Patent Publication Number: US-8116133-B2

Title: Maintenance operations for multi-level data storage cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 11/694,739 to Michael J. Cornwell and Christopher P. Dudte, entitled “Maintenance Operations for Multi-Level Data Storage Cells,” filed Mar. 30, 2007, and to U.S. Provisional Patent Application Ser. No. 60/800,357, filed on May 15, 2006, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     Various implementations may relate generally to non-volatile memory devices, and particular implementations may relate to systems and methods for operating multi-level flash cells. 
     BACKGROUND 
     As computing devices have increased in capabilities and features, demand for data storage devices has grown. Data storage devices have been used, for example, to store program instructions (i.e., code) that may be executed by processors. Data storage devices have also been used to store other types of data, including audio, image, and/or text information, for example. Recently, systems with data storage devices capable of storing substantial data content (e.g., songs, music videos, etc.) have become widely available in portable devices. 
     Such portable devices include data storage devices (DSDs) that have small form factors and are capable of operating from portable power sources, such as batteries. Some DSDs in portable devices may provide non-volatile memory that is capable of retaining data when disconnected from the power source. Portable devices have used various non-volatile data storage devices, such as hard disc drives, EEPROM (electrically erasable programmable read only memory), and flash memory. 
     Flash memory has become a widely used type of DSD. Flash memory may provide a non-volatile memory in portable electronic devices and consumer applications, for example. Two types of flash memory are NOR flash and NAND flash. NOR flash typically provides the capacity to execute code in place, and is randomly accessible (i.e., like a RAM). NAND flash can typically erase data more quickly, access data in bursts (e.g., 512 byte chunks), and may provide more lifetime erase cycles than comparable NOR flash. NAND flash may generally provide non-volatile storage at a low cost per bit as a high-density file storage medium for consumer devices, such as digital cameras and MP3 players, for example. 
     Typical flash memory stores a unit of information by storing an electrical charge in each memory cell at a voltage representative of a digital data value. Single level cells store one bit of information based on the cell being charged to a “high” voltage, or being discharged to a “low” voltage. NAND flash memory has been developed that stores up to two bits of information in a single cell by decoding the charge as being within one of four different voltage ranges. NOR flash memory has been developed that can store up to 8 bits of information in a single cell by decoding the charge as being within one of 256 different voltage ranges. 
     SUMMARY 
     Described apparatus and associated systems, methods and computer program products relate to multi-level data storage in flash memory devices. 
     In one general aspect, multi-level flash memory device that include multiple memory cells, are managed by detecting a voltage level from a first memory cell that stores a charge to a voltage level representing a data value. The data value represented by the voltage level is determined based at least partially on a resolution register entry corresponding to the first memory cell. Charge is applied to one or more of the memory cells to a target voltage representing the data value. The target voltage is determined based at least partially on a resolution register entry corresponding to the one or more memory cells. 
     Implementations may include one or more of the following features. The one or more memory cells is the first memory cell, and the operation of applying charge to the first memory cell includes applying additional charge to the first memory cell to adjust for voltage sag in the first memory cell. An amount of sag in the first memory cell is determined by detecting a reference voltage level stored in a reference cell. The reference cell is associated with a corresponding predetermined voltage level, and the amount of sag is determined by comparing the predetermined voltage level with the detected voltage level stored in the reference cell. The amount of sag is determined by detecting a stored voltage levels in multiple reference cells. An amount of additional charge applied to the first memory cell is determined based upon a correction function. 
     A signal is received from a host device to initiate a maintenance operation, and charge is applied to one or more of the memory cells to a target voltage representing the data value in response to the received signal. The signal from the host device indicates a power supply condition. The signal from the host device indicates whether the host device is supplied with AC power and/or whether a battery of the host device is charged to a predetermined charge level. The signal from the host device indicates a scheduled maintenance operation. 
     A signal is received from a flash memory processor to initiate a maintenance operation, and charge is applied to one or more of the memory cells to a target voltage representing the data value in response to the received signal. The signal from the flash memory processor indicates that the flash memory processor has sufficient bandwidth to perform a maintenance operation. The signal from the flash memory processor indicates that the flash memory processor is idle. 
     The resolution register entry corresponding to the first memory cell indicates a first resolution that corresponds to a first number of possible data values. A signal to write at a second resolution corresponding to a second number of possible data values is received, and a resolution register entry corresponding to the one or more memory cells is updated to indicate the second resolution. The target voltage is based upon the second resolution. The first number of possible data values is greater than the second number of possible data values, and the one or more memory cells includes more than one memory cell selected from the plurality of memory cells. Alternatively, the first number of possible data values is equal to the second number of possible data values, and the one or more memory cells includes a different memory cell than the first memory cell. The second number of possible data values exceeds the first number of possible data values. The first number of possible data values does not exceed 4 bits; for example, the first number of possible data values is 2 bits or 1 bit. The second number of possible data values is at least 4 bits; for example, the second number of possible data values is at least 8 bits. The data value stored in the first memory cell at the first resolution is a result of a writing operation in which data received from a host device is written to the flash memory device. 
     A signal is received from a host device to write data received from a host device to the flash memory device. The data value received from the host device is written to the first memory cell at a first resolution corresponding to a first number of possible data values, and the first resolution is recorded in the resolution register corresponding to the first memory cell. A signal to write at a second resolution corresponding to a second number of possible data values is received, and the second number of possible data values exceeds the first number of data values. A resolution register corresponding to the one or more memory cells is updated to indicate the second resolution, and the target voltage is based upon the second resolution. A signal is received from the host device indicating a power condition of the host device. 
     The resolution register entry corresponding to the first memory cell indicates a first resolution corresponding to a first number of possible data values. The resolution register entry associated with the first memory cell is updated to indicate a second resolution corresponding to a second number of possible data values, and the first number of possible data values is greater than the second number of possible data values. A second data value is written to the first memory cell at the second resolution. Updating the resolution register entry associated with the first memory cell and writing to the first memory cell at the second resolution are triggered by an error condition associated with a page of memory cells that includes the first memory cell. The page of memory cells is paired with a second page of downgraded memory cells. A logical addressing software code is updated to treat the pair of pages of memory cells as a single page of memory cells at the first resolution. 
     In another general aspect, a determination is made whether to perform a maintenance operation. Error information associated with a page of memory cells is identified in response to determining that a maintenance operation should be performed, and a determination if the error information meets an error criteria is made. One or more resolution registers corresponding to the page of memory cells are adjusted from a first resolution to a second resolution. The first and the second resolutions each define multiple voltage ranges, and each voltage range corresponds to a possible data value. The first resolution has more voltage ranges than the second resolution. 
     Implementations may include one or more of the following features. The determination of whether to perform the maintenance operation involves determining if a host device has a power supply that meets a predetermined condition, such as that the host device is receiving AC power or the host device has a battery charged to a predetermined charge level. The determination of whether to perform the maintenance operation involves determining if a processor has unused bandwidth exceeding a predetermined threshold. 
     In another general aspect, adjustments for sag in stored data values can be made by applying charge to multiple memory cells. Each memory cell is charged to a target voltage corresponding to a data value. The memory cells include a reference cell, which is charged to a predetermined voltage. A voltage level in the reference cell is detected, and voltage levels from a group of memory cells are detected. Additional charge is applied to the memory cells based upon the difference between the detected voltage level in the reference cell and the predetermined voltage. 
     Implementations may include one or more of the following features. A voltage level is detected in a second reference cell. The application of additional charge to the memory cells is further based upon the detected voltage level in the second reference cell and a predetermined second reference cell voltage. The memory cells are NAND flash memory cells or NOR flash memory cells. Each data value includes more than 4 bits. 
     In yet another general aspect, data is stored in multiple memory cells. Each memory cell is adapted to receive charge during a write operation to a voltage level corresponding to a data value having a specified number of bits. A resolution register associated with the memory cells includes entries each of which indicates a number of bits stored in one or more corresponding memory cells. A host interface is adapted to receive signals from a host device indicating a power supply condition for the host device. A processor is adapted to rewrite data values to the memory cells and to adjust the resolution registers from indicating a first number of bits to indicating a second number of bits in response to a signal indicating a predetermined power supply condition. 
     Implementations may include one or more of the following features. A logical addressing software code translates logical addresses received from the host device into physical addresses for use in accessing data. The host interface is further adapted to receive commands from a host device and for exchanging data with the host device. 
     In another general aspect, a determination whether to perform a maintenance operation is made. A maintenance log associated with multiple flash memory cells is read. A maintenance activity recorded on the maintenance log is performed. The maintenance activity can include one or more of the following: rewriting data values originally stored at a lower resolution at a higher resolution; reducing resolutions for groups of memory cells associated with a predetermined error condition; rewriting data values that exceed a predetermined error threshold at an identical resolution as the original data values; swapping most frequently accessed data with least frequently accessed data; or refreshing data values by applying additional charge to memory cells to correct for voltage sag. 
     Implementations may include one or more of the following features. The determination of whether to perform the maintenance operation includes determining if a host device has a power supply that meets a predetermined condition, such as the host device receiving AC power or the host device having a battery charged to a predetermined charge level. The determination of whether to perform the maintenance operation involves determining if an internal processor has unused bandwidth exceeding a predetermined threshold. Logical addressing software codes for data values rewritten to different physical memory cells during one or more of the maintenance activities are updated. 
     Some implementations may provide one or more advantages. For example, some implementations may provide high performance data storage functions. Storage density and/or capacity may be increased. Some examples may provide improved reliability and/or reduced data error rates. Various implementations may permit increased levels of integration, miniaturization, reduced electromagnetic noise and/or improved noise margins. Some implementations may realize lower system cost in auxiliary systems, such as voltage supplies to logic and/or programming/erase circuits. 
     The details of one or more implementations of the invention are set forth in the accompanying drawings and the description below. Other features of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of an architecture of a multi-chip package that includes a NAND flash memory die and a flash disk controller. 
         FIGS. 2A-2B  collectively show a mapping between a cell voltages and digital values stored in a memory cell. 
         FIGS. 3A-3B  show flow charts that illustrate examples of processes for reading pages of data from a multi-level cell flash memory. 
