Patent Publication Number: US-11651823-B2

Title: Fractional program commands for memory devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/TECHNICAL FIELD 
     This application is a Continuation that claims priority to U.S. application Ser. No. 15/956,647, filed Apr. 18, 2018, entitled “Fractional Program Commands For Memory Devices”, which is a Continuation that claims priority to U.S. application Ser. No. 12/990,945, filed Nov. 3, 2010, entitled “Fractional Program Commands For Memory Devices”, now U.S. Pat. No. 9,966,142, which claims priority from International Application No. PCT/US2009/042952, published as WO 2009/140112 A1 on Nov. 19, 2009, which claims priority from U.S. Provisional Application No. 61/052,710, filed May 13, 2008 and entitled “Fractional Program Commands For Memory Devices”. U.S. application Ser. Nos. 15/956,647, 12/990,945, International Application No. PCT/US2009/042952 and U.S. Provisional Application No. 61/052,710 are hereby incorporated by reference in their entirety. This disclosure relates generally to memory devices, and more specifically, to programming operations for memory devices that utilize incremental programming techniques. 
    
    
     BACKGROUND 
     To increase programming precision and to avoid over-programming, certain types of non-volatile memory are typically programmed using a sequence of program pulse and verify (PV) cycles. Flash memory and resistive random access memory (“ReRAM”) are two types of memory that often require the use of numerous consecutive PV cycles for programming. Each PV cycle typically includes a program pulse that incrementally increases the charge stored on the floating gates of memory cells by applying a program voltage to the memory cells for some duration, as well as a verify operation that determines whether the memory cells have been properly programmed. More specifically, during execution of a program command in a flash memory array, a first PV cycle is initiated that applies a first program pulse to the control gates of the addressed floating gate memory cells to incrementally program data therein, and then the contents of the addressed memory cells are examined to verify that the addressed memory cells are properly programmed. Any memory cells that are not properly (e.g., completely) programmed during the first PV cycle are identified, and then a second PV cycle is initiated for those identified memory cells. This process is repeated until all the memory cells addressed by the program command are completely programmed with the program data. 
     Once the sequence of PV cycles associated with a program command is initiated, all memory cells in the memory array are typically unavailable for other operations (e.g., read operations) until execution of the program command is complete. 
     Thus, there is a need for non-volatile memory that can be accessed in a faster and more efficient manner during programming operations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings, where: 
         FIG.  1 A  shows a simplified block diagram of a memory system including a memory controller and a memory device; 
         FIG.  1 B  shows a simplified block diagram of another memory system including a memory controller and a memory device; 
         FIG.  1 C  shows a simplified block diagram of yet another memory system including a memory controller and a memory device; 
         FIG.  2 A  is an illustrative flow chart depicting an exemplary method of creating a plurality of fractional program commands and creating a command queue; 
         FIG.  2 B  is an illustrative flow chart depicting an exemplary method of operation of the memory device of  FIG.  1 A ; 
         FIG.  3 A  depicts a plurality of independently executable fractional program commands stored in the command queue of  FIG.  1 A ; 
         FIG.  3 B  depicts a non-program command inserted at the top of the command queue of  FIG.  1 A ; 
         FIG.  3 C  depicts a non-program command inserted after a first fractional program command in the command queue of  FIG.  1 A ; 
         FIG.  4 A  depicts a plurality of separate and independently executable program pulse and verify commands stored in the command queue of  FIG.  1 A ; 
         FIG.  4 B  depicts a non-program command inserted between a program pulse and a corresponding verify command in the command queue of  FIG.  1 A ; 
         FIG.  5    shows a functional block diagram of one embodiment of the memory device of  FIG.  1 A ; and 
         FIG.  6    shows a block diagram of a computer system within which present embodiments can be employed. 
     
    
    
     Like reference numerals refer to corresponding parts throughout the drawing figures. 
     DETAILED DESCRIPTION 
     A memory system is disclosed that allows an array of memory cells to be accessed and otherwise utilized during a programming operation. In accordance with some embodiments, the programming operation is performed using a plurality of separate, independently executable fractional program commands that can be selectively provided to the array of memory cells in a pipelined manner via a command pipeline. Other non-program commands (e.g., read commands, erase commands, and so on) can be selectively inserted into the command pipeline between the fractional program commands so that the non-program commands can be executed in the memory array while the programming operation is in progress without interrupting execution of any of the fractional program commands. 
     As used herein, the term “non-volatile” refers to memory that retains data for an appreciable time after power loss, thereby enabling a host system to be powered off (power supplies fully discharged) and then back on (a power cycle) with a full expectation of data retention. Examples of non-volatile semiconductor memory technologies may include flash memory, phase change memory, resistance change memory, FRAM (ferroelectric RAM), MRAM (magnetic RAM) and other similar solutions. “Flash” memory includes both NAND and NOR core cell structures as well as floating gate and charge-trapping technologies (e.g. Silicon-Oxide-Nitride-Oxide-Silicon or “SONOS” technology.) Various effects from either aging or wear may lead to a semiconductor memory cell having progressively reduced retention interval, such that a particular memory may ultimately change in character from non-volatile to volatile. 
     Though discussed in the context of flash memories, the embodiments described herein can be extended to any technologies that rely on multiple steps which incrementally program a semiconductor memory cell (e.g., erase before write, multiple program steps, use of PV cycles, etc.), on significant programming time, or on programming in minimum unit sizes. 
