Patent Publication Number: US-2005128838-A1

Title: Non volatile memory and data processor

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a semiconductor device having an electrically programmable non-volatile memory, and a data processor. The present invention also relates to a technology for solving a delay in read access due to precharge and discharge operations of each bit line, which are necessary for the operation of reading memory information held in each non-volatile memory cells of the non-volatile memory, e.g., a technology effective for application to a microcomputer equipped with a flash memory in an on-chip form.  
      A flash memory, which is one electrically writable or programmable memory, has non-volatile memory cells each comprising one memory cell transistor having a floating gate, a control gate, a source and a drain, for example. In the memory cell transistor, a threshold voltage thereof rises when electrons are injected into the floating gate, whereas when the electrons are extracted from the floating gate, its threshold voltage drops. The memory cell transistor serves so as to store information corresponding to the magnitude or high and low levels of a threshold voltage relative to a word line voltage (voltage applied to control gate) for reading data. Although not restricted in particular, the state in which the threshold voltage of the memory cell transistor is low, is called an “erase state”, and the state in which the threshold voltage thereof is high, is called a “write state”.  
      In order to read information held in each of the non-volatile memory cells, the drain side (bit line side) of the selected non-volatile memory cell is clamped to a precharge voltage and a gate voltage is applied. If the electrons have been stored in the non-volatile memory cell, the threshold voltage is high and no current flows even if the gate voltage is applied. If no electrons are stored in the memory cell, the threshold voltage is low and the current flows when the gate voltage is applied.  
      A sense amplifier detects such a state. Now consider where the sense amplifier is of a current sense type sense amplifier, for example. When the sense amplifier causes a current equivalent to half of a memory cell current (corresponding to a current flowing in each memory cell held on) to flow, a bit line voltage of a memory cell high in threshold voltage becomes higher than a precharge voltage. A bit line voltage of a memory cell low in threshold voltage becomes lower than the precharge voltage. A logic value of memory information held in the corresponding memory cell is judged according to the difference between the above voltages. Even when the sense amplifier belongs to a non-current sense type configuration, e.g., it is of a differential sense amplifier of such a type that a bit line voltage is compared with a reference level, there is a need to precharge the corresponding bit line to the reference level or a level close to it in advance before the start of a read operation. In order to read the information of the memory cell in this way, there is a need to clamp the corresponding bit line to the precharge voltage.  
      On the other hand, let&#39;s look at the memory cell. If the voltage is always applied to the source side (source line side) or drain side (bit line side), such an electric field that electrons are stored in or extracted or drawn from the floating gate, is formed, thus causing a possibility that memory information will be damaged (disturbed). It is not desirable that the voltage like the precharge voltage is always applied to the drain of each memory cell via its corresponding bit line in order to prevent the fear of such disturb before happens.  
      Therefore, there is a need to set the source and drain sides of the memory cell as a ground potential when no reading is done. The bit line must be discharged because the precharge voltage is clamped before the reading and the voltages on the source and drain sides are set to the ground potential rapidly after the reading. In order to reduce capacitance parasitic on each bit line connected with the drain of the memory cell in particular, a main/sub bit line structure is frequently adopted. In the main/sub bit line structure, only a sub bit line connected with a memory cell intended for a read operation is connected to its corresponding main bit line.  
     SUMMARY OF THE INVENTION  
      However, in order to read memory information of a non-volatile memory cell, there is a need to carry out pre-reading bit line precharge and post-reading bit line discharge. Even if the operating time (corresponding to the time required to make a decision as to whether a main bit line voltage is larger or smaller than a precharge voltage) necessary to read memory information could be shortened owing to the configuration or the like of the sense amplifier, the shortening of the time intervals for the precharge and discharge have been not yet implemented. With the trend toward an increase in parasitic capacitive component due to an increase in storage capacity, the precharge and discharge time intervals become long in reverse.  
      An object of the present invention is to shorten the time necessary to charge and discharge a bit line to which a non-volatile memory cell is connected.  
      Another object of the present invention is to speed up the reading of memory information from a non-volatile memory cell without performing an improvement in the performance of a sense amplifier, etc.  
      A further object of the present invention is to provide a data processor capable of contributing to the speeding up of data processing in terms of the reading of memory information from a non-volatile memory cell at high speed.  
      The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.  
      Summaries of typical ones of the inventions disclosed in the present application will be described in brief as follows:  
      A semiconductor device according to the present invention has an electrically programmable non-volatile memory. The non-volatile memory includes a plurality of non-volatile memory cells (MC), word lines (X 00  through Xkn) respectively connected to select terminals of the non-volatile memory cells, sub bit lines (LB 00  through LBkm) respectively connected to data terminals of the non-volatile memory cells, main bit lines (MB 0  through MBm), sub bit line selection switch elements (Q 00  through Qkm) which selectively connect the sub bit lines to the main bit lines, a sense amplifier ( 30 ) connected to the main bit lines via column switch elements (QY 0  through QYm), and voltage supply elements (QPC 0  through QPCm) each of which supplies a predetermined clamp voltage to each of the main bit lines. The clamp voltage is a predetermined level between a high-level side attainment level outputted from the sense amplifier and a low-level side attainment level outputted therefrom. Each of the voltage supply elements supplies the clamp voltage to the corresponding main bit line during at least a period prior to and subsequent to a read operation for each of the non-volatile memory cells.  
      According to the above means, the clamp voltage is supplied from each of the voltage supply elements to its corresponding main bit line during at least the period prior to and subsequent to the read operation for each non-volatile memory cell. Accordingly, there is no need to precharge the main bit line from the ground level upon the operation of reading the memory information, and the read operating time can hence be shortened. Since the non-volatile memory becomes fast in operating speed, the operating speed or throughput of the whole data processing system using it can be enhanced.  
      The clamp voltage is a level desirable in terms of a sense operation of the sense amplifier. The clamp voltage is, for example, a criterion level of the sense amplifier or a level higher than the criterion level. A discharge for each sub bit line is not necessary either for a non-volatile memory cell having a device structure free of the occurrence of disturb at that level.  