         FIG. 4  shows a flow chart that illustrates an example of a process for reading a page of data from flash memory. 
         FIG. 5  shows a flow chart that illustrates an example of a process for performing error correction operations to correct a page of data containing bit errors. 
         FIGS. 6A-6C  collectively show examples of operations for executing an alternative value command. 
         FIGS. 7A-7B  show flow charts that illustrates an examples of processes for writing data to a flash memory page. 
         FIGS. 8A-8B  show flow charts illustrating examples of processes for adjusting cell resolution of a memory page. 
         FIG. 9  shows a flow chart illustrating an example of a maintenance process. 
         FIG. 10  shows a flow chart illustrating an example of a process of logical addressing in the flash disk controller. 
         FIG. 11  shows an example of a system that includes a charge pump and an analog to digital converter external to the NAND flash memory die. 
         FIG. 12  shows an example of a system that includes decoupled power input at a NAND flash memory die. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EXAMPLES 
     Various implementations relate to flash memory capable of storing information in deep multi-level cells (MLCs). Deep multi-level cells may encode at least several bits of data according to a cell voltage. Some implementations relate to architectures for implementing systems that include deep MLC flash memory. Some implementations relate to techniques for performing data storage operations with deep MLC flash memory. 
       FIG. 1  shows an example of a multi-chip package (MCP)  100  that provides data storage for a host device (not shown). The MCP  100  includes a NAND flash memory die  103  for storing data and a flash disk controller (FDC)  106  that facilitates access to the flash memory in response to read and/or write commands from the host. In some implementations, the NAND flash memory die  103  stores data in deep MLCs. For example, cells in the flash memory die  103  may hold 3, 4, 5, 6, 7, 8, 9, 10 or more bits of information. The MCP  100  may provide data storage in various portable devices, such as digital cameras, other image storing devices, portable audio devices, personal digital assistants (PDA), and digital video recorders, for example. Some implementations may also be used in other applications, examples of which may include desktop computers, servers, wireless routers, or embedded applications (e.g., automotive), particularly in situations where quick access to data is desirable. In general, apparatus and techniques according to examples described herein may be implemented to increase flash memory density and/or to realize high performance and/or reliable non-volatile data storage operations. 
     As an illustrative example, the MCP  100  may store a data file by storing a byte (i.e., 8 bits) of information in each cell in a group of cells (e.g., a page or a block) in a flash memory. Some other examples may have resolutions such as 16-bit, 32-bit, 64-bit, or more. In some implementations, resolution may be determined by single or multiple electron detection on a gate of a cell. In other implementations, any practical number of bits of information may be encoded in a voltage to which an individual flash memory cell is charged. 
     The FDC  106  includes a host interface  109 , a processor  112 , and a flash interface  115 . The FDC  106  receives commands and/or data (e.g., software code updates or user data) from and/or transmits data to a host device, such as a processor on a desktop computer, server, or portable computing device, via the host interface  109 . Communication with the host may use custom or standard protocols, such as Advanced Technology Attachment (ATA), Serial ATA (SATA), Block Abstracted NAND, Secure Digital (SD), or Multi-Media Card (MMC), for example. In some implementations, the MCP  100  may be part of the same product as the host device. In other implementations, the host device may be in operative communication with the MCP  100  through a communication link (e.g., USB, Firewire, Bluetooth) to at least one other processor-based device. For example, a host may access the MCP  100  remotely by sending control messages and sending and receiving data messages over at least one network, which may include wired, wireless, or fiber optic links, or a combination of these. Such networks may support packet-based communications, and may include a local network or a wide area network, such as the Internet. 
     The processor on the host device may read data from and/or write data to the NAND flash memory die  103  using a logical addressing scheme that is processed by the FDC  106  to identify physical addresses in the flash memory. In some implementations, the host interface  109  may be configured to communicate with the host device using an ATA/IDE interface. The processor  112  may process the received command and use the flash interface  115  to access the NAND flash memory die  103 . The FDC  106  may be configured to provide functionalities, such as wear management, block management, error correction, and logical addressing management functionalities, to improve performance of the NAND flash memory die  103 , such as increasing reliability, decreasing read and write time, improving power efficiency, and increasing capacity per chip volume. Certain techniques and apparatus described herein may be applicable to NAND and/or NOR flash memory, to other types of electrically erasable or electrically writable memory, or to memory in which data access resolution is in pages or blocks. 
     Although only one NAND flash memory die  103  is shown in  FIG. 1 , the MCP  100  may include more than one NAND flash memory die  103 . Some implementations may include any combination of non-volatile memories, which may include NAND flash, NOR flash, or electrically erasable programmable read only memory (EEPROM). In some illustrative examples, the MCP  100  can include two, three, four, or at least eight NAND flash memory dies  103 . For example, the MCP  100  may include a flash disk controller  106  on a die that is packaged with (e.g., in a stack) four NAND flash memory dies  103 . 
     In some implementations, the flash disk controller  106  and the flash memory die  103  may be implemented on a single die. In other implementations, one or more of the components in the flash disk controller  106  may be implemented in part or entirely external to a single die or the MCP  100 . For example, some or all of the synchronous dynamic random access memory (SDRAM)  151  and/or the non-volatile memory (NVM)  154  may be implemented external to the MCP  100 . In some implementations, some or all of the flash disk controller  106  may be packaged separately from the flash memory die  103 . In an illustrative example, the NVM  154 , the SDRAM  151 , the host interface  109 , and at least a portion of the processor  112  may each be implemented externally to the MCP  100 . In other implementations, the analog and/or digital signals between the flash interface  115  and the flash memory die  103  may be externally routed to an integrated package. 
     Remote or distributed transmission structures (e.g., shielded and/or controlled impedance signal paths) may be implemented to transport signals to and/or from at least one flash memory die  103 . In some implementations, memory expansion may be provided by installing additional packages of non-volatile memory. Buffering and/or routing devices may be used to support distribution of analog and/or digital signals to a variable number of memory dies  103 . Furthermore, functions of the processor  112  may be performed external to the MCP  100 . In various examples, the processor  112  may be implemented, in whole or in part, in a circuit on the same substrate (e.g., printed circuit board) or in the same product as the MCP  100 . The processor  112  may be implemented from another computing device in operative communication with the MCP  100  through a communication link (e.g., wired, wireless, fiber optic, or a combination of any of these). 
     The MCP  100  may have any practical memory size, such as up to at least 100 gigabytes or more. In the depicted example, the NAND flash memory die  103  is organized to include a number of flash memory blocks  118 . In some implementations, the NAND flash memory die  103  may include hundreds or thousands of flash memory blocks  118 . Each flash memory block  118  includes a number of flash memory pages  121 . As shown, each flash memory page  121  includes cells that may store data  124  and cells that may store error correction codes (ECCs)  127  associated with the data. As an example, the flash memory page  121  may store 2048 bytes of data and 64 bytes of ECC data. The data cells  124  store information received from the flash disk controller  106 . The ECC cells  127  store additional integrity meta-data (e.g., ECC data) that is associated with the data stored in the data cells  124 . In various implementations, the ECC data allows the flash disk controller  106  to detect and/or correct bit errors in the data. 
     In the illustrated example, each flash memory block  118  also includes one or more reference cells  130   a ,  130   b ,  130   c . In some implementations, the FDC  106  may monitor the voltage in the reference cells  130   a ,  130   b ,  130   c  to estimate the degree of voltage sag or drift, in the cells  124 ,  127 . In each block  118 , the reference cell  130   a  may be located at the beginning of the block  118 , and the reference cell  130   b  may be located at the end of the block  118 . Each flash memory page  121  may include the reference cell  130   c . In some implementations, a greater or lesser number of reference cells may be distributed in any pattern across the pages, blocks, and dies of the memory  103  to determine the likely performance of the cells  124 ,  127 . 
     In some implementations, reference cells may be located in or around cells that experience read/write usage levels that are representative of the usage level of certain data cells of interest. Compensation methods may be based on comparing non-reference cells to other non-reference cells. For example, if voltages in a number of cells in the same page or block are relatively low, then compensation may include adjusting thresholds (e.g., voltage thresholds between different value levels in a cell) downward according to the measured values so that read errors may be substantially reduced. Other examples include determining a correction function based upon the detected voltages in reference cells, the correction function adjusting the detected voltage prior to converting the detected voltage into the digital data value represented by the memory cell. 
     In some implementations, memory cells may be refreshed by applying additional charge to a plurality of cells to correct for detected voltage sag. For example, if the voltage level of one or more reference cells indicate more than some threshold amount of voltage drift, then the memory cells in the page(s) or block(s) associated with the reference cell(s) may be either adjusted by applying additional charge or rewritten to restored the cells to appropriate voltage levels according to the stored data. Such adjustments can be performed immediately upon detecting the voltage drift in the reference cell (s) or as part of a later maintenance operation. In some implementations, additional charge may be applied or the memory cells may be rewritten based on the difference between the detected reference cell voltage(s) and the target reference cell voltage(s), which may be assumed to indicate the approximate amount of voltage drift or sag for both the reference cell(s) and the corresponding data cells. 
     In other implementations, applying additional charge or rewriting of the memory cells may be performed by reading all of the cells, performing any necessary adjustments to thresholds (e.g., using a correction function based on the reference cell voltages and/or using other techniques described herein), and performing error correction on the detected data to obtain the stored data. Thereafter, the data can be used to determine appropriate voltage levels or how much additional charge is needed for the various memory cells to correct for the identified voltage drift or sag. In some implementations, the amount of additional charge applied may be determined based on a correction function that is the same or similar to the correction function used to adjust detected voltages prior to converting the detected voltages into digital data values. 