     For the exemplary memory systems described herein, a control circuit associated with the memory array creates a plurality of fractional program commands in response to a program command (e.g., received from an external source such as a host device, a host controller, a microprocessor, control software, and so on) that addresses a number of the array&#39;s memory cells for a programming operation to program data therein. The fractional program commands, which can be stored in a command queue (e.g., associated with the control circuit), are selectively provided to the memory array as independently executable commands via the command pipeline. In response to each fractional program command, a program pulse is applied to the control gates of the memory cells addressed by the program command to incrementally program data therein, and a verify operation is performed to verify that the addressed memory cells have been properly programmed. Verify data can be generated to indicate the results of the verify operation. If the verify data indicates that additional program pulses are needed, the control circuit can then apply the next fractional program command to the memory array. Conversely, if the verify data indicates that all the addressed memory cells are properly programmed, the control circuit can terminate the programming operation. For some embodiments, the control circuit can be provided within the memory device. For other embodiments, the control circuit can be provided within a memory controller coupled to the memory device. 
     The duration of the program pulse associated with each fractional program command is a selected fraction of the total programming time typically required to program the memory cells. Therefore, executing each fractional program command of the present embodiments requires only a fraction of the time necessary to perform the entire programming operation, and therefore requires only a fraction of the time typically required to execute a conventional program command (e.g., that applies a continuous sequence of PV cycles to the memory array). Accordingly, by selectively providing the fractional program commands to the memory array as separate, independently executable commands in a pipelined manner, the control circuit can retain control of and schedule the application of individual program pulses to the addressed memory cells during programming operations. As a result, the control circuit can schedule the execution of a non-program command in other portions (e.g., other pages) of the memory array not addressed by the program command while the programming operation is in progress by inserting the non-program command ahead of one or more of the fractional program commands in the command pipeline. For example, while a programming operation addressing a page of a flash memory array is in progress, data can be quickly read from memory cells in other pages of the memory array by inserting a read command ahead of all yet-to-be executed fractional program commands in the command pipeline. 
     Although programming operations are sometimes described herein as addressing one or more pages of memory, other minimal units of storage area of the memory array can be addressed, depending upon the organization of the memory cells within the memory array. 
     It should be noted that the various circuits disclosed herein may be described using computer aided design tools and expressed (or represented), as data and/or instructions embodied in various computer-readable media, in terms of their behavioral, register transfer, logic component, transistor, layout geometries, and/or other characteristics. Formats of files and other objects in which such circuit expressions may be implemented include, but are not limited to, formats supporting behavioral languages such as C, Verilog, and VHDL, formats supporting register level description languages like RTL, and formats supporting geometry description languages such as GDSII, GDSIII, GDSIV, CIF, MEBES and any other suitable formats and languages. Computer-readable media in which such formatted data and/or instructions may be embodied include, but are not limited to, non-volatile storage media in various forms (e.g., optical, magnetic or semiconductor storage media) that may be used to transfer such formatted data and/or instructions through wireless, optical, or wired signaling media or any combination thereof. Examples of transfers of such formatted data and/or instructions include, but are not limited to, transfers (uploads, downloads, e-mail, etc.) over the Internet and/or other computer networks via one or more data transfer protocols (e.g., HTTP, FTP, SMTP, etc.). 
     When received within a computer system via one or more computer-readable media, such data and/or instruction-based expressions of the above described circuits may be processed by a processing entity (e.g., one or more processors) within the computer system in conjunction with execution of one or more other computer programs. These computer programs may include, without limitation, net-list generation programs, place and route programs and the like, to generate a representation or image of a physical manifestation of such circuits. Such representation or image may thereafter be used in device fabrication, for example, by enabling generation of one or more masks that are used to form various components of the circuits in a device fabrication process. 
       FIG.  1 A  shows a simplified block diagram of an exemplary memory system  100 A. The memory system  100 A includes a flash memory controller  110  and a flash memory device  120 A, and is connected to a host device  150 . The flash memory controller  110  and the flash memory device  120 A are formed on the same integrated circuit (IC) chip, as depicted in  FIG.  1 A . For other embodiments, the flash memory controller  110  and the flash memory device  120 A can be formed on separate IC chips. 
     The host device  150  can be any suitable host device or controller that provides commands to the flash memory controller  110  for execution in the flash memory device  120 A. For example, the host device  150  can be the host controller in a computer system that controls communication between a central processing unit (CPU) and the flash memory system. The host device  150  can execute software that implements one or more of the present embodiments, as described in more detail below. 
     The flash memory controller  110  includes a first port connected to the host device  150  via a connection  101 , and a second port connected to the flash memory device  120 A via input/output (I/O) buses  102  and a control bus  103 . The I/O buses  102  include a data bus for transmitting data (DATA) between the flash memory controller  110  and the flash memory device  120 A, an address bus for providing address information (ADDR) to the flash memory device  120 A, and a command pipeline for providing commands (CMD) to the flash memory device  120 A. The control bus  103  routes control signals (CTRL) between the flash memory controller  110  and the flash memory device  120 A. For other embodiments, some of the I/O buses  102  and/or the control bus  103  can be combined. Further, although not shown for simplicity, clock and other control signals can be sent from the flash memory controller  110  to the flash memory device  120 A via the control bus  103 . 
