Abstract:
The invention relates to a non-volatile storage element comprising date connections (I 1 /O 1 , I 2 /O 2 ) and two MOS transistors ( 60,70 ) of a first conductivity type, the source connections of the transistors being connected to a second supply voltage connection ( 10 ). A drain connection of the MOS transistor ( 60 ) is connected to the data connection (I 1 ,O 1 ), and a drain connection of the MOS transistor ( 70 ) is connected to the data connection (I 2 /O 2 ), said MOS transistors ( 60, 70 ) being cross-coupled. The inventive storage element also comprises two cross-coupled floating gate MOS transistors ( 40, 50 ) of a second conductivity type, a drain connection of the MOS transistor ( 40 ) being connected to the data connection (I 1 /O 1 ), and a drain connection of the MOS transistor ( 50 ) being connected to the data connection (I 1 /O 1 ), and a drain connection of the MOS transistor ( 50 ) being connected to the data connection (I 2 /O 2 ). Said storage element further comprises two MOS transistors ( 20, 30 ) of a second conductivity type, the source connections thereof being connected to a first supply connection ( 12 ), the gate connections thereof being connected to a first common switching line ( 14 ), and the well connections thereof being connected to a third common control line ( 5 ). A drain connection of the MOS transistor ( 20 ) is connected to the source connection of the MOS transistor ( 50 ), and a drain connection of the MOS transistor ( 30 ) is connected to the source connection of the MOS transistor ( 50 ).

Description:
TECHNICAL FIELD 
       [0001]    This patent application relates to a non-volatile storage element. 
       BACKGROUND 
       [0002]    Storage elements as described in the present application are often known as “latches”. These are re-writable, non-volatile storage elements used for storing small amounts of data. Normally the amount of data ranges from 1 bit up to several 100 bits. The data is retained even if the power supply to the storage element is switched off. A large number of such storage elements are frequently used in electronic integrated circuits. The surface area occupied by one storage element should therefore be as small as possible. 
         [0003]    Non-volatile memories are already known. In these non-volatile standard memories, complex addressing is used with correspondingly complex circuit components. In addition, a sense amplifier is needed in order to be able to read out data from the non-volatile memory. The use of non-volatile standard memories is therefore relatively complex and occupies a large surface area. 
         [0004]    U.S. Pat. No. 6,411,545 discloses a non-volatile latch in which a source-coupled storage element and two write inputs are provided on the high-voltage side, plus an isolated data output. 
         [0005]    U.S. Pat. No. 5,648,930 discloses a non-volatile latch having drain-coupled storage elements as typically provided in a RAM. The latches comprise a data input and a data output. They are programmed via power busses. 
         [0006]    U.S. Pat. No. 5,523,971 discloses a non-volatile latch comprising eight transistors. The non-volatile latch is isolated from the supply voltage during the programming operation. 
         [0007]    U.S. Pat. No. 4,132,904 discloses a non-volatile latch, which has the disadvantage, however, that when data is read out again it is inverted compared to the previously written data. 
         [0008]    U.S. Pat. No. 4,348,745 discloses a non-volatile latch, which requires twelve transistors and a programming voltage of 17 V, which makes this latch relatively complex. 
       SUMMARY 
       [0009]    A non-volatile storage element uses a first transistor pair comprising floating-gate MOS transistors, with the floating gate of each transistor being capacitively coupled to the drain terminal of the other transistor of the transistor pair. The source terminals of the floating-gate transistors are connected to a second transistor pair of the same conductivity type. The gate terminals of this transistor pair are connected together. In write mode, the gate terminals are driven so that both transistors are high impedance and thus isolate the floating-gate transistors from the standard low-voltage supply. In order to achieve reliable isolation from the standard low-voltage supply, the well terminals of the second transistor pair are taken to the maximum potential during the write operation. In a read mode, both transistors are driven so that they are low impedance; the well terminals of the transistor pair are then connected to the low-voltage supply. The drain terminals of the floating-gate transistors are connected to the drain terminals of a third transistor pair, where these transistors have the opposite conductivity type. The gate terminals of the third transistor pair are cross-coupled to the drain terminals of the respective other transistor. The storage element requires only a small number of transistors and can be controlled easily, because for the write operation, only one voltage condition is required both to charge the one floating gate and to discharge the other floating gate. 
