Non-volatile memory

Abstract: A means (35) for latching stored data is disposed outside an array of non-volatile memory cells (1). Before erasure of the memory cells (1), some of the stored data items in the non-volatile memory cells are transferred to and held in the latch means (35). The saved data items are written again after erasure, or alternatively, the data items transferred to the latch means (35) are used to determine certain designated regions in which erasure is inhibited. This enables the conservation of certain stored items, and thus, the prevention of illicit use. The shortening of testing time is also made compatible with the conservation.

Description: The present invention relates to semiconductor storage technology, and more particularly to non-volatile memories which are electrically writable and erasable. It also relates to techniques which are effective when utilized for a non-volatile memory and a microcomputer having a non-volatile memory. An EEPROM (Electrically Erasable and Programmable Read Only Memory) is a non-volatile memory, and it is electrically rewritable. However, this feature gives rise to the problem that data which should be conserved can be rewritten. In order to protect the data in a non-volatile memory, a method has been proposed wherein a security bit is provided and, access from outside the memory is inhibited subject to the status of this bit. A data protection system based on a security bit is discussed in, for example, "Electronic Design", March 3, 1983, pp. 123-128. An example of such a system is a write-only security register, which includes a non-volatile memory cell. This is isolated from ordinary memory cells and is intended for rewriting data. Access to the memory cells is inhibited subject to the status of a specified bit of the register. The security register is constructed so that it can erase data only when the operation of complete and simultaneous erasure of the memory cells takes place. Thus, after information that indicates the protected data in the memory cells has been written into the security register, the memory cells cannot be accessed without destroying the data. Using this method the data can be kept secret. However, with this method, the information in the security register is lost when, to establish an intitial status, complete and simultaneous erasure of the memory cells takes place. This has caused concern that the memory will be used illegally after complete erasure of the memory cells. This is because after initialization of the memory by complete erasure of the memory cells, the memory can be reused by writing in false data. This has caused problems in cases where important information (such as information relating to money) is stored in, for example a cash card system. Furthermore, because the EEPROM requires long write/erase times, an increase in its storage capacity has resulted in a need to shorten the time required to perform the complete erase operation on the memory cells. Accordingly, it has become a requirement that the complete erasure of a memory is compatible with the inhibition of the unlawful intialization. Another proposal is an EEPROM having a column latch circuit disposed on every data line of memory cells, thus making it possible to write data items collectively in every row address (refer to "Hitachi Hyoron (Hitachi Review), Vol. 68, No. 7", pp. 75 - 78 issued on July 25, 1986 by Hitachi Hyoron Sha). The column latch makes it possible to simultaneously write or rewrite a plurality of data items; up to now this has been utilized only for increasing efficiency by shortening write/rewrite times per unit data. US 4648076 and its equivalent EP-A-0129054 discloses function data memory for determining the functions of data memory including non-volatile memory cells. An address line is connected to the memory cells in the data memory and also to memory cells to a function data memory. As described, the data in one row of the function data memory determines the function of the corresponding row of the data memory. The function data memory may be a mask programmable memory (ROM) or an electrically erasable programmable memory (EEPROM). JP 60059452 discloses a circuit which inhibits an input to an EEPROM at a specific address if a signal designating a specific area does not exist on a writing control signal line. The present invention seeks to provide a semiconductor memory which, by means of a comparatively simple circuit arrangement makes possible the protection of stored data in one part of a non-volatile memory and simultaneously, the writing or re-writing of data into or at the other part. According to the present invention there is provided a non-volatile memory device comprising: a memory array (M-ARY) including a first block (MBO) and a second block (MBI), each block including non-volatile memory cells; a control circuit (CONT) which controls an erasure operation on the memory array (M-ARY); selecting means (YDCR,XDCR) for selecting at least one non-volatile memory cell from the memory array (M-ARY); a designating circuit (DEC) which is arranged to designate whether the selected non-volatile memory cell is in the first memory block or in the second memory block; characterised in that the device further includes: a register circuit (REG) which includes a first control bit (EI) for controlling the erasure operation of the control circuit on the memory array, the first control bit selectively having a first state for inhibiting the erasure operation of the control circuit on the memory array and a second state for enabling the erasure operation of the control circuit on the memory array; and a gate circuit (G1) coupled to the designating circuit and to the register circuit for inhibiting the erasure operation of the control circuit on the second block regardless of the second state of the first control bit when the designating circuit designates that the selected non-volatile memory cell is in the second block. The present invention also relates to a microcomputer incorporating such a non-volatile memory and an integrated circuit card incorporating such a non-volatile memory. Embodiments of the invention will now be described, with reference to, and as illustrated in the accompanying drawings, in which: Fig 1 is a block diagram of non-volatile memory, being an arrangement useful in understanding the present invention, but not according to the present invention. Fig 2 is a circuit diagram showing in detail the arrangement of Fig 1; Fig 3 is a diagram of an address decoder in the circuit of Fig 2; Figs 4A to 4D are time charts for complete erasure in the arrangement of Fig 1; Fig 5 is a flow chart for complete erasure of the arrangement of Fig 2; Fig 6 is a diagram showing the fundamental arrangement of the type of non-volatile memory to which the present invention may be applied. Figs. 7 to 7E are time charts for the write operation of the memory in Fig. 6; Fig. 8 is a block diagram showing the first embodiment of the present invention; Fig. 9 is a circuit diagram of the first embodiment; Figs. 10A to 10N are diagrams of the operating waveforms of the first embodiment; Fig. 11 is a block diagram showing the second embodiment of the present invention; Fig. 12 is a block diagram-showing the third embodiment of an EEPROM according to the present invention; Before describing the embodiments of the present invention, the basic arrangement and operation of a typical electrically writable and erasable non-volatile memory will be first explained with reference to Fig. 6. In Fig. 6, numeral 1 indicates a group of non-volatile memory cells in which memory cells of, e.g., the MNOS (metal-nitride-oxide-semiconductor) type are disposed in an array (or matrix). Numeral 31 is an address decoder; numeral 35 is a column latch circuit which is disposed on every data line in the memory cell group 1; and numeral 36 is a write/erase control circuit. Data is written or rewritten by: applying address input signals AI to the address decoder 31, data signal DT to the memory cells 1, and a write enable signal WE to the write/erase control circuit 36. Similarly data (DT) may be read by applying address input signals AI and a read enable signal RE. Data of a designated address is erased by applying the address input signals AI and an erase enable signal ER. Erasure of all of the memory cells 1 ("complete" erasure) is performed by applying an all select signal AS to the address decoder 31 and the erase enable signal ER to the write/erase control circuit 36. When the all select signal AS is applied to the decoder 31, all the word lines in the memory cell group 1 are simultaneously selected. Column latches 35 are disposed on all the data lines, enabling data items to be collectively written in every row address. Fig. 7A - Fig. 7 E are time charts of a write or rewrite operation for the memory described in Fig. 6. As illustrated in Fig. 7A - Fig. 7E, the operation of writing data into the memory cells 1 is initiated by applying address input signals AI, data signals DT and a write enable signal WE. First, the internal mode of operation of the memory cell (group 1) is brought into a read status by the write/erase control circuit 36, and all data items at a designated row address are fetched and saved in the column latches 35. While the data inputs (i.e. the data to be written) are accepted, the contents of the column latches 35 are rewritten according to input column addresses and the input data items. The acceptance of the data input signals ends after the lapse of a fixed time interval, for example, 500 µsec, and the internal mode of operation becomes an erase status in which all the memory cells in designated row address are erased. The internal mode of operation then becomes a write status in which the contents of the column latches 35 are written into the memory cells in the designated row address. The reading - erasing-writing operations described above are carried out as a series which ends when the collective writing of data in row address units has finished. The necessary timing device is provided in the write/erase control circuit 36. The operation of erasing data from the memory cell group 1 is started by applying the erase enable signal ER, which brings the internal mode of operation into the erase status. In this status, when the all select signal AS is a "0", memory cells corresponding to a particular row address indicated by the address input signals AI are selected and erased; when the all select signal AS is a "1", all the memory cells are selected and erased. The erasure of some data items at a particular row address is carried out so that, in the foregoing write operation, data ("0" or "1") which is designated beforehand, is written into addresses which are to be erased. That is to say "0" is written when the erased condition of the address is designated as "0", and "1" is written when it is designated as "1". The components represented in Fig. 1 