     The flash memory controller  110  also includes a control circuit  111 , a command queue  112 , and a program address register  113 . Control circuit  111  receives commands from the host device  150 , and in response thereto generates and transmits commands and associated control signals to the memory device  120 A. For some embodiments, commands generated by the control circuit  111  can be queued in a desired order of execution in the command queue  112 , and thereafter selectively provided to the memory device  120 A in a pipelined manner via the command pipeline. The command queue  112  can be any suitable queue, register file, or other storage element. The program address register  113 , which is coupled to the control circuit  111 , can maintain an address list that indicates which pages of memory cells in the memory device  120 A are currently being programmed. 
     For exemplary embodiments described herein, the host device  150  and the flash memory controller  110  are separate devices, as illustrated in  FIG.  1 A . However, for other embodiments, the flash memory controller  110  can be included within the host device  150 , for example, so that the command queue  112  and the program address register  113  are included within and directly controlled by the host device  150 . For such embodiments, operating software executed by the host device  150  can control the creation and issuing of fractional program commands to the memory device  120 A. 
     The memory device  120 A includes I/O pins  121 , control pins  122 , a control circuit  130 , and a flash memory array  140 . The I/O pins  121  connect the control circuit  130  to the flash memory controller  110  via the I/O buses  102 , and the control pins  122  connect the control circuit  130  to the flash memory controller  110  via the control bus  103 . The memory array  140  can be any well-known memory array that includes a plurality of memory cells and associated control circuits (not shown in  FIG.  1 A  for simplicity). For example, the memory array  140  can include flash memory cells arranged in a NAND, NOR, or other suitable configuration. For exemplary embodiments described herein, the memory cells of memory array  140  are floating gate transistors that can be programmed by applying a plurality of program pulses to their control gates, where each program pulse is a suitably high program voltage that incrementally increases the Vt of the memory cells selected for programming. For other embodiments, the memory array  140  can include other types of non-volatile memory cells such as EPROM cells, EEPROM cells, and/or charge trapping device memory cells (e.g., SONOS flash cells). Further, the memory cells of the array  140  can be physically and/or logically divided into any number of banks, blocks, segments, sectors, or pages that can be independently operated. Moreover, although shown in  FIG.  1 A  as including one memory array  140 , the memory device  120 A can include a plurality of memory arrays  140 . 
     The control circuit  130 , which is coupled to the I/O pins  121 , to the control pins  122 , and to the memory array  140 , is configured to control operations (e.g., programming, reading, and erasing operations) to the memory array  140  in response to commands and/or control signals received from the flash memory controller  110 . The control circuit  130  also includes a status register  131  that stores status data indicating the progress of various operations in the memory array  140 . The status register  131  can also store verify data that indicates the results of program verify operations in the memory array  140 . The verify data can indicate whether the application of additional program pulses is needed to complete the programming operation, and can also include other information such as, for example, which memory cells are properly programmed, which memory cells are require additional programming, the number of program pulses already applied, and so on. 
     In addition, for some embodiments, during execution of a command in the memory array  140 , the control circuit  130  can assert a busy signal to alert the memory controller  110  that the memory device  120 A is not available, and upon completion of the command, the control circuit  130  can assert a ready signal to alert the memory controller  110  that the memory device  120 A is ready to accept another command. The busy and ready signals may be transmitted by way of a separate signal on the control bus  103 . 
     In accordance with present embodiments, the flash memory controller  110  is configured to control and selectively schedule the application of each of a plurality of individual program pulses to the memory cells of array  140  addressed by a programming operation. To initiate the programming operation in the memory array  140 , the host device  150  issues a program command to the flash memory controller  110  and loads the to-be-programmed data into memory (via a common or independent operation). The program command includes the addresses of the memory cells selected for programming. Typically, the smallest unit of storage area in the flash memory array  140  that can be selected for a programming operation is a page of memory cells. In response to the program command, the flash memory controller&#39;s control circuit  111  creates a number of fractional program commands that collectively embody the programming operation. Each fractional program command is an independent command that instructs the flash memory device  120 A to apply a program pulse to the addressed memory cells for a duration that is a selected fraction of the time typically required to completely program the memory cells. The fractional program commands can be queued in a desired order of execution in the command queue  112 , and then transmitted to the flash memory device  120 A as individually executable commands via the command pipeline CMD. For some embodiments, the control circuit  111  selectively transmits the fractional program commands to the flash memory device  120 A in response to the verify data generated by the flash memory device  120 A. 
     For some embodiments, the program address register  113  can store the page addresses associated with programming operations in progress. In this manner, the control circuit  111  can quickly determine which pages of memory cells in the array  140  are undergoing programming operations by accessing the program addresses stored in the program address register  113 . 
     When the flash memory device  120 A receives a fractional program command from the flash memory controller  110 , its control circuit  130  applies a program pulse of a predetermined duration to the control gates of the addressed memory cells of the memory array  140  to incrementally program data therein. As mentioned above, the program pulse has a duration that is a fraction of the time typically required to completely program the memory cells, and therefore execution of each fractional program command requires only a fraction of the time required to perform the entire programming operation to the memory device  120 A. After application of the program pulse to the addressed memory cells, the control circuit  130  performs a verify operation to determine whether any of the addressed memory cells require additional program pulses. 
