Static information storage and retrieval – Floating gate – Data security
Reexamination Certificate
2000-11-22
2003-03-18
Nelms, David (Department: 2818)
Static information storage and retrieval
Floating gate
Data security
C365S230090, C365S239000, C365S189011
Reexamination Certificate
active
06535420
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to non-volatile semiconductor memory devices, and, more particularly, to a non-volatile semiconductor memory device that has protection functions to prevent data damage due to a wrong write operation.
2. Description of the Related Art
Conventionally, a flash memory has a protection function to prevent data damage due to a wrong write operation. A memory cell array in the flash memory is divided into blocks that are erase units, and protection is provided for each erase unit. The size of each block is normally 64 Kbytes. In order to provide protection for an even smaller unit, there has been a demand to reduce the size of blocks. However, blocks are physically independent of each other. If the block size becomes too small, the chip size becomes too large. When the block size is 64 Kbytes, a 4-Mbit memory contains 8 blocks, and an 8-Mbit memory contains 16 blocks. A flash memory contains non-volatile memory elements that store the same number of protection information pieces as the number of blocks, and each block is protected using the non-volatile memory elements.
Meanwhile, as flash memories have been increasing in capacity, 32-Mbit flash memories and 64-Mbit flash memories have been developed. A 64-Mbit memory contains 164 blocks. Accordingly, as the number of blocks inside a memory increases, the same number of non-volatile memory elements that store protection information is required. As a result, the chip size also increases. Therefore, in a large-volume memory, protection is provided for a plurality of blocks at once, so as to reduce the number of non-volatile memory elements required and also reduce the chip size.
In a flash memory, a storage area that stores information such as manufacturer information is included, as well as a main storage area. Such a storage area is called a hidden block. Once protection is provided for the hidden block, the protected state cannot be canceled. The size of the hidden block varies with the type of the flash memory, for instance, in a range of 512 bytes to 64 Kbytes.
When a program is executed on a flash memory, the protection status of a block to be programmed should be read out from the memory elements prior to the execution of the program. If the block should be protected, a program voltage is generated so as to provide the protection.
FIG. 1
is a schematic view of the structure of a conventional flash memory
100
. This flash memory
100
comprises a main storage area
101
, a hidden block
104
, y-decoders
106
-
1
to
106
-
4
, an address buffer
110
, a block select decoder
111
, an x-predecoder
112
, a hidden block x-predecoder
113
, a protection status memory element group
114
, a program voltage generating circuit
115
, and a sense amplifier and input/output buffer unit
116
. The main storage area
101
comprises erase units (blocks)
101
-
1
to
101
-n, and x-decoders
102
and
103
. The hidden block
104
is provided with a hidden block x-decoder
105
.
When data is read from the flash memory
100
, an address is inputted into the address buffer
110
. The address buffer
110
then sends a block address to the block select decoder
111
in accordance with the inputted address. A block select signal outputted from the block select decoder
111
and a row address supplied from the address buffer
110
are decoded by the x-predecoder
112
, and the output of the x-predecoder
112
is sent to the x-decoders
102
and
103
. The x-decoders
102
and
103
select one of the word lines of memory cells. The y-decoders
106
-
1
to
106
-
4
then select a bit line in accordance with the block select signal and a column address. By doing so, the data stored in the selected memory cell is sent to a data bus line, and then outputted as output data via the sense amplifier and input/output buffer unit
116
.
When data is stored in the flash memory
100
, i.e., when a program is executed, a program execution command is inputted. Upon receipt of the program execution command, information stored in the protection status memory element corresponding to a block having a cell to be programmed is examined. If the information stored in the protection status memory element is in an unprotected state (i.e., a state that requires no protection), input data is stored in the cell selected in accordance with an address input by executing the program and by a program voltage being generated in the program voltage generating circuit
115
in accordance with the input data, as in the above-mentioned case of reading data from the flash memory. By contrast, if the information stored in the protection status memory element is in a protected state (i.e., a state that requires protection), the program voltage generating circuit
115
is not activated so as not to generate a program voltage. In such a case, a cell is also selected in accordance with an address input in the same manner as in the above-mentioned case of reading data. However, with no program voltage, the program is not executed on the selected cell.
Meanwhile, when reading is performed or a program is executed on the hidden block
104
, a hidden block access command
123
is inputted into hidden block memory elements in the hidden block x-predecoder
113
and the protection status memory element group
114
. A word line is then selected in the hidden block
104
via the hidden block x-decoder
105
, and reading is performed or the program is executed on the selected memory cell in the hidden block, in the same manner as in the above-mentioned case of performing reading or executing a program on a selected memory cell in the main storage area
101
.
FIGS. 2A
to
2
C show a conventional protection method. In this figure, the same components as in
FIG. 1
are denoted by the same reference numerals. As shown in
FIGS. 2A
to
2
C, the protection status memory element group
114
comprises a judgement circuit
202
and non-volatile memory elements
201
-
1
to
201
-
4
that store protection statuses of the blocks to be protected. The numbers (
1
) to (
4
) allotted to the non-volatile memory elements
201
-
1
to
201
-
4
and the blocks
101
-
1
to
101
-
4
indicate block addresses.
FIG. 2A
illustrates a state in which no data is stored in any of the blocks
101
-
1
to
101
-
4
, and no protection is provided. In this state, 64-Kbyte data is to be written in the block with the block address (
2
).
When a program is executed, information stored in the non-volatile memory element
201
-
2
of the block address (
2
) is first read from the protection status memory element group
114
. If the information stored in the non-volatile memory element
201
-
2
is “0”, the block with the block address (
2
) is in the unprotected state. If the information stored in the non-volatile memory element
201
-
2
is “1”, the block with the block address (
2
) is in the protected state. In the case shown in
FIG. 2A
, “0” is read out from the non-volatile memory element
201
-
2
. Accordingly, the block with the block address (
2
) is in the unprotected state, and a program voltage is generated by the program voltage generating circuit
115
. The block with the block address (
2
) is selected in accordance with an inputted address in the same manner as described with reference to
FIG. 1
, and the 64-Kbyte input data is written in the block with the block address (
2
). In the non-volatile memory element
201
-
2
in the protection status memory element group
114
, “1” is written so as to indicate the protected state.
FIG. 2B
shows a case where more data is to be added to the block with the block address (
2
), which already holds data and is protected. The information stored in the non-volatile memory element
201
-
2
with the block address (
2
) is read from the protection status memory element group
114
. Since “1” is read out from the non-volatile memory element
201
-
2
, the block with the block address (
2
) is in the protected state in this case. Accordingly, no program voltage is generated by th
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