Abstract:
A multi-bit-per-cell flash memory device supports a command such that each invocation of the command by the device&#39;s host changes respective values of one or more types of reference voltage (e.g., all read reference voltages and/or all program verify reference voltages) of the device to respective new values.

Description:
This patent application claims the benefit of U.S. Provisional Patent Application No. 60/806,477, filed Jul. 3, 2006 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to multi-bit-per-cell Flash memory devices, particularly NAND Flash memory devices, with an extended set of commands that enables efficient modification of Flash internal parameter values, in order to achieve improved device performance in “real life” operation. 
     NAND Flash memory devices, available in the market, are operated using a command set that allows execution of basic Flash operations, such as Page read, Page program, Block erase and Status and ID read. Different devices from different vendors may contain slight variations on this command set, but there is always a common group of commands, implemented in all the devices—this group of commands allows execution of the basic operations as mentioned above. As an example, a device MT29F2G08AABWP, manufactured by Micron Technology of Boise, Id., USA, may be considered. A data sheet of this device is available on request from Micron Technology. The following device commands are listed on page 22 of the MT29F2G08AABWP data sheet: 
     PAGE READ 
     PAGE READ CACHE MODE START 
     PAGE READ CACHE MODE START LAST 
     READ for INTERNAL DATA MOVE 
     RANDOM DATA READ 
     READ ID 
     READ STATUS 
     PROGRAM PAGE 
     PROGRAM PAGE CACHE 
     PROGRAM for INTERNAL DATA MOVE 
     RANDOM DATA INPUT for PROGRAM 
     BLOCK ERASE 
     RESET 
     In addition to the “standard” command set that is generally available for the applications, Flash vendors usually implement a set of additional “special” commands for setting values of internal parameters of the Flash operation. These commands are intended to ease debugging and evaluation processes of the Flash device, and as such address each Flash parameter separately, a single parameter per command. 
     As an example of internal Flash device parameters whose values are set by such “special” commands, one may consider programming and read parameters such as read reference voltages, program verify reference voltages, programming voltage steps, etc., as described in Chen et al., U.S. Pat. No. 7,020,017. This patent is incorporated herein by reference for all purposes as if fully set forth herein. 
     However, as the Chen et al. patent suggests, it is sometimes important to modify internal parameter values of a Flash memory device not only for development purposes, but also as a standard practice in a “real life” operation of the Flash memory device. The Chen et al. patent suggests using different settings of Flash parameter values when handling different pages of the Flash memory device, as a countermeasure for a phenomenon of different reliability exhibited by different Word Lines in the Flash memory. Specifically, the Chen et al. patent teaches using different values of read reference voltages and/or program verify reference voltages for different word lines of a multi-bit-per-cell Flash memory device. 
     Another example for the need to modify Flash parameter values in real life applications may be found in an U.S. patent application Ser. No. 11/651,483 “METHOD FOR GENERATING SOFT BITS IN FLASH MEMORIES” to Murin et al. This patent application is incorporated herein by reference for all purposes as if fully set forth herein. 
     As the Murin et al. patent application suggests, in order to obtain Soft Bit information for ECC computation, several readings of the Flash data with different read references for all voltages levels should be performed. 
     In all of the cases described above (changing of Word Line programming settings or reading information for computation of Soft Bits), modification of the values of the multiple Flash parameters (read voltages, program verify voltages, programming parameters) is currently done one-by-one, using “special commands” acceptable by the Flash. However, because numerous parameter values have to be changed for every mode change (especially in a multi-bit-per-cell device, in which the number of voltage level parameters grows exponentially with the number of bits stored in each cell), the operation of changing parameter values is quite cumbersome for the application and is also time consuming. 
     There is thus a widely recognized need for, and it would be highly advantageous to have, a way of changing a full set of Flash parameter values by an application in an efficient way, without the need of issuing a large number of special commands for effecting the mode change. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a multi-bit-per-cell flash memory device that supports a command such that each invocation of the command, by a host of the flash memory device, changes respective present values of all reference voltages, of at least one type, of the flash memory device to respective new values of the reference voltages. 
     Preferably, the command changes respective present values of all reference voltages of a single type. For example, in some preferred embodiments of the present invention the command changes respective present values of all read reference voltages or of all program verify reference voltages. 
     Alternatively, the command changes respective present values of all read reference voltages and of all program verify reference voltage. 
     In one class of preferred embodiments of the present invention, the command has a single argument. Preferably, the flash memory includes a plurality of stored sets of values of the reference voltages of the at least one type, and the single argument is an index of one of the stored sets, so that the new values are the values of the indicated stored set. Alternatively, the single argument is a value to be combined, by an arithmetic operation such as addition (or, equivalently, subtraction if the value is negative) or multiplication (or, equivalently, division if the value is less than  1  in absolute value), with a respective value associated with each reference voltage of the at least one type to yield the new values. Most preferably, the associated values are either default values of the reference voltages of the at least one type or present values of the reference voltages of the at least one type. 
     In another class of preferred embodiments of the present invention, the command has a plurality of arguments, equal in number to the reference voltages of the at least one type. Preferably, the arguments are the new values. Alternatively, the arguments are first respective values to be combined, by a common arithmetic operation such as addition or multiplication, with second respective values associated with the reference voltages of the at least one type to yield the new values. Most preferably, the second respective values are either default values of the reference voltages of the at least one type or present values of the reference voltages of the at least one type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       The sole FIGURE is a block diagram of a Flash memory device of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles and operation of a Flash memory device according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     In order to achieve the above-mentioned goal of concurrently modifying a set of Flash parameter values in an efficient manner, the present invention groups various related parameters, whose values are subject to concurrent change by an application, and assigns 
     to such a group of parameters a corresponding Flash command that, when issued by an application, modifies the values of all the parameters in the group in one operation. 
     Various parameter groups and various methods of the command operation may be implemented. The following are examples of the parameter grouping:
         A group of all read reference voltages in a multi-bit-per-cell flash memory.   A group of all program verify reference voltages in a multi-bit-per-cell flash memory.   A group combining both all read references voltages and all program verify reference voltages in a multi-bit-per-cell flash memory.       

