Abstract:
The inventive memory circuit comprises a plurality of memory cells. The memory circuit further comprises a memory access means being controlled by at least one control signal. In addition, a control means for generating the at least one control signal is contained in the memory circuit, with the control means comprising a delay means. The delay means delays a switching of the at least one control signal. The delay time is adjustable in view of the applied supply voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims foreign priority benefits under 35 U.S.C. §119 to co-pending European patent application number EP 04 020 080.0, filed 24 Aug. 2004. This related patent application is herein incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to memory circuits, particularly to SRAM circuits. 
   2. Description of the Related Art 
   SRAM (static random access memory) is internally organized as illustrated in  FIG. 1 . The SRAM-type memory comprises a memory cell array  15  of 1-bit memory cells  1  with m rows and n columns. Each memory cell is based on a flip-flop (typically CMOS-type) as the memory core cell with positive feedback for the storage of the data (not shown). The differential data outputs of the flip-flop are selectively coupled to differential bit lines bl j  and bln j  by memory cell internal select transistors (not shown), which are driven by a corresponding word line wl i . The bit lines bl j  and bln j  are usually precharged to a specific voltage level (usually the supply voltage V dd ) by precharging circuits P. 
   Each memory cell  1  is addressed by a row address signal  2  and a column address signal  3  with Id(m)- and Id(n)-bit word-length, respectively. The row address signal  2  is decoded in a row decoder  4  to select one of the m rows. In a similar way, the column address signal  3  is decoded in a column decoder  5  to select one of the n columns. Each row of memory cells is coupled to one word line wl i . In the event a row is selected, the corresponding word line wl i  exhibits a logically high potential. The selection of a word line is resetable by a signal wl_resetn which resets all word lines to a logically low level in case wl_resetn is high. This is realized by m AND-gates driving the m word lines, with each AND-gate being connected to one output signal a i  of the row decoder and the signal wl_resetn. For the selection of one of the n columns each of the n output signals of the column decoder  5  is connected to one of n total column switches  6 . In the event a column is selected, the corresponding pair of differential bit lines bl j  and bln j  is connected to the differential pair of data I/O lines  7  via the corresponding column switch  6 . 
   For read access and write access the data I/O lines  7  are connected to the differential input sa and san of a sense amplifier  8  and to the differential output of a write buffer  9 , respectively. In general, a dynamic sense amplifier  8  is employed, with its operation being controlled by a control signal saen. The signal saen activates the amplifier  8  when switching from low to high. The dynamic sense amplifier  8  is based on an internal positive feedback, which speeds up the signal detection compared to a static sense amplifier. In addition, a dynamic sense amplifier  8  also consumes less power compared to a static sense amplifier since it switches to zero power as soon as it has made a decision. Similar to the dynamic sense amplifier  8 , the operation of the write buffer  9  is controlled by a control signal irwb which activates or deactivates the driving of the I/O data lines  7  and the therewith connected bit lines bl j  and bln j , when switching from low to high level or from high to low level, respectively. 
     FIG. 2  illustrates the SRAM-internal timing during a memory read access. The transient voltage curves  10  and  11  are related to the differential inputs sa and san of the sense amplifier  8 , whereas the transient voltage curve  12  relates to the control signal saen for sense amplifier activation. At the beginning of the memory read process the inputs sa and san and the connected bit lines bl j  and bln j  are still precharged to V dd , with V dd  being the positive supply voltage. The recharge of the inputs sa and san with respect to the memory cell internal voltages is delayed because of parasitic capacitances (mainly parasitic capacitances of the bit lines bl j  and bln j ). At a time instant t a  the sense amplifier  8  is activated when the control signal saen switches from low to high. The differential voltage between the signals sa and san at the time instant t a  is defined as the read margin Δsa. The earlier the sense amplifier  8  is activated, the smaller Δsa is and the faster the read operation is. But the smaller Δsa is, the more critical the detection of the signal is that is read out of the memory cell. Since the recharging capability of a selected memory cell  1  may vary from memory circuit to memory circuit as well as from memory cell to memory cell and in addition the detection capability of a sense amplifier  8  may vary from memory circuit to memory circuit, the read margin Δsa is typically set as a value which provides a desired level of safe operation. Thus, there is a trade-off between memory speed and safety of operation when setting the read margin Δsa. At the end of the read operation the wordline wl i  is deactivated by switching the signal wl_resetn (cf.  FIG. 1 ) from high to low (not shown). 