         [0010]    In a first embodiment, the drain terminals of the floating-gate transistors are connected to a fourth transistor pair of the same conductivity, which are connected between the first transistor pair and the third transistor pair. The gate terminals of the transistors of the fourth transistor pair are connected together. Data terminals forming data outputs are provided between the transistors of the third and fourth transistor pairs. In write mode, the gate terminals are driven so that both transistors are high impedance and hence isolate the floating-gate transistors from the data outputs. In addition, the well terminals of the transistor pair are connected to the maximum potential during write mode. In read mode, both transistors are driven so that their channels are low impedance. At the same time, the well terminals of the transistor pair are connected to the standard low-voltage supply. The transistors of the second and fourth transistor pairs are driven by the same signal. This ensures that the outputs and the standard low-voltage supply are isolated from a high voltage applied to the inputs in write mode. 
         [0011]    In a second embodiment, additional transistors of the same conductivity type are arranged in parallel with each transistor of the third transistor pair. In write mode, these transistors are driven so that the outputs of the storage element are shorted to the reference potential. In read mode, both transistors are driven so that they are non-conducting. The gate terminals of both transistors are connected together. 
         [0012]    In write mode, the floating-gate transistors are isolated both from the standard low-voltage supply and from the outputs by the high-impedance transistors of the second and fourth transistor pairs. Data can be written to the storage element by applying a moderate high-voltage to the drain terminal, the source terminal and the substrate of the first floating-gate transistor and to the gate terminal of the second floating-gate transistor. In the second floating-gate transistor, the drain terminal, the source terminal and the substrate are taken to ground, and in the first floating-gate transistor the gate terminal is taken to ground. These voltage conditions enable electrons to tunnel from the drain, source and substrate regions to the floating-gate of the second floating-gate transistor, while in the first floating-gate transistor they enable electrons to tunnel from the floating gate to the drain, source and substrate regions. These current flows result in the floating gate of the first transistor being positively charged, and the floating gate of the second transistor being negatively charged. The outputs of the storage elements are meanwhile shorted together. 
         [0013]    In read mode, the second and fourth transistor pairs are switched to be conducting by connecting the gate terminals to ground. During write mode, the fifth transistor pair, which shorts the outputs to ground, is opened. If all the voltage conditions exist as described, the described structure forms two cross-coupled inverters having different trigger points depending on the charge on the floating gates. One floating-gate transistor starts to conduct before the other, with the result that the cross-coupled inverters go into a state in which one output is High and the other output is Low. 
         [0014]    An arrangement having a time-based self-monitoring facility can be implemented by the storage element according to the second embodiment by using an additional OR-gate that is connected to the data outputs of the storage element. In write mode the outputs are taken to ground, as described above, so that the output of the OR-gate is also at ground potential. In read mode, one output is at the operating voltage potential (High) and the other output is at ground potential (Low), according to the charge on the floating gates of the first and second floating-gate transistors. A change at the outputs of the storage element thus causes a change at the output of the OR-gate from ground potential (Low) to the operating-voltage potential (High). 
         [0015]    An automatic power-saving mode of the non-volatile storage element can be implemented using the output signal of the OR-gate and a few additional logic devices. The power-saving mode has the advantage of improved data-retention performance of the non-volatile storage element compared with the standard read mode. The automatic power-saving mode is described in more detail below with reference to an exemplary embodiment. 
         [0016]    The storage element forms a non-volatile storage element that consumes only a leakage current in write mode, in power-saving mode and also in read mode, once the outputs of the storage element have assumed the programmed state. 