     In some implementations, cells in the flash memory may be adaptively re-assigned. For example, reference cells may be added, removed, relocated, and/or redistributed as needed in response to read or write usage information, temperature, product age, supply voltage (e.g., low battery, AC-line powered), and/or detected error levels. If errors in certain blocks or pages of memory are low, then fewer cells may be assigned as ECC cells  127  and/or reference cells  130 , which allows for more data cells  124 . The relative assignments of cells to reference, data, and ECC functions, as well as the resolution of individual cells, may be dynamically adjusted based on current operating conditions, and/or according to predetermined conditions. For example, the resolution may be adjusted based on error rates, the number of ECC cells per page may be based on error rates and read and write history information, and the location and distribution of reference cells may be based on error rate and product age. This example merely illustrates that the controller  106  and the flash memory die may be dynamically adjusted according to various criteria. Other criteria may include criticality of the data, power source availability (e.g., AC line power, battery power), and defined criteria about the relative importance of maximizing memory size, speed performance, and data integrity. For example, maintaining a high cell resolution that requires a substantial number of software corrections may result in longer access times. The criteria may be tailored by the user, product manufacturer, or software, according to the needs of the application. 
     In some implementations, data that requires a substantial number of software corrections may be rewritten in a maintenance operation to correct for variations in charge associated with the passage of time or to correct for pages of memory cells that have begun to degrade. Typically, when changing the resolution of one or more memory cells, the data will be written to a different page of memory cells, and may be written at the same or a different resolution. In some implementations, the original page of memory cells will be downgraded to a lower resolution, which will often be required as the memory cells age and degrade. When rewriting of data is performed as a result of identified voltage drift or sag, it is possible to write the data to the same or a different page or block of memory cells. 
     The flash interface  115  provides direct control, handshaking, and data transfer access to the flash memory die  103 . The flash interface  115  includes a control interface  133  and an analog interface  136 . In some implementations, the control interface  133  may send control, address, and data signals to the flash memory die  103 . The commands and the memory addresses may be transmitted in digital signals or analog signals. The flash disk controller  106  can also receive analog signals from the flash memory die  103 . The flash disk controller  106  may include a processor for interfacing with flash memory logic on the flash memory die  103 , and this processor for interfacing with the flash memory logic on the flash die may be integrated into the flash interface  115 . 
     In response to a read command, the flash memory die  103  may output cell voltages representing data stored in individual data cells  124 . The flash disk controller  106  can receive the analog voltage signals output from each memory cell on the flash memory die  103 . These analog cell voltages or analog voltage signals may be transmitted to the analog interface  136  in the FDC  106 . In some implementations, the flash interface  115  may also include a data bus separate from the control interface  133  and analog interface  136  for communicating with the flash memory die  103 . 
     The analog interface  136  may include an analog front end (analog FE)  139  and an analog-to-digital converter (ADC)  142 . Upon receiving the analog signals, the analog FE  139  may condition the signals as needed, for example, to provide offset, corrective level shift, gain, buffering, filtering, or controlled impedance to minimize reflections. The analog FE may provide a high impedance input to minimize loading of the flash memory cell, and a low impedance output to drive a sample and hold or track and hold circuit that is coupled to an input of the ADC  142 . In some implementations, the analog FE  139  may further include an analog multiplexer (not shown) to select one of a number of analog output lines from one or more flash memory dies. 
     The ADC  142  processes the analog value to determine a corresponding digital data value representation of the voltage in the data cells  124 ,  127 . The ADC  142  receives the conditioned analog signal and convert the analog signal into a digital representation of the analog voltage. The ADC  142  (or a processor in the ADC) then converts the digital representation into a digital data value represented by the voltage stored on the memory cell based on, for example, a mapping function. The processor  112  could also be used to convert the digital representation into a digital data value. The digital representation of the analog voltage may include enough information to allow the ADC  142  or a processor to distinguish among a plurality of analog voltage levels each representing a particular digital data value. The digital representation may comprise a greater number of bits of data than the digital data value. In some implementations, the ADC  142  may be integrated into the flash memory die  103  rather than being included in the flash disk controller  106 . In such a case, the flash interface  115  may receive digital representations of cell voltages or digital data values from the flash memory die  103 . 
     An example of a mapping function  145  is shown. Based on the mapping function  145 , the ADC  142  or the processor  112  may convert an analog cell voltage into digital representation and/or a digital data value. For example, there may be a series of analog voltage thresholds that can be used to map an analog voltage to a digital representation and/or digital data value. Likewise, the mapping function  145  may also illustrate the conversion of a digital representation of the analog voltage into a digital data value. For example, one or more digital representations of the analog voltage may map to a particular digital data value, with each digital data value having a corresponding distinct set of one or more digital representations. 
     In some implementations, the ADC  142  or the processor  112  may receive parameters that change the mapping function  145 . For example, the FDC  106  may adapt the mapping function  145  based on current temperature, supply voltage, number of reads and write of the page data, and/or the voltage in the reference cells  130   a ,  130   b , and/or  130   c . In some implementations, adaptations to the mapping function may be based on voltage characteristics of neighboring data cells  124 , ECC cells  127 , and/or other cells. The mapping  145  between cell voltages and digital data values is described in further detail with reference to  FIGS. 2A-2B . In some implementations, the ADC  142  or a processor may also operate responsive to an alternative value command to retrieve alternative values for the received analog signals or digital representations of the analog signals. Example implementations of the alternative value command are described in further detail with reference to  FIGS. 6A-6C . 
     The flash disk controller  106  also includes an ECC engine  148 . In various implementations, the ECC engine  148  may perform hardware and/or software error checking and correction using ECC cells  127 . In some implementations, the ECC engine  148  may provide state machine-based data recovery. For example, the ECC engine  148  may detect the number of error bits in a page of data. Then, the ECC engine  148  may determine which ECC algorithm is used. As an example, the ECC engine  148  may be configured to first attempt a hardware ECC algorithm using, for example, Hamming or Reed-Solomon codes. If the hardware ECC algorithm is unsuccessful in recovering the page of data, then a software ECC correction may be attempted. An example method illustrating use of hardware ECC, software ECC, and other techniques in combination is described with reference to  FIG. 5 . In some implementations, the ECC engine  148  may provide error correction for up to at least about 10% or more of the size of a page of data. In some examples, a processor may determine which ECC algorithm to use. 
     In some implementations, the processor  112  will rewrite or refresh the data stored in a flash memory page if an ECC algorithm is used to recover data that includes more than some predetermined number or percentage of errors. In other implementations, the processor  112  will record the location, physical and/or logical, of data that included such errors in a maintenance log. The processor  112  will then rewrite or refresh that data during a maintenance operation (See  FIG. 9 ). Maintenance operations may be performed when the host device is operating under a predetermined power condition, when the processor  112  has a predetermined amount of excess bandwidth, and/or at scheduled intervals. 
     The flash disk controller (FDC)  106  may include dynamic random access memory (DRAM). The flash disk controller  106  of this example also includes a synchronous dynamic random access memory (SDRAM)  151 . For example, the SDRAM  151  may be a single data rate SDRAM or a double data rate SDRAM. In some implementations, the FDC  106  may use the SDRAM  151  as a high speed and high density buffer for storing temporary data such as output data for the host device and alternative digital values for a page of data, for example. FDC  106  may also include other types of RAM, such as DRAM. As an example, the FDC  106  may receive analog data from the NAND flash memory die  103 . 
     The FDC  106  may then convert detected analog voltages into digital data, including, in some cases, alternative digital data values for one or more of the cells. Then the ECC engine  148  checks and corrects the digital data, possibly checking multiple different combinations of data values and alternative data values for the cells on each flash memory page  121 . If the error correction is successful, then the processor  112  may store the digital data into a host output buffer in the SDRAM  151 . In some implementations, the host device may retrieve data from the host output buffer. Alternatively, the flash disk controller  106  may forward data from the host output buffer to the host device. The SDRAM  151 , or other cache memory, may further be used to store data to be written to the flash memory die  103 . 
     The FDC  106  also includes a non-volatile memory (NVM)  154 . In this example, the NVM  154  includes wear management software code  157 , block management software code  160 , logical addressing software code  163 , and cell resolution registers  166 , each of which contain instructions (or pointers to instructions in the flash memory) that, when executed by the processor  112 , perform certain operations. In some implementations, the NVM  154  may be separate from the NAND flash memory die  103 . For example, the NVM  154  may be a NOR flash memory or another NAND flash memory. In other implementations, the NVM  154  may be one or more pages in the NAND flash memory die  103 . In other implementations, the NVM  154  may store pointers or memory locations to the data stored in the NAND flash memory die  103 . In some implementations, the processor  112  may execute the wear management software code  157 , the block management software code  160 , and the logical addressing software code  163  to improve efficiency, performance, and/or reliability of the MCP  100 . 
     The processor  112  may use the wear management software code  157  to manage the wear of pages  121 , blocks  118 , or die  103  in the MCP  100 . For example, the wear management software code  157  may include instructions that, when executed by the processor  112 , perform operations that include load balancing operations to swap the data in the most frequently used memory page to a less used memory page. The swapping operations may also include an updating of the logical addressing software code  163 . 
     The wear management software code  157  may be activated during a maintenance operation. In some implementations, the physical and/or logical addresses of each read operation is recorded in a maintenance log. Each write operation may also be recorded in a maintenance log. The wear management software code  157  may then use predetermined threshold values for determining how to rearrange stored data among the pages of memory cells. These threshold values, for example, might include 100 or 1000 reads of the page of memory cells during the course of a week or month. In other implementations, the threshold values might be based upon a percentage of the total number of read operations, or based upon deviation from the average number of reads per page per time. An example of a maintenance operation is depicted in  FIG. 9 . 
     The block management software code  160  may include code for managing bad blocks in the flash memory die  103 . For example, the block management software code  160  may include historical error information about the flash memory blocks  118 . In some implementations, the error information may be used to maintain the cell resolution in each of the flash memory pages. An example of the block management software code is described in further detail with reference to  FIGS. 8A and 8B . 
     The block management software code  160 , possibly in conjunction with the logical addressing software code  163  and/or the cell resolution registers  166 , may also be used to pair sets of bad blocks or bad pages having reduced resolutions (updated in the cell resolution registers  166 ) in the flash memory die  103  and have the set of bad blocks or bad pages be treated for logical addressing purposes (perhaps updated in the logical addressing software code  163  and/or the cell resolution registers  166 ) as equivalent to a single block or single page of memory cells having the initial higher resolution. The block management software code  157  may be activated during a maintenance operation. An example of a maintenance operation is depicted in  FIG. 9 . 