     FIG. 3  illustrates the SRAM-internal timing during a memory write access. The transient voltage curves  20  and  21  are related to the signal on the selected word line wl i  and the signal irwb which controls the operation of the write buffer  9 , respectively. The transient voltage curves  22  and  23  relate to differential internal voltage levels c and cn of the flip-flop in the selected memory cell. The transient voltage curves  24  and  25  relate to the voltage levels on the differential bit line bl j  and bln j . For the write access, first the signal irwb is switched from low to high to activate the write buffer  9 . After the signal on the selected word line wl i  switches from low to high, a recharge of the bit lines bl j  and bln j  and the internal voltages c and cn of the selected flip-flop according to the data input signal of the write buffer  9  is initiated. After the recharge of the internal voltages c and cn is sufficient to switch the state of the flip-flop (due to flip-flop internal positive feedback), the write buffer  9  is deactivated by switching the signal irwb from high to low. The word line wl i  has to be maintained activated during the commutation of the flip-flop. Afterwards the wordline wl i  is deactivated by switching the signal wl_resetn (cf.  FIG. 1 ) from high to low (not shown). After closing the wordline (and typically not before) the differential bit line bl j  and bln j  is recharged to a precharging level by the precharging circuits P (cf.  FIG. 1 ). For the write access, the write margin Δt is a defined time-based value. The write margin Δt is defined by the time window which is described by the time instance t 1  when the flip-flop-internal node-voltage cn crosses V dd /2 and the time instance t 2  when the falling edge of the voltage on the word line wl i  crosses V dd /2 (word line deactivation). The larger the write margin Δt, the later the word line wl i  is deactivated and the more a correct commutation of the flip-flop is guaranteed. The smaller the write margin Δt is, the faster the write operation is. Thus, there is a trade-off between memory speed and safety of operation when setting the write margin Δt. 
   The document U.S. Pat. No. 5,936,905 describes a self-adjusting delay circuit in an SRAM circuit. The delay circuit relates only to read access and adjusts the internal timing with respect to the activation of a dynamic sense amplifier. The delay circuit comprises a dummy word line which replicates a corresponding word line and is connected to a number n of parallel dummy transistors. The number n of parallel dummy transistors is the same than the number of transistors connected to the corresponding word line. A subset m of these n dummy transistors drives a dummy bit line which replicates a corresponding bit line. The activation signal related to the dynamic sense amplifier is derived from the voltage level on the dummy bit line. To counteract process variation regarding the characteristics of the memory circuit and therewith related timing mismatch, the delay of the delay circuit is programmable by setting the number of transistors m driving the dummy bit line. 
   Due to these above-mentioned timing constraints, state-of-the-art memory circuits, particularly SRAM circuits, are only operational within a small range of the nominal supply voltage. In case the actual applied supply voltage is lower, the read margin Δsa and the write margin Δt might be too small and the operation of the memory circuit might fail. 
   In such SRAM circuits, the supply voltage may be constant. However, in some cases, a constant supply voltage may have undesirable dynamic and static power consumption. Accordingly, what is needed is a method and apparatus for reducing dynamic and static power consumption in a SRAM circuit. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a memory circuit which is flexibly usable within a broad range for the supply voltage, with the memory circuit guaranteeing safe operation for this range. 
   The memory circuit according to one embodiment of the invention comprises a plurality of memory cells, which are organized in rows and columns. The memory circuit further comprises a memory access means, which is controlled by at least one control signal. In addition, a control means for generating the at least one control signal is contained in the inventive memory circuit, with the control means comprising a delay means. The delay means delays a switching of the at least one control signal. The respective delay time is adjustable in view of the applied supply voltage. 