         [0017]    The storage element has a number of advantages in its various embodiments. A first advantage is that after switching on the storage element, a logic signal corresponding to the charges on the floating-gate transistors exists at its outputs. A second advantage is that during write mode, the memory cells are completely isolated from the standard low-voltage supply and the data outputs. A third advantage is that the memory cells can be read out using a time-based self-monitoring facility. This means that the storage element is itself able to identify when a logic decision is made. A further advantage is that the output voltage of the non-volatile storage element always lies within the range of the low-voltage supply in read mode and write mode. In read mode, there is no voltage drop V th . In write mode, a high voltage as used for programming does not appear at the data outputs of the non-volatile memory element, so that standard logic elements can be used at the outputs of the storage element. 
         [0018]    The storage element is described in further detail below with reference to exemplary embodiments. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]      FIGS. 1 and 2  show circuit diagrams of a non-volatile storage element, where the floating-gate transistors are isolated from the positive supply voltage, 
           [0020]      FIGS. 3 and 4  show circuit diagrams of an embodiment of the non-volatile storage element, where the floating-gate transistors are additionally isolated from the data outputs, 
           [0021]      FIG. 5  shows a circuit diagram of an embodiment of the non-volatile storage element having a precharge device, 
           [0022]      FIG. 6  shows a signal diagram depicting signal waveforms for the circuit of  FIG. 5 , 
           [0023]      FIG. 7  shows a circuit diagram of a development of the non-volatile storage element having an output-level monitoring facility in write mode, 
           [0024]      FIG. 8  shows an arrangement comprising the storage element of  FIG. 7 , an OR-gate and a flip-flop to form a time-based self-monitoring facility, and 
           [0025]      FIG. 9  shows a signal diagram depicting signal waveforms for the circuit of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION  
       [0026]    A storage element is described below with reference to exemplary embodiments, where the exemplary embodiments are in no way restrictive. Further embodiments of are possible and covered by the claims. 
         [0027]      FIGS. 1 and 2  show two circuit diagrams of storage elements of the type described herein. The circuit comprises a transistor pair having p-channel transistors  40  and  50 , which each have floating gates  44  and  54 . The transistors  40  and  50  have control gates  45  and  55 , which are capacitively coupled to the floating gates  44  and  54 . The control gate  45  of the first floating-gate transistor  40  is connected to the drain terminal of the other floating-gate transistor  50 , while the control-gate terminal  55  of the transistor  50  is connected to the drain terminal of the transistor  40 , resulting in a cross-coupled arrangement. The source terminals of the floating-gate transistors  40  and  50  are connected to their own n-type substrate. 
         [0028]    A third and fourth transistor  20  and  30  form a second transistor pair whose drain terminals are connected to the source terminals of the transistors  40  and  50 . The source terminals of the second transistor pair are connected to a supply-voltage terminal  12 , and a supply voltage VCC is supplied to them thereby. The gate terminals are connected together and connected to a first control-signal line  14 , whereby a first control signal WE can be applied to them. An external control unit (not shown) is provided to generate the control signal WE. 
         [0029]    The well terminals are connected together and connected to a third control-signal line  5 . In write mode, a moderate high-voltage, which is also used to program the floating-gate transistors, is applied to this terminal. In read mode, the well terminals are connected to the standard low-voltage supply VCC. An external control unit (not shown) is used to generate the control signal VWELL. 
         [0030]    The drain terminals of transistors  40  and  50  are connected to the drain terminals of a third transistor pair comprising a fifth transistor  60  and a sixth transistor  70 . The drain terminals of the first and fifth transistors  40  and  60 , and the gate terminal of the sixth transistor  70 , are connected to a first data terminal I 1 /O 1 , which forms both a data input and a data output. Correspondingly, the drain terminals of the second and sixth transistors  50  and  70 , and the gate terminal of the fifth transistor  60 , are connected to a second data terminal I 2 /O 2 , which likewise forms both a data input and a data output. The source terminals of the third transistor pair are connected to the reference-potential terminal  10 . 