      In order to stabilize the operation of sensing by the sense amplifier even with respect to the small amount of a signal read from the memory cell and thereby speed up the determination of the sense operation, the voltage supply element may preferably stop the operation of supply of the clamp voltage during the period of the read operation for the non-volatile memory cell.  
      In order to previously prevent the fear of disturb, discharge elements (QD 00  through QDkm) for respectively discharging the sub bit lines when the sub bit line selection switch elements are respectively held off, may preferably be adopted. That is, the precharge for each main bit line, i.e., the supply of the clamp voltage and the discharge for each sub bit line are performed in parallel. Supplying the clamp voltage to the main bit line so as to avoid mutual interference of such parallel operations may be performed when the sub bit line selection switch element is held off.  
      In order to shorten a precharge time prior to the reading of memory information with a main/sub bit line structure as a premise from the above, the main bit line is clamped to a precharge voltage in advance, and the drain (sub bit line) of each memory cell is maintained at a ground potential. There is therefore no problem of memory disturb. Since the time required to charge each main bit line becomes predominant, the time necessary for precharge can be shortened by clamping the main bit line to the precharge voltage in advance. While both the main bit and sub bit lines have heretofore been discharged and set as the ground potential after reading, only each sub bit line is intended for discharge. Clamping the main bit line to the precharge voltage simultaneously with its discharge makes it possible to shorten the time necessary for charge and discharge.  
      Since the above means does not depend on the configuration of the sense amplifier, it is adaptable even if sense amplifiers of any circuit configuration are used. It is not necessary to change the sense amplifier. The non-volatile memory cell may be a memory which stores multi-valued information as well as binary information.  
      As one aspect of the present invention, clamp voltage supply elements are disposed every main bit lines as an alternative to the conventional provision of the precharge elements every sense amplifiers. As an alternative to the provision of the discharge elements every sense amplifiers, they are also disposed every sub bit lines. In order to reduce the number of the clamp voltage supply elements, the voltage supply elements may be provided for a common data line (CD) commonly connected to the column switch elements. In this case, all the column switch elements sharing the sense amplifier are brought to an on state in a non-selected state of each memory cell to thereby enable clamp operations for the main bit lines. When the corresponding memory cell is selected, only the column switch to be selected is held in an on state and others are respectively set to an off state.  
      As one aspect of the present invention, the sub bit line selection switches can be switch-controlled separately every main bit lines sharing the common data line commonly connected to the column switch elements. Only the sub bit line selection switch connected to the main bit line selected by the corresponding column switch element is brought to an on state upon a read operation. The sub bit lines connected to the remaining sub bit line selection switches held off are respectively electrically isolated from their corresponding main bit lines, so that an electrical charge on each main bit line is not wastefully transferred to its corresponding sub bit line unsubjected to the read operation. As another aspect thereof, source line connection selection switch elements which selectively connect source lines of a plurality of non-volatile memory cells to a common source line every main bit lines sharing the common data line commonly connected to the column switch elements, are disposed. The source line connection selection switch element connected to the non-volatile memory cell for each main bit line selected by the corresponding column switch element is brought to an on state upon the read operation. It is thus possible to suppress the useless flowing of a charge current on each bit line in the common source line.  
      As one aspect of the present invention, the sense amplifier is configured as a current sense type circuit which supplies a current smaller than a current flowing in the corresponding non-volatile memory cell held on when selected by each of the word lines, to each bit line and detects a change in bit line level with respect to a criterion level.  
      A data processor according to the present invention has an electrically programmable non-volatile memory, and a CPU accessible to the non-volatile memory. The non-volatile memory includes a plurality of non-volatile memory cells, word lines respectively connected to select terminals of the non-volatile memory cells, sub bit lines respectively connected to data terminals of the non-volatile memory cells, main bit lines, sub bit line selection switch elements which selectively connect the sub bit lines to the main bit lines, a sense amplifier connected to the main bit lines via column switch elements, voltage supply elements each of which supplies a predetermined clamp voltage to each main bit line, and discharge elements which discharge the sub bit lines. Each of the voltage supply elements supplies the clamp voltage to the corresponding main bit line before the start and completion of a read operation effected on each non-volatile memory cell. The discharge elements discharge the sub bit lines after the completion of the read operation.  
      In a manner similar to the above, after the read operation, only the sub bit line is intended for discharge and each of the main bit lines is clamped to the precharge voltage simultaneously with its discharge. It is therefore possible to shorten the time required to perform the charge and discharge. Since the drain (sub bit line) of each memory cell is held at the ground potential when the main bit line is clamped to the precharge voltage, a memory disturb problem does not arise and the speeding up of the read operation can be realized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram showing the details of a flash memory according to one embodiment of the present invention with a memory mat as a principal base;  
       FIG. 2  is a block diagram schematically illustrating the whole of the flash memory according to one embodiment of the present invention;  
       FIG. 3  is a block diagram of a data processor according to one embodiment of the present invention;  
       FIG. 4  is a circuit diagram specifically illustrating part of the memory mat shown in  FIG. 1  and a sense amplifier;  
       FIG. 5  is an explanatory view showing a read operation of a current sense type in principle;  
       FIG. 6  is a timing chart depicting a read operation of the flash memory shown in  FIG. 1  together with a comparative example;  
       FIG. 7  is a circuit diagram illustrating a configuration of a flash memory according to the comparative example;  
       FIG. 8  is an explanatory view showing a precharge time, a discharge time and precharge/discharge operations between a comparative example and the present invention;  
       FIG. 9  is a circuit diagram showing a second detailed example of a flash memory;  
       FIG. 10  is a timing chart illustrating the operation of reading memory information of a memory cell MC 00  in a configuration of  FIG. 9 ;  
       FIG. 11  is a circuit diagram showing a third detailed example of a flash memory;  
       FIG. 12  is a circuit diagram depicting a fourth detailed example of a flash memory;  
       FIG. 13  is a timing chart showing a read operation executed in a configuration of  FIG. 12 ;  
       FIG. 14  is a circuit diagram illustrating a fifth detailed example of a flash memory;  
       FIG. 15  is a timing chart depicting a read operation executed in a configuration of  FIG. 14 ;  
       FIG. 16  is a circuit diagram showing a sixth detailed example of a flash memory;  
       FIG. 17  is a timing chart depicting a read operation executed in a configuration of  FIG. 16 ;  
       FIG. 18  is a circuit diagram showing a seventh detailed example of a flash memory;  
       FIG. 19  is a timing chart illustrating read operation timings of a memory cell of  FIG. 18 ;  
       FIG. 20  is a circuit diagram showing an eighth detailed example of a flash memory;  
       FIG. 21  is a timing chart illustrating write operation timings of the flash memory shown in  FIG. 20 ; and  
       FIG. 22  is a timing chart illustrating read operation and erase operation timings of the flash memory shown in  FIG. 20 . 
