Patent Publication Number: US-9417960-B2

Title: Preventing programming errors from occurring when programming flash memory cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application claims priority to provisional application Ser. No. 61/918,778, filed on Dec. 20, 2013, and entitled “PREVENTING PROGRAMMING ERRORS FROM OCCURRING WHEN PROGRAMMING FLASH MEMORY CELLS,” which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD OF INVENTION 
     The invention relates generally to flash memory and, more specifically, to preventing programming errors from occurring when programming flash memory cells. 
     BACKGROUND OF THE INVENTION 
     A flash memory is a non-volatile electrically erasable data storage device that evolved from electrically erasable programmable read-only memory (EEPROM). The two main types of flash memory are named after the logic gates that their storage cells resemble: NAND and NOR. NAND flash memory is commonly used in solid-state drives, which are supplanting magnetic disk drives in many applications. A NAND flash memory is commonly organized as multiple blocks, with each block organized as multiple pages. Each page comprises multiple cells. Each cell is capable of storing an electric charge. Some cells are used for storing data bits, while other cells are used for storing error-correcting code bits. A cell configured to store a single bit is known as a single-level cell (SLC). A cell configured to store two bits is known as a multi-level cell (MLC). In an MLC cell, one bit is commonly referred to as the least-significant bit (LSB), and the other as the most-significant bit (MSB). A cell configured to store three bits is known as a triple-level cell (TLC). Writing data to a flash memory is commonly referred to as “programming” the flash memory, due to the similarity to programming an EEPROM. 
     The electric charge stored in a cell can be detected in the form of a cell voltage. To read an SLC flash memory cell, the flash memory controller provides one or more reference voltages (also referred to as read voltages) to the flash memory device. Detection circuitry in the flash memory device will interpret the bit as a “0” if the cell voltage is greater than a reference voltage Vref and will interpret the bit as a “1” if the cell voltage is less than the reference voltage Vref. Thus, an SLC flash memory requires a single reference voltage Vref. In contrast, an MLC flash memory requires three such reference voltages, and a TLC flash memory requires seven such reference voltages. Thus, reading data from an MLC or TLC flash memory device requires that the controller provide multiple reference voltages having optimal values that allow the memory device to correctly detect the stored data values. 
     Determining or detecting stored data values using controller-provided reference voltages is hampered by undesirable physical non-uniformity across cells of a device that are inevitably introduced by the fabrication process, as such non-uniformity results in the reference voltages of different cells that store the same bit value being significantly different from each other. The detection is further hampered by target or optimal reference voltages changing over time due to adverse effects of changes in temperature, interference from programming neighboring cells, and numerous erase-program cycles. Errors in detecting stored data values are reflected in the performance measurement known as bit error rate (BER). The use of error-correcting codes (ECCs) can improve BER to some extent, but the effectiveness of ECCs diminishes as improved fabrication processes result in smaller cell features. 
     As illustrated in  FIG. 1 , an MLC flash memory has four cell voltage distributions  2 ,  4 ,  6  and  8  with four respective mean target cell voltages Vtarget0  12 , Vtarget1  14 , Vtarget2  16  and Vtarget3  18 . Such cell voltage distributions commonly overlap each other slightly, but such overlap is not shown in  FIG. 1  for purposes of clarity. During a read operation, to attempt to characterize or detect the two bits of cell data (i.e., the LSB and MSB) a flash memory device (not shown) uses three reference voltages it receives from a flash memory controller (not shown): Vref0  22 , Vref1  24  and Vref2  26 . More specifically, the flash memory device compares the cell voltage with Vref1  24  to attempt to detect the LSB. If the flash memory device determines that the cell voltage is less than Vref1  24 , i.e., within a window  28 , then the flash memory device characterizes the LSB as a “1”. If the flash memory device determines that the cell voltage is greater than Vref1  24 , i.e., within a window  30 , then the flash memory device characterizes the LSB as a “0”. 
     The flash memory device also compares the cell voltage with Vref0  22  and Vref2  26  to attempt to detect the MSB. If the flash memory device determines that the cell voltage is between Vref0  22  and Vref2  26 , i.e., within a window  32 , then the flash memory device characterizes the MSB as a “0”. If the flash memory device determines that the cell voltage is either less than Vref0  22  or greater than Vref2  26 , i.e., within a window  34 , then the flash memory device characterizes the MSB as a “1”. 
     To improve BER beyond what is commonly achievable with hard-decision decoded ECCs, flash memory controllers may employ soft-decision decoded ECCs, such as low density parity check (LDPC) ECCs. Soft-decision decoding is more powerful in correcting errors than hard-decision decoding, but soft input information must be provided to the ECC decoding logic. The ECC decoder soft input information is commonly provided in the form of log likelihood ratio (LLR) information. 
     MLC NAND flash memory is programmed in two stages, namely, a first stage during which LSB page programming is performed, and a second stage during which MSB page programming is performed. The first stage includes the following: (1) the flash memory controller sends LSB data to flash memory; (2) the flash memory loads the LSB data into an LSB page buffer portion of the flash memory; and (3) the flash memory uses the LSB data to program the corresponding LSB page of the flash memory. The second stage includes the following: (1) the flash memory controller sends the MSB data to be programmed to flash memory; (2) the flash memory loads the MSB data into an MSB page buffer portion of the flash memory; (3) logic of the flash memory reads the LSB page of the corresponding flash cells and loads the read LSB data into the LSB page buffer portion; (4) the logic uses the MSB and LSB value pairs held in the flash page buffer to determine the target reference voltage ranges to be programmed for the corresponding flash cells; and (5) the logic programs the target reference voltage ranges into the flash memory. 
     As flash memory technology improves, the sizes of the flash dies scale down, which results in the distance between neighboring flash cells becoming smaller. Because of the nearness of neighboring flash cells to one another, the programming of one flash cell can affect the charges stored on nearby flash cells, which contributes to the potentially noisy and unreliable nature of flash cells. Consequently, there can be errors in the LSB page data read out of the corresponding flash cells. Because the LSB page data read out of the flash cells is used in combination with the MSB page data to determine the target reference voltage ranges for the corresponding cells, such errors will typically cause the target reference voltage ranges to be incorrectly determined. This can cause the flash cells to be mis-programmed to improper reference voltage ranges when performing MSB page programming. The improper reference voltage ranges often will be far away from the borders of flash neighbor states and could provide incorrect, but highly confident, soft information. This, in turn, can significantly degrade the error correction performance of soft-decision decoding, such as LDPC decoding. 
