiplexes the user data and pilot data. In step  312 , a write module  143  writes to the memory  68  based on the multiplexed data. In step  314 , a read module  203  reads data back from the memory  68 . In step  316 , a demultiplexer module  204  demultiplexer user data from pilot data. In step  318 , user data and pilot data are buffered separately. In step  320 , an estimation module  216  recognizes the selected sequence and estimates parameters of the pilot cells in step  322  based on the sequence and disturbances that the pilot cells experienced. In step  324 , a signal processing module  220  estimates the data stored for cells in memory. In step  326 , a decoder module  221  decodes the data. 
     Referring now to  FIGS. 10A-10G , various exemplary implementations incorporating the teachings of the present disclosure are shown. 
     Referring now to  FIG. 10A , the teachings of the disclosure can be implemented to write and read back pilots to non-volatile memory  412  of a hard disk drive (HDD)  400 . The HDD  400  includes a hard disk assembly (HDA)  401  and an HDD printed circuit board (PCB)  402 . The HDA  401  may include a magnetic medium  403 , such as one or more platters that store data, and a read/write device  404 . The read/write device  404  may be arranged on an actuator arm  405  and may read and write data on the magnetic medium  403 . Additionally, the HDA  401  includes a spindle motor  406  that rotates the magnetic medium  403  and a voice-coil motor (VCM)  407  that actuates the actuator arm  405 . A preamplifier device  408  amplifies signals generated by the read/write device  404  during read operations and provides signals to the read/write device  404  during write operations. 
     The HDD PCB  402  includes a read/write channel module (hereinafter, “read channel”)  409 , a hard disk controller (HDC) module  410 , a buffer  411 , the nonvolatile memory  412 , a processor  413 , and a spindle/VCM driver module  414 . The read channel  409  processes data received from and transmitted to the preamplifier device  408 . The HDC module  410  controls components of the HDA  401  and communicates with an external device (not shown) via an I/O interface  415 . The external device may include a computer, a multimedia device, a mobile computing device, etc. The I/O interface  415  may include wireline and/or wireless communication links. 
     The HDC module  410  may receive data from the HDA  401 , the read channel  409 , the buffer  411 , nonvolatile memory  412 , the processor  413 , the spindle/VCM driver module  414 , and/or the I/O interface  415 . The processor  413  may process the data, including encoding, decoding, filtering, and/or formatting. The processed data may be output to the HDA  401 , the read channel  409 , the buffer  411 , nonvolatile memory  412 , the processor  413 , the spindle/VCM driver module  414 , and/or the I/O interface  415 . 
     The HDC module  410  may use the buffer  411  and/or nonvolatile memory  412  to store data related to the control and operation of the HDD  400 . The buffer  411  may include DRAM, SDRAM, etc. Nonvolatile memory  412  may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The spindle/VCM driver module  414  controls the spindle motor  406  and the VCM  407 . The HDD PCB  402  includes a power supply  416  that provides power to the components of the HDD  400 . 
     Referring now to  FIG. 10B , the teachings of the disclosure can be implemented to write and read back pilots to nonvolatile memory  423  of a DVD drive  418  or of a CD drive (not shown). The DVD drive  418  includes a DVD PCB  419  and a DVD assembly (DVDA)  420 . The DVD PCB  419  includes a DVD control module  421 , a buffer  422 , the nonvolatile memory  423 , a processor  424 , a spindle/FM (feed motor) driver module  425 , an analog front-end module  426 , a write strategy module  427 , and a DSP module  428 . 
     The DVD control module  421  controls components of the DVDA  420  and communicates with an external device (not shown) via an I/O interface  429 . The external device may include a computer, a multimedia device, a mobile computing device, etc. The I/O interface  429  may include wireline and/or wireless communication links. 
     The DVD control module  421  may receive data from the buffer  422 , nonvolatile memory  423 , the processor  424 , the spindle/FM driver module  425 , the analog front-end module  426 , the write strategy module  427 , the DSP module  428 , and/or the I/O interface  429 . The processor  424  may process the data, including encoding, decoding, filtering, and/or formatting. The DSP module  428  performs signal processing, such as video and/or audio coding/decoding. The processed data may be output to the buffer  422 , nonvolatile memory  423 , the processor  424 , the spindle/FM driver module  425 , the analog front-end module  426 , the write strategy module  427 , the DSP module  428 , and/or the I/O interface  429 . 
     The DVD control module  421  may use the buffer  422  and/or nonvolatile memory  423  to store data related to the control and operation of the DVD drive  418 . The buffer  422  may include DRAM, SDRAM, etc. Nonvolatile memory  423  may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The DVD PCB  419  includes a power supply  430  that provides power to the components of the DVD drive  418 . 
     The DVDA  420  may include a preamplifier device  431 , a laser driver  432 , and an optical device  433 , which may be an optical read/write (ORW) device or an optical read-only (OR) device. A spindle motor  434  rotates an optical storage medium  435 , and a feed motor  436  actuates the optical device  433  relative to the optical storage medium  435 . 
     When reading data from the optical storage medium  435 , the laser driver provides a read power to the optical device  433 . The optical device  433  detects data from the optical storage medium  435 , and transmits the data to the preamplifier device  431 . The analog front-end module  426  receives data from the preamplifier device  431  and performs such functions as filtering and A/D conversion. To write to the optical storage medium  435 , the write strategy module  427  transmits power level and timing data to the laser driver  432 . The laser driver  432  controls the optical device  433  to write data to the optical storage medium  435 . 