    
    
     DEAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      &lt;&lt;Flash Memory&gt;&gt; 
      A schematic configuration of a flash memory  1  illustrative of one embodiment of a semiconductor device according to the present invention is illustrated in  FIG. 2  by way of example. In the same drawing, each of memory arrays  2  has a memory mat, a data latch circuit and a sense latch circuit, etc. The memory mat has a large number of electrically erasable and programmable non-volatile memory cell transistors. The memory cell transistor (also described as “flash memory cell”) comprises, although not shown in the drawing in particular, a source and drain formed within a semiconductor substrate or a well, a floating gate formed in a channel region between the source and the drain with a tunnel oxide film interposed therebetween, and a control gate laminated on the floating gate with an interlayer insulating film interposed therebetween. The control gate is connected to its corresponding word line  4 , the drain is connected to its corresponding bit line  6 , and the source is electrically connected to its corresponding unillustrated source line. Although their detailed description is omitted herein, specific structures of the bit line and word line are determined according to the structure of the memory mat. For example, the word line  4  is configured as a main/sub word line structure, whereas the bit line  6  is configured as a main/sub bit line structure.  
      When electrons are injected into the floating gate of the memory cell transistor, the memory cell transistor rises in threshold voltage. On the other hand, when the electrons are extracted or drawn from the floating gate, the threshold voltage thereof is lowered. The memory cell transistor serves so as to store information corresponding to the magnitude or high and low levels of a threshold voltage relative to a word line voltage (voltage applied to control gate) for reading data. Although not restricted in particular, the state in which the threshold voltage of the memory cell transistor is low, is called an “erase state”, and the state in which the threshold voltage thereof is high, is called a “write state”. Incidentally, since the definition of writing and erasing is a relative concept, they may be defined contrary to the above.  
      External input/output terminals I/O 0  through I/O 7  of the flash memory  1  connected to a bus share the use of address input terminals, data input terminals, data output terminals and command input terminals. X address signals inputted from the external input/output terminals I/O 0  through I/O 7  are supplied to an X-address buffer  10  via a multiplexer  8 . An X-address decoder  12  decodes an internal complementary address signal outputted from the X-address buffer  10  to drive its corresponding word line  4 .  
      Although not restricted in particular, sense latch circuits are provided on the one-end sides of the bit lines  6  respectively, and data latch circuits are provided at their other ends respectively. The corresponding bit line  6  is selected by a Y switch array  16 , based on a select signal outputted from a Y-address decoder  14 . Y-address signals inputted from the external input/output terminals I/O 0  through I/O 7  are preset to a Y-address counter  18 , after which the address signals incremented sequentially with the preset values as starting points are supplied to the Y-address decoder  14 .  
      The corresponding bit line selected by the Y switch array  16  is made conductive or conducted to an input terminal of an output buffer  20  upon a data output operation. Upon a data input operation, the bit line is caused to conduct into an output terminal of a data control circuit  22  via an input buffer  21 . The multiplexer  8  controls electrical connections between the output buffer  20  and the input/output terminals I/O 0  through I/O 7  and between the input buffer  21  and the input/output terminals I/O 0  through I/O 7 . Commands supplied from the input/output terminals I/O 0  through I/O 7  are supplied to a mode control circuit  24  through the multiplexer  8  and the input buffer  21 .  
      A control signal buffer circuit  25  is inputted with a chip enable signal/CE, an output enable signal/OE, a write enable signal/WE, a serial clock signal SC, a reset signal/RES and a command enable signal/CDE as access control signals. Symbols/affixed immediately before signal names means that the corresponding signals are low enable. The mode control circuit  24  controls the function of interfacing signals with the outside via the multiplexer  8  according to the states of these signals, etc. The command inputs sent from the input/output terminals I/O 0  through I/O 7  are synchronized with the command enable/CDE. Each data input is synchronized with the serial clock SC. The input of address information is synchronized with the write enable signal/WE. When the start of an erase or write operation is instructed by a command code, the mode control circuit  24  asserts a ready/busy signal R/B indicative of the erase or write operation being in execution and outputs it to the outside during its instruction period.  
      An internal power circuit (internal voltage generator)  26  generates operating power supplies  27  set as various internal voltages for write, erase, verify and read, etc. and supplies them to the X-address decoder  12  and the memory cell array  2 , etc.  
      The mode control circuit  24  generally controls the flash memory according to input commands. The operation of the flash memory  1  is basically determined according to commands. As the commands for the flash memory  1 , may be mentioned, respective commands for read, erase, write, etc. For instance, the read command includes a read command code, a read X address and a necessary Y address. The write command includes a write command code, an X address, a necessary Y address and write data.  
      The flash memory  1  has a status register  28  to indicate its internal state. The contents thereof can be read from the input/output terminals I/O 0  through I/O 7  according to the assertion of the signal/OE.  
      The erasing of memory information from the flash memory  1  is enabled in block units. The minimum unit of each erase block is set as a flash memory cell group which shares the use of a source line. The designation of each block intended for erasing is determined according to a set value of an unillustrated erase block designation register included in the mode control circuit. The erase block designation register can be accessed from outside via the input/output terminals I/O 0  through I/O 7 .  
      &lt;&lt;First Detailed Example of Flash Memory&gt;&gt; 
      A memory mat of the flash memory  1  is illustrated in  FIG. 1  by way of example. A configuration corresponding to one sense amplifier (SA)  30  is illustrated in the same drawing by way of example. In the same drawing, CD indicate a common data line, QY 0  through QYm indicate main bit line selection switches (column switch elements), MBO through MBm indicate main bit lines, LBOO through LBkm indicate sub bit lines, Q 00  through Qkm indicate sub bit line selection switches, and MCs indicate the flash memory cells, respectively. The drains of the flash memory cells MCs are connected to their corresponding sub bit lines LB 00  through LBkm, and the sources thereof are connected to their corresponding common source lines LS 0  through LSi. In the present example, the erasing minimum unit is defined as a common source line unit.  