       FIG. 2  illustrates cell voltage distributions and target reference voltage ranges for different LSB and MSB states and demonstrates the manner in which the reference voltage ranges are selected based on the values of the MSB and LSB pair. Cell voltage distribution  42  represents the LSB and MSB erased program state. Cell distributions  43  and  44  represent LSB “1” and “0” programmed states, respectively. Cell distributions  45 ,  46 ,  47 , and  48  represent MSB programmed states of “1,” “0,” “0,” and “1,” respectively. 
     If the MSB and LSB values of the MSB, LSB pair contained in the flash page buffer are both “1,” then the reference voltage range that will be programmed into the flash memory for the corresponding flash cells is selected to be range A, which is the range of reference voltages that is less than Vref0. If the MSB and LSB values of the MSB, LSB pair contained in the flash page buffer are “1” and “0,” respectively, then the reference voltage range that will be programmed into the flash memory device for the corresponding flash cells is selected to be range D, which is the range of reference voltages that is greater than Vref2. If the MSB and LSB values of the MSB, LSB pair contained in the flash page buffer are “0” and “1,” respectively, then the reference voltage range that will be programmed into the flash memory device for the corresponding flash cells is selected to be range B, which is the range of reference voltages that is greater than Vref0 and less than Vref1. 
     As can be seen from the above, if the LSB values that are read from the flash memory cells are inaccurate, which is possible for the reasons described above, then the reference voltage ranges will likely be mis-programmed. Accordingly, a need exists for a way to ensure that the reference voltage ranges are correctly determined and programmed. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention relate to a data storage system, a flash memory IC for use in the data storage system, and methods used therein for preventing programming errors from occurring when programming flash memory. In accordance with an illustrative embodiment, the data storage system comprises a host system and a solid state drive (SSD) that are interfaced with one another. The SSD includes an SSD controller and at least one nonvolatile memory (NVM). The SSD controller includes at least one SSD processor, a tier 1 error-correcting ECC encoder/decoder, and a tier 2 ECC encoder/decoder. The NVM includes at least a first flash memory having a plurality of flash memory cells, reference voltage range determination logic and tier 2 ECC decoding logic. The SSD controller receives write data from the host system to be programmed into flash cells of the NVM. The write data comprises at least a first MSB page of data and at least a first LSB page of data. The tier 1 ECC encoder/decoder performs tier 1 ECC encoding of the first LSB page of data to produce a tier 1-encoded first LSB page of data. The tier 2 ECC encoder/decoder performs tier 2 ECC encoding of the tier 1-encoded first LSB page of data to produce a tier 1/tier 2-encoded first LSB page data. The SSD controller sends the tier 1/tier 2-encoded first LSB page of data to the first flash memory. The tier 1 ECC encoding/decoding logic performs tier 1 ECC encoding of the first MSB page of data to produce a tier 1-encoded first MSB page of data. The SSD controller sends the tier 1-encoded first MSB page of data to the first flash memory. The tier 1 ECC encoding and the tier 2 encoding are different types of encoding. The tier 2 decoding logic of the first flash memory is adapted to perform tier 2 decoding of LSB page data in the first flash memory. The tier 2-decoded first LSB page of data is subsequently used in conjunction with the tier 1-encoded MSB page data in the first flash memory to determine reference voltage ranges for the flash memory cells 
     In accordance with an illustrative embodiment, the flash memory IC die comprises a plurality of flash memory cells for storing data and ECC decoding logic. The flash memory IC die is configured to receive write data from an interface that interfaces an SSD controller with the first flash memory IC die. The received write data comprises at least a first LSB page of data and at least a first MSB page of data. The first LSB page of data is encoded with a tier 1 ECC encoding and with a tier 2 ECC encoding. The first MSB page of data is encoded with the tier 1 encoding. The flash memory IC die programs the first LSB page of data to a first LSB page of the flash memory cells. Prior to the first flash memory IC die programming the first MSB page of data to a first MSB page of the flash memory cells, the first flash memory IC die causes the encoded first LSB page of data to be read from the first LSB page of the flash memory cells and sent to the ECC decoding logic. The ECC decoding logic performs tier 2 ECC decoding of the encoded LSB page data to produce a tier 2-decoded first LSB page of data that is subsequently used in conjunction with the tier 1-encoded MSB page data to determine reference voltage ranges for the flash memory cells. 
     These and other features and advantages of the invention will become apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plot of cell voltage distributions in a flash memory device, as known in the art, and demonstrates the known manner in which LSB and MSB values are determined. 
         FIG. 2  is a plot of cell voltage distributions in a flash memory device, as known in the art, and demonstrates the manner in which LSB and MSB value pairs are used to determine the reference voltage ranges. 
         FIG. 3  illustrates a block diagram of a storage system in accordance with an illustrative embodiment that includes one or more instances of an SSD device that is suitable for implementing the invention. 
         FIG. 4  illustrates a block diagram of an illustrative embodiment of one of the SSDs shown in  FIG. 3  including the SSD controller shown in  FIG. 3  that performs flash cell programming in a way that ensures that programming errors do not occur when programming the reference voltage ranges of the flash cells. 
         FIG. 5  illustrates a block diagram of a plurality of flash cells disposed in a portion of one of the flash dies shown in  FIG. 4 . 
         FIG. 6  illustrates a block diagram of a portion of one of the flash dies shown in  FIG. 4  that includes reference voltage range determination logic, a flash memory buffer and tier 2 ECC decoding logic. 
         FIGS. 7A-7C  illustrate an LSB page data frame before and after being concatenated with tier 1 and tier 2 parity bits. 
         FIGS. 8A-8C  illustrate an MSB page data frame before and after being concatenated with tier 1 parity bits and other data bits. 
         FIG. 9  illustrates a flow diagram that represents the method in accordance with an illustrative embodiment for preventing errors from occurring when determining reference voltage ranges for flash memory cells of a flash die. 
     
    
    
     DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT 
     In accordance with exemplary, or illustrative, embodiments, the LSB values that are used in conjunction with the MSB values to determine the proper reference voltage ranges are error corrected by ECC decoding logic inside of the flash memory before being used in conjunction with the MSB values to determine the proper reference voltage ranges. Error correcting the LSB page data prior to using it in combination with the MSB page data to determine the reference voltage ranges ensures that the reference voltage ranges will be properly determined and programmed into the flash cells. 