     The logical addressing software code  163  may include code to convert a logical address in a host command to physical addresses in the NAND flash memory die  103 . In some examples, a logical page may be associated with multiple physical memory pages in the NAND flash memory die  103 . The logical addressing software code  163  manages the conversion and update of the logical address table in the NVM  154 . In an example, the logical addressing software code  163  may dynamically maintain links between logical block addresses from the host and physical page addresses as the pages are downgraded from 10 bit resolution to 8 bit resolution, for instance, or as the mapping of logical block addresses to different physical page addresses are changed for purposes of wear management. Intermediate forms of addresses may be generated in the process of converting between logical and physical addresses, for example. Intermediate address forms may be generated, processed, stored, used, and/or otherwise manipulated to perform various non-volatile memory operations. An example of the logical addressing software code is described in further detail with reference to  FIG. 10 . 
     The cell resolution registers  166  store information about cell resolution in each flash memory page  121 . For example, the NAND flash memory die  103  may be an 8-bit MLC flash memory. In some implementations, some of the flash memory block  118  may be downgraded or up-graded in response to various conditions. Illustrative examples of such conditions include error performance, temperature, voltage conditions, number of read or write cycles of individual cells, groups of cells, pages, cells in a neighboring location, reference cells, cells with comparable read and/or write usage history, or other factors, such as age of the device. Information about some or all of these conditions may be stored in a data storage device, or determined or estimated from one or more other bits of stored information. In one example, stored information may include historical read and write usage data that represents usage levels for at least some of the cells in the memory die  103 . The processor  112  may update the cell resolution registers  166  to reduce a cell resolution of a down-graded memory page to, for example, 4-bit, so that the flash memory page  121  may still be usable with a smaller memory size. In other implementations, the cell resolution registers  166  may also store the cell resolution for each flash memory block  118 . 
     In some implementations, the cell resolution registers  166  are downwardly adjusted to a single bit resolution or another low number bit resolution prior to transferring data from a host device to memory cells in the MCP  100 . This process is depicted in further detail in  FIG. 7B . Lowering of the cell resolution registers  166  prior to transferring data may allow for faster data transfer rates because less precision is needed in charging each memory cell. The transferred data may subsequently be rewritten to memory cells at a higher resolution. In some implementations, the transferred data may be rewritten at a higher resolution during a maintenance operation (e.g., during a later time when sufficient processing resources are available and the rewriting does not interfere with other reading or writing operations). In some implementations, a record of the low cell resolution data transfer is made in a maintenance log. 
     In some implementations, the logical addressing software code  163 , the resolution registers  166 , and/or the block management software code  160  will group down-graded memory pages (or down-graded memory blocks) together and treat the group for logical addressing purposes as a single non-down-graded memory page (or block). The memory pages of the group of down-graded memory pages do not need to be adjacent memory pages. The group of down-graded memory pages can include memory pages from different blocks and even from different memory dies. In some implementations, each down-graded memory page or block in a group of down-graded memory pages or blocks is down-graded in response to an error condition associated with the page or block. 
       FIGS. 2A-2B  collectively show mappings between cell voltages and digital data values stored in the memory cell. As shown in  FIG. 2A , an illustrative digital data value distribution  200  of an 8-bit memory cell is shown. An 8-bit memory cell would include 256 possible digital data values; a 4-bit memory cell would include 16 possible data values. The number of possible data values is equal to 2 n  (where n equals the number of bits), but the number of possible digital data values need not correspond to an n-bit number of possible digital data values. Each memory cell could have any integer number of possible digital data values greater than 1, for example, some memory cells could have 10 possible data values. The digital value distribution  200  includes digital value distribution curves  205 - 210  that represent the voltage distribution for each digital data value. Each digital value distribution curve (e.g.,  205 - 210 ) represents a range of digital voltage values corresponding to voltage levels associated with each possible digital data value. 
     During a write operation, each memory cell receives a charge to an analog voltage corresponding to a digital data value selected from one of the possible digital data values. This corresponding voltage typically falls within the distribution curves  205 - 210  for the desired digital data value. This corresponding voltage could also be a target voltage corresponding to the digital data value. For example, if a cell voltage lies within the distribution  207 , then the digital value stored in the cell may be 02 H . During a read operation, an analog voltage signal is detected from each cell. The ADC  136  then converts the analog voltage signal into a digital representation of the analog voltage signal. This digital representation is then compared with at least one digital value distribution curve to determine the digital data value represented by the analog voltage stored in the read memory cell. 
     The digital data value distribution  200  includes grey areas  215  between the digital data value distribution curves  205 - 210 . In some implementations, when the ADC  142  receives a cell voltage or detects an analog voltage signal that lies within one of the grey areas  215 , the ADC  142  may, for example, convert the cell voltage to the nearest adjacent digital data value. For example, if the ADC  142  receives a cell voltage substantially near a voltage level  220 , then the ADC  142  may resolve to the nearest adjacent digital data value, namely FE H . In some implementations, the FDC  106  may also include an alternative value command that instructs the ADC  142  to resolve to an alternative value other than the nearest adjacent value based on some parameters. 
     In some implementations, the FDC  106  may use both the nearest adjacent digital data value and one or more alternative values in an error correction process that attempts to resolve a page or block of data values. Furthermore, the FDC  106  may assign an uncertainty to particular cell voltages or corresponding data values based on the location of the cell voltage within the digital data value distribution curves  205 - 210  or the grey areas  215 . The assigned uncertainty may be used by an algorithm that attempts to resolve a page or block of data values. Some examples of these parameters may include one or more of temperature, number of reads to the cell, number of writes to the cell, supply voltage, and voltage in the reference cells  130   a ,  130   b ,  130   c . In some examples, the cell voltage may drop below a minimum cell voltage (Vmin). The FDC  106  may implement a correction by adding an offset to the received cell voltage. This offset may be added by either the analog FE  139  or added digitally by either the ADC  142  or the processor  112 . 
     In some implementations, the FDC  106  may dynamically adjust locations and the widths of the grey areas  215  by altering the digital data value distribution  200 . For example, the FDC  106  may include maintenance software code that adjusts the grey areas  215  based on parameters such as one or more reference cell voltages, the usage of the memory cell, and other heuristics that may be preloaded in the NVM  154 . The maintenance software code may also perform updating of the cell resolution registers  166 . For example, each die  103 , analog interface  135 , and/or MCP  100  may be characterized at manufacturing time and a linearization table, correction factors, or other corrective adjustment may be stored in non-volatile memory in the MCP  100 . In some cases, the maximum and minimum voltage levels (Vmax and Vmin) as well as the digital value distribution curves  205 - 210  may be adjusted and/or redistributed based on empirical testing of the cells during their lifetime of use. 
     As shown in  FIG. 2B , a cell voltage to digital value graph  250  is shown. The graph  250  includes an ideal voltage characteristic  255  that the ADC  142  uses to convert analog voltages to digital values. In some examples, the data cell  124  may store digital values according to non-ideal voltage characteristics  260 ,  265  due to, for example, the heuristics of temperature, age of the cell, charge pump or supply voltage tolerances, non-linearity of the ADC  136 , detected errors in the memory cell, and/or the number of reads and writes of the cell. The FDC  106  may compensate in various ways for the voltage characteristics  260 ,  265  to be closer to the ideal characteristics  255 . Example compensation methods are described with reference to  FIGS. 3-6 . 
       FIGS. 3A and 3B  show flow charts that illustrate examples of processes  350  and  300  for reading a page of data from a NAND flash memory. The processes  350  and  300  include operations that may be performed generally by the processor  112 . In some implementations, the processes  350  and  300  may also be performed, supplemented, or augmented by other processing and/or control elements that may be incorporated with the ADC  142 . For example, there may be a controller or compensator in the analog interface  136  that performs some or all of the operations in the processes  350  and  300 . 
       FIG. 3A  depicts a process of converting detected voltage levels from multi-level memory cells into digital data values. The process  350  begins with detecting an analog voltage level from a multi-level memory cell (step  355 ). This voltage may be detected by the analog interface  136 , for example. The analog interface  136  may include an input operable to receive analog signals from a flash memory die  103 . The flash disk controller  106  may further include a control module to select memory cells from which the input receives analog signals. In step  360 , the analog voltage signal is converted into a digital representation of the detected analog voltage. This conversion may be performed by the ADC  142 . The digital representation may have sufficient data to allow for the ADC  142  or the processor  112  to distinguish the level of the analog voltage stored by a memory cell among a plurality of possible voltage levels representing a digital data value. This may be accomplished by having a digital representation comprising more bits of data than the digital data value represented by the voltage stored on the memory cell. 
       FIG. 2A  helps illustrate this concept. The range of possible analog cell voltages may be segregated into multiple segments (e.g., such as represented by voltage level  220 ) that each correspond to a digital representation of the analog cell voltage. Each digital value distribution curve  205 - 210  and each grey area  215  may include multiple such segments, allowing for the use of digital representations having a higher resolution than the digital value distribution curves  205 - 210 , which can provide additional information relating to, for example, where a cell voltage lies within a digital value distribution curve  205 - 210  or a grey area  215 . 
     In step  365 , the digital representation is converted into a digital data value based upon a digital data value distribution. The digital data value distribution may be stored in the cell resolution registers  166  and may be the digital data value distribution  200  shown in  FIG. 2A . In step  335 , a processor or controller determines whether there are more memory cells to read. If so, then the process returns to step  355 . Otherwise, process  350  ends. 