         [0031]    In the embodiment of  FIGS. 1 and 2 , in write mode, the programming voltage is applied to the first data terminal I 1 /O 1  or to the second data terminal I 2 /O 2  depending on the programming required. In read mode, the output signal at the first data terminal I 1 /O 1  is determined by the drain terminals of the transistors  40  and  60 , while the signal at the second data terminal I 2 /O 2  is determined by the drain terminals of the transistors  50  and  70 . 
         [0032]    In write mode, a supply voltage VCC equal to the standard operating-voltage for the technology used is applied to the first supply-voltage terminal  12 . A programming voltage VPP provides the control signal WE at the first control terminal  14  and the control signal VWELL at the third control-signal line  5 , so that the third transistor  20  and the fourth transistor  30  are non-conducting. The programming voltage equals 10 V to 14 V, depending on the coupling factor between the floating gates and the control gates, and on the thickness of the tunnel oxide layer. The programming voltage VPP must be applied to the first data terminal I 1 /O 1  in order to save data signals in the storage element. When this is the case, the n-channel transistor  70  starts to conduct, the floating-gate transistor  50  stops conducting, and the second data terminal I 2 /O 2  is pulled to ground. By terminal I 2 /O 2  being at ground potential, the first floating-gate transistor  40  starts to conduct, while the n-channel transistor  60  is non-conducting. No DC current flows through the storage-element arrangement in this situation. The control gate  45  of the floating-gate transistor  40  is pulled to ground, although the voltage can also be applied externally via the second data terminal I 2 /O 2 . The control gate  55  of the second floating-gate transistor  50  is meanwhile at the programming voltage VPP. The potential difference causes electrons to tunnel from the floating gate  44  to the channel of the transistor  40 , so that the floating gate becomes positively charged. 
         [0033]    At the same time, the programming voltage VPP is applied to the control gate  55  of the second floating-gate transistor  50 , and the drain terminal is pulled to ground. The potential difference between the control gate  55  and the floating gate  54  causes electrons to tunnel from the drain region to the floating gate  54 , whereby it becomes negatively charged. 
         [0034]    Owing to the symmetry of the circuit, for the write operation only one voltage condition is required both to charge the one floating gate and to discharge the other floating gate. 
         [0035]    Two additional data terminals I 3  and I 4 , which form data inputs and hence are also referred to below as input terminals, are provided in the embodiment of  FIG. 2 . In write mode, the data terminal I 3  is connected by a low-impedance path to the data terminal I 1 /O 1 , while the data terminal I 4  is connected by a low-impedance path to the data terminal I 2 /O 2 . Both substrates are thereby at the potential for write mode. If data is written into the storage element in the manner described, a tunnel current flows from the channel of the one transistor to the associated floating gate, and from the floating gate of the other transistor to the channel belonging to this transistor. 
         [0036]    At the end of the write access, the floating gate  44  of the first transistor  40  is positively charged, and the floating gate  54  of the second transistor  50  is negatively charged, according to the described example. 
         [0037]    In read mode, the control signal VWELL is switched to the supply voltage VCC, the control signal WE is taken to ground, and the second transistor pair comprising transistors  20  and  30 , which is connected to the first supply-voltage terminal  12 , starts conducting. The transistor having the positively charged floating gate does not conduct, while the transistor having the negatively charged floating gate is conducting. In the above example, transistors  20  and  30  are conducting when the control signal WE is taken to ground, and the control signal VWELL is connected to the supply voltage VCC. The second transistor  50  is conducting and the first transistor  40  is non-conducting. The current flowing through the second transistor  50  charges the node  16  at the second data terminal I 2 /O 2  with a positive charge, so that the fifth transistor  60  starts to conduct. The control gate  55  of transistor  50  is thereby pulled to ground, because transistor  40  is non-conducting. When the control gate  55  is at ground potential, the transistor  70  is non-conducting, and the full voltage VCC at the first supply terminal  12  is applied to the control gate  45 . 
         [0038]    If the floating gates were charged with the opposite charge by swapping over the input signals to the data terminals I 1 /O 1  and I 2 /O 2 , then in read mode, inverse signal states are obtained also as output signals at the data terminals I 1 /O 1  and I 2 /O 2 . 