are formed on a single semiconductor chip by well-known semiconductor techniques. When all the data other than protected data in the memory (EEPROM) of the arrangement shown in Fig. 1 is to be erased, an erase enable signal ER is applied and an all select signal AS is set at "1". A series of operations are then executed and these are as follows: First, data items at a previously-designated row address (for example, the first row 11 in Fig. 1 ) in the group 1 of non-volatile memory cells are fetched and held in column latches 35. Subsequently, the overall and simultaneous erasure of the group of memory cells 1 is performed. Thereafter, the contents of the column latches 35 are written into the row address 11 again. Thus, in this arrangement, when complete erasure occurs, the reading - erasing - rewriting are carried out at the designated row address, and complete erasure is carried out at the other row addresses. Fig. 2 is a diagram of the memory in an arrangement useful in understanding the present invention. Numeral 31 denotes an address decoder, numerals 321 - 329 high-voltage control circuits, numeral 33 a sense amplifier, numeral 34 an output buffer, numeral 35 the column latch, numeral 36 a write/erase control circuit, numerals 111,112,121 and 122 transistors which constitute the memory cells, and numerals 113 and 123 transistors which turn "on" and "off" the gate voltages of the memory cells. Input logic gates 51 - 53 are provided for the write enable signal WE and the erase enable signal ER which are applied to the write/erase control circuit 36 in the non-volatile memory of Fig. 6. A signal line 471 is provided which supplies a control signal from the write/erase control circuit 36 to the address decoder 31, and a line 48 for a control signal from the column latch 35 to the write/erase control circuit 36. In the memory of Fig. 2, the voltage conditions listed in Table 1 are applied to the sources S, isolated gates I, high-voltage word lines WH, selection word lines W, drains D and well regions WELL of the memory cells 111,112,121 and 122 where the operations of reading out data, writing "0" or "1" and erasing data are carried out. In a read operation, a power source voltage V cc (for example, 5 V) is applied to the word line W by the address decoder 31, and the high-voltage word line WH is set at 0 V through the high-voltage control circuit, thereby selecting the memory cells. The contents (D out ) of the cells are fetched from the drains D to data lines 25. In a write operation, the voltage V cc is applied to the word line W by the address decoder 31, and the same voltage V cc is applied from the high-voltage control circuit 321 or 322 to the high-voltage word line WH, a minus high-voltage -V pp (for example, -12 V) is applied from the high-voltage control circuit 324 to the well WELL, and the voltages-V pp /V cc are respectively applied to the data lines (D) according to whether the write data items are "0" or "1", whereby "0"/"1" can be written into selected memory cells. In the case of an erase operation in row address unit, the voltage V cc is applied to one of the word lines W which corresponds to an address designated by the address decoder 31. The voltage -V pp is applied from the high-voltage control circuit 321 or 322 to the high-voltage word line WH corresponding to the address, and the voltage V cc is applied from the high-voltage control circuit 324 to the well WELL, whereby the contents of selected memory cells are erased. In the write or rewrite operation of this memory, fetching the contents of designated row address(es) to save them in the colum latches 35, rewriting the contents of the column latches 35, erasing the contents of the selected row address, and writing the contents of the column latches 35 into the selected row address(es) are performed as a series of operations under the control of the write/erase control circuit 36. This is done in the same manner as in the circuit shown in Fig. 6. By comparison with the fundamental device of Fig. 6, the present embodiment is constructed with logic gates 51-53 which are added so that the write/erase control circuit 36 also controls the address decoder 31. Thus, in case of overall and simultaneous erasure, the memory is controlled so that the data items of a previously-designated row address are saved in the column latches before the erase operation and that the contents of the column latches are written into the row address after the erase operation. For example, in order to execute complete simultaneous erasure in the device in Fig. 2, both the all select signal AS and erase enable signal ER are set at "1" so that a control signal C1 through the logic gates 51 and 52 becomes "1," and a control signal C2 through the logic gate 53 becomes "0,". Thus, an operation similar to the write or rewrite operation described above is instructed to the write/erase control circuit 36. Thus, the internal operation of the memory can be described by the series of operations of saving - erasing - writing. Fig. 3 shows the arrangement of the principal parts of the address decoder 31 of this arrangement. AI stands for address input signals, AS the all select signal, and CS the control signal which is the output from the write/erase control circuit 36 to the address decoder 31 (through the signal line 471). When the memory is in ordinary operation, that is, when the all select signal AS is in a "0" status, a signal for selecting the word line W1 or W2 is formed in accordance with the address input signals AI by gates 311 and 312. The first row address designated beforehand is a row address corresponding to the word line W1, and it is selected whenever the memory is in the overall and simultaneous erase operation, that is whenever the all select signal AS is in the status of a logical "1". The control signal CS which is supplied from the write/erase control circuit 36 becomes a logical "1" only in the erase operation and becomes a logical "0" in the save and write operations. Accordingly, in the all select status, a row address corresponding to the word line W2 is selected only in the erase operation, so that only the erasure of the data items of that address is carried out. Figs. 4A - 4D show the time charts of the overall and simultaenous erase operation in this arrangement. In the overall and simultaneous erase operation, saving the contents of the first row address in the column latches 35, simultaneously erasing the contents of all the non-volatile memory cells of the cell group 1 and rewriting the contents of the column latches 35 into the designated row address (first row address) are performed as a series of operations in accordance with the control of the write/erase control circuit 36, thereby making it possible to conserve the contents of the designated row address. Thus, the word line W1 of the first row is held in the selected status (level "1") continually throughout the overall erase operation, and the word line W2 of the second row is held at the select level only during the erase operation. The memory may be constructed so that data inputs are accepted in the save operation and that the contents saved in the column latches 35 are written again. Alternatively, it may be constructed so that the data inputs are inhibited, in other words, so that rewriting is inhibited. This type of construction is appropriate in cases where the stored contents are secret and require protection. The arrangement in Fig. 2 is constructed so that protected information is included in the contents saved in the column latches 35, and the saved contents that are rewritten after the overall and simultaneous erasure are selected according to the content of the protected information. The protected information is stored in the memory cell 112, and the data held in the column latch 35 corresponding to the memory cell 112 is supplied to the write/erase control circuit 36 by the signal line 48 so as to make the operation thereof different. By way of example, when the content of the memory cell 112 is "0," it brings the signal line 48 to "0" and instructs the write/erase control circuit 36 to rewrite the saved contents, and when the content of the memory cell 112 is "1," it instructs the control circuit 36 to end the operation thereof with the overall and simultaneous erasure and to perform no rewriting. Fig. 5 shows the flow chart of a control process which is executed by the write/erase control circuit 36 in the overall and simultaneous erase operation of the present arrangement. As illustrated in the figure, in the overall erasure, the contents of a designated address are first saved in the column latches at a step S1. Subsequently, whether protective information is "0" or "1" is decided at a step S2. If the decided result is "0," all the memory cells are simultaneously erased at a step S3. Thereafter, the contents of the column latches are written into the designated address again at a step S4. In contrast, if the decided result at the step S2 is "1," the control process shifts to a step S5 and ends upon performing only the overall and simultaneous erasure. Incidentally, erasure in word line unit (row address unit) can be performed in the same manner as in Fig. 6. According to this arrangement even a non-volatile memory capable of only the overall and simultaneous erasure is permitted to conserve some of stored contents in case of an erase operation. Further, a memory capacity in which the stored contents are conservable can be increased in such a way that a plurality of column latch trains are provided and that the save and rewrite operations described before are carried out a plurality of times. By the way, the arrangement fails to conserve the stored contents in a case where the power source voltage lowers or is cut off after the start of the overall and simultaneous erasure and before the end of the rewriting. In order to cope with this situation, a capacitor for backup is provided so as to hold necessary electric power therein, whereby the memory keeps operating even after the lowering or cutoff of the power source voltage, and the conservation can be effected. Alternatively, a non-volatile memory cell is further provided outside the group of memory cells 1, and only the fact of the occurrence of the lowering or cutoff of the power source voltage before the end of the writing is recorded in the non-volatile memory cell so as to inhibit all the subsequent operations. With this construction, the capacity of the capacitor can be made small, and the protection of secrecy can be achieved. However, a period of time required for the erasing and the writing is 10 - 50 msec, and the lowering or cutoff of the power source voltage will, in general, pose no problem. Fig. 8 shows a block diagram of an EEPROM device which is the first embodiment of this invention. Using the known manufacturing techniques of semiconductor integrated circuits, various circuit blocks in the figure are formed on a