     In some embodiments, if the verify operation identifies any of the addressed memory cells that require additional programming, the control circuit  130  can de-assert a verify signal, and in response thereto the flash memory controller  110  can send the next fractional program command (e.g., queued in command queue  112 ) to the flash memory device  120 A. The next fractional program command causes the control circuit  130  to apply another program pulse to the identified memory cells to further program data therein, and to perform another verify operation. This process is repeated until all of the addressed memory cells are properly programmed. When the verify operation indicates that all the addressed memory cells are properly programmed, the control circuit  130  can assert the verify signal. In response thereto, the flash memory controller  110  does not send additional fractional program commands associated with the programming operation to the flash memory device, and the programming operation is terminated. Further, upon completion of the programming operation, the control circuit  111  can remove the corresponding program address from the program write register  113 . 
     If a non-program command (e.g., a read command) is received by the flash memory controller  110  during the programming operation, its control circuit  111  can selectively insert the non-program command into any location in the command queue  112 . For some embodiments, upon receiving the non-program command, the control circuit  111  accesses the program address register  113  and compares the program addresses stored therein with the address associated with the non-program command to determine whether the memory page addressed by the non-program command is currently being programmed with data. If there is not a match, which indicates that the memory page addressed by the non-program command is not currently being programmed, the control circuit  111  can insert the non-program command in the command queue  112 . 
     The non-program command provided by the host device  150  can include priority information that indicates the priority of the non-program command relative to any programming operation being performed in the memory device  120 A. For one example, if the priority information indicates a highest priority, the control circuit  111  can insert the non-program command at the top of the command queue  112  (e.g., ahead of any remaining fractional program commands previously queued therein) so that upon completion of the current fractional program command, the non-program command is sent to the memory device  120 A for immediate execution therein. For another example, if the priority information indicates a lowest priority, the control circuit  111  can insert the non-program command at the bottom of the command queue  112  (e.g., after all remaining fractional program commands previously queued therein) so that the non-program command is sent to and executed by the memory device  120 A after completion of the entire programming operation. Alternatively, the control circuit  111  can be configured to automatically insert the non-program command at the top of the command queue  112 . 
     Conversely, if there is a match with a program address stored in the program address register  113 , which indicates that the memory page addressed by the non-program command is currently being programmed, the control circuit  111  can “hold” the non-program command until the programming operation to the addressed memory page is complete (e.g., as indicated by the removal of the corresponding address entry in the program address register  113 ), and thereafter insert the non-program command at the top of the command queue  112 . In this manner, the memory cells of the array  140  being programmed are not accessed for data retrieval until the programming operation is complete, thereby preventing the retrieval of partially programmed (e.g., and therefore possibly incorrect) data from the memory array  140 . Alternatively, a copy of the data being programmed in the memory cells may be temporarily stored for access while the programming operation is in progress. 
     By creating a plurality of individual fractional program commands in response to a single program command received from the host device  150 , the programming operation is effectively divided into a number of shorter, independently executable commands that each tie up the memory device  120 A for less time than would the original program command. In this manner, the memory controller  110  can selectively schedule the execution of non-program commands in the memory device  120 A during programming operations thereto, which in turn allows the memory controller  110  to quickly access some portions of the memory device  120 A during programming operations to other portions of the memory device  120 A. For example, by inserting a read command ahead of a number of fractional program commands previously queued in the command queue  112 , the read command can be sent to and executed in the flash memory device  120 A before the fractional program commands. In this manner, data can be read from some pages of the memory array while a programming operation to other pages of the memory array is in progress without interrupting the execution of any of the fractional program commands associated with the programming operation. 
     In addition, the ability to selectively schedule the execution of non-program commands in the flash memory device during programming operations can maximize utilization of the flash memory device  120 A. For example, during a programming operation to a selected page of memory cells, if a series of read commands are received that address data stored in other pages of memory cells, the read commands can be sent to and executed in the flash memory device  120 A immediately upon completion of the current fractional program command. Thereafter, any remaining fractional program commands associated with the programming operation can be executed in the flash memory device while the retrieved data is being processed (e.g., by the host device  150 ). In this manner, the programming operation can be completed in the flash memory device  120 A while the host device  150  is processing the retrieved data, thereby minimizing idle time of the flash memory device  120 A. 
     As mentioned above, during a programming operation, the flash memory device  120 A generates program result verify data that indicates whether the memory cells have been properly programmed. For some embodiments, the flash memory controller  110  can read the verify data from the flash memory device&#39;s status register  131  during the programming operation (e.g., after the verify operation of each fractional program command associated with the programming operation). For other embodiments, the flash memory device  120 A can be configured to send a verify signal to the flash memory controller  110 A upon completion of each fractional program command. For example, if the verify operation identifies any of the memory cells addressed by the program command that are not completely programmed after application of the corresponding program pulse, the flash memory device  120 A can de-assert the verify signal to instruct the flash memory controller  110  to send the next fractional program command to the flash memory device  120 A. Conversely, if the verify operation does not identify any un-programmed memory cells in the array  140 , the flash memory device  120 A can assert the verify signal to alert the flash memory controller  110  that the programming operation is complete, that additional fractional program commands are not needed, and that the page addresses corresponding to the programming operation can be removed from the program address register  113 . In this manner, the flash memory controller  110  may be continually alerted to the incremental progress of each programming operation within the flash memory device  120 A. This is in contrast to conventional flash memory systems in which the flash memory controller delegates the entire programming operation to the flash memory device, during which the entire flash memory array is typically unavailable until the whole programming operation is completed. 