         [0039]      FIGS. 3 and 4  show an embodiment of the circuit arrangement described herein. Whereas in the embodiment shown in FIGS.  1  and  2 , the first and second data terminal I 1 /O 1  and I 2 /O 2  each have a function both as a data input and as a data output, in the embodiments shown in  FIGS. 3 and 4 , additional, isolated data terminals are provided. The data terminals connected to the drain terminals of the floating-gate transistors  40  and  50 , and which are labeled I 1 /O 1  and I 2 /O 2  in  FIGS. 1 and 2 , only have a data-input function in the storage elements described below, and are hence also labeled as input terminals with the references I 1  and I 2  in this exemplary embodiment, while separate data terminals O 1  and O 2  are provided that form outputs and hence are also referred to below as output terminals. The outputs O 1  and O 2  can thereby be isolated from the programming voltage VPP. 
         [0040]    In both the circuit arrangements shown, an additional fourth transistor pair is provided for this purpose comprising p-channel transistors  80  and  90 . The transistors  80  and  90  are arranged in a similar manner to the second transistor pair comprising transistors  20  and  30 , which isolates the floating-gate transistors  40  and  50  from the supply voltage of the first supply-voltage terminal  12 . The gate terminals of the additional transistors  80  and  90  are connected together, and are also driven by the control signal WE supplied via the control-signal line  14 . The well terminals of the two transistors are connected together and are similarly driven by the control signal VWELL supplied via the control-signal line. The source terminals of these two transistors are connected to the drain terminals of the cross-coupled floating-gate transistors  40  and  50 . The drain terminals of transistors  80  and  90  are connected to the drain terminals of the cross-coupled n-channel transistors  60  and  70 . 
         [0041]    In write mode, the control signals WE and VWELL are at the programming voltage VPP, and the second and fourth transistor pairs comprising transistors  20 ,  30 ,  80  and  90  are non-conducting. The cross-coupled floating-gate transistors  40  and  50  are fully isolated from the supply voltage VCC at the first supply-voltage node  12 . They are also isolated from the output terminals O 1  and O 2 . The voltages at the two input terminals I 1  and I 2  must be set externally as described above. By the outputs being isolated from the programming voltage VPP, the cross-coupled n-channel transistors  60  and  70  can be implemented as standard low-voltage transistors, so that they can be fabricated in a base process. The output levels at the output terminals O 1  and O 2  always lie within a low-voltage range between ground and the supply voltage. Logic devices connected to the outputs of the storage element can thus be implemented by standard logic gates, which can also be fabricated in the base process. 
         [0042]    Input terminals I 3  and I 4  have been added in  FIG. 4 . In write mode, the input terminal I 3  is connected by a low-impedance path to the input terminal I 1 , and the input terminal I 4  is connected by a low-impedance path to the input terminal I 2 . In read mode, both inputs are at a floating potential. 
         [0043]      FIG. 5  shows a circuit arrangement according to another embodiment, which employs a precharge approach in order to obtain a more sensitive storage element. An n-channel transistor  100  is added to the circuit arrangement of  FIG. 4 . The drain terminal of this transistor is connected to the output terminal O 2 , and the source terminal to the output terminal O 1 . A second control signal WE_LV, which is also generated by the external control unit, is applied to the gate via a second control-signal line  9 . The waveforms of the control signals WE and WE_LV and of the signals at the output terminals O 1  and O 2  are shown in  FIG. 6 . 