single semiconductor substrate such as single-crystal silicon though this is not especially restrictive. A memory array M-ARY is divided into a memory block MBO which is enabled to write and rewrite (also erase) data in ordinary operating statuses, and a memory block MB1 which is inhibited from writing and erasing data. The memory block MB1 is used as a storage area for data to-be-protected by having the write and erase operations inhibited as stated above. As will be described later, the memory array M-ARY includes a plurality of non-volatile memory cells which are arranged in the shape of a matrix, a plurality of word lines which are laterally extended, and a plurality of data lines which are vertically extended. In this embodiment, the memory blocks MB0 and MB1 are so defined that some of the plurality of word lines belong to the memory block MB0, while the others belong to the memory block MB1. Although not especially restricted, address signals which are supplied from external terminals AX0 - AXm are fed to an X-address buffer XADB. Upon receiving the address signals supplied from the external terminals, the X-address buffer XADB forms complementary internal address signals which consist of internal address signals inphase with the received signals and internal address signals antiphase thereto and which are supplied to an X-address decoder circuit XDCR. Besides, the address buffer XADB transmits predetermined address signals to a decoder DEC in order to identify accesses to the memory blocks MB0 and MB1. By way of example, in a case where the memory array M-ARY is divided in four in an X-direction and where three of the four divided memory areas are allotted to the memory block MB0, while the remaining one is allotted to the memory block MB1, the X-address signals axm and axm-1 of the upper 2 bits are supplied to the decoder DEC and decoded therein. The X-address decoder circuit XDCR decodes the complementary internal address signals, and performs the operation of selecting one of the word lines. In the erase and write operations thereof, the non-volatile memory cell requires comparatively high voltages which differ from a voltage for the read operation thereof. Therefore, the X-address decoder circuit XDCR decodes the internal complementary address signals, and in accordance with a control signal C1 (or control signals) supplied from a control circuit CONT, it selects the word line of the memory array M-ARY under the state of the comparatively high voltage in the erase or write operation and brings the same into a selected status under the state of the comparatively low voltage in the read operation. Although not especially restricted, address signals which are supplied from external terminals AY0 - AYn are fed to a Y-address buffer YADB. Upon receiving the address signals supplied from the external terminals, the Y-address buffer YADB forms complementary internal address signals which consist of internal address signals inphase with the received signals and internal address signals antiphase thereto and which are supplied to a Y-address decoder circuit YDCR. The Y-address decoder circuit YDCR decodes the complementary internal address signals, and performs the data line select operation of connecting at least two of the data lines to an input/output circuit I/O in order to write/read data in plural-bit unit. It is to be understood that, for this purpose, the memory array M-ARY includes a Y-gate or column switch circuit by which two or more of the data lines are selectively connected to a plurality of common data lines connected to the input/output circuit I/O. Since the non-volatile memory cell needs to supply a comparatively high voltage to the data lines in the write operation thereof, the Y-address decoder circuit YDCR has the function of forming select signals at the high voltage. The input/output circuit I/O includes a writing circuit which receives write signals supplied from external terminals D0 - D7 in, for example, 8-bit unit and transmits them to the data lines to-be-selected, and a reading circuit by which data items read out in 8-bit unit are delivered to the external terminals D0 - D7. The writing circuit and reading circuit included in the input/output circuit I/O are selectively brought into an operating status in accordance with a control signal C3 (or control signals) which is supplied from the control circuit CONT. Basically, the control circuit CONT receives control signals supplied from external terminals, for example, a chip enable signal CE , an output enable signal OE and a write enable signal WE , as well as a writing high voltage -Vpp, and it identifies an operation mode and generates control signals and timing signals corresponding thereto. In this case, when the write mode is instructed by the combination of the control signals CE , OE and WE , the control circuit CONT executes the following three operatins in time series before the execution of the operation of writing data items into memory cells, though they are not especially restrictive: They are the first operation in which the stored information items of memory cells belonging to a word line to be selected are fetched and are held in latch circuits disposed in correspondence with data lines, the second operation in which the data items to be written are substituted into the latch circuits, and the third operation in which the erase operations of the memory cells corresponding to the word line are performed. Thereafter, it is carried out as the fourth operation that the data items held in the latch circuits are actually written into the memory cells corresponding to the word line. To this end, the control circuit CONT includes a timer circuit for executing the respective operations time-serially. Owing to such operations, the EEPROM can be externally accessed similarly to a static RAM. In this embodiment, a register REG is disposed in order to add the function of selectively inhibiting the operations of erasing and writing data from and into the memory blocks MB0 and MB1 as described above. Although not especially restricted, the register REG has 2-bit stored information consisting of a write inhibit signal WI and an erase inhibit signal EI. The write inhibit signal WI is supplied to the control circuit CONT. Even when the write enable signal WE supplied from the external terminal to the control circuit CONT is at a low level instructive of the write operation, this control circuit CONT invalidates the acceptance of the supplied signal in a case where the write inhibit signal WI indicates a status inhibiting the write operation. That is, the write inhibit signal WI is preferred to the write enable signal WE . The erase inhibit signal EI and the output signal of the decoder DEC are supplied to an OR gate circuit G1. The output signal EI' of the OR gate circuit G1 is supplied to the control circuit CONT. In a case where this signal EI' indicates a status inhibiting the erase operation, the control circuit CONT does not execute the erase operation even when the erase mode is instructed by the combination of the control signals or when the erase operation during the write operation stated before is encountered. An address selection circuit and a data input circuit for the register REG are omitted from the illustration. By way of example, the register REG is selected by setting one or more specified address terminals at a level higher than the ordinary high level, and the information items to be held are supplied from any two of the data terminals D0 thru D7. By setting the address terminal or terminals at the voltage higher than the ordinary high level as stated above, the assignment of a special address to the register REG is dispensed with in an arrangement for designating this register REG, and the operation of selecting the register can be distinguished from the select operation of the memory array M-ARY. Incidentally, when the register REG is selected by setting the address terminal or terminals at the high voltage, the select operation of the memory array M-ARY is inhibited. Alternatively, the control signals WI and EI may well be supplied from external terminals. Further, in a case where the EEPROM is built in a microcomputer of single chip, the control signals WI and EI can be supplied from the predetermined register of the microcomputer. The control signals WI and EI are basically signals which instruct the write inhibit and erase inhibit for all the cells of the memory array M-ARY without the distinction between the memory blocks MBO and MB1. In the ordinary operating statuses, therefore, both the signals WI and EI are held in reset statuses (logic "0"). The controls with such signals WI and EI are effective for, e. g., the overall protection of the stored data items of the memory array M-ARY in a specified operating status. Further, although not illustrated in the figure, a read inhibit signal RI may well be provided in the register REG at need, thereby to add the function of selectively inhibiting the read operation. Regarding this function of selectively inhibiting the read operation, reading data items which need to be kept secret can be inhibited in such a way that an identifying code, for example, is included in a procedure for clearing the read inhibit signal (for establishing a readable status). Fig. 9 shows a circuit diagram of one embodiment of the memory array M-ARY as well as the decoder circuit of the EEPROM illustrated in Fig. 8. In order to facilitate the ensuing description, Fig. 9 depicts only one input terminal D in , only one output terminal D out and circuit portions associated with them. Although the other seven input and output terminals and circuit portions associated therewith are not shown, they have arrangements similar to those depicted in Fig. 9. In this regard, however, the control circuit CONT and an oscillator circuit OSC are common to the eight unit portions (each of which is constructed of the input terminal, the output terminal and the associated circuit portions). Although not especially restricted, the EEPROM device is operated by a comparatively low power source voltage V cc of, e. g., +5 V and a minus high voltage -V pp of, e. g., -12 V which are supplied from outside the device. The X-address decoder XDCR, etc. constituting the selection circuit are constructed of CMOS circuits. The CMOS circuits are operated by being fed with the comparatively low power source voltage V cc of, e. g., +5 V. Accordingly, select/unselect signals which are formed by the address decoders XDCR and YDCR have their high level set at substantially +5 V and their low level set at substantially 0 V being the ground potential of the circuitry. The structures of elements themselves constituting the illustrated EEPROM device are not directly pertinent to the present invention and are therefore omitted from the drawings, but they will now be outlined. The whole illustrated device is formed on a semiconductor substrate