       FIG. 3B  depicts in greater detail a process of storing identified digital data values and marking the location of uncertain digital data values. The process  300  begins when, for example, the processor  112  receives a command to retrieve a page of data from the NAND flash memory die  103 . In step  305 , the processor  112  retrieves cell resolution information for a page from the cell resolution registers  166 . Then, in step  310 , the processor  112  receives from the ADC  142  a digital output value for a data cell. The digital output value for the data cell is a digital representation of the voltage detected from the data cell. The ADC  142  determines the received digital data value based on stored thresholds in step  310 . In some implementations, the processor  112  may use information in the cell resolution registers  166  to determine which set of thresholds are used. These thresholds may relate to the digital value distribution curves  205 - 210  discussed above in regard to  FIG. 2A . For example, the processor  112  may use one set of thresholds for an 8-bit cell and another set of thresholds for a 2-bit cell. In some cases, the processor  112  may use one set of thresholds for one 8-bit cell and another set of thresholds for a different 8-bit cell. Each set of thresholds may correspond to a possible digital data value distribution and may constitute ranges of digital representations of analog voltages that correspond to possible digital data values. 
     In step  320 , the processor  112  determines whether the digital data values for a received analog voltage values are uncertain. In some implementations, the processor  112  may determine that a digital data value is uncertain if the cell voltage lies in a grey zone  215  of the digital value distribution  200  or if the cell voltage is near the boundary between a digital value distribution curve  205 - 210  and a grey zone  215 . In some implementations, different levels of uncertainty can be assigned depending on where the cell voltage falls within the digital value distribution  200  (e.g., higher voltages may tend to have greater uncertainty and/or uncertainty may be higher for cell voltages that are closer to the middle of a grey zone  215 ). In step  320 , if the processor  112  determines that the received digital values are not uncertain, then the processor  112  stores the received digital value in a host output buffer in step  325 . If the processor  112  determines that the received digital value is uncertain in step  320 , then the processor  112  may mark the location of the uncertain digital value in a mask table in step  330 , and then executes step  325 . In some implementations, one or more alternative values may also be stored for subsequent use in resolving which value (e.g., the uncertain value or one of the alternative values) is correct. 
     After the processor  112  stores the received digital value, the processor  112  determines, in step  335 , whether there are more cells to read. For example, the processor  112  may check whether the end of the memory page is reached. If there are more cells to read, then the process returns to step  310 . If there are no more cells to read, the process  300  ends. In some implementations, the process will also record the number of uncertain data values associated with a page or block of memory cells in a maintenance log. In other implementations, the process will record the location, physical and/or logical, of a page and/or block of memory cells if the number of uncertain data values exceeds a predetermined threshold. 
       FIG. 4  shows a flow chart that illustrates an example of a process  400  for reading a page of data from an MLC flash memory, such as the NAND flash memory die  103 , using a correction function to adjust mapping of the cell voltages to the digital values. The process  400  may be performed by the processor  112 , for example. The process  400  begins in step  405  when the processor  112  determines whether a read command is received. For example, the FDC  106  may receive a read command from the host device through the host interface  109 . If, in step  405 , the processor  112  determines that no read command is received, then step  405  is repeated. 
     If the processor  112  determines that a read command is received in step  405 , then the processor  112  updates a correction function in step  410  based on temperature, number of reads or writes in the memory page, supplied voltage, and/or other operating conditions of the NAND flash memory die  103 . In some implementations, the ADC  142  or the analog interface  136  may use the correction function to adjust measured cell voltages at the analog front end  139  before the cell voltages are converted into digital values. In other implementations, the processor  112  may use the correction function to adjust the thresholds in the mapping function, so the ADC  142  may convert analog voltage into adjusted digital values. The correction function can be different for different cells. For example, memory cells having higher detected voltages can have a greater adjustment due to the correction function. 
     Next, the processor  112  selects a reference cell in step  415 . For example, the processor  112  may select one of the reference cells  130   a ,  130   b , or  130   c . Then, the processor  112  reads, in step  420 , a reference voltage stored in the selected reference cell. In step  425 , the processor  112  updates the correction function based on the reference voltage. For example, if a reference voltage appears to be sagging by ten percent, then the processor  112  may adjust the correction function to compensate the sag voltage in the data. In some implementations, the correction function will non-linearly adjust detected voltage levels. The correction function may adjust higher detected voltages levels more than lower detected voltage levels. The correction function may adjust detected voltages at different voltage levels by different adjustment amounts or by different adjustment percentages. 
     In some implementations, thresholds may be dynamically adjusted on the fly during operation. In some implementations, the processor  112  may store a fixed number of previous samples, such as one hundred samples, of previously read reference voltages and use a moving average of the stored reference voltages to update the correction function. The correction function may also be updated based on other functions, which may involve mean, median, mode, or weighted averaging, for example. For example, a weighted moving average may be used. The processor  112  then, in step  430 , determines whether to select another reference cell. As an example, the processor  112  may determine whether there is enough information to adjust the correction function. As another example, the processor  112  may be configured to read all the reference cells in some memory blocks as well as in some memory pages based on the read command. 
     If, in step  430 , the processor  112  determines that there is another reference cell to be read, then the process  400  returns to step  415 . In some implementations, the process of adjusting the correction function by reading the voltage in reference cells is triggered by detected errors in data retrieved from a group of memory cells. In other implementations, detected errors will result in a shift of thresholds for determining a data value associated with a detected voltage. These thresholds in some implementations may be automatically shifted down, but in other implementations the thresholds are adjusted based upon the voltage in one or more reference cells. The error may be detected by the use of ECC  127  associated with the group of memory cells. 
     If the processor  112  determines in step  430  that there is no other reference cell to be read, then the processor  112 , in step  435 , selects a page to read based on the read command. Then, in step  440 , the processor  112  reads the selected page of data from flash memory using, for example, the process  300  ( FIG. 3B ). In step  445 , the processor  112  corrects the page data using the correction function. For example, the processor  112  may set some parameters in the analog interface  136  to adjust the mapping function. As another example, the processor  112  may adjust the digital representation, output from the ADC  142 , using the correction function. Next, the processor  112  can perform error checking operations to check if there is any error in the page in step  450 . In some implementations, the error checking operations may be done in the ECC engine  148  using hardware error detection circuits. In other implementations, the error checking operations may be done in software, where the processor  112  may execute an error detection code stored in the NVM  154  to check for errors in the page. After the error checking operations, in step  455 , the processor  112  can determine if any error is detected. 
     If there is no error detected, then the processor  112  may, in step  460 , transmit the read data to the host device. Then the processor  112  may determine whether there is another page to read in step  465 . If there are more pages to read, then the step  435  is repeated. Otherwise, the process  400  ends. If there are one or more errors detected in step  455 , then, in step  470 , the processor  112  may perform error correction operations, an example of which is described with reference to  FIG. 5 . Then the processor  112  may, in step  475 , determine whether the error correcting operation is successful. If the error correcting operation is successful, then the step  460  is repeated. If the error correcting operation is not successful, then the processor  112  may store error information (e.g., an error log) in the NVM  154  in step  480  and the process may continue at step  465 . The error information may also be stored in a maintenance log. The stored error information may be used for block management operations, for which an example is described with reference to  FIG. 8 . In some implementations, the processor  112  will record the variation in sag between reference cells in a page or block of memory cells in a maintenance log in NVM  154 . In other implementations, the processor  112  will only record the location, physical and/or logical, of a page and/or block of memory cells in a maintenance log if the degree of sag in reference cells meets a predetermined condition. For example, if the sag in the reference cell exceeds 10% or if the difference between the degree of sag in different reference cells exceeds 10%, the data stored in the page and/or block of memory cells may be refreshed by applying additional charge to the memory cells or by completely rewriting the page during a maintenance operation. An example of a maintenance operation is described with reference to  FIG. 9 . 
       FIG. 5  shows a flow chart that illustrates an example of a process  500  for performing error correction operations to correct a page of data containing bit errors. The process  500  begins when, for example, the processor  112  detects bit errors in a page of data read from the flash memory and sends a command to the ECC engine  148  to perform a hardware ECC algorithm to correct the bit errors in step  505 . In some implementations, the ECC engine  148  and the ADC  142 , and/or the analog interface  136  may cooperate to correct the bit errors. 
     Next, the ECC engine  148  may check, in step  510 , whether the hardware ECC algorithm is successful. If the hardware ECC algorithm is able to correct all the errors in the page of data, then the hardware ECC algorithm is successful. Then, in step  515 , the ECC engine  148  stores the ECC result in, for example, the SDRAM  151 . Next, the ECC engine  148  generates a message to indicate “Error correction successful” in step  518  and the process  500  ends. 
     If the number of existing error bits exceed the number of error bits that the hardware ECC algorithm can correct, then the ECC engine  148  sends a message to the analog interface  136  to re-read, in step  520 , the page of data from the flash memory. Next, in step  525 , the ECC engine  148  performs a hardware ECC algorithm again. In step  530 , the ECC engine  148  checks whether the hardware ECC algorithm is successful. If hardware ECC algorithm can correct, then the hardware ECC algorithm is successful, and the process continues with step  515 . 
     In step  530 , if the ECC engine  148  determines that the number of existing error bits exceed the number of error bits that the hardware ECC algorithm can correct, then the ECC engine  148  executes an alternative value command to correct the bit errors. Example implementations of the alternative value command are described with reference to  FIGS. 6A-6C . Then, the ECC engine  148  may check whether the alternative value command corrects the bit errors in step  535 . If the ECC engine  148  determines that the bit errors are corrected, then the process continues with step  515 . 
     If the ECC engine  148  determines that the bit errors are not corrected, then the ECC engine  148  may perform an extended software ECC algorithm in step  540  to recover the page of data. For example, the extended software ECC algorithm may include deeper ECC algorithms that use more ECC bits. For example, the hardware ECC algorithm may require four ECC bits and the extended software ECC algorithm may use 128 ECC bits. Then, the ECC engine  148  may check whether the extended software ECC algorithm is successful in step  550 . If the ECC engine  148  determines that the extended software ECC algorithm is successful, then the process continues with step  515 . If, in step  550 , the extended software ECC algorithm is not successful, then the ECC engine  148  generates, in step  555 , a message: “Error correction unsuccessful” and the process  500  ends. 
       FIG. 6A  shows a flow chart that illustrates an example of a process  600  for generating and using alternative data values. The processor  112 , the ECC engine  148 , the flash interface  115 , or other combinations of the above and other elements may perform the operations in the process  600 . At step  605 , the processor  112  retrieves information from a mask table to identify uncertain digital data values in a data page (see, e.g.,  FIG. 3 , step  330 ) and, in some cases, to retrieve information regarding a degree of uncertainty. 