     Referring now to  FIG. 10C , the teachings of the disclosure can be implemented to write and read back pilots to memory  441  of a high definition television (HDTV)  437 . The HDTV  437  includes an HDTV control module  438 , a display  439 , a power supply  440 , the memory  441 , a storage device  442 , a network interface  443 , and an external interface  445 . If the network interface  443  includes a wireless local area network interface, an antenna (not shown) may be included. 
     The HDTV  437  can receive input signals from the network interface  443  and/or the external interface  445 , which can send and receive data via cable, broadband Internet, and/or satellite. The HDTV control module  438  may process the input signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. The output signals may be communicated to one or more of the display  439 , memory  441 , the storage device  442 , the network interface  443 , and the external interface  445 . 
     Memory  441  may include random access memory (RAM) and/or nonvolatile memory. Nonvolatile memory may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The storage device  442  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The HDTV control module  438  communicates externally via the network interface  443  and/or the external interface  445 . The power supply  440  provides power to the components of the HDTV  437 . 
     Referring now to  FIG. 10D , the teachings of the disclosure may be implemented to write and read back pilots to memory  449  of a vehicle  446 . The vehicle  446  may include a vehicle control system  447 , a power supply  448 , the memory  449 , a storage device  450 , and a network interface  452 . If the network interface  452  includes a wireless local area network interface, an antenna (not shown) may be included. The vehicle control system  447  may be a powertrain control system, a body control system, an entertainment control system, an anti-lock braking system (ABS), a navigation system, a telematics system, a lane departure system, an adaptive cruise control system, etc. 
     The vehicle control system  447  may communicate with one or more sensors  454  and generate one or more output signals  456 . The sensors  454  may include temperature sensors, acceleration sensors, pressure sensors, rotational sensors, airflow sensors, etc. The output signals  456  may control engine operating parameters, transmission operating parameters, suspension parameters, etc. 
     The power supply  448  provides power to the components of the vehicle  446 . The vehicle control system  447  may store data in memory  449  and/or the storage device  450 . Memory  449  may include random access memory (RAM) and/or nonvolatile memory. Nonvolatile memory may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The storage device  450  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The vehicle control system  447  may communicate externally using the network interface  452 . 
     Referring now to  FIG. 10E , the teachings of the disclosure can be implemented to write and read back pilots to memory  464  of a cellular phone  458 . The cellular phone  458  includes a phone control module  460 , a power supply  462 , the memory  464 , a storage device  466 , and a cellular network interface  467 . The cellular phone  458  may include a network interface  468 , a microphone  470 , an audio output  472  such as a speaker and/or output jack, a display  474 , and a user input device  476  such as a keypad and/or pointing device. If the network interface  468  includes a wireless local area network interface, an antenna (not shown) may be included. 
     The phone control module  460  may receive input signals from the cellular network interface  467 , the network interface  468 , the microphone  470 , and/or the user input device  476 . The phone control module  460  may process signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. The output signals may be communicated to one or more of memory  464 , the storage device  466 , the cellular network interface  467 , the network interface  468 , and the audio output  472 . 
     Memory  464  may include random access memory (RAM) and/or nonvolatile memory. Nonvolatile memory may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The storage device  466  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The power supply  462  provides power to the components of the cellular phone  458 . 
     Referring now to  FIG. 10F , the teachings of the disclosure can be implemented to write and read back pilots to memory  483  of a set top box  478 . The set top box  478  includes a set top control module  480 , a display  481 , a power supply  482 , the memory  483 , a storage device  484 , and a network interface  485 . If the network interface  485  includes a wireless local area network interface, an antenna (not shown) may be included. 
     The set top control module  480  may receive input signals from the network interface  485  and an external interface  487 , which can send and receive data via cable, broadband Internet, and/or satellite. The set top control module  480  may process signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. The output signals may include audio and/or video signals in standard and/or high definition formats. The output signals may be communicated to the network interface  485  and/or to the display  481 . The display  481  may include a television, a projector, and/or a monitor. 
     The power supply  482  provides power to the components of the set top box  478 . Memory  483  may include random access memory (RAM) and/or nonvolatile memory. Nonvolatile memory may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The storage device  484  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). 
     Referring now to  FIG. 10G , the teachings of the disclosure can be implemented to write and read back pilots to memory  492  of a mobile device  489 . The mobile device  489  may include a mobile device control module  490 , a power supply  491 , the memory  492 , a storage device  493 , a network interface  494 , and an external interface  499 . If the network interface  494  includes a wireless local area network interface, an antenna (not shown) may be included. 
     The mobile device control module  490  may receive input signals from the network interface  494  and/or the external interface  499 . The external interface  499  may include USB, infrared, and/or Ethernet. The input signals may include compressed audio and/or video, and may be compliant with the MP3 format. Additionally, the mobile device control module  490  may receive input from a user input  496  such as a keypad, touchpad, or individual buttons. The mobile device control module  490  may process input signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. 
     The mobile device control module  490  may output audio signals to an audio output  497  and video signals to a display  498 . The audio output  497  may include a speaker and/or an output jack. The display  498  may present a graphical user interface, which may include menus, icons, etc. The power supply  491  provides power to the components of the mobile device  489 . Memory  492  may include random access memory (RAM) and/or nonvolatile memory. 
     Nonvolatile memory may include any suitable type of semiconductor or solid-state memory, such as flash memory (including NAND and NOR flash memory), phase change memory, magnetic RAM, and multi-state memory, in which each memory cell has more than two states. The storage device  493  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The mobile device may include a personal digital assistant, a media player, a laptop computer, a gaming console, or other mobile computing device. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.