         [0044]    In write mode, the control signal WE_LV is at the supply voltage VCC, the transistor  100  is conducting, and the output terminals O 1  and O 2  are connected together by a low-impedance path, so that both output terminals O 1  and O 2  have the same potential. The data is stored in the non-volatile storage element, as described with reference to  FIG. 4 . To switch from write mode to read mode, the signal WE is taken to ground, the control signal VWELL is taken to the supply voltage VCC, and the transistors  20 ,  30 ,  80  and  90  of the second and fourth transistor pairs are conducting. The floating-gate transistor  40  has a positive charge, and the transistor  50  has a negative charge. The floating-gate transistor  50  thereby starts to conduct, and the potential at the output terminals O 1  and O 2  rises. The cross-coupled transistor pair comprising transistors  60  and  70  thus starts to conduct. The second control signal WE_LV is taken to ground after a short delay. The gate of the n-channel transistor  60  is driven by VCC, and the output terminal O 1  is thereby pulled to ground. By the output terminal O 1  being at ground potential, the n-channel transistor  70  stops conducting, and the second output terminal O 2  is raised to VCC. 
         [0045]    Very small charge differences on the floating gates of the transistors  40  and  50  can be evaluated by the circuit arrangement described above. The read-out speed is thereby increased for a storage element designed according to this embodiment. 
         [0046]      FIG. 7  shows an embodiment of the circuit arrangement employing a time-based self-monitoring facility for initializing a storage element, which can detect a signal change at its outputs by very simple means. The described, non-volatile storage element is also more sensitive to small differences in charge on the floating gates than the circuit arrangement of  FIG. 4 . A comparison of the circuit arrangement in  FIG. 5  and the circuit arrangement in  FIG. 7  shows that two additional n-channel transistors  110  and  120  are provided. The ninth transistor  100  is no longer provided. The drain terminal of the transistor  120  is connected to the output terminal O 1 , and the source terminal is connected to ground. The drain terminal of the transistor  110  is connected to the output terminal O 2 , and the source terminal is connected to ground. The second control signal WE_LV is applied via second control-signal lines  9  to the gate terminals of the two transistors  110  and  120 . 
         [0047]    In write mode, the control signal WE_LV is at the supply voltage VCC, both transistors  110  and  120  are conducting, and the outputs are at ground potential. Data can be written to the storage element in the manner described with reference to  FIG. 4 . In the described example, the floating-gate transistor  50  is negatively charged, and the floating gate of the transistor  40  is positively charged. 
         [0048]    At the end of a write pulse, the signal WE goes to ground potential, the control signal VWELL goes to the supply voltage, and the second and fourth transistor pairs comprising transistors  20 ,  30 ,  80  and  90  start to conduct. When the second control signal WE_LV goes to VCC, the potential at the respective drain and gate terminals of the floating-gate transistors  40  and  50  go to ground potential. The transistor  50  is conducting and the transistor  40  is non-conducting. 
         [0049]      FIG. 8  shows a circuit in which a simple circuit has been added to the storage element described previously. In this case, the two output signals from the non-volatile storage element are taken to a single OR-gate  200 . The output signal from the OR-gate  200  is “0” when both input signals are at “0”. 
         [0050]    At the end of a write pulse, once the first control signal WE has gone to ground potential, and the third control signal has gone to the supply voltage, after a short delay the second control signal WE_LV also goes to ground potential. The waveforms are shown in  FIG. 9 . The voltage at the second output terminal O 2  thereby rises to VCC, while the voltage at the first output terminal O 1  falls to ground potential. This causes the output signal of the OR-gate to switch from “0” to “1”, because now one of the input signals is at “1”, and the other at “0”. A signal change at the outputs of the storage element described can thereby be detected. 
         [0051]    This fact can be used to transfer the non-volatile storage element into a power-saving mode, once the stored data has been transferred to a standard flip-flop  210  connected to the storage element. For example, the output signal of the OR-gate  200  can trigger a flip-flop  210 , which adopts the data from the non-volatile storage element as shown in  FIG. 8 . 
         [0052]    In order to explain how it works, it is assumed that the first output terminal O 1  is at the “0” level, and the second output terminal O 2  is at the “1” level. The D-input  211  of the flip-flop  210  is at a level “1” corresponding to that at the connected output terminal O 2 . The two output signals of the storage element are OR-ed by the OR-gate  200 , so that if the output signals from the storage element change, a signal having a rising edge is produced by the OR-gate  200 , which is supplied as a clock signal to the clock input  212  of the flip-flop  210 . Owing to the switching delay of the OR-gate  200 , the clock signal reaches the clock input  212  after the data signal reaches the input  211 . The flip-flop  210  is thus set with confidence. 