     Then the processor  112  may, in step  610 , retrieve correction data based on parameters (e.g., temperature, number of reads from the data page, number of writes to the data page, information in the cell resolution registers  166 , supply voltage, charge pump voltage, the reference voltage in the data page, etc.). For example, the processor  112  may compute a correction function to determine the correction data for the data page. In addition or as an alternative, the processor  112  uses the correction data to determine alternative digital values for each uncertain data value in step  615 . The alternative digital values for each uncertain data value will often include the nearest adjacent digital value and the next nearest adjacent digital value. It might also include the digital data values two digital data values away from the digital representation of the detected analog voltage of the memory cell. Typically, not every memory cell will have an uncertain data value. In step  620 , the processor  112  stores the identified alternative digital values in a buffer along with stored digital data values for memory cells having certain digital data values. 
     After the alternative digital values are stored, the processor  112  selects, in step  625 , a combination of alternative digital values from the buffer. The combination of alternative digital values may itself be selected based on an algorithm that, for example, attempts to identify those alternative digital values more likely to be correct. This selection algorithm may use data relating to a degree of uncertainty associated with each digital data value. Moreover, regardless of whether such a selection algorithm is used, the selected combination of alternative digital values need not include all of the possible alternative digital values. In other words, even among the data values identified as being uncertain, some of the original data values may be used along with some subset of alternative data values. 
     Next, the processor  112  stores the page data in a buffer using the selected combination of alternative digital values in step  630  along with the digital data values determined with adequate certainty. Then, the processor  112  performs ECC algorithm on the stored page data in step  635 . For example, the processor  112  may perform the operations as described in the process  500 . In some cases, the execution of an ECC algorithm may result in changes to one or more of the alternative digital values and even to one or more of the digital data values determined with some presumption of certainty. In step  640 , the processor  112  determines whether the ECC algorithm is successful. If the processor  112  determines that the ECC algorithm is successful, then, in step  645 , the processor  112  stores the page data with the result of the successful ECC and the process  600  ends. 
     In step  640 , if the processor  112  determines that the ECC algorithm is not successful, then, in step  650 , the processor  112  determines whether another combination of alternative values is available to try. The number of possible combinations of alternative values will depend upon the number of memory cells with uncertain digital data values and the number of identified alternative digital values. Typically, most of the memory cells will not have uncertain digital data values. If the processor  112  determines another combination of alternative values is available to try, then the process returns to step  625 . 
     If, in step  650 , the processor  112  determines that all alternative combinations have been tried, then the processor  112  generates an error message in step  655  and the process  600  ends. In some implementations, it may also be possible to generate additional alternative values and/or to adjust voltage thresholds for reading the various data values and to retry performing the ECC algorithm to identify correct values for the page data. For example, alternative values may be identified for voltage levels that were previously determined to represent a particular value with adequate certainty but that are relatively near a threshold for one of the digital value distribution curves  205 - 210  (discussed above in regard to  FIG. 2A ). Alternatively, the voltage thresholds for the various digital value distribution curves  205 - 210  can be adjusted as discussed above, and the data values can be regenerated, including identifying new alternative values. 
     In some implementations, the error message in step  655  is recorded in a maintenance log in NVM  154 . Then, during a maintenance operation, such as that shown in  FIG. 9 , it may also be possible to generate additional alternative values and/or to adjust voltage thresholds for reading the various data values and to retry performing the ECC algorithm to identify correct values for the page data. The identified correct values then may be used to rewrite the data. 
       FIG. 6B  shows a flow chart that illustrates another example of a process  660  for generating and using alternative values. The process  660  has some steps in common with the process  600 . In this example, after identifying uncertain data values through use of a mask table or otherwise at step  605 , the processor  112  determines, in step  665 , an alternative value for each uncertain value using the nearest adjacent digital value. For example, the processor  112  may use the digital value distribution  200  ( FIG. 2A ) and select a second nearest adjacent digital value instead of the nearest adjacent digital value to the cell voltage. Then, the processor  112  continues the process  660  by performing operations described in connection with  FIG. 6A  beginning with step  620 . 
       FIG. 6C  shows a flow chart that illustrates another example of a process  670  for generating and using alternative values. In this example, the processor  112  does not necessarily retrieve uncertain digital value information from the mask table. The process  670  begins in step  672  when the processor  112  receives a command to perform an alternative value identification and analysis on a selected page (see, e.g.,  FIG. 5 , step  535 ). 
     The processor  112  initiates, in step  674 , reading of the selected page. In step  676 , the processor  112  selects a cell in the page to read a cell voltage. In step  678 , the processor  112  determines whether the cell voltage is uncertain. For example, the processor  112  may use the digital value distribution  200  as shown in  FIG. 2A  to determine whether the received cell voltage is in one of the grey areas  220 . If the processor  112  determines that the cell voltage is in the grey area, then the processor  112  determines a digital data value of the cell using a second closest digital data value in step  680 . In other implementations, the processor  112  determines a digital data value of the cell using the first closest digital data value. Next, the processor  112  stores, in step  682 , the digital data values in a buffer. 
     If, in step  678 , the processor  112  determines that the cell voltage is not in the grey area, then the processor  112  determines a digital data value of the cell based on stored thresholds in step  684  and the processor  112  performs the step  682 . After the step  682 , in step  686 , the processor  112  determines whether to read another cell in the page. If the processor  112  determines to read another cell, then the process returns to step  676 . If the processor  112  determines that there are no further cells to read, then the process  670  ends. 
       FIG. 7A  shows a flow chart that illustrates an example of a process  700  for writing data to the flash memory page  121  using the reference cells  130   a ,  130   b ,  130   c . The process  700  may be generally performed by the processor  112 . The process  700  begins in step  705  when the processor  112  receives a write command. For example, the write command may include a write instruction, data to be written, and a memory address that the data is going to be written to, which may be received, for example, as a logical block address from the host. Then, based on the write command, the processor  112  selects a memory page in the flash memory in step  710 . 
     Next, the processor  112  may copy, in step  715 , the data to be written to a buffer, such as the SDRAM  151 . The data may either be transferred from an external host device or from another memory page. In some implementations, the data stored on the selected memory page is copied into the buffer for recopying back into the selected page. In other implementations, the data to be written to a selected memory page is not copied into the buffer, but rather written directly from the data source (either from an external host device or from other memory cells) to the selected memory page. 
     Then, the processor  112  erases, in step  725 , any data stored in the selected page. In step  730 , the processor  112  writes the data from the buffer to the selected memory page by, for example, applying charges to the data cells  124  and the reference cell  130   c . Step  730  applies different amounts of charge to the memory cells depending on the desired data value and corresponding analog voltage level for each cell. In some implementations, a charge pump may be used to apply charges to memory cells in the selected memory page. Then, the processor  112  reads a reference voltage in the reference cell  130   c  of the selected page in step  735 . The reference voltage is read by detecting a voltage level in the reference cell  130   c . The processor  112  checks, in step  740 , whether the reference voltage is less than a target voltage. If the processor  112  determines that the reference voltage is less than the target voltage, then the process returns to step  730  to apply additional charge and increase the voltage stored in the cells in the selected memory page. The amount of applied additional charge may be scaled depending upon how the desired voltage level compares percentage-wise to the voltage of the reference cell(s) (e.g., if the detected reference cell voltage is 10% lower than targeted and a particular memory cell should have a voltage level double that of the reference cell, then the amount of additional charge applied to the particular memory cell may be twice that applied to the reference cell). 
     In step  740 , if the processor  112  determines that the reference voltage is not less than the target voltage, then the processor  112 , in step  745 , selects a data cell and reads voltage of the selected data cell in step  750 . Then, in step  755 , the processor  112  determines whether the read voltage is too high. For example, the processor  112  may compare the read cell voltage to the digital value distribution and check whether the cell voltage lies within a voltage range of the targeted digital value. If the processor  112  determines that the voltage is not too high, then the processor determines, in step  760 , whether to select another data cell. If the processor  112  determines that it is not necessary to select another data cell, then the process  700  ends. Otherwise, the process  700  returns to step  745  to test an additional data cell. 
     In some implementations, it may also be possible to test the data cells selected at step  745  to determine if they are too low. If so, the process  700  may return to step  730  to apply additional charge to one or more of the data cells. In some implementations, once the testing of one or more reference cells at step  740  is complete, the voltage level of all data cells may be selected at step  745  (or in iterative repetitions of step  745 ) to determine if the levels are too high and/or too low. In this manner, the reference cells may be used to perform an initial charging of the page or block, followed by testing and possible tweaking of voltage levels in the cells. Furthermore, in some implementations, the target voltages for reference cells used at step  740  may be set a little lower than the threshold voltage for a desired data value to attempt to avoid overcharging, followed by checking actual data cell values and tweaking the voltage levels to reach voltage levels corresponding to the desired data values for the actual data cell 
     If, in step  755 , the processor  112  determines that the voltage is too high, then the processor  112  determines whether it is necessary to rewrite the selected page. For example, the processor  112  may compare the number of bit errors to a threshold that is less than or equal to the number of correctable errors using one of the correction algorithms described with reference to  FIG. 5 . If the number of bit errors is greater than the threshold, then the selected page is re-written. Otherwise, the processor  112  may determine that a rewrite of the selected page is not required. In step  765 , if the processor  112  determines that rewrite of the page is not required, then the process  700  continues with step  760 . If the processor  112  determines that rewrite of the page is required in step  765 , then the process  700  returns to step  725  to reinitiate writing of the memory page. In some implementations, the target voltage may be decreased incrementally after step  765  to reduce the likelihood of overshooting the target voltage. 
       FIG. 7B  shows a flow chart illustrating an example of a process  770  that achieves a higher data transfer rate between a host device and the MCP  100 . The process  770  begins in step  772  when the processor  112  receives a write command from a host device. For example, the write command from a host device may include a write instruction, data to be written, and a memory address that the data is going to be written to, which may be received, for example, as a logical block address from the host. 