     Embodiments of the invention can be implemented in a number of ways, and therefore a few illustrative embodiments are described below with reference to  FIGS. 3-9 , in which like reference numerals in the figures identify like features, components or elements. Before describing specific embodiments for ensuring that programming errors do not occur when programming the reference voltage ranges of the flash cells, the components of the storage system in accordance with an illustrative embodiment and the functions that they perform will be generally described with reference to  FIGS. 3 and 4 . 
       FIG. 3  illustrates a block diagram of a storage system in accordance with an illustrative embodiment that includes one or more instances of a solid state drive (SSD)  101  that implements the invention. The SSD  101  includes an SSD controller  100  coupled to NVM  199  via device interfaces  190 . As will be described below in more detail with reference to  FIG. 4 , the NVM  199  comprises one or more flash memory dies, each of which comprises a plurality of flash cells. The storage system may include, for example, a host system  102 , a single SSD  101  coupled directly to the host system  102 , a plurality of SSDs  101  each respectively coupled directly to the host system  102  via respective external interfaces, or one or more SSDs  101  coupled indirectly to a host system  102  via various interconnection elements. As an exemplary embodiment of a single SSD  101  coupled directly to the host system  102 , one instance of SSD  101  is coupled directly to host system  102  via external interfaces  110  (e.g., switch/fabric/intermediate controller  103  is omitted, bypassed, or passed-through). 
     As an exemplary embodiment of a plurality of SSDs  101  being coupled directly to the host system  102  via respective external interfaces, each of a plurality of instances of SSD  101  is respectively coupled directly to host system  102  via a respective instance of external interfaces  110  (e.g., switch/fabric/intermediate controller  103  is omitted, bypassed, or passed-through). As an exemplary embodiment of one or more SSDs  101  coupled indirectly to host system  102  via various interconnection elements, each of one or more instances of SSD  101  is respectively coupled indirectly to host system  102  via a respective instance of external interfaces  110  coupled to switch/fabric/intermediate controller  103 , and intermediate interfaces  104  coupled to host system  102 . 
     The host system  102  includes one or more processors, such as, for example, one or more microprocessors and/or microcontrollers operating as a central processing unit (CPU)  102   a , and a host memory device  102   b  for storing instructions and data used by the host CPU  102   a . Host system  102  is enabled or configured via the host CPU  102   a  to execute various elements of host software  115 , such as various combinations of operating system (OS)  105 , driver  107 , application  109 , and multi-device management software  114 . The host software  115  is stored in a memory device  102   b  of the host system  102  and is executed by the host CPU  102   a . Dotted-arrow  107 D is representative of host software   I/O device communication, e.g., data sent/received to/from one or more of the instances of SSD  101  and from/to any one or more of OS  105  via driver  107 , driver  107 , and application  109 , either via driver  107 , or directly as a VF. 
     OS  105  includes and/or is enabled or configured to operate with drivers (illustrated conceptually by driver  107 ) for interfacing with the SSD. Various versions of Windows (e.g., 95, 98, ME, NT, XP, 2000, Server, Vista, and 7), various versions of Linux (e.g., Red Hat, Debian, and Ubuntu), and various versions of MacOS (e.g., 8, 9 and X) are examples of OS  105 . In various embodiments, the drivers are standard and/or generic drivers (sometimes termed “shrink-wrapped” or “pre-installed”) operable with a standard interface and/or protocol such as SATA, AHCI, or NVM Express, or are optionally customized and/or vendor specific to enable use of commands specific to SSD  101 . 
     Some drives and/or drivers have pass-through modes to enable application-level programs, such as application  109  via optimized NAND Access (sometimes termed ONA) or direct NAND Access (sometimes termed DNA) techniques, to communicate commands directly to SSD  101 , enabling a customized application to use commands specific to SSD  101  even with a generic driver. ONA techniques include one or more of: use of non-standard modifiers (hints); use of vendor-specific commands; communication of non-standard statistics, such as actual NVM usage according to compressibility; and other techniques. DNA techniques include one or more of: use of non-standard commands or vendor-specific providing unmapped read, write, and/or erase access to the NVM; use of non-standard or vendor-specific commands providing more direct access to the NVM, such as by bypassing formatting of data that the I/O device would otherwise do; and other techniques. Examples of the driver are a driver without ONA or DNA support, an ONA-enabled driver, a DNA-enabled driver, and an ONA/DNA-enabled driver. Further examples of the driver are a vendor-provided, vendor-developed, and/or vendor-enhanced driver, and a client-provided, client-developed, and/or client-enhanced driver. 
     Examples of the application-level programs are an application without ONA or DNA support, an ONA-enabled application, a DNA-enabled application, and an ONA/DNA-enabled application. Dotted-arrow  109 D is representative of application I/O device communication (e.g. bypass via a driver or bypass via a VF for an application), e.g. an ONA-enabled application and an ONA-enabled driver communicating with an SSD, such as without the application using the OS as an intermediary. Dotted-arrow  109 V is representative of application I/O device communication (e.g. bypass via a VF for an application), e.g. a DNA-enabled application and a DNA-enabled driver communicating with an SSD, such as without the application using the OS or the driver as intermediaries. 
     Some of the embodiments that include switch/fabric/intermediate controller  103  also include card memory  112 C coupled via memory interface  180  and accessible by the SSDs  101 . In various embodiments, one or more of the SSDs  101 , the switch/fabric/intermediate controller  103 , and/or the card memory  112 C are included on a physically identifiable module, card, or pluggable element (e.g., I/O Card  116 ). In some embodiments, SSD  101  (or variations thereof) corresponds to a SAS drive or a SATA drive that is coupled to an initiator operating as host system  102 . 
     In some embodiments lacking the switch/fabric/intermediate controller, the SSD  101  is coupled to the host system  102  directly via external interfaces  110 . In various embodiments, SSD Controller  100  is coupled to the host system  102  via one or more intermediate levels of other controllers, such as a RAID controller. In some embodiments, SSD  101  (or variations thereof) corresponds to a SAS drive or a SATA drive and switch/fabric/intermediate controller  103  corresponds to an expander that is in turn coupled to an initiator, or alternatively switch/fabric/intermediate controller  103  corresponds to a bridge that is indirectly coupled to an initiator via an expander. In some embodiments, switch/fabric/intermediate controller  103  includes one or more PCIe switches and/or fabrics. 