      Control gates of the flash memory cells MCs are connected to their corresponding main word lines X 00  through Xkn in row units. The sub bit line selection switches Q 00  through Qkm are connected to their corresponding sub word lines Z 00  through Z 0 k in row units.  
      The main bit line selection switches QY 0  through QYm are connected to their corresponding column select signal lines YS 0  through YSm.  
      In the example of  FIG. 1 , n-channel type clamp MOS transistors (precharge MOS transistors) QPC 0  through QPCmg for respectively clamping the main bit lines MB 0  through MBm to a precharge voltage are connected to their corresponding main bit lines MB 0  through MBm in advance. The gates of the clamp MOS transistors QPC 0  through QPCm are controlled by a precharge signal PC. Theoretical clamp levels of the main bit lines MB 0  through MBm are respectively set as potentials reduced by threshold voltages of the clamp MOS transistors QPC 0  through QPCm from the level of the precharge signal PC.  
      n-channel type discharge MOS transistors (discharge switch) QD 00  through QDkm connected to a circuit ground potential GND are provided for the sub bit lines LB 00  through LBkm. The gates of the discharge MOS transistors QD 00  through QDkm are respectively controlled by discharge signals D 00  through D 0 k.  
      A specific example illustrative of part of the memory mat and sense amplifier  30  shown in  FIG. 1  is shown in  FIG. 4 . Although not restricted in particular, the sense amplifier  30  shown in the same drawing has a current voltage transformation type circuit configuration. A sense operation of the current voltage transformation type sense amplifier (current sense type circuit) is intended to supply a current (½ current) equivalent to substantially one-half the current flowing in a flash memory cell held in an on state when selected by the corresponding word line, to its corresponding bit line to thereby detect a change in bit line level with respect to a criterion level. More specifically, the bit line is precharged to the criterion level of the sense amplifier  30  as illustrated in  FIG. 5  by way of example. When the sense amplifier  30  is activated, the sense amplifier supplies the current equivalent to substantially one-half the current flowing in the flash memory cell held in the on state, to the corresponding bit line. When, at this time, the corresponding flash memory cell intended for selection is in a write state in which its threshold voltage is relatively high, the flash memory cell is brought to an off state, so that the level of the bit line gradually rises from the criterion level. When the flash memory cell intended for selection is in a write state in which its threshold voltage is relatively low, the flash memory cell is brought to an on state, so that the level of the bit line gradually drops from the criterion level. The sense amplifier  30  detects a change in the level of the bit line.  
      According to  FIG. 4 , the sense amplifier  30  has as an output circuit, a CMOS inverter comprising a p-channel type MOS transistor QSA 0  and an n-channel type MOS transistor QSA 1 . A common data line CD is connected to the input of the CMOS inverter via series-connected n-channel type MOS transistors QS 0  and QS 2 . Upon operation non-selection of the sense amplifier  30  (SASEL=“H (High level)”), an input node of the CMOS inverter is precharged to a voltage VSA via a p-channel type MOS transistor QS 3 . Upon operation selection (SASEL=“L (low level)”) of the sense amplifier  30 , the half current defined under conductance control of a p-channel type MOS transistor QM is supplied to the common data line CD via the MOS transistors QS 1 , QS 2  and QS 0 . VSA indicates an operation power supply voltage of the sense amplifier, and VOM indicates a conductance control voltage for obtaining the ½ current.  
      A timing chart showing a read operation is illustrated in  FIG. 6  by way of example. Timings provided when the memory cell MC 00  of  FIG. 4  is intended for reading in adherence to such a configuration as shown in  FIGS. 1 and 4 , are illustrated in the column of the present invention in the same drawing.  
      According to the configuration shown in  FIGS. 1 and 4 , when the memory cell is in a state prior to memory cell reading at a time antecedent to a time t 0 , the precharge MOS transistors QPC 0  through QPCm are respectively set to an on state, the main bit line selection switches QY 0  through QYm and the sub bit line selection switches Q 00  and Qkm are respectively set to an off state, and a clamp power supply VP is connected to the main bit lines MB 0  through MBm. As to the sub bit lines LB 00  through LBkm, the discharge switches QD 00  through QDkm are respectively brought to an on state so as to take a ground potential GND. Thus, the main bit lines MB 0  through MBm are respectively clamped to a potential approximately equal to the criterion level of the sense amplifier  30 .  
      When the read operation is instructed at the time t 0 , the precharge switches QPC 0  through QPCm are respectively brought to an off state, and the discharge switches QD 00  through QD 0 m are respectively brought to an off state. In order to select the flash memory cell MC 00  intended for access, select signals YS 0  and Z 00  are thereafter set to a select level to thereby allow the sub bit line LB 00  to conduct into the main bit MB 0 . Thus, the sense amplifier  30  supplies the ½ current to the drain of the memory cell. When the select signal Z 00  is set to the select level to make the sub bit line LB 00  conductive to the main bit line MB 0 , the main bit line MB 0  has been clamped to the criterion level in advance, and the parasitic capacitance of the sub bit line LB 00  is smaller than that of the main bit line MB 0 . Therefore, there is no possibility that the level of the main bit line MB 0  will be undesirably lowered under the influence of the sub bit line LB 00  brought into conduction. Immediately thereafter, the sense amplifier is thus capable of proceeding to the operation of selecting the memory cell MC 00  by a main word line control signal X 00 . During a read period shown in  FIG. 6 , the memory cell MC 00  is selected by the main word line control signal X 00 . When no electrons are stored in the memory cell MC 00 , the threshold voltage of the memory cell MC 00  is low. When a control signal is supplied to the main word line X 00 , a current flows in the memory cell MC 00 . On the other hand, when the electrons are stored in the memory cell MC 00 , the threshold voltage of the memory cell MC 00  is high, and no current flows in the memory cell MC 00  even when the control signal is applied to the main word line X 00 .  