     As mentioned above, the duration of the program pulse applied to the array&#39;s memory cells in response to each fractional program command is a fraction of the total program time typically required to completely program the memory cells of the array  140 . The duration of each program pulse can be selected by a pulse width (PW) signal. The PW signal can be provided to the flash memory controller  110  with the program command, or alternately provided to the flash memory controller  110  or to the flash memory device  120 A by another suitable circuit. For one example, the PW signal can specify a predetermined duration for the program pulse generated in response to each fractional program command. For another example, the flash memory controller  110  and/or the flash memory device  120 A can be configured to incrementally increase the program pulse duration for sequential fractional program commands associated with a singular programming operation. 
     Further, for some embodiments, the program command sent to the flash memory controller  110  by the host device  150  can specify how many fractional program commands are to be executed in the flash memory device  120 A in response to the program command. For one embodiment, the memory controller  110  can send a fixed number of fractional program commands to the flash memory device  110 A. For another embodiment, the memory controller  110  can be configured to send a minimum number of fractional program commands to the flash memory device  120 A prior to checking the status of the verify signal. For other embodiments, the program command sent to the flash memory controller  110  by the host device  150  can include a limit value (LIM) indicating a maximum number of fractional program commands that the flash memory controller  110  creates in response to the program command. For another embodiment, the flash memory controller  110  can be configured to create only one fractional program command in response to the program command received from the host device  150 , and to subsequently create additional fractional program commands as necessary to properly program the array&#39;s memory cells (e.g., in response to the verify data). 
     In addition, the program command issued by the host device  150  can include grouping information that instructs the flash memory controller  110  to group a minimum number of fractional program commands together for execution in the flash memory device  120 A, for example, to prevent an intervening non-program command from being inserted between the grouped fractional program commands in the command queue  112 . The grouping of fractional program commands can ensure that the memory cells addressed for a programming operation are programmed by some predetermined minimal amount prior to the execution of a non-program command to other memory cells of the array  140  while the programming operation is in progress. 
     An exemplary operation of the flash memory device  120 A is described below with respect to the illustrative flow chart of  FIGS.  2 A- 2 B . First, the host device  150  issues a program command that provides program data and address information to the flash memory controller  110  (step  201 ). In response thereto, the flash memory controller  110  creates a plurality of fractional program commands (step  202 ). As described above, execution of each independent fractional program command applies a single program pulse to the addressed memory cells in the memory array  140 , where the duration of each program pulse represents a fraction of the total time typically required to program the array&#39;s memory cells. Then, the fractional program commands are stored in the command queue  112  in the desired order of execution, and the addresses of the memory cells to be programmed are stored in the program address register  113  (step  203 ). For example,  FIG.  3 A  depicts a plurality of fractional program commands  301 ( 1 )- 301 ( n ) stored in corresponding locations  112 ( 1 )- 112 ( n ) of the command queue  112 . 
     If the flash memory controller  110  receives another, intervening non-program command (e.g., a read command or an erase command) from the host device  150 , as tested at step  204 , its control circuit  111  compares the program addresses stored in the program address register  113  with the address associated with the non-program command (step  205   a ). If there is not a match, as tested at step  205   b,  which indicates that the memory page addressed by the non-program command is not currently being programmed, the control circuit  111  selectively inserts the other command into the command queue  112  (step  205   c ). 
     For one example,  FIG.  3 B  depicts a non-program command  302  inserted at the top of the command queue  112  ahead of all fractional program commands  301 ( 1 )- 301 ( n ). For another example,  FIG.  3 C  depicts a non-program command  302  inserted between the first fractional program command  301 ( 1 ) and the second fractional program command  301 ( 2 ) in the command queue  112 . As described above, for some embodiments, the flash memory controller  110  can be configured to selectively insert the non-program command at any location in the command queue  112  (e.g., as depicted in  FIG.  3 C ), and for other embodiments the flash memory controller  110  can be configured to insert the non-program command at the top of the command queue  112  (e.g., as depicted in  FIG.  3 B ). 
     Conversely, if there is a match with a program address stored in the program address register  113 , as tested at  205   b,  which indicates that the memory page addressed by the non-program command is currently being programmed, the control circuit  111  “holds” the conflicting non-program command until the programming operation to the addressed memory page is complete (step  205   d ). For some embodiments, the control circuit  111  can include a buffer (not shown for simplicity) to hold the conflicting non-program commands. For one embodiment, a copy of the data being programmed in the memory cells may be temporarily stored for access while the programming operation is in progress. 
     Thereafter, the control circuit  111  can periodically poll the program address register  113  to determine whether the programming operation to the addressed memory page is complete (step  205   a ). If the programming operation is complete, the control circuit  111  can insert the non-program command into the command queue  112  (step  205   c ). Otherwise, the control circuit  111  continues to hold the conflicting non-program command (step  205   d ). Alternatively, the program address register  113  can send a completion flag to control circuit  111  when the programming operation to the addressed memory page is complete. 
     Then, the flash memory controller  110  forwards the command stored at the top of the command queue  112  to the flash memory device  120 A (step  206 ). If the received command is a fractional program command, as tested at step  207 , the memory device&#39;s control circuit  130  executes the fractional program command to apply a single program pulse to incrementally program data into the addressed memory cells of the array  140  (step  208 ), and then verifies the program data (step  209 ). Then, the flash memory device  120 A generates verify data indicative of the results of the verify operation (step  210 ). 