     In various embodiments, such as some of the embodiments where host system  102  is a computing host (e.g., a computer, a workstation computer, a server computer, a storage server, a SAN, a NAS device, a DAS device, a storage appliance, a PC, a laptop computer, a notebook computer, and/or a netbook computer), the computing host is optionally enabled to communicate (e.g., via optional I/O &amp; Storage Devices/Resources  117  and optional LAN/WAN  119 ) with one or more local and/or remote servers (e.g., optional servers  118 ). The communication enables, for example, local and/or remote access, management, and/or usage of any one or more of SSD  101  elements. In some embodiments, the communication is wholly or partially via Ethernet. In some embodiments, the communication is wholly or partially via Fibre Channel. LAN/WAN  119  is representative, in various embodiments, of one or more Local and/or Wide Area Networks, such as any one or more of a network in a server farm, a network coupling server farms, a metro-area network, and the Internet. 
     In various embodiments, an SSD controller and/or a computing-host flash memory controller in combination with one or more NVMs are implemented as a non-volatile storage component, such as a USB storage component, a CF storage component, a MultiMediaCard (MMC) storage component, an eMMC storage component, a Thunderbolt storage component, a UFS storage component, an SD storage component, a memory stick storage component, and an xD-picture card storage component. 
     In various embodiments, all or any portions of an SSD controller (or a computing-host flash memory controller), or functions thereof, are implemented in a host that the controller is to be coupled with (e.g., host system  102 ). In various embodiments, all or any portions of an SSD controller (or a computing-host flash memory controller), or functions thereof, are implemented via hardware (e.g., logic circuitry), software and/or firmware (e.g., driver software or SSD control firmware), or any combination thereof. 
       FIG. 4  illustrates a block diagram of an illustrative embodiment of one of the SSDs  101  shown in  FIG. 3  including the SSD controller  100  shown in  FIG. 3  that performs flash cell programming in a way that ensures that programming errors do not occur when programming the reference voltage ranges of the flash cells. Prior to describing an illustrative embodiment of the manner in which the SSD controller  100  performs flash cell programming, the configuration of the SSD controller  100  that is suitable for performing the methods will be described with reference to  FIG. 4 . 
     SSD controller  100  is communicatively coupled via one or more external interfaces  110  to the host system  102  ( FIG. 3 ). According to various embodiments, external interfaces  110  are one or more of: a SATA interface; a SAS interface; a PCIe interface; a Fibre Channel interface; an ethernet interface (such as 10 Gigabit Ethernet); a non-standard version of any of the preceding interfaces; a custom interface; or any other type of interface used to interconnect storage and/or communications and/or computing devices. For example, in some embodiments, SSD controller  100  includes a SATA interface and a PCIe interface. 
     SSD controller  100  is further communicatively coupled via one or more device interfaces  190  to NVM  199 , which includes one or more flash devices  192 . According to various illustrative embodiments, device interfaces  190  are one or more of: an asynchronous interface; a synchronous interface; a single-data-rate (SDR) interface; a double-data-rate (DDR) interface; a DRAM-compatible DDR or DDR2 synchronous interface; an Open NAND Flash Interface (ONFI) compatible interface, such as an ONFI 2.2 or ONFI 3.0 compatible interface; a Toggle-mode compatible flash interface; a non-standard version of any of the preceding interfaces; a custom interface; or any other type of interface used to connect to storage devices. 
     Each flash device  192  includes one or more individual flash dies  194 . According to type of a particular one of flash devices  192 , a plurality of the flash dies  194  in the particular flash device  192  are optionally and/or selectively accessible in parallel. Flash device  192  is merely representative of one type of storage device enabled to communicatively couple to SSD controller  100 . In various embodiments, any type of storage device is usable, such as an SLC NAND flash memory, MLC NAND flash memory, NOR flash memory, flash memory using polysilicon or silicon nitride technology-based charge storage cells, two- or three-dimensional technology-based flash memory, read-only memory, static random access memory, dynamic random access memory, ferromagnetic memory, phase-change memory, racetrack memory, ReRAM, or any other type of memory device or storage medium. 
     According to various embodiments, device interfaces  190  are organized as: one or more busses with one or more of flash device  192  per bus; one or more groups of busses with one or more of flash device  192  per bus, where busses in a group are generally accessed in parallel; or any other organization of one or more of flash device  192  onto device interfaces  190 . 
     The SSD controller  100  typically, but not necessarily, has one or more modules, such as, for example, host interfaces module  111 , data processing module  121 , buffer module  131 , map module  141 , recycler module  151 , ECC module  161 , device interface logic module  191 , and CPU  171 . The specific modules and interconnections illustrated in  FIG. 4  are merely representative of one embodiment, and many arrangements and interconnections of some or all of the modules, as well as additional modules not illustrated, are possible, and fewer than all of the modules shown in  FIG. 4  may be included in the SSD controller  100 . In a first example, in some embodiments, there are two or more host interfaces  111  to provide dual-porting. In a second example, in some embodiments, data processing module  121  and/or ECC module  161  are combined with buffer module  131 . In a third example, in some embodiments, host interfaces module  111  is directly coupled to buffer module  131 , and data processing module  121  optionally and/or selectively operates on data stored in buffer module  131 . In a fourth example, in some embodiments, device interface logic module  191  is directly coupled to buffer module  131 , and ECC module  161  optionally and/or selectively operates on data stored in buffer module  131 . 
     Host interfaces module  111  sends and receives commands and/or data via external interfaces  110 . For example, the commands include a read command specifying an address (such as a logical block address (LBA)) and an amount of data (such as a number of LBA quanta, e.g., sectors) to read; in response the SSD  101  provides read status and/or read data. As another example, the commands include a write command specifying an address (such as an LBA) and an amount of data (such as a number of LBA quanta, e.g., sectors) to write; in response the SSD  101  provides write status and/or requests write data and optionally subsequently provides write status. For yet another example, the commands include a de-allocation command (e.g., a trim command) specifying one or more addresses (such as one or more LBAs) that no longer need be allocated. 
     According to various embodiments, one or more of: Data processing module  121  optionally and/or selectively processes some or all data sent between buffer module  131  and external interfaces  110 ; and data processing module  121  optionally and/or selectively processes data stored in buffer module  131 . In some embodiments, data processing module  121  uses one or more engines  123  to perform one or more of: formatting; reformatting; transcoding; and any other data processing and/or manipulation task. 
     Buffer module  131 , which is optional, stores data sent to/from external interfaces  110  from/to device interfaces  190 . In some embodiments, buffer module  131  additionally stores system data, such as some or all map tables, used by SSD controller  100  to manage one or more of the flash devices  192 . Buffer module  131  is typically a portion of the local memory of the SSD controller  100  that has been allocated for use as temporary storage for storing MSB and LSB page data to be written to flash memory cells of the flash die  194 . The buffer module  131  typically, but not necessarily, also includes a direct memory access (DMA) engine (not shown) that is used to control movement of data to and/or from the buffer module  131  and ECC-X engine (not shown) that is used to provide higher-level error correction and/or redundancy functions. 