      When the current equivalent to one-half the memory cell current is caused to flow in the main bit line MB 0 , the ½ current flows in the memory cell MC 00  where the threshold voltage of the memory cell MC 00  is low, so that the voltage on the main bit line MB 0  becomes lower than the clamp voltage and changes as indicated by a broken line in the column of the present invention in  FIG. 6 . Since the current equivalent to ½ the memory cell current flows in the main bit line MB 0  where the threshold voltage of the memory cell MC 00  is high, the voltage on the main bit line becomes higher than the clamp voltage and changes as indicated by a solid line in the column of the present invention in  FIG. 6 .  
      The sense amplifier  30  determines a logic value of information stored in each memory cell according to whether the main bit line voltage becomes higher or lower than the clamp voltage, and outputs the result of its determination as data SAOUT.  
      After the reading of the memory information (time t 1 ), the main word line control signal X 00  is returned to a non-select level (low level) to set the memory cell MC 00  to a non-selected state, whereby the selected main bit line MB 0  is separated from the common data line CD by the signal YS 0 , and the selected sub bit line LB 00  is separated from the main bit line MB 0  by the signal Z 00 . Thereafter, the sub bit lines LB 00  through LB 0 m are made conductive to the ground potential GND via the discharge switches QD 00  through QD 0 m by the signal D 00 , and the precharge switches QPC 0  through QPCm are turned on by the signal PC to thereby precharge the main bit lines MB 0  through MBm to the corresponding clamp voltage. Since, at this time, the precharge control signal PC is a high voltage of such a degree that substrate effects of the precharge switches QPC 0  through QPCm can be neglected, the voltages are reduced via the precharge switches if the voltages on the main bit lines are higher than the clamp voltage, whereas if the voltages on the main bit lines are lower than the clamp voltage, the voltages rise via the precharge switches. Thus, a voltage clamp operation is performed so that the main bit lines reach the clamp voltage.  
      Operation timings described in the column of the comparative example of  FIG. 6  are ones showing operations of a flash memory having a configuration according to a comparative example of  FIG. 7 . In  FIG. 7  as compared with the configuration shown in  FIGS. 1 and 4 , the precharge switches QPC 0  through QPCm and discharge MOS transistors QD 00  through QD 1 m are omitted.  
      A description will be made of a case in which, for example, the memory cell MC 00  is intended for access. When a read operation is instructed in the circuit configuration of the comparative example, a sub bit line LB 00  is made conductive to its corresponding main bit line MB 0  via a switch MOS transistor Q 00 , so that the main bit line MB 0  is made conductive to a common data line CD via a selection MOS transistor QY 0 . In parallel with it, a precharge MOS transistor QPC is turned on so that a precharge operation based on a voltage VP for each of the main bit line MB 0  and sub bit line LB 00  is started. The precharge operation corresponds to an operation for charge-up from a ground potential GND. As compared with the case in which the main bit lines are clamped to the clamp voltage in advance as shown in the timing chart according to the present invention, the precharge operation needs a great long time to obtain a predetermined precharge potential. When the precharge voltage is reached, the precharge switch MOS transistor QPC is turned off. Afterwards, the memory cell MC 00  is selected based on a main word line control signal X 00 , and memory information of the memory cell is detected from the magnitude of the voltage of the main bit line MB 0  when a current flows from the sense amplifier  30  side to the memory cell, followed by output of the memory information. After the reading of the memory information, the main word line control signal X 00  is inverted to a non-select level so that the corresponding memory cell is brought to a non-selected state. Thereafter, a discharge MOS transistor QD is turned on to discharge both the sub bit line LB 00  and the main bit line MB 0  to the ground potential GND. If the ground potential GND is reached, the main bit line selection MOS transistor QY 0  and the sub bit line selection switch Q 00  are respectively brought to an off state to separate the sense amplifier  30  and the memory cell from each other.  
      As is apparent from a comparison between a signal waveform on the main bit line MB 0  in the comparative example of  FIG. 6  and a signal waveform on the main bit line MB 0  in the present invention, the present invention is capable of shortening necessary precharge and discharge times.  
      A precharge time, a discharge time and precharge/discharge operations between a comparative example and the present invention are respectively illustrated in  FIG. 8  by comparison. As to the precharge time, the comparative example must wait for the time required to complete the step-up or boosting of the voltage from a ground level GND to a criterion level. On the other hand, the present invention may simply await the time required to form a clamp voltage used as a criterion level from voltages V 3  and V 4  upon completion of reading.  
      As to the discharge time, the corresponding main bit line and the corresponding sub bit line must be discharged from voltages V 1  and V 2  to the ground level GND upon completion of reading in the case of the comparative example. In the case of the present invention, only the corresponding sub bit line may be discharged from the voltages V 3  and V 4  to a ground level GND upon completion of reading.  
      As to the precharge/discharge operations, the comparative example performs a precharge operation before the reading of memory information from each memory cell and carries out a discharge operation after the reading of the memory information. In the present invention, a clamp operation based on precharge for each main bit line and a discharge operation for each sub bit line can be performed in parallel after the reading of memory information.  
      Thus, when respective read operation periods of two cycles are compared as illustrated in the waveform diagrams of the main bit lines MB 0  of  FIG. 6 , a time reduction can be realized a few 10% in the present invention as compared with the comparative example.  
      Since the drain (sub bit line) of each memory cell is held at the ground potential, there is no problem of memory disturb. Since the time required to charge the main bit line becomes predominant, the time necessary for its precharge can be shortened by clamping the main bit line to the precharge voltage in advance. While both the main bit and sub bit lines have heretofore been discharged and set as the ground potential after the completion of reading, only each sub bit line is discharged. Clamping the main bit line to the precharge voltage simultaneously with its discharge makes it possible to shorten the time necessary for charge and discharge. Since the discharge and precharge are simultaneously performed, the reading speed of a non-volatile semiconductor memory device becomes fast. Since the operating speed of the non-volatile semiconductor memory device becomes fast, the whole system can be speeded up.  
      &lt;&lt;Second Detailed Example of Flash Memory&gt;&gt; 
      A second detailed example of a flash memory is shown in  FIG. 9 . The second detailed example is different from the configuration of  FIG. 1  in that a precharge MOS transistor QPC is disposed at a common data line CD.  