         [0053]    For a “1” at the output  213  of the flip-flop  210 , a “1” is applied to the inputs I 1  and I 3 , while a “0” is applied to the inputs I 2  and I 4 . This corresponds to the original programming, so that the state of the storage element is retained. 
         [0054]    Once the data has been saved in the flip-flop  210 , the non-volatile storage element is taken into a power-saving mode. In this mode, the first control signal WE and the third control signal VWELL are taken to the supply voltage VCC, the transistors  20 ,  30 ,  80  and  90  are thereby non-conducting and the floating-gate transistors  40  and  50  are isolated from the supply voltage VCC. The transistors  40  and  50  are also isolated from the output terminals O 1  and O 2 . If the second control signal WE_LV goes to VCC, both n-channel transistors  110  and  120  start to conduct and the output terminals O 1  and O 2  go to ground potential. The levels of control signals WE and WE_LV thus equal the levels in write mode. 
         [0055]    The output  213  of the flip-flop  210  is connected to the input terminals I 1  and I 3 . The complementary output  214  of the flip-flop  210  is connected to the input terminals I 2  and I 4 . The switches  220  and  230  shown in  FIG. 8  are closed for this purpose. In the power-saving mode described above, the non-volatile storage element consumes only a leakage current. 
         [0056]    The power-saving mode comes to an end when modified data is to be written to the storage element. To do this, the switches  220  and  230  are first opened to prevent a high voltage, which is used for the write operation and is applied to the input terminals I 1 , I 2 , I 3  or I 4  of the storage element, from reaching the outputs  213  and  214  of the flip-flop  210 . The new data can then be written as described above. 
         [0057]    The drive signals for the switches  220  and  230 , like the control signals WE, VWELL and WE_LV, are provided by an external control unit. 
         [0058]    As an alternative to the terminal connections described for the storage element and the flip-flop  210 , the third data terminal O 1  can obviously also be connected to the data input  211  of the flip-flop  210 . The connections from the outputs  213  and  214  would need to be swapped over accordingly. 
         [0059]    To understand a further advantage of the power-saving mode regarding data retention, it is important to consider the voltage conditions in the standard read mode and in the power-saving mode. In the power-saving mode, the control gate  55  of the transistor  50 , which is connected to the node  18 , is taken to the supply voltage VCC, and the channel region is connected to ground. The floating gate  54  remains negatively charged, corresponding to its programming in the aforementioned example, even if the floating gate is discharged to VCC after a long period of time as a result of a gate-channel leakage current. The control gate of the transistor  40 , which is connected to the node  16 , is taken to ground potential, and the channel region to VCC. The floating gate  44  of the transistor  40  remains positively charged, even if the floating gate is discharged to ground after a long period of time as a result of leakage currents. The originally programmed data is retained in power-saving mode despite leakage currents and thermal stresses. 
         [0060]    In standard read mode, the control gate  55  of the transistor  50  is at ground potential, and the channel region is at VCC. The floating gate  54  of the transistor  50  will become positively charged after a long period of time owing to leakage currents between the gate and the channel. The floating gate  54  in the aforementioned example was originally negatively programmed, however. The control gate  45  of the transistor  40  lies at VCC, and the channel region at ground potential. The floating gate  44  will become negatively charged after a long period of time owing to gate-channel leakage currents, although it was originally positively programmed. This results in a reversal of the originally programmed states owing to the gate-channel leakage current and thermal stresses, so that the stored data is lost. 
         [0061]    Thus unlike the standard read mode, in power-saving mode there are no problems with retaining data over a long period of time. 
         [0062]    The Exemplary embodiments and applications have been described for standard CMOS n-type substrate processes. It is equally possible to fabricate storage elements as n-channel floating-gate transistors in a p-type substrate CMOS process. All embodiments can be adapted easily in order to retain the same functions in a p-type substrate process.