     ECC module  161  processes some or all data sent between buffer module  131  and device interfaces  190 . ECC module  161  optionally and/or selectively processes data stored in buffer module  131 . In some embodiments, ECC module  161  is used to provide lower-level error correction and/or redundancy functions in accordance with one or more ECC techniques. In some embodiments, ECC module  161  implements one or more of: a CRC code; a Hamming code; an RS code; a BCH code; an LDPC code; a Viterbi code; a trellis code; a hard-decision code; a soft-decision code; an erasure-based code; any error detecting and/or correcting code; and any combination of the preceding. 
     ECC module  161  includes one or more ECC encoders for performing ECC encoding and one or more ECC decoders for performing ECC decoding. In accordance with an illustrative embodiment, the ECC module  161  includes two tiers of encoding/decoding logic, namely, a tier 1 encoding/decoder  161   a  and a tier 2 encoder/decoder  161   b . The tier 1 encoder/decoder  161   a  performs a stronger, or more robust, level of ECC encoding/decoding than the level of ECC encoding/decoding performed by the tier 2 encoding/decoding logic  161   b . For example, the tier 1 encoder/decoder  161   a  may use LDPC codes whereas the tier 2 encoder/decoder  161   b  may use BCH codes. The tier 1 encoding/decoding is typically soft-decision ECC encoding/decoding, whereas the tier 2 encoding/decoding is typically a hard-decision ECC encoding/decoding. The reasons for using these different levels of ECC encoding/decoding is described below in detail. The invention is not limited with respect to the types of soft- and hard-decision ECC encoding/decoding techniques that are used as the tier 1 and tier 2 ECC encoding/decoding techniques, respectively. As will be understood by those of skill in the art in view of the description being provided herein, a variety of soft- and hard-decision ECC encoding/decoding techniques are suitable for use with the invention as the tier 1 and tier 2 ECC encoding/decoding techniques, respectively. Because such techniques are well known in the art, they will not be described herein in detail. 
     Device interface logic module  191  controls instances of flash device  192  via device interfaces  190 . Device interface logic module  191  is enabled to send data to/from the instances of flash device  192  according to a protocol of flash device  192 . Device interface logic module  191  typically includes scheduling logic  193  that selectively sequence controls instances of flash device  192  via device interfaces  190 . For example, in some embodiments, scheduling logic  193  is enabled to queue operations to the instances of flash device  192 , and to selectively send the operations to individual ones of the instances of flash device  192  (or flash die  194 ) as individual ones of the instances of flash device  192  (or flash die  194 ) become available. 
     Map module  141  converts between data addressing used on external interfaces  110  and data addressing used on device interfaces  190 , using table  143  to map external data addresses to locations in NVM  199 . For example, in some embodiments, map module  141  converts LBAs used on external interfaces  110  to block and/or page addresses targeting one or more flash die  194 , via mapping provided by table  143 . In some embodiments, map module  141  uses table  143  to perform and/or to look up translations between addresses used on external interfaces  110  and data addressing used on device interfaces  190 . According to various embodiments, table  143  is one or more of: a one-level map; a two-level map; a multi-level map; a map cache; a compressed map; any type of mapping from one address space to another; and any combination of the foregoing. According to various embodiments, table  143  includes one or more of: static random access memory; dynamic random access memory; NVM (such as flash memory); cache memory; on-chip memory; off-chip memory; and any combination of the foregoing. 
     In some embodiments, recycler module  151  performs garbage collection. For example, in some embodiments, instances of flash device  192  contain blocks that must be erased before the blocks are re-writeable. Recycler module  151  is enabled to determine which portions of the instances of flash device  192  are actively in use (e.g., allocated instead of de-allocated), such as by scanning a map maintained by map module  141 , and to make unused (e.g., de-allocated) portions of the instances of flash device  192  available for writing by erasing them. In further embodiments, recycler module  151  is enabled to move data stored within instances of flash device  192  to make larger contiguous portions of the instances of flash device  192  available for writing. 
     In some embodiments, instances of flash device  192  are selectively and/or dynamically configured, managed, and/or used to have one or more bands for storing data of different types and/or properties. A number, arrangement, size, and type of the bands are dynamically changeable. For example, data from a computing host is written into a hot (active) band, while data from recycler module  151  is written into a cold (less active) band. In some usage scenarios, if the computing host writes a long, sequential stream, then a size of the hot band grows, whereas if the computing host does random writes or few writes, then a size of the cold band grows. 
     CPU  171  controls various portions of SSD controller  100 . CPU module  171  typically includes CPU Core  172 , which is, according to various embodiments, one or more single-core or multi-core processors. The individual processor cores in CPU core  172  are, in some embodiments, multi-threaded. CPU core  172  includes instruction and/or data caches and/or memories. For example, the instruction memory contains instructions to enable CPU core  172  to execute programs (e.g. software sometimes called firmware) to control SSD Controller  100 . In some embodiments, some or all of the firmware executed by CPU core  172  is stored on instances of flash device  192 . 
     In various embodiments, CPU  171  further includes: command management logic  173  for tracking and controlling commands received via external interfaces  110  while the commands are in progress; buffer management logic  175  for controlling allocation and use of buffer module  131 ; translation Management logic  177  for controlling map module  141 ; coherency management module  179  for controlling consistency of data addressing and for avoiding conflicts such as between external data accesses and recycle data accesses; device management logic  181  for controlling device interface logic  191 ; identity management logic  182  for controlling modification and communication of identity information, and optionally other management units. None, any, or all of the management functions performed by CPU  171  are, according to various embodiments, controlled and/or managed by hardware, by software (such as firmware executing on CPU core  172  or on host system  102  ( FIG. 3 ) connected via external interfaces  110 ), or any combination thereof. 
     In various embodiments, all or any portions of an SSD Controller  100  are implemented on a single IC, a single die of a multi-die IC, a plurality of dice of a multi-die IC, or a plurality of ICs. For example, buffer module  131  may be implemented on a same die as other elements of SSD controller  100 . As another example, buffer module  131  may be implemented on a different die than other elements of SSD controller  100 . The SSD controller  100  and one or more of the flash devices  192  may be implemented on the same die, although they are typically implemented on separate dies. 