      A timing chart showing the operation of reading memory information of a memory cell MC 00  in a configuration of  FIG. 9  is illustrated in  FIG. 10  by way of example. During a non-selection period of the memory cell, i.e., during a period in which all of sub bit line selection switches are respectively held in an off state, all the main bit line selection switches QY 0  through QYm are brought to an on state, so that a clamp voltage is supplied from a precharge MOS transistor QPC via them. Upon the selecting operation of each memory cell, only the main bit line selection MOS transistor for the selected main bit line is held in the on state, and the others are respectively inverted to an off state. According to such a configuration, the number of the precharge MOS transistors can be reduced.  
      &lt;&lt;Third Detailed Example of Flash Memory&gt;&gt; 
      A third detailed example of a flash memory is shown in  FIG. 11 . The third detailed example is different from the configuration of  FIG. 1  in that sub bit line selection switches Q 00  through Qkm are selected by individual control signals Z 00  through Zkm. The present configuration is intended to suppress needless power consumption of each access-nonselected memory cell.  
      That is, in the configuration shown in  FIG. 1 , when, for example, the memory cell MC 00  is selected, the main bit line selection switch QY 0  selects the main bit line MB 0  to bring the sub word line control signal Z 00  and main word line control signal X 00  to the select level. Since, at this time, all the sub word line selection switches Q 00  through Q 0 m sharing the sub word line control signal Z 00  are held on, the corresponding memory cell low in threshold voltage, of other memory cells MCs sharing the use of the main word line control signal X 00  together with the memory cell MC 00  is not intended for access but turned on to discharge an electrical charge on the corresponding main bit line from the common source line. Such a discharge operation will cause an increase in power consumption and an increase in main bit line precharge operating time after the completion of accessing.  
      In the configuration of  FIG. 11 , the sub bit line selection switches Q 00  through Qkm can be selected every sub bit lines. When, for example, a memory cell MC 00  is intended for access, a main bit line selection switch QY 0  selects a main bit line MB 0  to thereby bring a sub word line control signal Z 00  and a main word line control signal X 00  to a select level. Consequently, the sub bit line selection switch brought to an on state corresponds to Q 00  alone. Other sub bit line selection switches Q 01  through Q 0 m disposed in the same row are respectively brought to an off state. Thus, even if other memory cells MCs sharing the use of the main word line control signal X 00  together with the memory cell MC 00  are turned on, electrical charges on the main bit lines are not discharged into a common source line via them.  
      &lt;&lt;Fourth Detailed Example of Flash Memory&gt;&gt; 
      A fourth detailed example of a flash memory is shown in  FIG. 12 . The fourth detailed example is different from the configuration of  FIG. 1  in that source discharge switches QDS 00  through QDSim are provided every source lines S 00  through Sim and respectively selected by individual control signals QDS 00  through QDSim. The present configuration is intended to suppress needless power consumption of each access-nonselected memory cell on the source line side.  
      In the configuration shown in  FIG. 12 , the source discharge switches QDS 00  through QDSim are capable of on operation every source lines S 00  through Sim. When, for instance, a memory cell MC 00  is intended for access, a main bit line selection switch QY 0  selects a main bit line MB 0  to bring a sub word line control signal Z 00  and a main word line control signal X 00  to a select level. At this time, the source discharge switch brought to an on state is QDS 00  alone. Other source discharge switches QDS 01  through QDS 0 m disposed in the same row are respectively brought to an off state. Thus, even if other memory cells MCs sharing the use of the main word line control signal X 00  together with the memory cell MC 00  are turned on, electrical charges on the main bit lines are not discharged into a common source line LSO via them.  FIG. 12  is compared with  FIG. 11 . Even if each memory cell not intended for access is brought to an on state, the electrical charge on each main bit line is not discharged toward its corresponding sub bit line in the configuration shown in  FIG. 11 . In the configuration of  FIG. 12  on the other hand, when each memory cell not intended for access is turned on, an electrical charge on the corresponding main bit line is discharged from its corresponding sub bit line to a source line but does not reach the common source line. In this sense, the configuration of  FIG. 11  is superior to that of  FIG. 12  in the reduction in power consumption.  
      Timings provided for a read operation made to the memory cell MC 00  in the configuration of  FIG. 12  are illustrated in  FIG. 13 . When the memory cell MC 00  is accessed according to the select levels of YS 0 , Z 00  and X 00 , DS 00  for switch-controlling QDS 00  connected with the corresponding access-aimed memory cell MC 00  is brought to a select level, and other signals each typified by DS 01  are respectively brought to a non-select level. Data read from the memory cell MC 00  is determined, and YS 0 , Z 00  and X 00  are respectively inverted to a non-select level. Thus, all the switches QDS 00  through QDSim are temporarily turned on in response to the discharge of their corresponding sub bit lines to initialize the source lines S 00  through Sim into a ground potential GND.  
      &lt;&lt;Fifth Detailed Example of Flash Memory&gt;&gt; 
      A fifth detailed example of a flash memory is shown in  FIG. 14 . Read operation timings of a memory cell MC 20  shown in  FIG. 14  are illustrated in  FIG. 15  by way of example. The example shown in  FIG. 14  is one wherein a clamp voltage is set to such a level as to reduce disturb of the memory cell. In short, a flash memory cell relatively strong in disturb resistance is used. Thus, the discharge of each sub bit line becomes unnecessary and hence discharge switches for the sub bit lines can be abolished.  
      &lt;&lt;Sixth Detailed Example of Flash Memory&gt;&gt; 
      A sixth detailed example of a flash memory is shown in  FIG. 16 . Read operation timings of a memory cell MC 20  shown in  FIG. 16  are illustrated in  FIG. 17  by way of example. A configuration shown in  FIG. 16  is different from the configuration of  FIG. 4  in that a sense amplifier  30   a  having such a configuration that the current equivalent to one-half the current flowing in the memory cell MC held on is not supplied to each main bit line, is adopted. In short, the transistors QM and QS 1  in  FIG. 4  are omitted in the sense amplifier  30   a  shown in  FIG. 16 . If the current equivalent to one-half the memory cell current does not flow in the main bit line upon the read operation, there is little voltage variation where the threshold voltage of each memory cell is high as in the case of a main bit line MB 0  illustrated in  FIG. 17  by way of example. When the threshold voltage of the memory cell is low, the voltage on the main bit line drops. If a sense level is set in between the voltage variations developed where the threshold voltage of the memory cell is high and low, the sense amplifier  30   a  can read information held in the corresponding memory cell.  
