Coding method of multi-level memory cell

[0002] This application claims the priority benefit of Taiwan application serial no. 90113161, filed May 31, 2001.

[0003] 1. Field of the Invention

[0004] The invention relates in general to a coding method for a data storage medium, and more particularly, to a coding method of a multi-level memory cell.

[0005] 2. Description of the Related Art

[0006] Multi-level memory cell is a kind of memory cell that comprises n (N>1) bits and has 2ⁿlevels to store one of 2ⁿcodes. Each code is formed with n bits. The value of the 2ⁿcodes is 0 to 2ⁿ⁻1. “Multi-level” means that the memory cell can have multiple threshold voltages V_T. For a certain kind of multi-level memory cell, the threshold voltage is set as one of the 2ⁿlevels for programming. The threshold voltage is then used to determine the stored code for reading. This kind of multi-level memory cell includes electrically erasable programmable read only memory (EEPROM) or mask read only memory (Mask ROM). On the other hand, for the multi-level memory cell in the dynamic random access memory (DRAM), the gate voltage is set to 2ⁿV_T, SO that the capacitor has a certain voltage, and the certain voltage is retrospectively referred to obtain the stored data for reading.

[0007] The conventional coding method of a multi-level memory cell is to arrange according to the values of 2ⁿcodes, and then correspond to each level from low to high sequentially. In FIG. 1, a multi-level memory cell with 2 bits is used as an example. In the conventional method, the codes are (11), (10), (01) and (00) with respect to the first to the fourth levels from low to high. In FIG. 2, a multi-level memory cell with 3 bits is used as an example. The codes are (111), (110), (101), (100), (011), (010), (001) and (000) with respect to the first level 201 to the eighth levels 208, from low to high.

[0008] Although the multi-level memory cell can store massive amounts of information, the large number of levels causes errors of storing another code of the level before or after the exact level of which a specified code is to be stored. Thus, the error code correction (ECC) is very important. Such error code correction compares the actual storage value to the predetermined storage value after programming the memory cell. The error address of the memory cell and the bit record requiring correction are saved into an additional memory cell.

[0009] A common error code correction rule includes a 2^q+2q−1 rule. That is, 2q−1 additional spaces are given to store the error code correction data in each 2^qbits of data of a memory, whereas, 2q−2 bits are used to stored the address of the error memory cells and the remaining one bit is used to store the corrected bit. When the probability of the error occurrence for data storage is high, the value of q has to be reduced to increase the proportion of the error code correction memory cells ((2q−1)/2^q). That is, with the same data storage spaces, more additional spaces are consumed to store error data, and vice versa.

[0010] In FIG. 1, in the conventional coding method of a 2 bit multi-level memory cell, the bit difference between two codes of two neighboring levels is 1, 2, 1, sequentially. That is, if one mistakenly writes (01) as (10) or (10) as (01), the corrected bit number to be stored is 2. Referring to FIG. 2, in the conventional coding method of a 3 bit multi-level memory cell, the bit difference between two codes of two neighboring levels is 1, 2, 1, 3, 1, 2, 1 in sequence. The corrected bit number is even more. In the conventional coding method, more than one error bits are generated when one memory cell is wrongly written. More storage spaces of the memory are thus consumed.

[0011] The invention provides a coding method of a multi-level memory cell applied to a programming operation of a multi-level memory cell. The multi-level memory cell can store n bits and has 2ⁿlevels with respect to 2ⁿcodes. Each code comprises n bits. In the coding method, a code to be stored is provided. According to a relationship between the code and level, a specified level of the code to be stored is obtained by the multi-level memory cell. The relationship between the code and the level is a correspondence between the 2ⁿcodes and the 2ⁿlevels. Two codes of any two corresponding neighboring levels have a one bit difference.

[0012] In the above coding method of multi-level memory cell, the range of n is 2≦n≦4.

[0013] The invention further provides a programming method of a multi-level memory cell. Data is written to a multi-level memory cell using the above coding method. An error code inspection step is performed. If the actual stored code of any memory cell has one bit different from the predetermined stored code, the address of this multi-level memory cell and the different bit are written into an additional error code correction memory cell.

[0014] As mentioned above, since two codes of two neighboring levels only have one bit difference, whatever n is, when a specified code of a corresponding level is mistakenly written as another code of the level right before or after such certain level, only one additional correction bit is required. The number for a correction bit is thus decreased, and the space occupied by storing the correction bit is saved.

[0015] Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0016] FIG. 1 shows a coding method of a 2-bit multi-level memory cell and the difference in bit number between two neighboring levels according to conventional art;

[0017] FIG. 2 shows a coding method of a 3-bit multi-level memory cell and the difference in bit number between two neighboring levels according to conventional art;

[0018] FIG. 3 shows a coding method of a 2-bit multi-level memory cell and the difference in bit number between two neighboring levels according to this invention; and

[0019] FIG. 4 shows a coding method of a 3-bit multi-level memory cell and the difference in bit number between two neighboring levels according to this invention.

[0020] As the adjusting and modifying process of the multi-level memory cell is not the features of the invention, and a description has been given as above, the description of the invention is focused on the relationship between the code-level used in the coding method hereinafter.

[0021] FIG. 3 shows the relationship between the code and the level for a 2 bit multi-level memory cell and the bit difference of the two-bit codes of two neighboring two levels. As shown in FIG. 3, the codes of the first level 301 to the fourth level 304 include (00), (10), (11) and (01) from low to high. The codes between two neighboring levels has a one-bit difference.

[0022] FIG. 4 shows the relationship between the code and level for a 3-bit multi-level memory cell, and the bit difference between the codes corresponding to two neighboring levels according to this invention. As shown in FIG. 4, the codes corresponding to the first level 401 to the eighth level 408 are (000), (001), (011), (010), (110), (111), (101) and (100), from low to high. There is only a one-bit difference between the codes of two neighboring levels.

[0023] To the 2-bit multi-level memory cell, the code corresponding to the lowest level can be (01), (10) or (11) as long as the code corresponding to each level has a one bit difference from the code corresponding to its previous level. Similarly, to the 3-bit multi-level memory cell, the code corresponding lowest level can be (001), (010), (011), (100), (101), (110) or (111) only if the code corresponding to each level has a one-bit difference from the code corresponding to its previous level.

[0024] Furthermore, the coding method provided by this invention can be applied to a multi-level memory cell comprising the error code correction function, for example, the multi-level flash EEPROM, flash erasable programmable read only memory (flash EPROM), EEPROM, Mask ROM, or dynamic random access memory (DRAM).

[0025] As mentioned above, as two codes of two neighboring levels have only a one-bit difference, whether n is 2 or 3, when one specific code is wrongly written as another of the level ahead or behind, only one correction bit is stored for the error code correction. The total number of correction bit can thus be saved, and the space occupied for ECC memory cell can also be saved. The previously mentioned ECC design rule for a “2^q+(2q−1) bits ECC memory cell” is used as an example, such that the advantages of this invention can be illustrated by actual numbers.

[0026] When the error rate for memory cell programming (level adjustment) is the same, the error bit proportion caused by the coding method of this invention is lower than that of the conventional coding method. Thus, if q has to be 8 in the conventional coding method, that is, a 15 bits error data storage space is required for each 256 bits storage space, the value of q can be increased to 9 in this invention. That is, each 512 bits storage space requires only a 17 bits error storage space to store all the error data. Therefore, using the coding method provided by this invention, the ECC memory has a proportion of 17/512=0.033, which is about a half of that (15/256=0.059) for the conventional coding method.

[0027] Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.