made of, e. g., N-type single-crystal silicon. An MNOS transistor is of the N-channel type, and it is formed on a P-type well region or P-type semiconductor region formed in the surface of the semiconductor substrate. An N-channel type MOSFET is similarly formed on the P-type semiconductor region. A P-channel type MOSFET is formed on the semiconductor substrate. Although not especially restricted, one memory cell is constructed of one MNOS transistor and two MOSFET's connected in series therewith. In each memory cell, the MNOS transistor and the two MOSFET's are configured into, for example, a so-called stacked gate structure in which the gate electrodes of the two MOSFET's overlap the gate electrode of the MNOS transistor, respectively. Thus, the MNOS transistor and the two MOSFET's constituting the memory cell are, in effect, made a unitary structure, so that the size of the memory cell is reduced. Although not especially restricted, the memory cells are formed on a common well region. An N-channel MOSFET for constructing the CMOS circuit such as the X-decoder or Y-decoder is formed on a P-type well region which is independent of the common P-type well region for the memory cells. In this structure, the N-type semiconductor substrate forms a body gate common to a plurality of P-channel MOSFET's which are formed thereon, and it is set at the level of the power source voltage V cc of the circuitry. A well region as the body gate of an N-channel MOSFET for constructing the CMOS circuit is maintained at the ground potential, 0 V of the circuitry. Referring to Fig. 9, a memory array M-ARY includes a plurality of memory cells which are arranged in the shape of a matrix. One memory cell is constructed of an MNOS transistor Q2, an addressing MOSFET Q1 which is interposed between the drain of the transistor Q2 and a data line (bit line or digit line) D1, and an isolating MOSFET Q3 which is interposed between the source of the MNOS transistor Q2 and a common source line though this is not especially restrictive. By the way, in a case where the stacked gate structure as stated before is adopted, the channel forming regions of the MOSFET's Q1 and Q3 are directly adjacent to the channel forming region of the MNOS transistor Q2. Therefore, it is to be understood that the drain and source of the MNOS transistor Q2 are terms for convenience' sake. The gates of the respective addressing MOSFET's Q1 etc. of the memory cells arranged in an identical row are connected to the first word line W11 in common, and the gates of the MNOS transistors Q2 etc. corresponding thereto are connected to the second word line W12 in common. Likewise, the gates of the addressing MOSFET's and MNOS transistors of the memory cells arranged in another identical row are respectively connected to the first word line W21 and second word line W22 in common. The drains of the addressing MOSFET's Q1 etc. of the memory cells arranged in an identical column are connected to the data line D1 in common. Likewise, the drains of the addressing MOSFET's of the memory cells arranged in another identical column are connected to a data line D2 in common. The sources of the isolating MOSFET's Q3 etc. in the memory cells are made common, to construct the common source line CS. The memory array M-ARY of this embodiment is operated essentially by potentials to be explained below. First, in a read operation, the potential V w of the well region WELL is set at a low level which is substantially equal to 0 volt being the ground potential of the circuitry. The common source line CS is set at the low level which is substantially equal to the ground potential. A control line coupled to the gates of the isolating MOSFET's Q3 is set at a high level which is substantially equal to the power source voltage V cc , so as to bring these MOSFET's Q3 into "on" statuses. The second word lines W12 - W22 each of which is coupled to the gate electrodes of the corresponding MNOS transistors, are set at a potential substantially equal to the ground potential, that is, a voltage intermediate between the high threshold voltage and low threshold voltage of the MNOS transistors. The first word line to be selected from among the first word lines W11 - W21 is brought to a select level or high level which is substantially equal to the power source voltage V cc , whereas the remaining first word lines, namely, unselected word lines are held at an unselect level or low level which is substantially equal to the ground potential. The data line to be selected from among the data lines D1 - D2 is fed with a sense current. If the MNOS transistor in the memory cell selected by the first word line has the low threshold voltage, this memory cell forms a current path for the data line with which it is coupled. If the MNOS transistor in the selected memory cell has the high threshold voltage, this memory cell forms, in effect, no current path. Accordingly, the data of the memory cell is read out by detecting the sense current. In a write operation, the well region WELL is set at a minus high voltage which is substantially equal to -V pp , and the control line coupled to the gate electrodes of the isolating MOSFET's Q3 is set at a minus high potential so as to bring these MOSFET's Q3 into "off" statuses. The first word lines W11 - W21 is set at the unselect level or low level which is substantially equal to the ground potential. One of the second word lines W12 - W22 is brought to a select level which is substantially equal to the power source voltage V cc , whereas the remaining second word lines are held at a minus high voltage which is close to the voltage -V pp . The data lines are set at a high level substantially equal to the power source voltage V cc or a low level having a minus high voltage close to the minus voltage -V pp , in accordance with data items to be written into the memory cells. In an erase operation, the well region WELL and the common source line CS are set at an erase level or high level which is substantially equal to the power source voltage V cc . For the purpose of erasure, the first word lines W11 - W21 and the second word lines W12 - W22 are basically set at the level substantially equal to the power source voltage V cc of the circuitry and the level substantially equal to the voltage -V pp , respectively. According to this embodiment, however, the levels of the first and second word lines are determined so as to permit the memory cells to be erased every memory row though this is not especially restrictive. Among the first word lines W11 - W21, the first word line corresponding to the memory row which needs to be erased is set at an erase level substantially equal to the power source voltage V cc , whereas the first word lines corresponding to the memory rows which need not be erased are set at an unerase level substantially equal to the ground potential of the circuitry. Among the second word lines W12 - W22, the second word line corresponding to the first word line which is set at the erase level is brought to an erase level substantially equal to the minus voltage -V pp , whereas the second word lines corresponding to the first word lines which are set at the unerase level are held at an unerase level substantially equal to the power source voltage V cc . According to this embodiment, as stated before, there is adopted the construction in which the stored information items of the MNOS transistors are erased by applying the power source voltage V cc to the well region, namely, the body gates of the MNOS transistors. On the other hand, the body gates of the N-channel MOSFET's constituting the CMOS circuits need to be set at a potential of, for example, 0 volt independently of the body gates of the MNOS transistors. As stated before, therefore, the body gates of the memory cells, namely, the semiconductor region WELL formed with the memory array M-ARY are/is electrically isolated from the semiconductor region (well region) in which the N-channel MOSFET's constituting the peripheral circuits such as X-decoder and Y-decoder are formed. The first and second word lines W11 - W21 and W12 - W22 are driven by the X-decoder XDCR. Although not especially restricted, the X-decoder XDCR is composed of a plurality of unit decoder circuits which correspond to the memory rows of the memory array M-ARY in one-to-one relationship. As shown by way of example in the figure, one unit decoder circuit is constructed of a NOR gate circuit NOR1 receiving address signals, a gate circuit G and a level conversion circuit LVC. The gate circuit G is so constructed as to transmit the output of the corresponding NOR gate circuit to the corresponding first word line in, at least, the read operation and to set the first word line at the level substantially equal to the ground potential of the circuitry irrespective of the output of the corresponding NOR gate circuit in the write operation. According to this embodiment, in order to permit the selective erase operation stated before, the gate circuit G is so constructed as to transmit the output of the corresponding NOR gate circuit to the corresponding first word line in the erase operation besides in the read operation. In the write operation, the level conversion circuit LVC brings the corresponding second word line to the select level substantially equal to the power source voltage V cc in response to the fact that the output of the corresponding NOR gate circuit is the select level of the high level, and it brings the second word line to the unselect level substantially equal to the minus voltage -V pp in response to the fact that the output of the NOR gate circuit is the unselect level of the low level. Besides, in the erase operation, the level conversion circuit LVC brings the corresponding second word line to the erase selection level substantially equal to the minus voltage -V pp in response to the fact that the output of the corresponding NOR gate circuit is the select level of the high level, and it brings the second word line to the erase unselection level substantially equal to the power source voltage V cc in response to the fact that the output of the NOR gate circuit is the unselect level of the low level. The gates of the isolating MOSFET's Q3 etc. are coupled in common to the control line which is supplied with a control voltage V ig formed by a control voltage generator circuit V ig -G. The sources of these isolating MOSFET's Q3 etc. are made common to construct the common source line CS. The control voltage V ig which is supplied to the isolating MOSFET Q3 is set at a low potential of, e. g., about -10 V in order that, in the operation of writing data into the MNOS transistor as will be described later, the MOSFET Q3 may be brought into the "off" status when one of the second word lines W12 - W22 with which the memory cell to be selected is coupled is set at the high level (5 V), the well region WELL as the body gate is set at about -12 V and the data