     For some embodiments, the flash memory device  120 A generates additional status information indicative of the progress of the programming operation (e.g., which memory cells have been completely programmed, which memory cells have not been completely programmed, how many program pulses have been applied to the memory cells, and so on). The verify data and/or the status information can be stored in the flash memory device&#39;s status register  131 , as described above. Further, for some embodiments, the verify data can be used by the flash memory controller  110  to remove the corresponding fractional program command from the command queue  112  and to remove the program address from the program address register  113 . 
     Also, for some embodiments, the flash memory device  120 A can be configured to assert a busy signal during execution of each command in its memory array  140 , and to assert a ready signal upon completion of the command. For such embodiments, the asserted busy signal can instruct the flash memory controller  110  not to send additional commands to the flash memory device  120 A, and the asserted ready signal can alert the flash memory controller  110  that the flash memory device  120 A is ready to accept additional commands. 
     If the verify data indicates that one or more of the addressed memory cells are not completely programmed, as tested at step  211 , processing continues at step  204 . For some embodiments, the flash memory device  120 A can de-assert a verify signal that causes the flash memory controller  110  to send the next fractional program command to the flash memory device  120 A. For other embodiments, the flash memory controller  110  can read the verify data from the flash memory device  120 A. Otherwise, if the verify data indicates that all the memory cells in array  140  addressed by the program command are properly programmed, any remaining fractional program commands associated with the programming operation are removed from the command queue  112 , and the corresponding program address is removed from the program address register  113  (step  212 ). 
     If the command received from the flash memory controller  110  is a non-program command, as tested at step  207 , the flash memory device  120 A executes the non-program command (step  213 ), the non-program command is removed from the command queue  112  (step  214 ), and processing continues at step  204 . For example, if the non-program command is a read command, the flash memory device  120 A executes the read command to read data from the memory cells in the array  140  addressed by the read command, provides the read data to the flash memory controller  110 , and thereafter is ready to accept the next command from the command queue  112 . 
     For other embodiments, the flash memory controller  110  generates only one fractional program command in response to the program command received from the host device  150 , and can selectively create and send additional fractional program commands to the flash memory device  120 A if additional program pulses are required to properly program the addressed memory cells, in which case step  212  can be eliminated. 
     For the exemplary embodiments described above, each fractional program command instructs the flash memory device  120 A to apply a single program pulse to incrementally program the addressed memory cells in the array  140  with the program data, and also instructs the flash memory device to perform a verify operation that compares the data stored in the memory cells being programmed with the original program data to determine whether any of the addressed memory cells require additional program pulses. For other embodiments, each fractional program command generated by the flash memory controller  110  in response to a program command received from the host device  150  can instruct the flash memory device  120 A only to apply the program pulse to the addressed memory cells. For such other embodiments, the flash memory controller  110  can be configured to generate a separate verify command associated with each fractional program command, where each verify command instructs the flash memory device  120 A to verify the data programmed into the addressed memory cells by a corresponding fractional program command. The verify commands can be stored along with the fractional program commands in the command queue  112 . 
     For example,  FIG.  4 A  depicts storing a plurality of fractional program pulse commands  401 ( 1 )- 401 ( n ) and a corresponding plurality of separate verify commands  402 ( 1 )- 402 ( n ) in the command queue  112 , where execution of each fractional program pulse command  401  applies a program pulse to the memory cells addressed for the programming operation, and execution of each verify command  402  verifies the results of the corresponding fractional program pulse command  401 .  FIG.  4 B  illustrates the insertion of a non-program command  302  between the first pair of fractional program pulse and verify commands  401 ( 1 )- 402 ( 1 ) and the second pair of fractional program pulse and verify commands  401 ( 2 )- 402 ( 2 ) in the command queue  112 . Although it is possible to insert the non-program command  302  between a program pulse command  401  and its corresponding verify command  402 , the verify command, in and of itself, is generally not very time consuming, and it may be architecturally preferred to complete a single PV cycle (including verification) before permitting an interleaved command. 
     For other embodiments, the host device  150  and/or software executed by the host device  150  can be configured to generate the fractional program commands associated with a programming operation, to queue the fractional program commands in a command queue, and to selectively send the fractional program commands to the flash memory device  120 A in a pipelined manner (e.g., via flash memory controller  110 ). For such embodiments, the host device  150  and/or software executed by the host device  150  can be configured to selectively insert non-program commands such as read commands into any location in the command queue, for example, so that the non-program commands can be executed in the flash memory device  120 A during a programming operation without interrupting the execution of any program-related commands. 
     For another embodiment, the flash memory device  120 A can be configured to generate the fractional program commands in response to a programming command issued by the host device  150 . For example,  FIG.  1 B  shows a memory system  100 B including the flash memory controller  110  and a flash memory device  120 B in accordance with other embodiments. The flash memory device  120 B is similar to the flash memory device  120 A, except that the control circuit  130  of the flash memory device  120 B includes a command queue  132  and a program address register  133 . The command queue  132  and program address register  133  of  FIG.  1 B  are similar to the command queue  112  and program address register  113  of  FIG.  1 A , respectively. For the embodiment of  FIG.  1 B , the memory device&#39;s control circuit  130  receives the program command issued by the host device  150  (e.g., via the flash memory controller  110 ), and in response thereto generates the fractional program commands. The flash memory controller can queue the fractional commands in the command queue  132  and, as needed, forward these commands to the memory array  140  to apply corresponding program pulses in a manner similar to that described above with respect to  FIG.  1 A . 