     As described above in the Background of the Invention, flash memory is typically programmed in two stages, namely, a first stage during which LSB page programming is performed and a second stage during which MSB page programming is performed. As also described above, during the second stage, errors in the LSB page data read out of the flash cells will typically cause the target reference voltage ranges to be incorrectly determined, and therefore mis-programmed. Because the time interval between LSB page programming and MSB page programming is typically short, the LSB page data does not contain retention errors, but may contain errors caused by other factors (i.e., program, erase and/or read errors). However, such errors can be easily corrected by using known hard-decision (i.e., relatively weak) ECC decoding techniques, which can be relatively easily and inexpensively implemented on a flash die  194 . Because the SSD controller  100  should be capable of handling all types of errors (i.e., retention, program, erase and/or read errors), soft-decision (i.e., relatively strong) ECC encoding/decoding techniques are implemented in the SSD controller  100  by the ECC module  161  for all data that is written to and read from the flash dies  194 , as will be described below in detail. 
     In accordance with an illustrative embodiment, tier 2 ECC decoding logic is implemented on the flash die. During the MSB page data programming stage, the associated LSB page data is read from the flash cells and is error corrected by the tier 2 ECC decoding logic of the flash die. The error-corrected LSB page data is then used in conjunction with the MSB page data in the known manner to determine the proper reference voltage ranges to be programmed for the flash cells. The manner in which the MSB page data programming stage has been altered to perform the ECC decoding process on the flash die  194  to ensure that the LSB data that is used in determining the reference voltage values does not contain errors will now be described with reference to  FIGS. 3-9 . 
       FIG. 5  illustrates a block diagram of a plurality of flash cells  195  disposed in a portion  194   a  of one of the flash dies  194  shown in  FIG. 4 . A plurality of word lines  196   a - 196   d  and bit lines  197  are used to address LSB and MSB pages of the flash cells  195  ( FIG. 5 ). An example of the manner in which the LSB and MSB pages are programmed is provided below in conjunction with a description of the manner in which the MSB page data programming stage has been altered to perform the ECC decoding process on the flash die  194 . 
       FIG. 6  illustrates a block diagram of a portion of one of the flash dies  194  that includes reference voltage determination logic  200  for determining the reference voltage ranges for the flash cells  195 , a flash memory buffer  201  for holding MSB data and LSB data to be written to the flash cells  195 , and tier 2 ECC decoding logic  210  for performing tier 2 decoding of LSB page data to be used in determining the reference voltage ranges for the flash cells  195 . The logic  200  and  210  are typically state machines, but could be some other type of logic, such as one or more processors, for example. The invention is not limited with respect to the manner in which the logic  200  and  210  are implemented in the flash die  194 . The manner in which the reference voltage determination logic  200 , the flash memory buffer  201  and the tier 2 ECC decoding logic  210  operate is described below in conjunction with a description of the manner in which the MSB page data programming stage has been altered to perform the ECC decoding process on the flash die  194 . 
       FIGS. 7A-7C  illustrate LSB data frames before and after being concatenated with tier 1 and tier 2 parity bits.  FIGS. 8A-8C  illustrate LSB data frames before and after being concatenated with tier 1 parity bits and other data. An example of the manner in which the data is encoded/decoded is described below in conjunction with a description of the manner in which the MSB page data programming stage has been altered to perform the ECC decoding process on the flash die  194 . 
     In accordance with an illustrative embodiment, the following process occurs when writing data to the flash die  194 : (1) MSB and LSB page data to be written to flash memory is received in the SSD controller  100  ( FIG. 3 ) from the host system  102  ( FIG. 3 ). For exemplary purposes, it will be assumed that the SSD controller  100  temporarily places the LSB page data in buffer  131  ( FIG. 4 ), although the buffer  131  and the buffering step are optional because some SSD controllers do not include such buffers. The MSB and LSB page data to be written is loaded into the buffer  131  ( FIG. 4 ) of the SSD controller  100 ; (2) the LSB page data ( FIG. 7A ) held in the buffer  131  ( FIG. 4 ) is subjected to tier 1 (i.e., relatively strong) ECC encoding ( FIG. 7B ); (3) the tier 1-encoded LSB page data ( FIG. 7B ) is then subjected to tier 2 (i.e., relatively weak) ECC encoding ( FIG. 7C ); (4) the tier 1/tier 2-encoded LSB page data ( FIG. 7C ) is sent to the flash die  194  ( FIG. 4 ), which loads it into the LSB page buffer portion  201   b  ( FIG. 6 ) of the flash memory buffer  201 ; (5) the tier 1/tier 2-encoded LSB page data contained in the LSB page buffer portion  201   b  ( FIG. 6 ) is written, or programmed, to the corresponding LSB page of the flash cells  195  ( FIG. 5 ); (6) the MSB page data ( FIG. 8A ) contained in the buffer  131  ( FIG. 4 ) is subjected to tier 1 ECC encoding; (7) the tier 1-encoded MSB page data ( FIG. 8B ), with other data bits concatenated to it ( FIG. 8C ) to make it the same size as the concatenated LSB frame shown in  FIG. 7C , is then sent to the flash die  194 , which loads the tier 1-encoded MSB page data into the MSB page buffer portion  201   a  ( FIG. 6 ); (8) the tier 1/tier 2-encoded LSB page data is read from the LSB page of flash memory cells  195  ( FIG. 5 ) and sent to tier 2 ECC decoding logic  210  ( FIG. 6 ) of the flash die  194 ; (9) the tier 2 ECC decoding logic  210  ( FIG. 6 ) of the flash die  194  ( FIGS. 4 and 5 ) performs tier 2 ECC decoding to produce an error-corrected tier 1-encoded LSB page data and loads the error-corrected tier 1-encoded LSB page data into the LSB page buffer portion  201   b ; (10) the error-corrected tier 1-encoded LSB page data and the tier 1-encoded MSB page data contained in the LSB page buffer  201   b  and the MSB page buffer portion  201   a , respectively, are used by the reference voltage determination logic  200  ( FIG. 6 ) of the flash die  194  to determine the proper reference voltage ranges for the corresponding flash cells  195  ( FIG. 5 ) of the die  194 ; and (11) the reference voltage determination logic  200  ( FIG. 6 ) programs the corresponding flash cells  195  ( FIG. 5 ) to the proper reference voltage ranges. 
     An example of the manner in which steps (1)-(11) are performed during LSB and MSB page programming in accordance with an illustrative embodiment will now be described. The SSD controller  100  receives data to be written to the NVM  199  from the host system  102 . For example purposes, it will be assumed that eight pages (LSB pages 0, 1, 3, and 5 and MSB pages 2, 4, 6, and 8) of data are to be written to the flash die portion  194   a  ( FIG. 5 ). The eight pages of data are received in the SSD controller  100  from the host system  102 . The invention is not limited with respect to the size of the data frames that are transferred from the host system  102  to the SSD controller  100 . It will also be assumed that the data is temporarily stored in the buffer  131 , although the buffer  131  and the buffering step are optional. 
