Patent Publication Number: US-7917832-B2

Title: Apparatus for improving data access reliability of flash memory

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for improving data access reliability of flash memories and, more particularly, to an apparatus applicable to flash memories for improving data access reliability by hardware encoding, comparison, arbitration and decoding. 
     2. The Related Arts 
     Flash memory is widely used in computer and consumer electronic digital equipments, such as flash memory pen, MP3 player. However, the current technology for access control for the flash memory allows about 1,000,000 erases to a flash memory area. Beyond the limit, the data stored in that flash memory area may not be able to read. When an area in the flash memory cannot be read because it exceeds the erase limit, often referred as tolerance, it is sometimes referred as penetrated. 
     Because of the above constrain on the lifespan of the flash memory, it is an important issue to assure that the penetrated flash memory can still be read. The conventional approaches to this problem are either to improve the hardware technology, namely raising the tolerance, or to use the error correction code (ECC) to correct the errors. The capability of ECC approach is constrained by the flash memory page size and the spare area size. For example, to detect two-bit error, and correct one-bit error for a 512-byte page, it requires 24 bits of ECC. 
       FIG. 1  of the attached drawings shows a schematic view of a conventional structure of flash memory data access. To write data A 1  into a data area A 2 , the access control also generates an ECC A 3  to be stored in the reserved area A 4  after the data area A 2 . When this type of data access structure is applied in the even more unreliable multi-level cell (MLC) flash memory, the problem is even more severe. This is because a cell of an MLC flash memory stores two or more bits; and therefore, the value of the bit can be erroneously determined. Although increasing the size of reserved area to provide more storage space for ECC can slightly alleviate the problem, this approach must change the entire system and hardware design of the current flash memory, which is not economically viable. 
     In addition, Taiwan Patent Publication No. 575806 disclosed a method for enhancing the ECC capability of flash memory and encrypting data. However, the disclosed method is not applicable to the two-bit error correction capability, and may lead to erroneous data determination. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an apparatus for improving data access reliability of flash memory. Through encoding the written data by an encoder and performing error correction computation on the encoded data by an error code generator, the apparatus improves the decoding efficiency as to be applied to the situation in which the data access to flash memory must meet the requirements of two-bit error detection and one-bit error correction. 
     Another object of the present invention is to provide an apparatus for improving data access reliability of flash memory through a comparator, an arbitrator and a decoder. The result from the comparison with the ECC in the reserved area by the comparator is used to select the decoder cells. The arbitrator informs the decoder of the suspicious bits. When the decoder selects the cells to recover the original data, the decoder tries to avoid the suspicious cells as to improve the decoding efficiency. When applied in MLC flash memory, the apparatus enables the correct judgment of the bit value and storage to improve the data access reliability. 
     A further object of the present invention is to provide an apparatus for improving the data access reliability of flash memory through an arbitrator which can further differentiate voltage level in MLC flash memory application. When storing data in the two-bit MLC flash memory, it is necessary to differentiate four voltage levels, represented by 11, 10, 01, 00, to eliminate the bit value mis-judgment. The arbitrator informs the decoder of the suspicious bit value, the decoder can try to avoid the suspicious cells when selecting cells to recover the original data. 
     To achieve the aforementioned objects, the present invention provides an apparatus for improving the data access reliability of flash memory, comprising an instruction register, an address register, a flash memory control circuit, a data register, an encoder, an error correction code (ECC) generator, a signal converter, a comparator, an arbitrator, and a decoder. The instruction register and the address register are connected to a flash memory respectively for storing the access instructions and the addresses. The flash memory control circuit is connected to both instruction register and address register for controlling the access to the flash memory. The data register is connected to flash memory control circuit for loading data to be written to the flash memory. The encoder encodes the written data, and the ECC generator generates an ECC, which is written to the flash memory through the signal converter. The comparator and the arbitrator provide the comparison with ECC and informing decoder f suspicious bit values when data is read from the flash memory. The decoder is for correctly decoding data and avoiding the suspicious bit values to be read. Thus, the object improving the data access reliability of flash memory is achieved. 
     These and other objects, features and advantages of the invention will be apparent to those skilled in the art, from a reading of the following brief description of the drawings, the detailed description of the preferred embodiment, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be understood in more detail by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic view of the error code generation and storage structure of a conventional flash memory; 
         FIG. 2  shows a block diagram of an apparatus for improving the data access reliability of flash memory of according the present invention; and 
         FIG. 3  shows a voltage logic diagram of the arbitrator selecting the suspicious cells according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings and in particular to  FIG. 2 , which shows an apparatus constructed in accordance with the present invention for improving data access reliability of a flash memory, generally designated with reference numeral  100 , the apparatus  100  comprises an instruction register  10 , an address register  20 , a flash memory control circuit  30 , a data register  40 , an encoder  50 , an ECC generator  60 , a signal converter  70 , a comparator  80 , an arbitrator  90  and an decoder  95 . The instruction register  10  and the address register  20  are connected to a flash memory  200 . The instruction register  10  stores the access instructions to the flash memory  200 , and identifies whether the instruction is read instruction or write instruction. The address register  20  stores the target address for access to the flash memory  200 . 
     The flash memory control circuit  30  is connected to the instruction register  10  and the address register  20  for controlling read or write access to the flash memory  200  according to the contents of the instruction register  10  and the address register  20 . The date register  40  is connected to the flash memory  200  for storing the data read from or written into the flash memory  200 . 
     The encoder  50  is connected to the data register  40  for encoding the written data in the data register  40  and generate additional information for later error correction. 
     The ECC generator  60  is connected to the encoder  50  for performing ECC computation on the encoded data. The ECC code does not only for error correction but also for improving decoding efficiency. 
     The signal converter  70  is connected to the encoder  50 , the ECC generator  60  and the flash memory  200  for converting the encoded data from the encoder  50  and ECC from the ECC generator  60 , and then storing into the data area and reserved area in the flash memory  200 . 
     The comparator  80  is connected to the ECC generator  60  and the signal converter  70  for comparing the ECC stored in the reserved area in the flash memory  200  and the ECC from the ECC generator  60  when the data is read from the flash memory  200 . Based on the comparison result, the comparator  80  outputs a comparison control instruction  81  to assist in selecting the cells from the flash memory  200  for reading data. 
     Also referring to  FIG. 3 , the arbitrator  90  is connected to the signal converter  70  for further differentiating voltage levels in MLC application when reading data from the flash memory  200 ; for example, the four voltage levels can be identified as 11, 10, 01, and 00. Then, the arbitrator  90  picks out the suspicious bit value. For example, the x-axis in  FIG. 3  is the logic voltage VT, and the y-axis is the number of cells.  FIG. 3  shows a bit A has the value between 11 and 10, which is determined as suspicious, and outputted by the arbitrator  90 . 
     The decoder  95  is connected to the comparator  80 , the arbitrator  90  and the data register  40 . The decoder  95  receives the comparison control instruction  81  from the comparator  80  and suspicious bits from the arbitrator  90 . If the decoded data from the cells according to the information is correct, the decoded data is stored in the data register  40 . Otherwise, another set of cells of the flash memory  200  are selected for decoding until the correct data is obtained or an iteration threshold is exceeded. By this, the data access reliability of the flash memory  200  is improved and suspicious cells are avoided to take erroneously read as correct. 
     While the invention has been described in connection with what is presently considered to the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the appended claims.