Abstract:
There exists a tradeoff between the fidelity of data storage and the number of bits stored in a memory cell. The number of bits may be increased per cell when fidelity is less important. The number of bits per cell may be decreased when fidelity is more important. A memory, in some embodiments, may change between storage modes on a cell by cell basis.

Description:
This is a continuation of prior application Ser. No. 09/955,282, filed Sep. 18, 2001 now U.S. Pat. No. 6,643,169. 

   BACKGROUND 
   This invention relates generally to memory devices and particularly to memory devices with a multi-level cell architecture. 
   A multi-level cell memory is comprised of multi-level cells, each of which is able to store multiple charge states or levels. Each of the charge states is associated with a memory element bit pattern. 
   A flash EEPROM memory cell, as well as other types of memory cells, is configurable to store multiple threshold levels (V t ). In a memory cell capable of storing two bits per cell, for example, four threshold levels (V t ) are used. Consequently, two bits are designated for each threshold level. In one embodiment, the multi-level cell may store four charge states. Level three maintains a higher charge than level two. Level two maintains a higher charge than level one and level one maintains a higher charge than level zero. A reference voltage may separate the various charge states. For example, a first voltage reference may separate level three from level two, a second voltage reference may separate level two from level one and a third reference voltage may separate level one from level zero. 
   A multi-level cell memory is able to store more than one bit of data based on the number of charge states. For example, multi-level cell memory that can store four charge states can store two bits of data, a multi-level cell memory that can store eight charge states can store three bits of data, and a multi-level cell memory that can store sixteen charge states can store four bits of data. For each of the N-bit multi-level cell memories, various memory element bit patterns can be associated with each of the different charge states. 
   The number of charge states storable in a multi-level cell, however, is not limited to powers of two. For example, a multi-level cell memory with three charge states stores 1.5 bits of data. When this multi-level cell is combined with additional decoding logic and coupled to a second similar multi-level cell, three bits of data are provided as the output of the two-cell combination. Various other multi-level cell combinations are possible as well. 
   The higher the number of bits per cell, the greater the possibility of read errors. Thus, a four bit multi-level cell is more likely to experience read errors than a one bit cell. The potential for read errors is inherent in the small differential voltages used to store adjacent states. If the stored data is potentially lossy, sensitive data stored in relatively high-density multi-level cells may be subject to increased error rates. 
   In many applications, the nonvolatile memories store a large amount of data that is tolerant to a small number of bit errors. Applications may also have a small amount of data that is not tolerant to bit errors. Examples of such applications may include control structures, header information, to mention a few examples. These typical applications, where a relatively small amount of the overall storage requires higher fidelity, may include digital audio players, digital cameras, digital video recorders, to mention a few examples. 
   Thus, there is a need for a way to store a large amount of data in dense multi-level cells while ensuring that sensitive data is stored in a fashion that sufficiently reduces the possibility of read errors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block depiction of one embodiment of the present invention; 
       FIG. 2  is a depiction of a cell in accordance with one embodiment of the present invention; 
       FIG. 3  is a depiction of another cell in accordance with another embodiment of the present invention; 
       FIG. 4  is a depiction of still another cell in accordance with one embodiment of the present invention; and 
       FIG. 5  is a flow chart for software in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a processor  100  may be coupled through a bus  102  to a multi-level cell memory  104 . The memory  104  contains an interface controller  105 , a write state machine  106  and a multi-level cell memory array  150 . The processor  100  is coupled by the bus  102  to both the interface controller  105  and the memory array  150  in one embodiment of the present invention. The interface controller  105  provides control over the multi-level cell memory array  150 . The write state machine  106  communicates with the interface controller  105  and the memory array  150 . The interface controller  105  passes data to be written into the array  150  to the state machine  106 . The state machine  106  executes a sequence of events to write data into the array  150 . In one embodiment, the interface controller  105 , the write state machine  106  and the multi-level cell memory array  150  are located on a single integrated circuit die. 
   Although embodiments are described in conjunction with a memory array  150  storing one, two or four bits per cell, any number of bits may be stored in a single cell, for example, by increasing the number of threshold levels, without deviating from the spirit and scope of the present invention. Although embodiments of the present invention are described in conjunction with a memory array  150  of flash cells, other cells such as read only memory (ROM), erasable programmable read only memory (EPROM) conventional electrically erasable programmable read only memory (EEPROM), or dynamic random access memory (DRAM), to mention a few examples, may be substituted without deviating from the spirit and scope of the present invention. 
   Referring to  FIG. 2 , a cell may include only one bit of data at the first and last states of the cell. In the embodiments shown in  FIGS. 2 ,  3  and  4 , the actual storage of data is indicated by an X and empty states are indicated by dashes. A similarly sized cell, shown in  FIG. 3 , may store two bits per cell at every fifth level within the cell. Likewise, as shown in  FIG. 4 , the same sized cell may store four bits per cell using every single state or level of the sixteen available states in this example. 
   Thus, in some embodiments of the present invention, the number of bits per cell may be changed to increase the fidelity of the stored data. Thus, if density is more important than fidelity, the scheme shown in  FIG. 4  or other higher density schemes may be utilized. Conversely, when fidelity is more important, the data may be spread in the cell, decreasing the density per cell and increasing the number of cells required to store all of the data. With wider spacing between the states that are utilized, the integrity of the data storage will be improved. This is because it is easier to discern the differential voltage between significantly nonadjacent levels. In fact, the greater the distance between the levels, the easier it is to discern a differential voltage. 
   Thus, in the embodiment shown in  FIG. 2 , only two levels are used, and in the embodiment shown in  FIG. 3 , four levels are used. In the embodiment shown in  FIG. 4 , all sixteen levels are utilized in accordance with some embodiments of the present invention. 
   Thus, in some embodiments, data may be stored in varying numbers of bits per cell depending on the type of data involved. Thus, some data may be packed closely as indicated for example in FIG.  4  and other data may be spread farther apart, requiring additional numbers of cells to complete the data storage. 
   Thus, turning to  FIG. 5 , the write algorithm  122 , which may be implemented in software or hardware, initially identifies the number of bits per cell. The number of bits per cell may be derived from information included with the data indicating the desired fidelity. Based on the number of bits per cell, the packing of bits into each given cell may be adjusted. Thus, in some cases, denser packing may be utilized, for example as shown in  FIG. 4 , and in other cases, looser or more spread apart packing may be utilized as shown in FIG.  2 . Once the number of bits per cell has been determined as indicated in block  124 , the packing of bits into each cell is adjusted as indicated in block  126 . Finally the bits are written to the cells as indicated in block  128 . The number of bits per cell may be changed on the fly from cell to cell. 
   The read process simply reverses the flow, ignoring the missing levels, and simply reading the actual data out of each cell. The spread apart data may then be repacked into a continuous data string. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.