line, for example, D1 is set at about -10 V. Thus, even when the data line D2 is set at the high level of, e. g., +5 V, current is prevented from flowing from the data line D2 to the side of the memory cell into which the data is to be written. The common source line CS is coupled to the output terminal of a common source line driver circuit DVR. Basically, the driver circuit DVR may have such output characteristics that, in the erase operation, the common source line CS can be driven to the level substantially equal to the power source voltage V cc , while in the read operation, the common source line CS can be driven to the level substantially equal to the ground potential of the circuitry. Thus, when the well region WELL is set at the level of the power source voltage V cc in the erase operation, the junction between the well region WELL and the electrode of the MOSFET Q3 coupled to the common source line CS can be prevented from being forward-biased. Moreover, a current path required for the read operation can be formed between the common source line CS and the ground point of the circuitry. Although not especially restricted, the driver circuit DVR is constructed, as shown in Fig. 9 , of a MOSFET Q6 which is interposed between the power source terminal V cc of the circuitry and the common source line CS, MOSFET's Q7 and Q8 which are connected in parallel between the common source line CS and the ground point of the circuitry, and a CMOS inverter circuit IV. The gates of the MOSFET's Q7 and Q8 are supplied with a control signal er, and the gate of the MOSFET Q6 with a signal obtained by inverting the control signal er by means of the inverter circuit IV. Thus, the MOSFET's Q7, Q8 and the MOSFET Q6 are complementarily turned "on" and "off" in accordance with the level of the control signal er. Basically, the control signal er is set at a high level substantially equal to the power source voltage V cc , so as to bring the MOSFET Q6 into the "on" status and the MOSFET's Q7 and Q8 into the "off" statuses in the erase operation, and it is set at a low level substantially equal to 0 volt, in the read and write operations. According to this embodiment, the control signal er has its output timing controlled in correspondence with the change timing of the potential of the well region WELL in order that PN-junctions defined by the MOSFET's etc. formed in the well region may be prevented from falling into forward-biased statuses. According to this embodiment, MOSFET's Q4 and Q5 are respectively interposed between the second word lines W12 and W22 and the common source line CS. These MOSFET's Q4 and Q5 are switched and controlled by a control signal er/we . Although not especially restricted, the control signal er/we has its high level set at a level substantially equal to the power source voltage V cc and has its low level set at a level substantially equal to the ground potential. The MOSFET's Q4 and Q5 are made the P-channel type so as to be favorably turned "off" even when the minus potential is applied to the corresponding second word lines W12 and W22. In the read operation, the switching MOSFET's Q4 - Q5 are brought into "on" statuses so as to short-circuit the gates of the MNOS transistors Q2 etc. and the common source line CS thereby to equalize the potentials of them. These switching MOSFET's Q4 - Q5 are interposed between the corresponding second word lines and the common source line CS, for the following reason: The MOSFET's Q7 and Q8 in the driver circuit DVR are brought into the "on" statuses in the read operation in accordance with the control signal er which is set at the low level substantially equal to 0 volt. In this case, the MOSFET's Q7 and Q8 have unnegligible "on" resistances though they are connected in parallel as shown in the figure. As a result, the common source line CS has its potential raised by a current which flows through the resistances in the read operation. Especially in a case where the MOSFET's Q7 and Q8 are of the P-channel type, they do not have a driving ability sufficient for changing the common source line CS to the ground potential of the circuitry, and hence, the floating magnitude of the potential of the common source line CS becomes large. More specifically, the current transfer electrodes of the MOSFET's Q7 and Q8 coupled to the common source line CS act as source electrodes with respect to a plus potential which is applied through the memory array M-ARY as well as the common source line CS. Therefore, the MOSFET's Q7 and Q8 are substantially turned "off" when the common source line CS becomes a potential below the threshold voltages of the respective MOSFET's. Such rise in the potential of the common source line CS incurs increase in an effective threshold voltage attributed to the substrate effect of the MNOS transistor, and lowers the conductance of the MNOS transistor which ought to have the low threshold voltage. In other words, a read current which flows through the MNOS transistor having the low threshold voltage is reduced. The short-circuiting MOSFET's Q4 and Q5 render the potentials of the corresponding second word lines W12 and W22 substantially equal to the potential of the common source line CS in the read operation, thereby to prevent the increase of the effective threshold voltage of the MNOS transistor. The well region WELL in which the memory array M-ARY is formed is supplied with the control voltage V w which is formed by a control voltage generator circuit V w -G. This voltage V w is set at a minus high voltage of, e. g., about -12 V in the write operation, at a potential of about +5 V in the erase operation, and at about 0 V at any other time. In this embodiment, to the end of raising the speed of the read operation, the respective data lines D1 and D2 of the memory array M-ARY are provided with N-channel MOSFET's Q11 and Q12 which electrically isolate these data lines D1 and D2 from corresponding column switch MOSFET's Q9 and Q10. More specifically, the MOSFET's Q11, Q12 etc. and the N-channel MOSFET's Q9, Q10 etc. as a Y gate (column switch) circuit C-SW are respectively interposed in series form between the corresponding data lines D1, D2 etc. and the common data line CD. The data line isolating MOSFET's Q11 - Q12 are formed in the same P-type well region WELL as that of the MNOS transistors. The gates of these MOSFET's Q11 - Q12 are supplied with a control voltage V c which is formed by a control voltage generator circuit V c -G. This control voltage V c is set at a minus high voltage of, e. g., -12 V in only the status of the write operation, and at a high level of, e. g., the power source voltage V cc in the other statuses of the read and erase operations. Thus, the MOSFET's Q11 - Q12 are turned "off" in the status of the write operation. Besides, the MOSFET's Q11 - Q12 are turned "off" in the status of the erase operation in accordance with the fact that the well region WELL is set at the high level of, e. g., the power source voltage V cc . Therefore, the MOSFET's Q11 - Q12 are turned "on" only in the status of the read operation. In this manner, the MOSFET's Q11, Q12 etc. are held in the "off" statuses in the write operation, so that even when the potentials of the data lines are brought to the minus high voltage, the nodes between the MOSFET's Q11, Q12 etc. and the column switch MOSFET's Q9, Q10 etc. to be explained below are held in floating statuses. Thus, the sources and drains of the switch MOSFET's Q9, Q10 etc. coupled to the mutual nodes and the well region in which these MOSFET's are formed can be prevented from being forward-biased. The gates of the MOSFET's Q9 - Q10 constituting the column switch circuit C-SW are supplied with the output signals of a Y-decoder YDCR. In the read operation, the outputs of the Y-decoder YDCR are set at a select level substantially equal to the power source voltage V cc or at an unselect level substantially equal to 0 volt. The common data line CD is coupled to the output terminal of a data input circuit DIB which constitutes an input/output circuit IOB, and to the input terminal of a data output circuit DOB which is composed of a sense amplifier SA and an output buffer circuit OBC. The input terminal of the data input circuit and the output terminal of the data output circuit constituting the input/output circuit IOB are coupled to an external terminal I/O. According to this embodiment, each of the data lines D1 - D2 is provided with a latch circuit FF which serves to hold preceding stored information before an erase/write mode, and with a level conversion circuit LVC which brings the potential of the data line to the minus high voltage -V pp selectively according to the stored information of the latch circuit FF in the write operation. These circuits realize an automatic rewrite operation to be explained later, and the simultaneous writing of data items into a plurality of memory cells coupled to one selected word line. The control circuit CONT receives the chip enable signal, write enable signal and output enable signal respectively supplied to the external terminals CE , WE and OE and the write voltage supplied to the external terminal -V pp as stated before. Thus, it discriminates various operation modes and produces various control signals for controlling the operations of the circuits such as the gate circuits G, level conversion circuits LVC, control voltage generator circuits V ig -G, V c -G and V w -G, driver circuit DVR, data input circuit DIB and data output circuit DOB. Among the control signals which are formed by the control circuit CONT, principal ones are illustrated in Fig. 9. In addition, the waveform diagrams of the principal control signals are exemplified in Fig. 10A- Fig. 10N. Although not especially restricted, a read operation mode is instructed by the low level, low level and high level of the respective signals (hereinbelow, expressed as signals CE , WE and OE ) at the external terminals CE , WE and OE , and a standby operation mode is instructed by the high level of the signal CE . The first write operation mode for writing data items into the latch circuits FF in Fig. 9 is instructed by the low level, low level, high level and low level of the respective signals CE , WE , OE and -V pp , while the second write operation mode for writing data items into the memory cells is instructed by the low level, low level, high level and high level of the respective signals CE , WE , OE and -V pp . An erase operation is instructed for a predetermined interval when the second write operation mode has been instructed. The various control signals to be delivered from the control circuit CONT are provided in time series in accordance with this embodiment. The oscillator circuit OSC in Fig. 9 is operated by the power source voltage V cc of, e. g., +5 volts which is applied between the external terminals V cc and GND of the EEPROM device. If necessary for lowering the power consumption of the circuitry, it