     Next, the processor  112  determines whether to proceed to a quick write process (e.g., writing at a single level cell resolution or other relatively low resolution) or to use a more time, power, and processor intensive writing process of writing at a higher resolution. In step  774 , the processor  112  determines if there is a command from a host interface to perform a quick write. In some implementations, a host device may also specify the resolution of the quick write. If a host device does not specify a quick write, then the processor  112  may independently determine if a whether a quick write is warranted. Step  776  then determines if the MCP  100  or the host device connected to the MCP  100  meets a predetermined power supply conditions. In the shown implementation, step  776  determines whether the host device is supplied with AC power. In other implementations, step  776  instead determines if a battery supplying power to the host device is charged to a predetermined charge. In some implementations, step  776  will determine whether a battery supplying charge to the host device is charged to maximum capacity or at least 90% of capacity. Step  778  then determines whether the processor  112  has excess bandwidth meeting a predetermined bandwidth condition. In some implementations, step  778  is satisfied if the processor  112  is otherwise idle. In other implementations, step  778  is satisfied if a predetermined percentage of bandwidth of the processor  112  be unused. If both  776  and  778  are satisfied, then the process uses process  700  to write to memory cells at a high resolution, step  780 . If one or both of conditions  776  and  778  are unsatisfied, then the process proceeds to a quick write procedure. 
     In a quick write procedure, the processor  112  in step  782 , selects one or more available memory cell pages to write data from the host device to. In some implementations, the processor may copy data from the host device into a buffer, such as the SDRAM  151 . In other implementations, data from the host device is not copied into the buffer, but rather written directly to the selected memory page in step  786  after steps  725  and  784 . In step  725 , the processor erases any data stored in the selected page(s). In step  774 , the processor  112  updates any cell resolution registers associated with the selected memory page(s) to a low resolution. In some implementations, the low resolution will be a resolution of one bit per memory cell. In other implementations, the low resolution will be 2, 3, or 4 bits per cell. Writing at a lower resolution when copying data from a host device to the MCP  100  increases the data transfer rate because less precision is needed when charging each memory cell and, thus, the degree of care and the amount of voltage adjustments needed when writing to the memory cells can be reduced. After writing the data to the memory cell(s) at a low resolution, step  784  will record a maintenance log entry indicating that the data stored in the selected memory page should be rewritten at a higher resolution during a maintenance process (process  900 ). 
       FIG. 8A  shows a flow chart illustrating an example of a process  800  for adjusting a cell resolution of a memory page. The process  800  may perform the operations in the process  800  when, for example, the processor  112  executes a maintenance program to update the cell resolution registers  166 . The process  800  begins in step  805  when the processor  112  reads stored error information in step  805 . The error information may be stored during read errors or write errors, for example, as described at step  480  in  FIG. 4 . Next, the processor  112  selects a page in step  810 . In step  815 , the processor  112  determines whether the error count of the selected page is greater than a threshold. If the error count of the selected page is not greater than the threshold, then the processor  112  checks whether there are more pages to check in step  820 . If the processor  112  determines that there are no more pages to check, then the process  800  ends. If, in step  820 , the processor  112  determines that there are more pages to check, then the process returns to step  810 . In some implementations, the processor  112  may check all the memory pages with errors. In other implementations, the processor  112  may only check memory pages with new errors recorded in the error information. 
     In step  815 , if the error count of the selected page is greater than the threshold, then the processor  112  copies a page of data from the selected page into a buffer in step  825 . Next, the processor  112  updates the cell resolution registers  166  to reduce the cell resolution of the selected page. For example, the flash interface  115  may check the cell resolution registers  166  to find that the cell resolution is reduced and the flash interface  115  may then read and write to the selected page using the new reduced cell resolution. 
     Then, the processor  112  can assign physical addresses for the copied data in step  835 . Depending on available memory pages, the processor  112  may assign one, two, four, or other number of physical memory pages to store the copied data. Next, the processor  112  updates in step  840  a logical address table to correspond to the assigned physical addresses. The logical address table may be used to map a logical page to one or more physical pages. An example use of the logical address table during a memory access operation is described with reference to  FIG. 10 . In step  845 , the processor  112  moves the copied data from the buffer to the pages at the assigned physical addresses. Next, the processor  112  determines whether there are more pages to check instep  820 . If so, the process  800  returns to step  810 . Otherwise, the process  800  ends. 
       FIG. 8B  depicts a similar process to that in  FIG. 8A  that is focused on downgrading groups of pages or blocks of memory cells and logically treating the group as a single page or block of memory cells having the original resolution. In  FIG. 8B  the process  860  also reads stored error information  805 , selects a page  810 , and determines whether an error count associated with the page is in excess of a threshold  815 . If the error count associated with the page is in excess of a threshold, then the data stored in the page is copied to a buffer  825  and the cell resolution register(s) associated with the page are updated to reduce the resolution of the page  830 . In the implementation depicted in  FIG. 8B , however, a processor also selects another page of data having a reduced cell resolution  855  and updates the block management code and/or the logical addressing code to pair the two pages together. The two pages having reduced cell resolution are then logically treated as a single page with the higher original resolution. This process may group together more than two pages of memory cells. 
     In some implementations, this process will downgrade entire blocks of memory cells and pair or otherwise associate them. In some implementations, each paired page will have the same downwardly adjusted cell resolution and include the same number of memory cells. For example, a page of memory cells downgraded from each memory cell storing 8 bits of data to each memory cell storing 4 bits of data is grouped with another page of memory cells with each memory cell storing 4 bits of data. The combination of these two pages of memory cells is then logically treated by the flash disk controller as a single page (or as a single block) storing 8 bits of data per memory cell. These paired pages of memory cells need not be on the same block and could possibly be on different flash memory dies. The process  860  next performs step  820  of determining whether there are more memory pages to check and proceeds in the same manner as described in  FIG. 8A . 
       FIG. 9  is a flow chart that illustrates a maintenance process  900 . One possible function for maintenance process  900  is for rewriting, at a relatively high resolution, data stored in a flash memory at a relatively low resolution (e.g., see  FIG. 7B ). The maintenance process can be used, for example, to maximize the battery life of a host device while also maximizing data storage capacity. In some implementations, maintenance process  900  is triggered by the processor as part of a routinely scheduled maintenance operation. In some implementations, maintenance process  900  is triggered by a signal from a host device signaling that the host device is supplied with AC power. In other implementations, other conditions may cause a host device or the processor  112  to trigger the maintenance process  900 , such as, an idle processor  112 . 
     Process  900  begins with step  905 , which may determine whether the MCP  100  is operating under a predetermined power condition. In some implementations, this power condition is met by a host device receiving AC power. In some implementations, this power condition is met by a host device battery meeting a predetermined amount of charge, for example, the battery being fully charged. A fully charged battery may indicate that a host device is being supplied with AC power. If the MCP  100  does not meet the predetermined power condition, process  900  ends. 
     Next, in step  910 , the processor  112  may determine whether the processor  112  has sufficient bandwidth to fully perform the maintenance process  900 . In some implementations, the maintenance operation merely runs as a background process which requires minimal bandwidth. In some implementations, the maintenance operation requires an idle processor  112 . In other implementations, process  900  does not determine whether the processor  112  has sufficient bandwidth. In some implementations, the bandwidth requirement changes based upon the need for the maintenance operations, which may be measured by the time between successful maintenance processes or by the amount of available space on the flash memory. If the processor  112  does not have sufficient bandwidth, process  900  ends. 
     If the predetermined power condition is met and the processor  112  has sufficient bandwidth, process  900  then may read stored maintenance logs, step  915 . In some implementations, the stored maintenance logs are stored in NVM  154 . In some implementations, stored maintenance logs indicate the priority of possible maintenance operations. In some implementations, the stored maintenance logs are used to determine whether any maintenance steps (such as steps  920 ,  925 ,  930  &amp;  935 ) can be performed in a simplified operation. For example, maintenance logs may indicate the need to both downgrade a particular page of memory cells and to rewrite the data on the same page of memory cells. In other implementations, the maintenance operations are a predetermined sequence of, for example, rewriting transferred data at a higher resolution (step  920 ); downwardly adjusting cell resolutions and pairing groups of pages, e.g., by performing processes  800  and  850  (step  925 ); rewriting data that meet a predetermined error condition (e.g., using process  700 ) (step  930 ); swapping the most frequently accessed data with the least frequently used data using the wear management software code  157  (step  935 ); updating the logical addressing software code  163  for each maintenance operation that moved data from one physical location to another physical location (e.g., using process  1000 ), (step  940 ); and refreshing pages of data that exceed a threshold amount of voltage sag by applying additional charge to the page of memory cells (step  945 ). Other sequences including some, all, or additional operations may also be used. In some implementations, the process  900  repeats step  905  and/or step  910  between each maintenance step  920 ,  925 ,  930 ,  935 , or  945 , and may end if either of condition  905  or  910  change. Process  900  then ends. 
       FIG. 10  is a flow chart that illustrates an example of a process  1000  of logical addressing in the FDC  106 . For example, the FDC  106  may map a received read or write command with a logical address to one or more physical pages. In some implementations, the FDC  106  may dynamically map a logical page to one or more variable physical page(s). For example, the FDC  106  may change the mapping to balance the load of a physical memory page. In some implementations, the mapping between logical pages and physical pages may be stored in a logical address table. In some implementations, the process  1000  may be performed by the processor  112  when the processor  112  is executing the logical addressing code  163 . The process  1000  begins when the FDC receives a command from the host device that a memory page is to be accessed (e.g., read, written to, or erased). Then, in step  1005 , the processor  112  receives a logical page address to access a page in the flash memory. 
     Next, the processor  112  determines, in step  1010 , one or more physical page addresses associated with the received logical address. In one example, the received logical page address may be associated to only one physical page address. In another example, the received logical page address may be associated with two or more physical pages because the physical pages have a lower cell resolution than normal, or the physical pages are not contiguous in the flash memory, or they are in different blocks or on different dies. 