     The SSD controller  100  loads LSB page 0, LSB page 1, MSB page 2 and MSB page 4 data into buffer  131 . The ECC module  161  uses the tier 1 encoder/decoder  161   a  and the tier 2 encoded/decoder  161   b  to perform tier 1 and tier 2 ECC encoding, respectively, on the LSB page 0 data such that the LSB page 0 data has the configuration shown in  FIG. 7C . The SSD controller  100  sends the tier 1/tier 2-encoded LSB page 0 data to the flash die  194 , which loads the LSB page 0 data into the LSB page buffer portion  201   b  ( FIG. 6 ) of flash die  194 . In a typical configuration of the flash die  194 , the MSB and LSB page buffer portions  201   a  and  201   b  ( FIG. 6 ), respectively, each have a capacity for holding a single page of MSB and LSB data, respectively, although the invention is not limited with respect to the storage capacity of buffer portions  201   a  and  201   b . Logic (not shown) inside of the flash die  194  then writes the tier 1/tier 2-encoded LSB page 0 data held in LSB page buffer portion  201   b  to the corresponding flash cells  195  ( FIG. 5 ) connected to word line  196   a  ( FIG. 5 ). 
     The ECC module  161  uses the tier 1 encoder/decoder  161   a  and the tier 2 encoded/decoder  161   b  to perform tier 1 and tier 2 ECC encoding, respectively, on the LSB page 1 data such that the LSB page 1 data has the configuration shown in  FIG. 7C . The SSD controller  100  sends the tier 1/tier 2-encoded LSB page 1 data to the flash die  194 , which loads the LSB page 1 data into the LSB page buffer portion  201   b  ( FIG. 6 ) of flash die  194 , thereby overwriting the LSB page 0 data. Logic (not shown) inside of the flash die  194  then writes the LSB page 1 data to corresponding flash cells  195  connected to word line  196   b  ( FIG. 5 ). 
     The ECC module  161  ( FIG. 4 ) uses the tier 1 encoder/decoder  161   a  to perform tier 1 ECC encoding on the MSB page 2 data such that the MSB page 2 data has the configuration shown in  FIG. 8B . Additional data bits are added to the MSB page 2 data such that it has the configuration shown in  FIG. 8C . The SSD controller  100  sends the tier 1-encoded MSB page 2 data to the flash die  194 , which loads the tier-1-encoded MSB page 2 data into the MSB page buffer portion  201   a  ( FIG. 6 ) of flash die  194 . The tier 2 decoding logic  210  reads the tier 1/tier 2-encoded LSB page 0 data from the LSB page of flash cells  195  ( FIG. 5 ), performs tier 2 ECC decoding on the LSB page 0 data to produce error-corrected tier 1-encoded LSB page 0 data, and loads the error-corrected tier 1-encoded LSB page 0 data into the LSB page buffer portion  201   b  ( FIG. 6 ). The reference voltage determination logic  200  ( FIG. 6 ) then uses the tier 1-encoded MSB page 2 data and the error-corrected tier 1-encoded LSB page 0 data contained in the MSB and LSB page buffer portions  201   a  and  201   b , respectively, to determine the proper reference voltage ranges to be programmed for the flash cells  195  connected to word line  196   a  ( FIG. 5 ). Because the manner in which the LSB and MSB data pair is used to determine the proper reference voltage ranges has been described above with reference to  FIG. 2 , it will not be described again herein in the interest of brevity. 
     The ECC module  161  ( FIG. 4 ) uses the tier 1 encoder/decoder  161   a  to perform tier 1 ECC encoding on the MSB page 4 data. The SSD controller  100  then sends the tier 1-encoded MSB page 4 data ( FIG. 8C ) to the flash die  194 , which loads the encoded MSB page 4 data into the MSB page buffer portion  201   a  ( FIG. 6 ). The tier 2 decoding logic  210  reads the tier 1/tier 2-encoded LSB page 1 data from the LSB page of flash cells  195  ( FIG. 5 ), performs tier 2 ECC decoding on the LSB page 1 data to correct errors in the LSB page 1 data, and loads the error-corrected LSB page 1 data into the LSB page buffer portion  201   b  ( FIG. 6 ). The reference voltage determination logic  200  ( FIG. 6 ) then uses the error-corrected tier 1-encoded LSB page 1 data and the tier 1-encoded MSB page 4 data contained in the LSB and MSB page buffer portions  201   b  and  201   a , respectively, ( FIG. 6 ) to determine the proper reference voltage ranges to be programmed for the flash cells  195  connected to word line  196   b  ( FIG. 5 ). 
     The SSD controller  100  loads LSB pages 3 and 5 and MSB pages 6 and 8 into buffer  131  ( FIG. 4 ). The ECC module  161  ( FIG. 4 ) uses the tier 1 encoder/decoder  161   a  and the tier 2 encoded/decoder  161   b  to perform tier 1 and tier 2 ECC encoding, respectively, on the LSB page 3 data and sends the tier 1/tier 2-encoded LSB page 3 data to the flash die  194 , which loads the LSB page 3 data into the LSB page buffer portion  201   b  ( FIG. 6 ) of flash die  194 . Logic (not shown) inside of the flash die  194  then writes the tier 1/tier 2-encoded LSB page 3 data held in LSB page buffer portion  201   b  ( FIG. 6 ) to the corresponding flash cells  195  ( FIG. 5 ) connected to word line  196   c  ( FIG. 5 ). 
     The ECC module  161  uses the tier 1 encoder/decoder  161   a  and the tier 2 encoded/decoder  161   b  to perform tier 1 and tier 2 ECC encoding, respectively, on the LSB page 5 data and sends the tier 1/tier 2-encoded LSB page 5 data to the flash die  194 , which loads the LSB page 5 data into the LSB page buffer portion  201   b  ( FIG. 6 ) of flash die  194 , thereby overwriting the LSB page 3 data. Logic (not shown) inside of the flash die  194  then writes the LSB page 5 data to corresponding flash cells  195  connected to word line  196   d  ( FIG. 5 ). 