      &lt;&lt;Seventh Detailed Example of Flash Memory&gt;&gt; 
      A seventh detailed example of a flash memory is shown in  FIG. 18 . Read operation timings of a memory cell MC 20  shown in  FIG. 18  are illustrated in  FIG. 19  by way of example. A configuration shown in  FIG. 18  adopts a differential amplification type sense amplifier  30   b.  Bit line selection switches QY(2i) and QY(2i+2) sharing the use of a select signal YSi (where i=even number) are connected to common data lines SAIN 0  and SAIN 1  that constitute the input of the sense amplifier  30   b.  Similarly, bit line selection switches QY (2i−1) and QY (2i+1) sharing the use of a select signal YSi (where i=odd number) are connected thereto. p-channel type transistors QM 0  and QM 1  for selectively supplying the ½ current are connected to the common data lines SAIN 0  and SAIN 1  and switch-controlled by control signals SELPC 0  and SELPC 1 , respectively. The common data lines SAIN 0  and SAIN 1  are connected to their corresponding differential input terminals of differential amplifier circuits DIF 0  and DIF 1 . An inverted output and a non-inverted output taken out in single-end form from the differential amplifier circuits DIF 0  and DIF 1  are supplied to a sense latch LAT as complementary outputs, from which differential sense outputs SAOUT and SAOUTB are formed with complementary output levels as signals levels between a circuit ground level and a circuit power supply voltage VCC.  
      If a memory cell MC 20  is intended for access as illustrated in  FIG. 19 , a control signal YS 0  is set to a high level so that main bit lines MB 0  and MB 2  are made conductive to the common data lines SAIN 0  and SAIN 1 . At this time, a transistor QM 1  on the common data line SAIN 1  side to which the access-aimed memory cell MC 20  is connected, is turned on to allow a ½ current to be supplied to the memory cell MC 20 . In the common data line SAIN 0  on the opposite side, a VP-based clamp level is set to a criterion level. As a matter of course, sub bit line selection switches Q 00  and Q 01  for the main bit lines MB 0  and MB 1  connected to the common data line SAIN 0  on the criterion side are held off. Sub bit line selection switches Q 02  and Q 03  for the main bit lines MB 2  and MB 3  connected to the common data line SAIN 1  on the read signal side are respectively brought to an on state.  
      &lt;&lt;Eighth Detailed Example of Flash Memory&gt;&gt; 
      An eighth detailed example of a flash memory is shown in  FIG. 20 . Write operation timings provided for the flash memory of  FIG. 20  are illustrated in  FIG. 21  by way of example, and read and erase operation timings provided for the flash memory shown in  FIG. 20  are illustrated in  FIG. 22  by way of example.  
      A configuration for writing, whose illustration has been omitted up to now, is also shown in  FIG. 20 . Latches DLAT 0  and DLAT 1  are connected to their corresponding main bit lines typified by MB 0  and MB 1  via transfer switches TR 0  and TR 1 . An initializing discharge switch QD 0  is connected to a common data line CD, and write data DIN can be inputted via an input gate ING.  
      A common source line typified by LSO is rendered floating (poen) upon writing. Further, the common source line is set to a ground potential GND upon a read operation, and the common source line is set to an erase negative voltage VSSE upon an erase operation.  
      As illustrated in  FIG. 21  by way of example, a main word line selected for writing is set to a negative high voltage VSSW, and its corresponding main bit and sub bit lines are set to a positive write high voltage VCCW or like to thereby inject hot electrons into the corresponding floating gate from a drain, whereby the writing is performed. The minimum unit for the erase operation is set as a source line unit. As illustrated in  FIG. 22  by way of example, a negative high voltage VSSE is applied to the corresponding source line, and a positive high voltage VCCE or the like is applied to the corresponding main word line to thereby pull out or draw out electrons from the floating gate to the source, whereby the easing is performed.  
      As apparent from  FIGS. 21 and 22 , the main bit lines typified by MB 0  and MB 1  are respectively set to the ground voltage GND and the high voltage VCCW upon the write and erase operations. In such a case, it is needless to say that much charge time is required to no small extent until each of the main bit lines is clamped from such a voltage to a VP-based clamp voltage as compared with the read operation being continuously performed. Incidentally, a relationship between respective voltages in  FIGS. 21 and 22  is represented as follows: Voltages designated at PC, VCCZ, VCCW and VCCYW are set as voltages unaffected by substrate effects of respective MOS switches. Further, a relationship of VSSE, VSSW&lt;GND is established, and a relationship of GND&lt;VP&lt;VCC ≦VCCR&lt;VCCZ, VCCYW, VCCE is established.  
      &lt;&lt;Data Processor&gt;&gt; 
      A data processor  41  showing one example of a semiconductor integrated circuit according to the present invention is shown in  FIG. 3 . The data processor  41  shown in the same drawing is formed on a single semiconductor substrate (semiconductor chip) like monocrystalline silicon by, for example, a CMOS integrated circuit manufacturing technology.  
      The data processor  41  includes a central processing unit (CPU)  42 , a DMA controller (DMAC)  43 , a read only memory (ROM)  44  corresponding to a program memory which stores a processing program, etc. for the CPU  42 , a random access memory (RAM)  45  used for a working area of the CPU  42  and to temporarily store data, a shift operating module  46  capable of shift-operating arbitrary plural bits, a bus controller  47 , a clock generator (CPG)  48 , an interrupt controller  50 , a timer counter (TMR)  51 , a serial communication interface controller (SCI)  52 , the flash memory (FLSH)  1 , a pulsewise modulator (PWM)  56 , a watchdog timer (WDT)  57 , and input/output ports (PORT)  58  through  60 . The CPU  42 , DMAC  43 , ROM  44 , RAM  45 , shift operating module  46  and bus controller  47  are connected to a CPU bus  61 . The CPU bus  61  is interfaced to a peripheral bus  62  via the bus controller  47 . The interrupt controller  50 , TMR  51 , SCI  52 , FLAH  1 , PWM  56 , and WDT  57  are connected to the peripheral bus  62  as peripheral circuits. The CPU bus  61  and peripheral bus  62  respectively include a data bus, address bus and a control bus (control signal bus). The peripheral bus  62  is interfaced to an external bus (not shown) via the input/output port  58 . The CPU bus  61  interfaces to the peripheral bus  62  via the bus controller  47  and further to the external bus via the input/output port  58 . The input/output ports  59  and  60  respectively function as external interface buffers used for the peripheral circuits.  