     Then the processor  112  selects a first of the determined physical page addresses in step  1015 . In step  1020 , the processor  112  reads the page data at the selected physical address. The processor  112  then stores, in step  1025 , the page data in the host output buffer. In step  1030 , the processor  112  determines whether it is necessary to access another memory page. For example, if there is more than one physical page addresses associated with the logical page address, then the processor  112  may access another memory page. If, in step  1030 , the processor  112  determines that it is necessary to access another memory page, then the processor, in step  1035 , selects a next determined physical page address and the process returns to step  1020 . Otherwise, the process  1000  ends. 
       FIG. 11  shows an example system  1100  that includes multiple NAND flash memory dies  103  and the FDC  106 . The FDC  106  includes a multiplexer (MUX)  1105  in the analog interface  115  and a charge pump  1110 . Although the system  1100  is shown using NAND flash memory dies  103 , some of the techniques used in the system  1100  may also be applicable to NOR flash memory dies, or a combination of NAND and NOR dies. The system  1100  may be implemented using discrete ICs, or it may be partially or fully integrated in a single package. 
     The FDC  106  receives analog data from the NAND flash memory dies  103  through the analog interface  115 . In this example, the MUX  1105  receives multiple analog inputs. In some implementations, the MUX  1105  receives the multiple analog inputs from multiple flash memory dies  103 . The analog interface  115  can control the MUX  1105  to select one analog input to be transmitted to the ADC  142 . For example, the analog interface  115  may control the MUX  1105  based on a received read command. During a write operation, the FDC  106  uses the charge pump  1110  to apply charges to the memory cells in one of the NAND flash memory dies  103 . In some implementations, the charge pump  1110  is adapted to supply charge to memory cells on a plurality of flash memory dies  103 . For example, the FDC  106  may send a control signal to select a designated memory die to receive charges from the charge pump  1110 . Then, when the charge pump  1110  applies charges, the selected memory die receives the charges. 
     By sharing the ADC  1105  and the charge pump  1110  between multiple dies  103 , the storage size of the memory dies  103  may be increased. Additionally, the flash memory dies  103  may be manufactured with a lower cost without the ADC  142  and the charge pump  1110 . In some implementations, the charge pump  1110  may be integrated on a die with the FDC  106  or separately mounted on a different die or on a different substrate, such as a printed circuit board. 
     In order to facilitate the use of an ADC  1105  and a charge pump  1110  adapted to be used with multiple flash memory dies  103 , some flash memory dies  103  may include an input adapted to receive a programming charge from an external supply node. The flash memory dies  103  are then not required to include any additional circuitry to alter or regulate the supplied programming charge. The flash memory dies  103  may also include an output adapted to send an analog voltage signal to a flash disk controller  106 . 
     In some implementations, the FDC  106  may also include a charge pump interleaving method to write data to the memory dies  103 . 
       FIG. 12  shows an example system  1200  illustrating an architecture to separately provide programming and logic-level power to the NAND flash memory die  103 . The system  1200  includes the charge pump  1110  and a low dropout regulator (LDO)  1205  that receive electrical power from a power supply  1210 . 
     As shown, the NAND flash memory die  103  includes two power inputs. A power input for charge pump voltage (V cp ) and a power input for logic voltage (V logic ). In some examples, the V cp  may be substantially higher than the V logic . For example, the V cp  may be approximately 12-20 V or approximately 12-30 V and the V logic  maybe approximately 1-3 V. In some implementations, the regulation and current requirements for the V cp  may be substantially different from those for the V logic . 
     As an example, the NAND flash memory die  103  may require that the V logic  to have a tightly regulated (e.g., 0.5%, 1.0%, 5%) voltage tolerance at a low logic voltage to minimize power consumption, switching times, etc. Furthermore, the logic voltage may call for high frequency bypass capacitance at a low voltage level. In contrast, the charge pump supply regulation requirements may be between about 5% and 10%, with a need for substantially low frequency, higher voltage capacitance. 
     In order to facilitate the system of  FIG. 12 , the flash memory die  103  may include a first interface for receiving power for selectively programming each flash memory cell, and a second interface for receiving power supplied to logic level circuitry to perform the selection of flash memory cells to be supplied with power from the first input during a write operation. The flash disk controller  106  may comprise a first power source for supplying power to the first interface at a programming voltage and a second power source for supplying logic-level power to the second interface. The first and second power sources may be external to the flash memory die  103 . 
     Although various implementations of processes and techniques have been described, other implementations may perform the steps in different sequence, or a modified arrangement to achieve the same primary function. In addition, although operations in the various processes are sometimes described as being performed by a particular device or component, such devices or components are merely examples, and the operations can be performed using alternative devices or components in some implementation 
     In some examples, the NAND flash memory die  103  may also have any practical number of bits of resolution, such as, for example, 6, 7, 10, 12 bits of resolution. Various implementations may be used to perform ECC operations with flash memory that may include NAND flash memory, NOR flash memory, or a combination of these or other non-volatile memories. Flash memory die of one or more types may be stacked and/or mounted adjacent each other in the MCP  100 . Those of ordinary skill in the art will recognize that some examples of techniques described herein may be applied to particular advantage with NAND flash technology, and some methods described herein may be generally applicable to non-volatile memories such as NAND and/or NOR flash. 
     Although an example of a system, which may be portable, has been described with reference to the above figures, other implementations may be deployed in other processing applications, such as desktop and networked installations. 
     Although particular features of an architecture have been described, other features may be incorporated to improve performance. For example, caching (e.g., L1, L2, etc. . . . ) techniques may be used in the FDC  106 . Random access memory may be included, for example, to provide scratch pad memory and or to load executable code or parameter information stored in the flash memory for use during runtime operations. Other hardware and software may be provided to perform operations, such as network or other communications using one or more protocols, wireless (e.g., infrared) communications, stored operational energy and power supplies (e.g., batteries), switching and/or linear power supply circuits, software maintenance (e.g., self-test, upgrades, etc. . . . ), and the like. One or more communication interfaces may be provided in support of data storage and related operations. 
     In some implementations, one or a combination of methods may be used to improve data integrity. For example, cell voltage errors may be addressed by adjusting thresholds and/or rewriting cells at least once. Cell re-writing may be performed in response to deviations from ideal cell voltage and/or as a background activity. For example, multi-level cell voltages may be rewritten to refresh the voltage in one or more lossy cells in a page. For cells that have been characterized as tending to lose voltage over time, the voltage level to which such cells are charged may be boosted to near an upper threshold of each cell&#39;s range to compensate for anticipated loss of charge in those cells over time. The boosted voltage level may initially be near or above the upper threshold of the intended range, which may be in a gray zone between ranges. Based on estimated or determined loss rates, the data may be re-written frequently enough to substantially maintain the cell voltages within a desired range. Similar compensation may be used to compensate for cells characterized as having an upward drift. Such rewriting procedures may be performed, for example, as a low priority background process that is executed as resources are available. For data identified as high value data, rewriting may be scheduled to occur frequently enough to maintain the cell voltages within a desired range, the frequency being based on an expected voltage drift rate and the size of the voltage range associated with each bit level. In some implementations, rewriting may be configured to be performed more frequently when a portable device is coupled to an external power source, such as a power source derived from the electric utility grid. Rewriting operations may be performed in response to being coupled to such a power source. In addition, rewriting may be configured to be performed less frequently under certain conditions, such as, for example, while in a power conservation mode, during a low battery condition, or when storing short duration or non-critical data (e.g., streaming audio/video). 
     Some systems may be implemented as a computer system that can be used with implementations of the invention. For example, various implementations may include digital and/or analog circuitry, computer hardware, firmware, software, or combinations thereof. Apparatus can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by a programmable processor; and methods can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating an output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and/or at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. 
     Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, which may include a single processor or one of multiple processors of any kind of computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Generally, a computer will also include, or be operatively coupled to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and, CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     In some implementations, each system  100  may be programmed with the same or similar information and/or initialized with substantially identical information stored in volatile and/or non-volatile memory. For example, one data interface may be configured to perform auto configuration, auto download, and/or auto update functions when coupled to an appropriate host device, such as a desktop computer or a server. 
     In some implementations, one or more user-interface features may be custom configured to perform specific functions. The invention may be implemented in a computer system that includes a graphical user interface and/or an Internet browser. To provide for interaction with a user, some implementations may be implemented on a computer having a display device, such as a CRT (cathode ray tube) or LCD (liquid crystal display) monitor for displaying information to the user, a keyboard, and a pointing device, such as a mouse or a trackball by which the user can provide input to the computer. 
     In various implementations, the system  100  may communicate using suitable communication methods, equipment, and techniques. For example, the system  100  may communicate with compatible devices (e.g., devices capable of transferring data to and/or from the system  100 ) using point-to-point communication in which a message is transported directly from the source to the receiver over a dedicated physical link (e.g., fiber optic link, point-to-point wiring, daisy-chain). The components of the system may exchange information by any form or medium of analog or digital data communication, including packet-based messages on a communication network. Examples of communication networks include, e.g., a LAN (local area network), a WAN (wide area network), MAN (metropolitan area network), wireless and/or optical networks, and the computers and networks forming the Internet. Other implementations may transport messages by broadcasting to all or substantially all devices that are coupled together by a communication network, for example, by using omni-directional radio frequency (RF) signals. Still other implementations may transport messages characterized by high directivity, such as RF signals transmitted using directional (i.e., narrow beam) antennas or infrared signals that may optionally be used with focusing optics. Still other implementations are possible using appropriate interfaces and protocols such as, by way of example and not intended to be limiting, USB 2.0, Firewire, ATA/IDE, RS-232, RS-422, RS-485, 802.11 a/b/g, Wi-Fi, Ethernet, IrDA, FDDI (fiber distributed data interface), token-ring networks, or multiplexing techniques based on frequency, time, or code division. Some implementations may optionally incorporate features such as error checking and correction (ECC) for data integrity, or security measures, such as encryption (e.g., WEP) and password protection. 
     A number of implementations of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, advantageous results may be achieved if the steps of the disclosed techniques were performed in a different sequence, if components in the disclosed systems were combined in a different manner, or if the components were replaced or supplemented by other components. The functions and processes (including algorithms) may be performed in hardware, software, or a combination thereof, and some implementations may be performed on modules or hardware not identical to those described.