is also allowed to control the oscillator circuit OSC so as to operate only when the write voltage is applied to the terminal -V pp by way of example. Now, an example of the second write operation mode of the EEPROM of this embodiment will be described with reference to the timing charts shown in Fig. 10A - Fig. 10N. In case of rewriting data, the first write mode not illustrated is carried out before the second write mode. In the first write mode, the stored information items of all the memory cells coupled to one addressed word line are once fetched and are held in the latch circuits FF shown in Fig. 9 . Subsequently, a data signal supplied from the external terminal is put into the latch circuit corresponding to the data line of the memory cell into which the data item is to be written. By way of example, in a case where all the bits of the memory cells coupled to the word line are to be rewritten, the Y-addresses are sequentially switched, whereby the write signals composed of a plurality of bits and supplied from the external terminal are sequentially put into the respectively corresponding latch circuits. Thereafter, the second write mode illustrated in the charts is carried out. The erase operations of the MNOS transistors coupled to the word line are performed, whereupon the memory cells for one word line are simultaneously subjected to write operations in accordance with the information items of the latch circuits. Owing to the above operations, a write operation which is similar to that of a static RAM when viewed from outside can be executed. In the second write mode which is instructed by the low level, low level, high level and high level of the respective external signals CE , WE , OE and -V pp not shown, a control signal EW is raised from a low level to a high level. Internal signals er , ert and erts are respectively changed from high levels to low levels with predetermined time differences since the rise of the signal EW to the high level. The MOSFET Q6 of the driver circuit DVR in Fig. 13 is brought into the "on" status by the low level of the internal signal er (high level of er), so that the common source line CS of the memory array M-ARY is set at the high level of, e. g., +5 V. Owing to the time difference between the internal signals er and ert , a reset signal cr is changed from +5 V to a low level of, e. g., -4 V at which it is temporarily held. Thus, the output terminals of the level conversion circuits LVC (the word lines W12 etc.) are reset to the ground potential and are thereafter set at the low level (0 V) in the floating statuses. Besides, owing to the time difference between the internal signals er and erts , a reset signal cu is changed from +5 V to a low level of, e. g., -4 V at which it is temporarily held. Thus, loads having comparatively large parasitic capacitances, such as the well WELL and isolating MOSFET's, are subjected to reset operations similar to the above. In response to the low level of the internal signal ert , the X-decoder XDCR starts the level changing operation thereof. By way of example, the potential of the selected second word line, in other words, the gate potentials of the MNOS transistors to be erased is/are lowered to the minus high voltage of about -10 V as explained before. The potentials of the word lines not to be selected, in other words, the gate voltages of the MNOS transistors to be inhibited from the erase operations are set at the high level of, e. g., +5 V as understood from the above-stated operations though this situation is not illustrated. Thereafter, the control voltage generator circuit V w -G for forming the drive voltage of the body gate of the memory array M-ARY, in other words, the well region WELL sets its voltage V w at the high level of, e. g., +5 V in response to the low level of the internal signal erts . Thus, the minus high voltage is supplied between the body gate and the gates of the MNOS transistors coupled to the selected word line. As a result, information charges gathered in the floating gates of the MNOS transistors are restored into the body gate by the tunnel effect based on the high electric field. Incidentally, since the body gate and the gates of the MNOS transistors coupled to the unselected word lines are held at the identical potential, the erase operations of these MNOS transistors are not performed. In ending the erasure, the internal signals are respectively changed from the low levels to the high levels with time differences in the order of erts , ert and er reverse to the order for the start of the erasure. Consequently, the original statuses are restored in the order of the well region WELL, the second word line and the data lines. In addition, reset signals cr , cu and cw are formed by the aforementioned internal signals. With the above operation timings, in starting the erasure, the potential of the P-type well region WELL is lastly raised to the high level of, e. g., the power source voltage V cc , and in ending the erasure, it is first lowered. Therefore, the PN-junctions between the well region WELL and the N-type drains and sources of the addressing MOSFET's or isolating MOSFET's formed in this well region WELL can be maintained in reverse-biased statuses. The above erase operation is followed by the write operation. Internal signals we ' and wets are successively changed from high levels to low levels with a time difference. In response to the low level of the internal signal we ', the control voltage generator circuit V w -G brings its voltage V w to the minus high voltage -V pp of, e. g., -12 V. Thus, the well region WELL formed with the memory array M-ARY is first lowered to the minus high voltage -V pp . In synchronism therewith, also the control voltage generator circuit V ig -G brings its voltage V ig to the minus high voltage of, e. g., about -12 V. Thus, the isolating MOSFET's of the memory cells are turned "off." Likewise, the voltage V c is brought to the minus high voltage of, e. g., -12 V as stated above. Thus, the data line isolating switch MOSFET's Q11 - Q12 are turned "off." Moreover, the gate circuits G of the X-decoder XDCR are enabled in accordance with the low level of the internal signal we ', whereby the first word line of the selected memory cells is set at the high level (+5 V), and the unselected word lines are set at the ground potential (0 V) of the circuitry (not illustrated). Subsequently, in synchronism with the low level of the internal signal wets , the X-decoder XDCR brings the selected second word line to the high level (+5 V) and the unselected ones to the low level. Upon receiving the high level and low level, the level conversion circuits LVC bring the corresponding second word line to the high level of, e. g., +5 V for the select signal of the high level and the corresponding second word lines to the minus high voltage of, e. g., -10 V for the unselect signals of the low level though the latter case is not illustrated. In addition, the level conversion circuits LVC coupled to the respective data lines are brought into operating statuses. Then, according to the stored information items of the corresponding latch circuits FF, by way of example, the level conversion circuit to write logic "1" is set at the minus high voltage of about -10 V, and the level conversion circuit to write logic "0" (to be inhibited from writing) is set at the high level of about +5 V. Accordingly, regarding the MNOS transistor into which the logic "1" is to be written, the gate voltage becomes about +5 V, the voltage of the body gate (well region WELL) becomes about -12 V, and the drain (data line) voltage becomes about -10 V, so that a high electric field of, e. g., about 15 V acts between the channel in the body gate and the gate electrode, and electrons are injected by the tunnel effect. In contrast, regarding the MNOS transistor into which the logic "0" is to be written, the drain voltage is brought to about +5 V, so that no high voltage is applied between the gate and the channel, and the injection of electrons does not take place. In ending the write operation, the internal signals are respectively changed from the low levels to the high levels with a time difference in the order of wets and we ' reverse to the order for the start of the write operation. Consequently, the original statuses are restored in the order of the data lines as well as the second word lines and the well region. In addition, the reset signals cr , cu and cw are formed by the aforementioned internal signals. With the above operation timings, in starting the writing, the potential of the P-type well region WELL is first lowered to the minus high voltage, and in ending the writing, it is lastly restored. Therefore, the PN-junctions between the well region WELL and the N-type drains and sources of the addressing MOSFET's or isolating MOSFET's formed in this well region WELL can be maintained in the reverse-biased statuses. In this embodiment, as illustrated in Fig. 8 , the control circuit CONT inhibits the memory block MB1 from the erase operation by the use of the signal formed by the decoder DEC, irrespective of the erase inhibit signal EI. That is, even when the memory block MB1 is addressed and is instructed to rewrite data, the erase operation is inhibited, so that only the write operation is performed. In other words, the various control signals for the erase operation shown in Figs. 10A - 10N are not generated, so that only the write operation is validated and executed. More specifically, in response to the erase inhibit signal EI' rendered the high level, the control circuit CONT forms various signals indicated by broken lines in an erase period shown in Figs. 10A - 10N. On the other hand, the control circuit CONT forms signals indicated by solid lines in a write period. Thus, the erase operation is inhibited, and only the write operation is executed. In this way, only the first write operation is, in effect, validated for the memory block MB1. Accordingly, data to be protected (for example, a normal identifying code or ID information) is permitted to be effectively written at first. After such identifying code or ID information has been written, the erasure is inhibited, with the result that merely a change from the logic "1" (unwritten status) to the logic "0" (written status) is executed by the write operation. Thus, even when it is intended by an unlawful manipulation to rewrite the data of the memory block MB1 into any identifying code or ID information, only the write operation proceeds, with the result that merely the bits corresponding to the logic "1" are changed to the logic "0," with the bits of the logic "0" remaining unchanged. Therefore, merely the number of bits of the logic "0" within one data increases, and the stored data is, in effect, changed to a meaningless identifying code or ID information. With note taken of this fact, it becomes possible to discriminate whether or not data has been