     Thus, for the embodiments of  FIG.  1 B , the flash memory device&#39;s control circuit  130  controls the generation, scheduling, and execution of the fractional program commands for the memory array  140 . 
     Embodiments of the flash memory devices described above with respect to  FIGS.  1 A- 1 B  are configured to execute a plurality of independent fractional program commands that collectively embody a programming operation to the memory cells of the array  140 . For other embodiments, the flash memory device can include additional circuitry that can execute the program commands issued by the host device  150  in a conventional manner (e.g., by applying a sequence of continuous PV cycles to the memory cells addressed by the program command). For example,  FIG.  1 C  shows a memory system  100 C including the flash memory controller  110  and a flash memory device  120 C. The flash memory device  120 C includes a control circuit  160  and a mode register  170 . Mode register  170 , which can be any suitable storage element, is coupled to control circuit  160  and can be configured to store program mode data. The control circuit  160  includes all the elements of the control circuit  130  of  FIG.  1 B , and also includes well-known sequence logic  161  that allows the flash memory device  120 C to receive and execute a program command (e.g., received from host device  150  via the flash memory controller  110 ) in a conventional manner. The program mode data can be generated by the host device  150 , by the flash memory controller  110 , or by any other suitable circuit. The mode data is used to instruct the flash memory device  120 C whether to execute fractional program commands received from the flash memory controller  110 , to create and execute fractional program commands in response to a program command received from the flash memory controller  110 , or to implement conventional programming operations in response to a program command received from the flash memory controller  110 . For some embodiments, the mode data is provided to the flash memory device  120 C via the control pins  122  and stored in the mode register  170  (as depicted in  FIG.  1 C ). For other embodiments, the mode data can be provided to and stored in the mode register  170  using dedicated pins (not shown for simplicity) or other suitable circuitry. 
     More specifically, when the program mode data is set to a first state to indicate a first fractional programming mode, the control circuit  160  is configured to receive a fractional program command from the flash memory controller  110 . The control circuitry responsively applies a single program pulse to the addressed memory cells of the array  140  and performs a corresponding verify operation, for example, in the manner described above with respect to  FIG.  1 A . After execution of the fractional program command, the flash memory device  120 C provides a verify signal to the flash memory controller  110  indicating whether additional fractional program commands are needed. The first state of the program mode data can also be used to disable the flash memory device&#39;s command queue  132 , program address register  133 , and sequence logic  161 . 
     When the program mode data is set to a second state to indicate a second fractional programming mode, the control circuit  160  is configured to receive a conventional program command from the flash memory controller  110 . In response thereto, the control circuit  160  generates one or more independently executable fractional program commands that can be queued in the memory device&#39;s command queue  132  and selectively provided to the memory array  140  for execution therein, for example, in the manner described above with respect to  FIG.  1 B . The second state of the program mode data can also be used to enable the memory device&#39;s command queue  132  and program address register  133 , and to disable its sequence logic  161 . 
     When the program mode data is set to a third state to indicate a conventional programming mode, the control circuit  160  is configured to receive a conventional program command from the flash memory controller  110 , and in response thereto instructs the sequence logic  161  to initiate a sequence of PV cycles to program the addressed memory cells of the array  140  according to well-known techniques, during which the entire memory array  140  is typically unavailable. The third state of the program mode data can also be used to disable the flash memory device&#39;s command queue  132  and program address register  133 , and to enable its sequence logic  161 . 
     In this manner, the flash memory device  120 C of  FIG.  1 C  can perform programming operations either using fractional program commands in accordance with present embodiments or using conventional programming techniques, thereby allowing the flash memory device  120 C to be employed in conventional memory systems or in memory systems configured in accordance with present embodiments. 
     Further, for other embodiments, the mode register  170  and associated control circuitry can be added to the flash memory device  120 A of  FIG.  1 A  so that the flash memory device  120 A can either execute fractional program commands received from the flash memory controller  110  or perform conventional programming operations in response to a conventional program command received from the flash memory controller  110  (e.g., depending upon the state of the program mode data). 
       FIG.  5    shows a functional block diagram of a flash memory device  500  that is one embodiment of the flash memory device  120 A of  FIG.  1 A . The memory device  500  includes an I/O control circuit  510 , a control register circuit  520 , command interface logic  530 , a program and erase controller  540 , array control logic  550 , a high-voltage circuit  560 , data buffers  570 , and a memory array  580 . Together, the I/O control circuit  510 , the control register circuit  520 , the command interface logic  530 , the program and erase controller  540 , the array control logic  550 , the high-voltage circuit  560 , and the data buffers  570  form one embodiment of the control circuit  130  of  FIG.  1 A , and the memory array  580  forms one embodiment of the memory array  140  of  FIG.  1 A . 
     The I/O control circuit  510  is coupled to the memory device&#39;s I/O pins, to the control register circuit  520  via control lines  511 - 513 , and to the data buffers  570  via data lines  514 . The I/O control circuit  510 , which is well-known, receives data to be programmed into the memory array  580  via the I/O pins, and provides data read from the memory array  580  to the I/O pins. The control register circuit  520  includes an address register  521 , a status register  522 , and a command register  523 . The address register  521  is coupled to the memory array  580 , the status register  522  is coupled to the command interface logic  530 , and the command register  523  is coupled to the command interface logic  530 . 