     The ECC module  161  uses the tier 1 encoder/decoder  161   a  to perform tier 1 ECC encoding on the MSB page 6 data. The SSD controller  100  sends the tier 1-encoded MSB page 6 data to the flash die  194 , which loads the MSB page 6 data into the MSB page buffer portion  201   a  ( FIG. 6 ) of flash die  194 . The tier 2 decoding logic  210  reads the tier 1/tier 2-encoded LSB page 3 data from the LSB page of flash cells  195  ( FIG. 5 ), performs tier 2 ECC decoding on the LSB page 3 data to correct errors in the LSB page 3 data, and loads the error-corrected tier 1-encoded LSB page 3 data into the LSB page buffer portion  201   b  ( FIG. 6 ). The reference voltage determination logic  200  ( FIG. 6 ) then uses the tier 1-encoded MSB page 6 data and the error-corrected tier 1-encoded LSB page 3 data contained in the MSB and LSB page buffer portions  201   a  and  201   b , respectively, to determine the proper reference voltage ranges to be programmed for the flash cells  195  connected to word line  196   c  ( FIG. 5 ). 
     The ECC module  161  uses the tier 1 encoder/decoder  161   a  to perform tier 1 ECC encoding on the MSB page 8 data and sends the tier 1-encoded MSB page 8 data to the flash die  194 . Logic inside of the flash die  194  loads the tier 1-encoded MSB page 8 data into the MSB page buffer portion  201   a  ( FIG. 6 ). The tier 2 decoding logic  210  reads the tier 1/tier 2-encoded LSB page 5 data from the corresponding LSB page of flash cells  195  ( FIG. 5 ), performs tier 2 ECC decoding on the LSB page 5 data to correct errors in the LSB page 5 data, and loads the error-corrected tier 1-encoded LSB page 5 data into the LSB page buffer portion  201   b  ( FIG. 6 ). The reference voltage determination logic  200  ( FIG. 6 ) then uses the error-corrected tier 1-encoded LSB page 5 data and the MSB page 8 data contained in the LSB and MSB page buffer portions  201   b  and  201   a , respectively, ( FIG. 6 ) to determine the proper reference voltage ranges to be programmed for the flash cells  195  connected to word line  196   d  ( FIG. 5 ). 
     It should be noted that the invention is not limited with respect to the order in which data is transferred from the host system  102  to the SSD controller  100 , ECC encoded in the SSD controller  100 , transferred from the SSD controller  100  to the flash die  194 , written to the flash cells  195 , read from the flash cells  195 , decoded by the tier 2 ECC decoding logic  210 , loaded into and unloaded from the buffer portions  201   a  and  201   b , loaded into the reference voltage determination logic  200 , transferred from the flash die  195  to the SSD controller  100 , ECC decoded in the SSD controller  100 , and transferred from the SSD controller  100  to the host system  102 . The above example assumes that LSB pages and MSB pages are programmed in a particular order, but the invention is not limited to the LSB and MSB pages being programmed in any particular order, as will be understood by those of skill in the art in view of the description being provided herein. 
       FIG. 9  illustrates a flow diagram that represents the method in accordance with an illustrative embodiment for preventing errors from occurring when programming the reference voltage ranges. Data to be written to flash memory is received in the SSD controller  100  from the host system  102 , as indicated by block  221 . Tier 1 and tier 2 ECC encoding is then performed on the LSB page data, as indicated by block  223 . The tier 1/tier 2-encoded LSB page data is then sent to the flash die  194 , as indicated by block  224 . In the flash die  194 , the tier 1/tier 2-encoded LSB page data is written to the flash cells, as indicated by block  225 . 
     In the SSD controller  100 , tier 1 encoding is then performed in the MSB page data, as indicated by block  226 . The SSD controller  100  then sends the tier 1-encoded MSB page data to the flash die  194 , as indicated by block  227 . In the flash die  194 , the previously-written LSB page data is read from the flash cells and sent to the tier 2 ECC decoding logic  210  of the flash die  194 , as indicated by block  228 . The tier 2 ECC decoding logic  210  then performs tier 2 decoding on the LSB page data to correct errors in the LSB page data, as indicated by block  229 . The error-corrected LSB page data and the MSB page data are then used by the reference voltage determination logic  200  to determine the proper reference voltage ranges for the flash cells  195 , as indicated by block  231 . The flash cells  195  are then programmed with the determined reference voltage ranges, as indicated by block  232 . 
     It should be noted that the tier 2 ECC decoding process performed by logic  210  is only performed when data is being written to the flash cells  195 . During a normal read operation, the LSB and MSB page data are transferred from the flash die  194  to the SSD controller  100  without any ECC decoding being performed in the flash die  194 . In the case of a normal LSB page read operation, the tier 2 parity bits that were concatenated onto the LSB page data in the ECC module  161  are discarded from the LSB page read data. The ECC module  161  performs tier 1 ( 161   a ) ECC decoding on the LSB page read data to correct errors in the LSB page data. In the case of a normal MSB page read operation, when the MSB page read data is received in the SSD controller  100 , the ECC module  161  performs tier 1 ECC ( 161   a ) decoding on the MSB page data to correct errors in the MSB page data. Thus, the MSB page read operation is performed in the known manner, whereas the LSB page read operation includes the additional operation of discarding the tier 2 parity bits before performing the tier 1 decoding operation. The tier 2 parity bits may be discarded in the flash die  194 , in the SSD controller  100  or at any location in between the flash die  194  and the SSD controller  100 . 
     It should be understood that the flow diagram of  FIG. 9  is intended only to be exemplary or illustrative of the above-described method. In view of the descriptions herein, persons skilled in the art readily will be capable of programming or configuring an SSD controller or similar system in any of various ways to effectuate the above-described method and similar methods. The process represented by the blocks described above with regard to  FIG. 9  is intended only as an example, and in other embodiments the steps or acts described above and similar steps or acts can occur in any other suitable order or sequence. Steps or acts described above can be combined with others or omitted in some embodiments. Similarly, the logic elements or components described above with regard to  FIGS. 3-6  are intended only as examples of suitable configurations for performing the above-described method. Also, it should be understood that the combination of software instructions or similar logic and the memory in which such software instructions or similar logic is stored or embodied in non-transitory form comprise a “computer-readable medium” or “computer program product” as that term is used in the patent lexicon. 
     It should be noted that the invention has been described with reference to one or more exemplary embodiments for the purpose of demonstrating the principles and concepts of the invention. The invention is not limited to these embodiments. For example, although the above-described exemplary embodiment relates to MLC NAND flash memory, other embodiments can relate to TLC or any other suitable type of flash memory. As will be understood by persons skilled in the art, in view of the description provided herein, many variations may be made to the embodiments described herein and all such variations are within the scope of the invention.