illicitly written. It is assumed by way of example that the identifying code or ID information be unit data composed of 8 bits. Although not especially restricted, the number of the bits of the logic "0" is determined to be four in the unit data. Then, when the unlawful rewriting as exemplified above has been done, the logic "1" in the remaining 4 bits is changed to the logic "0," and the number of bits of the logic "0" exceeds 4. Thus, when the identifying code or ID information etc. are read out, whether or not the unlawful rewrite operation has been done can be simply decided by counting the number of the bits of the logic "0." When the number of bits of the logic "0" is limited to 4 in the unit data, codes expressive of characters or numerals which can be constructed of signals of 8 bits are in a number of 70 which is small as compared with the number of 256 in the case of employing all of the 8 bits. The 70 sorts, however, suffice for the characters or numerical values. The smaller number of sorts can also be solved in such a way that the memory capacity of the memory block MB1 is enlarged to increase the digits of the characters or numerical values of the identifying code or ID information. Alternatively to setting the number of "0's" for the unit data, one identifying code or ID information is constructed of a plurality of data items, for example, 8 bytes, in which the summation of "0's" may be set at a number of 32. By endowing the stored contents with a redundancy, errors in which the logic "1" is read out as the logic "0" due to malfunctions based on noise or the like can also be remedied by software or hardware. For testing the write operation and erase operation for the memory block MB1, it is desirable to add the function of validating the erase operation of the memory block MB1 in only the overall erase operation. To this end, the control circuit CONT is additionally provided with the function of instructing the overall erasure and the function of responsively invalidating the erase inhibit signal EI' delivered from the gate circuit G1. Besides, the control circuit CONT is supplied with the write inhibit signal WI. In response to the write inhibit signal WI rendered the high level, the control circuit CONT forms various signals indicated by dot-and-dash lines in the write period shown in Figs. 14A - 14N. Thus, the operation of writing data into the EEPROM is inhibited. That is, the erase operation is possible, but the write operation becomes impossible. Of course, in a case where the inhibit signals EI' and WI are respectively held at the high levels, neither the erase operation nor the write operation can be executed for the memory block MB1, and the EEPROM can be regarded as a kind of masked ROM. Shown in Fig. 11 is a block diagram of an EEPROM device which is the second embodiment of this invention. In the embodiment of Fig. 8, the memory block MB1 is inhibited from erasure, and hence, the write/ erase operation tests of the memory block MB1 are inconvenient. According to this embodiment, therefore, the EEPROM in Fig. 8 is further furnished with circuits to be explained below: A decision circuit LOG is disposed which receives the signal of the internal signal line (common data line) between the memory array M-ARY and the input/output circuit I/O. This decision circuit LOG has the function of counting the number of "0's" in unit data read out of the memory array M-ARY. By way of example, the number of "0's" in the unit data written in the memory block MB1 is set at 4 as described before. Then, the decision circuit LOG forms a decision signal ER which is set at a high level (logic "1") when the number of "0's" in the read data is at least 4, and which is set at a low level (logic "0") when the number of "0's" is less than 4. This decision signal ER is supplied to one input of an AND gate circuit G2, the other input of which is fed with the output signal of the decoder DEC, as the control signal of the gate circuit G2. The output signal of the AND gate circuit G2 is supplied to one input of the OR gate circuit G1, the other input of which receives the erase inhibit signal EI from the register REG. In this arrangement, when the address of the memory block MB1 to be rewritten is designated by addressing, reading the address is automatically instructed. If the number of "0's" of data thus read out is less than 4, the decision circuit LOG brings the output signal ER to the low level. Thus, even when the decoder DEC has formed the signal detecting the access to the memory block MB1, the erase inhibition by this decoder DEC can be invalidated. Accordingly, the write test or erase test for the memory block MB1 can be conducted at will in such a way that the number of "0's" in data to be written is set to be less than 4. After the end of such tests, when data to be protected is written, the number of "0's" in unit data may be set at 4 as explained before. Let's consider a case where the memory block MB1 is addressed to instruct rewriting after an identifying code, ID information or the like composed of a plurality of data items each including the four logical "0's" as stated above has been written. In this case, an internal read operation is performed beforehand, and the output signal ER of the decision circuit LOG is set at the high level (logic "1") because the number of logical "0's" is 4. Therefore, the output signal of the decoder DEC is validated to inhibit the erase operation as in the foregoing. Data read out by the internal read operation is not delivered to the external terminals D0 - D7 at all for the reason that the input/output circuit I/O is put in a non-operating status. According to this embodiment, the addition of the decision circuit LOG and gate circuit G2 of comparatively simple arrangement makes it possible to easily perform the rewrite test of the memory block MB1 which is inhibited from the erase operation. Besides, the decision circuit can be endowed with the function of informing the exterior of the fact that data has been illicitly written. More specifically, the function of detecting that the set number of logical "0's" (4 in the above example) is exceeded is added as a count deciding function, and an alarm signal indicative of the illicit writing is generated. The circuit blocks and the internal circuits and operations thereof omitted from the above description are similar to those elucidated with reference to Fig. 8 - Fig. 10N. Fig. 12 shows the third embodiment of the present invention. This embodiment is a single-chip microcomputer for an IC card in which the EEPROM of the first or second embodiment stated above is built. Although not especially restricted, various circuit blocks enclosed with a dot-and-dash line A in the figure are formed on one semiconductor chip such as single-crystal silicon substrate. Although no special restriction is meant, the single-chip microcomputer of this embodiment includes a microprocessor unit (hereinbelow, termed "CPU") 81 which controls the internal execute unit etc. thereof in accordance with programs so as to perform desired processes, a read only memory (ROM) 82 in which the operation program of the CPU 81, etc. are stored, a random access memory (RAM) 83 which principally offers the working area of the CPU 81, and a serial communication interface 84 which sends and receives data to and from an external apparatus such as terminal equipment, these circuits being interconnected through an internal system bus 85. In this embodiment, the read only memory 82 is constructed of an unrewritable masked ROM into which data is written by masking in the course of a manufacturing process. An EEPROM 86 in which data items proper to a user, such as the money information of a bank and the identifying code of the individual, are stored, and a booster circuit 87 which generates a write voltage V pp required in the operation of writing the data items into the EEPROM 86, are packaged on the chip A separately from the masked ROM 82. The EEPROM 86 is so constructed that it can be usually accessed by only the CPU 81. For the purpose of permitting a manufacturer side to test the EEPROM 86 in a short time, however, the IC card is provided with a testing bus 88 separately from the internal system bus 85, and with a mode setting circuit 89 for making it possible to access the EEPROM 86 from outside directly without the intervention of the CPU 81 in accordance with an external control signal TEST and by the use of the testing bus 88, as well as a terminal 90 for inputting the mode control signal TEST. This terminal 90 for mode control, and a terminal 91 for inputting a read enable signal RE, a write enable signal WE, an erase enable signal ER and an all select signal AS for the testing bus 88 as well as the EEPROM 86, are not connected to external terminals as the IC card. Connected to the external terminals of the IC card are only five terminals; an I/O terminal 92 for serial communications, power source terminals 93 and 94 for applying power source voltages V cc and V ss to the chip, respectively, an input terminal 95 for a reset signal RES, and an input terminal 96 for a clock CLK. Accordingly, after the single-chip microcomputer of this embodiment has been encapsulated in the IC card, the EEPROM 86 cannot be directly accessed using the testing bus 88. Moreover, when the EEPROM 86 is so constructed that some of data items can be conserved by rewriting even in the overall erasure mode as in the foregoing embodiment, the unlawful initialization of the EEPROM cannot be performed. Thus, it is possible to prevent the illicit use of the IC card ascribable to the alterations of the important data items in the EEPROM, such as money information and identifying code. The ROM 82 stores therein the instructions or programs according to which the CPU 81 executes the rewriting of data for the EEPROM 86 in the ordinary operation modes. In this regard, by partly modifying the address decoder of the EEPROM 86 by way of example, the IC card can also be so constructed that the CPU 81 cannot access the first row address or a designation row address containing protective information. The control signals such as the read enable signal RE, write enable signal WE and erase enable signal ER, which are required when data items are read out of and written into the EEPROM 86 by the CPU 81 in the ordinary modes, are afforded from the CPU 81 to the EEPROM 86. Although the embodiment has been described as to the case where the single-chip microcomputer with the built-in EEPROM is used for the IC card, a microcomputer for a card can also be constructed by combining an EEPROM chip with a single-chip microcomputer in which a ROM and a RAM are built. Functional effects which are produced by the foregoing embodiments are as stated below: (1) In case of overall and simultaneous erasure, some of stored data items are conserved, so that the prevention of an illicit use and the shortening of a testing time can be