     All three groups may be used to advantage in conjunction with different word lines, as described in the Chen et al. patent. The first group (setting all read reference voltages for all voltage levels) may be used to advantage when obtaining data from the Flash memory for “Soft Bit” computation, as described in the Murin et al. patent application. 
     The following are possible methods of configuring the special commands that affect the multiple parameter value setting operation:
         The command selects one set of parameter values out of several sets, where all parameter values of all sets are preset in advance. For example, the command instructs the flash device to activate set No. 3 of read reference voltages out of 5 possible sets, and the device is designed to know which value of each read reference voltage corresponds to set No. 3.   The command selects an additive or multiplicative offset value for each one of the parameters in the set, where the offset value is specified in the command as a part of the command syntax. The offset may be relative to a default value or relative to the current value. The offset may be the same for all parameters in the group or different for different parameters. For example, the command instructs the Flash device to increase all current read reference voltages by 100 millivolts, or instructs the Flash device to multiply all current read reference voltages by 1.05.   The command explicitly specifies values for each one of the parameters in the group, where these values are part of the command syntax.       

     It should be noted, that the present invention may be implemented in different ways—by designing new commands for a Flash memory device, by writing corresponding data (i.e. parameter set selection or parameter values) to a configuration register in the Flash memory device, or by using existing commands with newly implemented command prefixes that specify specific non-default sets of parameters to be used by the Flash memory device. 
     The FIGURE, that is based on  FIG. 5  of the Chen et al. patent, is a block diagram of one embodiment of a (preferably) multi-bit-per-cell Flash memory device of the present invention. A memory cell array  302  is controlled by a column control circuit  304 , a row control circuit  306 , a c-source control circuit  310  and a p-well control circuit  308 . Column control circuit  304  is connected to the bit lines (BLs) of memory cell array  302  for reading data stored in the memory cells of memory cell array  302 , for determining a state of the memory cells during a program operation, and for controlling potential levels of the bit lines to promote the programming or to inhibit the programming. Row control circuit  306  is connected to the word lines (WLs) of memory cell array  302  to select one of the word lines, to apply read voltages, to apply program voltages combined with the bit line potential levels controlled by column control circuit  304 , and to apply an erase voltage. A C-source control circuit  310  controls a common source line connected to the memory cells. A P-well control circuit  308  controls the p-well voltage. 
     The data stored in the memory cells are read out by column control circuit  304  and are output to external I/O lines via a data input/output buffer  312 . Program data to be stored in the memory cells are input to data input/output buffer  312  via the external I/O lines, and are transferred to column control circuit  304 . The external I/O lines are connected to a controller  318 . 
     Command data for controlling the flash memory device are input to controller  318 . The command data informs the flash memory of what operation is requested. The input command is transferred to a state machine  316  that controls column control circuit  304 , row control circuit  306 , c-source control circuit  310 , p-well control circuit  308  and data input/output buffer  312 . State machine  316  can also output status data of the flash memory such as READY/BUSY or PASS/FAIL. 
     Controller  318  is connected to or connectable with a host system (not shown) such as a personal computer, a digital camera, or personal digital assistant, etc. Controller  318  communicates with the host that initiates commands, such as to store or read data to or from the memory array  302 , and provides or receives such data. Controller  318  converts such commands into command signals that can be interpreted and executed by command circuits  314  that are in communication with state machine  316 . Controller  318  typically contains buffer memory for the user data being written to or read from memory cell array  302 . 
     Controller  318  supports all the prior art host commands that are described in the Field and Background section above. Controller  318  also supports multi-parameter commands of the present invention. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.