      In the data processor  41 , bus master modules correspond to the CPU  42  and DMAC  43 . The CPU  42  includes an instruction controller which fetches an instruction from, e.g., the ROM  44  or FLSH  1  and decodes the fetched instruction, and an execution unit which performs arithmetic processing using a general-purpose register, an arithmetic and logic unit, etc. according to the result of instruction decoding by the instruction controller. The DMAC  43  includes a data transfer condition initially set by the CPU  42  and performs data transfer control in response to a data transfer request sent from each of the peripheral circuits or the like.  
      The bus controller  47  effects arbitration on competition in bus right request between the CPU  42 , DMAC  47  and external bus master corresponding to the bus master modules. Arbitration logic indicates arbitration control based on precedence, for example. As a result of arbitration, the bus master module given the bus right outputs a bus command, and the bus controller  47  controls the corresponding bus based on the bus command. The bus controller  47  outputs address signals and an access strobe signal to the outside via the input/output port  58  where an address signal outputted from the bus master module means an external address space of the data processor  41 .  
      The interrupt controller  50  inputs an interrupt request signal (not shown) outputted from the circuit module of the SCI  52  or the like connected to the peripheral bus  62  and effects priority control and mask control on the input interrupt request signal to thereby accept an interrupt request. The interrupt controller  50  outputs an interrupt request signal (not shown) to the CPU  42  when it has accepted an interrupt. When the CPU  42  is given the interrupt request signal, the CPU  42  discontinues or interrupts processing being in execution and causes a branch to a predetermined processing routine corresponding to an interrupt factor. At the last of the processing routine corresponding to the branch destination, a return instruction is executed and the interrupted processing can be resumed by executing the instruction.  
      In addition, the data processor  1  has external terminals for a ground level (Vss) and a power supply voltage level (Vcc) or the like as power terminals. Besides, the data processor  1  includes respective terminals for a reset input (RES), standby (input STBY), mode control inputs (MDO, MD 1 ), and a clock input (EXTAL, XTAL) as dedicated control terminals.  
      Although not restricted in particular, the CPG  48  generates a system clock signal φ, based on an external clock signal inputted to a crystal oscillator or EXTAL terminal connected to the terminals EXTAL and XTAL.  
      When the data processor  41  is supplied with the reset signal RES, the on-chip circuit modules such as the CPU  42 , etc. are brought to a reset state. When the reset state based on the reset signal RES is released, the CPU  42  reads an instruction from a predetermined start address and starts execution of a program. According to it, the CPU  42  fetches data from the RAM  45 , for example, and effects arithmetic processing on the fetched data. Based on the result of its processing, the CPU  42  performs the input/output of signals to and from the outside through the use of the SCI  52  or the like to thereby control various devices.  
      Since the data processor  41  can be utilized while an operand and the like are being held in the flash memory  1  whose read cycle has been reduced, the speeding up of data processing becomes easy.  
      While the invention made above by the present inventors has been described specifically by the illustrated embodiments, the present invention is not limited to them. It is needless to say that various changes can be made thereto within the scope not departing from the substance thereof.  
      For instance, the memory mat configuration of the flash memory cell is not limited to such a NOR type as described above and may take a NAND type or the like. The circuit configuration of the sense amplifier is not limited to the current sense type and the differential amplification type or the like. The specific configuration of the flash memory is not limited to  FIG. 2 . The circuit module with the data processor being on-chipped is not limited to the above. The non-volatile memory cell may be one which stores multi-valued information of four value or more.  
      Advantageous effects obtained by typical ones of the inventions disclosed in the present application will be described in brief as follows:  
      Each of main bit lines is supplied with a clamp voltage from a voltage supply element during at least a period prior to and subsequent to a read operation with respect to a non-volatile memory cell. Accordingly, there is no need to precharge the main bit line from a ground level upon the operation of reading memory information. A read operation time can hence be shortened. Since the non-volatile memory becomes fast in operating speed, the operating speed or throughput of the whole data processing system using it can be improved.  
      Owing to the stop of supply of the clamp voltage by the voltage supply element during the read operation for the non-volatile memory cell, the operation of sensing by a sense amplifier is made stable even with respect to the small amount of a signal read from the memory cell, and hence the determination of the sense operation can be speeded up.  
      If a discharge element for discharging each of the sub bit lines when each of the sub bit line selection switch elements is brought to an off state, is adopted, the fear of disturb can be prevented before happens. That is, the precharge of the main bit line, i.e., the supply of the clamp voltage and the discharge of the sub bit line are performed in parallel. Supplying the clamp voltage to the main bit line when the sub bit line selection switch element is held off, avoids mutual interference of the parallel operations.  
      In order to shorten a precharge time prior to the reading of memory information with a main/sub bit line structure as a premise, the main bit line is clamped to a precharge voltage in advance, and the drain (sub bit line) of each memory cell is maintained at a ground potential. There is therefore no problem of memory disturb. Since the time required to charge each main bit line becomes predominant, the time necessary for precharge can be shortened by clamping the main bit line to the precharge voltage in advance. While both the main bit and sub bit lines have heretofore been discharged and set as the ground potential after reading, only each sub bit line is intended for discharge. Clamping the main bit line to the precharge voltage simultaneously with its discharge makes it possible to shorten the time necessary for charge and discharge.  
      Since the above means does not depend on the configuration of the sense amplifier, it is adaptable even if sense amplifiers of any circuit configuration are used. It is not necessary to change the sense amplifier.