made compatible. (2) A method for the conservation is so constructed that some of stored data items are transferred and held outside a group of non-volatile memory cells before the overall-and simultaneous erasure, and that the stored data items are written again after the overall and simultaneous erasure, whereby the invention becomes applicable also to an overall erasure type non-volatile memory. (3) Stored data to be held outside the group of non-volatile memory cells is endowed with protective information, thereby making it possible to select whether or not the conservation is performed. (4) On the basis of the protective information, erasure is withheld in row address unit, whereby the addresses and capacity of stored data to be conserved can be set at will in row address unit. (5) In case of simultaneous erasure, the row addresses to be withheld from the erasure are fixed, whereby some stored data items stated above can be transferred outside the group of non-volatile memory cells, and a simpler construction is realized. (6) A memory area in a non-volatile memory circuit which is electrically writable and erasable is divided, and the function of inhibiting a write or erase operation for the whole or partial memory area is added, thereby to attain the effect that the unlawful rewriting of data can be made impossible. (7) Addressing to a memory area in which data to be protected is stored is detected, and an erase operation is inhibited according to the detection, thus to adopt a construction in which while the data to be protected is permitted to be written, it is destroyed by multiple writing thereon, thereby to attain the effect that the data can be substantially protected. (8) By adding a simple circuit for detecting addressing to a memory area in which data to be protected is stored, there is attained the effect that the protection of the data can be realized. (9) By adopting a construction in which the number of bits in a written status is previously set for data to-be-protected, there is attained the effect that whether or not data has been unlawfully written can be decided. (10) By adopting a construction in which the number of bits indicative of a written status in readout data is decided, and an erase operation is granted when the number of the written bits is not larger than a fixed number, there is attained the affect that write, erase and rewrite tests for a memory area in which data to be protected is stored can be simply conducted while the protection of the data is achieved. (11) Owing to the combination between an erase inhibit signal and addressing to a memory area in which data to be protected is stored, there is attained the effect that while writing for the memory area is inhibited with the first writing of the data to-be-protected made possible, rewriting for another memory area can be made possible. (12) A select bit is internally provided so as to select memory blocks in accordance with this select bit, and the erase operation of the memory block selected in correspondence with the set status of the select bit is inhibited, thus to adopt a construction in which while data to be protected is permitted to be written, it is destroyed by multiple writing thereon, thereby to attain the effect that the data can be substantially protected. (13) By providing a select bit, a decoder for addressing is dispensed with, and hence, there is attained the effect that the protection of the data as stated above is realized by a simpler circuit arrangement. (14) By adopting a construction in which two memory blocks having an identical address assigned thereto by the desingation of a select bit are alternatively brought into selected statuses, it becomes possible that when the writing or reading of data into or from one memory block has been instructed, the data of the other memory block at the identical address is previously or simultaneously read out to decide whether the instructed operation is granted or inhibited. Thus, there is attained the effect that protection is permitted in the unit of the data. Although, in the above, the invention made by the inventor has been concretely described in conjunction with embodiments, it is needless to say that the present invention is not restricted to the foregoing embodiments, but that it can be variously altered within a scope not departing from the purport thereof. By way of example, an EEPROM may well be such that an erase operation and a write operation are independently designated and that a memory array is erased or written according to the designated operation. Besides, the first write operation mode may well be shifted to the second write operation mode automatically within an EEPROM. Alternatively, overall erasure may be carried out every memory block by dividedly isolating a well region in which a memory array is constructed. Further, the capacity of a memory block in which data to be protected is stored may well be made freely designated by a master slice system. In, for example, the embodiment of Fig. 8, the memory area of the memory block MB1 can be simply changed by altering the address signals which are supplied to the decoder. Alternatively to the system based on the division of word lines, the system for dividing memory blocks may be based on the division of data lines or based on the combination of the divisions of the word and data lines. Adopting the division of the data lines in this manner can be realized by, for example, disposing a decoder which receives address signals from a Y-address buffer or supplying a select bit to a Y-address buffer or a decoder. Further, the practicable arrangements of a memory array and its peripheral circuits can adopt various modifications. By way of example, although the high voltage -V pp has been described as being supplied from outside, a high voltage generator circuit may well be built in. Alternatively to the peripheral circuits constructed of the CMOS circuits as stated before, they may be constructed of only N-channel MOSFET's or P-channel MOSFET's. Further, as high voltages for writing/erasing, various aspects of performance can be adopted, including a method in which a substrate formed with MNOS transistors is fixed at the ground potential of circuitry, whereupon a plus high voltage and a minus high voltage are used. An EEPROM according to this invention is constructed as a single non-volatile memory as in any of the embodiments, and besides, it can be comparatively simply built in a digital integrated circuit, for example, a microcomputer of single chip because a circuit scale for realizing the data protective function as stated above is small. The single-chip microcomputer which has the built-in EEPROM endowed with the protective function as stated above is suitable for an IC card. More specifically, the IC card needs to have its physical strength increased for the purpose of protecting a built-in semiconductor integrated circuit device. If the single-chip microcomputer having the built-in EEPROM as described above can be realized, the physical size of the semiconductor integrated circuit device built in the IC card can be made small, so that the increase of the physical strength is simplified and that the cost of the IC card can be lowered. A non-volatile memory element which is electrically writable and erasable may be any other than the MNOS transistor, such as an element of FLOTOX (floating-gate tunnel oxide) structure. This invention can be extensively utilized for semiconductor integrated circuit devices each including a non-volatile memory circuit which is electrically writable and erasable.

Claims: A non-volatile memory device comprising: a memory array (M-ARY) including a first memory block (MBO) and a second memory block (MB1), each memory block including non-volatile memory cells; a control circuit (CONT) which controls an erasure operation on the memory array; selecting means (XDCR, YDCR) for selecting at least one non-volatile memory cell from the memory array; a designating circuit (DEC) which is arranged to designate whether the selected non-volatile memory cell is in the first memory block or in the second memory block: characterised in that the device further includes: a register circuit (REG) which includes a first control bit (EI) for controlling the erasure operation of the control circuit on the memory array, the first control bit selectively having a first state for inhibiting the erasure operation of the control circuit on the memory array and a second state for enabling the erasure operation of the control circuit on the memory array; and a gate circuit (G1) coupled to the designating circuit and to the register circuit for inhibiting the erasure operation of the control circuit on the second block regardless of the second state of the first control bit when the designating circuit designates that the selected non-volatile memory cell is in the second block. A non-volatile memory device according to claim 1, wherein each of the non-volatile memory cells includes an electrically erasable and programmable memory element. A non-volatile memory device according to claim 1 or claim 2, wherein the control circuit also controls a write operation on the memory array; and wherein the register circuit further includes a second control bit (WI) for controlling the write operation of the control circuit on the memory array, the second control bit having a third state for inhibiting the write operation of the control circuit on the memory array and a fourth state for enabling the write operation of the control circuit on the memory array. A single-chip microcomputer comprising a microprocessor (81) and a non-volatile memory device according to any one of claims 1 to 3, wherein each of the states of the first and second control bits in the register circuit is set by the microprocessor. An integrated circuit card comprising a microcomputer according to claim 4. A non-volatile memory device according to claim 3, wherein the control circuit controls a read operation on the memory array; and wherein the register circuit further includes a third control bit (RI) for controlling the read operation of the control circuit on the memory array, the third control bit having a fifth state for inhibiting the read operation of the control circuit on the memory array and a sixth state for enabling the read operation of the control circuit on the memory array. A non-volatile memory device according to claim 1, further comprising: a decision circuit (LOG) which forms a decision signal (ER) on the basis of data read out from the second memory block (MB1) and which supplies the gate circuit (Gl) with the decision signal to invalidate a designating function by the designating circuit.

Inventors: MATSUBARA KIYOSHI, MITSUISHI NAOKI, OZAWA YOSHIYUKI, TAKAMORI YOH, MATSUBARA, KIYOSHI, MITSUISHI, NAOKI, OZAWA, YOSHIYUKI, TAKAMORI, YOH
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G06F12
IPC codes: G11C16, G06F12