     Memory array  580  includes an array  581  of non-volatile memory cells (not shown for simplicity), a row decoder  582 , and page buffers  584 . The memory cells contained in the array  581  can be any suitable non-volatile memory cells such as, for example, those formed using floating gate transistors. The row decoder  582  selects a row of the memory cells for array operations (e.g., program and read operations) in response to address information provided by the address register  521 . The page buffers  584  store data to be programmed into the memory array  581  during programming operations and store data read from the memory array  581  during read operations. 
     The command interface logic  530  is coupled to the device control pins, and is also coupled to the status register  522 , to the program and erase controller  540 , to the array control logic  550 , to the data buffers  570 , and to the page buffers  584 . In operation, the command interface logic  530  receives the fractional program commands from the flash memory controller  110  (see also  FIG.  1 A ), and in response thereto provides control signals to the array control logic  550 , to the program and erase controller  540 . The command interface logic  530  also responsively provides control over the data buffers  570  that generate and apply the program pulse to the memory cell array  581 . For other embodiments, the command interface logic  530  includes or is associated with the command queue  132  (not shown in  FIG.  5   ), and is configured to create a plurality of fractional program commands in response to a program command received from the flash memory controller  110 . 
     The program and erase controller  540  includes an input to receive control signals from the command interface logic  530  and includes an output to provide control signals to the array control logic  550 . The array control logic  550  receives commands from the command interface logic  530  and control signals from the program and erase controller  540 , and includes outputs coupled to the HV circuit  560 , to the row decoder  582 , and to the page buffers  584 . 
     The HV circuit  560  includes inputs coupled to the array control logic  550 , and includes outputs coupled to the memory array cells  581  and to the row decoder  582 . The HV circuit  560  provides high-voltage signals to the memory array  580  during program and erase operations in response to control signals provided by the array control logic  550 . 
     The data buffers  570  are coupled to the command interface logic  530  and to the I/O control circuit  510 . In operation, the data buffers  570  receive control signals from the command interface logic  530 , and either receive data from the I/O control circuit  510  during program operations or provide data read from the memory array  580  to the I/O control circuit  510  during read operations. The data buffers  570  are used to buffer data during program and read operations, for example, because the width of the data bus connected between the flash memory controller  110  and the I/O control circuit  510  is typically much less than the width of a memory page in the array  581 . For example, in some embodiments, the memory device  500  can receive only one byte (e.g., eight data bits) at a time from the flash memory controller  110 , and the page buffer  584  can be used as a cache to accumulate data for simultaneous page-sized program or read operations with the memory array  581 . 
       FIG.  6    shows an exemplary computer system  600  within which present embodiments can be employed. The system  600  includes a central processing unit (CPU)  611 , a host controller  612 , an I/O controller  613 , a DRAM  620 , I/O devices  630 , the flash memory controller  110 , and the flash memory device  120 . The CPU  611  is connected to the host controller  612  and the I/O controller  613  via well-known buses. The CPU  611 , the host controller  612 , and the I/O controller  613  are well-known, and together form a host device  610  that is one embodiment of the host device  150  of  FIGS.  1 A- 1 C . For some embodiments, the CPU  611  and the host controller  612  can be formed on the same chip. For other embodiments, the CPU  611 , the host controller  612 , and the I/O controller  613  can be formed on the same chip. 
     The I/O controller  613  is coupled to the I/O devices  630 , the latter potentially including well-known components such as, for example, a monitor, keyboard, mouse, and other I/O devices or circuits. The host controller  612  may be coupled to a DRAM system  620 , which is employed as main memory for the computer system  600 , as well as to the flash memory device  120  via the flash memory controller  110 . For other embodiments, the flash memory controller  110  can be connected to the I/O controller  613  instead of the host controller  612 . The flash memory device  120  can also function as main memory for the exemplary computer system shown in  FIG.  6   . For other embodiments, the DRAM  620  can be eliminated, and the flash memory device  120  can be employed as the primary main memory component. 
     Employing present embodiments of the flash memory controller  110  and the flash memory device  120  as main memory in the computer system  600  has several advantages over conventional flash memory systems. First, by programming the flash memory device  120  using a plurality of fractional program commands that are independently executable, the flash memory controller  110  can access the flash memory device  120  for other operations (e.g., read operations) during a programming operation. More specifically, because each fractional program command can be configured to apply a single program pulse to the addressed memory cells for a duration that is significantly shorter than the total duration of the programming pulse required to completely program the memory cells, the fractional program command requires significantly less time than a complete sequence of PV cycles. As a result, during a programming operation to addressed portions of the memory array, the flash memory controller  110  can quickly access other portions of the memory array by issuing an appropriate command (e.g., that accesses the other portions of the memory array) prior to issuing any more fractional program commands associated with the programming operation. Second, because the flash memory controller  110  schedules the application of individual program pulses to the memory cells during a programming operation, the flash memory controller  110  is aware of the progress of the programming operation, and therefore can also schedule the execution of other non-program commands in the memory device  120  in an efficient manner. This is in contrast to conventional flash memory systems in which the flash memory controller relinquishes control of programming operations to the flash memory device, during which time none of its memory cells are typically available for other operations. 
     In the above description, numerous specific details are set forth such as examples of specific, components, circuits, and processes to provide a thorough understanding of the present embodiments. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present embodiments. In other instances, well known components or methods have not been described in detail in order to avoid unnecessarily obscuring the present embodiments. Thus, although described above in the context of interleaving non-program operations with PV cycles of flash memory devices, the present embodiments are equally applicable to any type of memory device that utilizes incremental programming techniques.