A multi bit/level per cell memory includes multi level cells, each of which is able to store multiple charge states or levels. Each charge state is associated with a memory element bit pattern. A multi bit/level per cell memory is able to store more than one bit of data per cell based on the number of charge states for the cell. For example, a four charge state per cell multi bit/level per cell can store two bits of data per cell. Generally speaking, the number of charge states required to store “n” bits of data is 2n. However, the number of charge states storable in a multi bit/level per cell is not limited to powers of two, and a memory cell with three charge states is operable to store 1.5 bits of data, for example.
A flash EEPROM memory cell, as well as many other types of memory cells, may be configurable to store multiple bits per cell by maintaining multiple threshold voltage levels (Vt) at each cell. In a memory cell capable of storing two bits per cell, for example, four threshold voltage levels (Vt) are used. Consequently, two bits are designated for each threshold level. In one implementation, the multi level cell may store four charge states with level three having a higher charge than level two, level two a higher charge than level one and level one 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 voltage reference may separate level one from level zero.
With greater numbers of bits per cell, there is a greater possibility of read errors. Thus, a four bit multi bit/level per 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 bit/level per cell devices may be subject to increased error rates.