   The delay time being adjustable in view of the supply voltage is employed to counteract the dependency of the characteristics of the memory circuit on the applied supply voltage. Generally, the circuit-internal signal detection capability is reduced with decreasing supply voltage. During read access a sense amplifier being part of the memory access means typically needs a larger voltage swing for signal detection, which is equivalent to a larger read margin. On the other hand, during write access a selected flip-flop in a memory cell may require a higher voltage swing before deactivating the write buffer and the selected word line, which results in a desire to increase the write margin. Thus, both the read and the write margin should be increased when reducing the supply voltage, thereby reducing the operating speed of the memory circuit. 
   Additionally, the read and write margins may be increased with decreasing supply voltage because of the increasing impact of the threshold-voltage mismatch and other mismatches in the memory core cells, i.e., in the flip-flops. In general, if the supply voltage is reduced, the signal swing is also reduced (e.g., in CMOS circuits). This results in a decreased signal-to-noise-ratio (the threshold-voltage variance can be regarded as noise). 
   In one embodiment, a memory circuit provides the capability to adapt those margins to the actual applied supply voltage by adjusting the timing for the at least one control signal, e.g., an activation signal for a dynamic sense amplifier. Therefore, in one embodiment, the memory circuit guarantees safe operation even for low supply voltages. 
   By adjusting the delay and thereby the read and/or write margins the memory circuit may be applied to a low supply voltage resulting in low power consumption and low operating speed in case the operating speed is not crucial. Alternatively, by choosing an appropriate adjustment the memory circuit may be applied to a high supply voltage in case the maximum operating speed is required. 
   In one embodiment, every control signal of the data access unit is derived from the delay unit. In addition, in one embodiment, the delay time may not be constant where more than one control signals are derived for the delay unit. One embodiment may provide different adjustable delay times assigned to different control signals. 
   In one embodiment, the memory access means supports a memory read access mode and a memory write access mode and the delay time is independently selectable for the memory read access mode and for the memory write access mode. This provides a further degree of freedom since the delay time is not only adjustable with respect to supply voltage but also with respect to read or write mode. This added adjustability with respect to read or write mode is based on the recognition that the supply voltage driven impact on the memory characteristics, particularly the timing characteristics, may be different for read and write access. Therefore, the timing of the at least one control signal in the advantageous memory circuit may be independently optimized for read and write access. 
   In one embodiment, the memory circuit comprises a plurality of bit lines each coupled to a column of the plurality of memory cells. Further, the delay means comprises a dummy bit line, the signal transmission behavior of which is essentially characteristic of the signal transmission behavior of each of the bit lines. The dummy bit line is coupled to dummy memory cells which replicate the load of a bit line, i.e., the plurality of memory cells assigned to a bit line. The at least one control signal is generated in dependency of the signal, particularly the voltage signal, on the dummy bit line. The delay means further comprises a driver unit which drives the dummy bit line and discharges the dummy bit line. 
   In some cases, an a priori unknown delay variation for accessing a memory cell via a capacitive bit line transfers to a more or less identical delay variation with respect to the delay of the dummy bit line in case the bit line and the dummy bit line are matched to each other (for instance, where the load and the driving circuit of both lines are matched). Matching between the bit line and the dummy bit line means that the main characteristic structure features of both transmission lines which impact the line capacitance and the propagation delay of the transmission line are identical, e.g., same length, same width, same thickness and same dielectricity of the isolation. In case the delay for reading the content of a memory cell via a bit line is increased, e.g., caused by semiconductor process variations, the activation of a sense amplifier by the control signal is also delayed by the same additional delay. This leads to a self-adjusting timing architecture. In case the number of dummy memory cells which are connected to the dummy bit line and the number of the memory cells which are connected to the selected bit line are the same, the delay matching is improved. 
   It is noted that a bit line may be implemented as a differential bit line with two metal lines. The dummy bit line may be implemented as a differential dummy bit line or only as a single bit line with one metal line. 
   In one embodiment, the driving capability of the driver unit is adjustable. Driving capability means the capability of the driver unit to recharge the load of the driver unit. The higher the driving capability is, the faster the recharge process is. Thus, the driving capability is equivalent to the current driving capability. The driving capability of a driver circuit is determined by the parasitic elements of its equivalent circuit. In general, the larger the transistor area of a driving transistor is, the higher the driving capability is. It should be noted that in some cases, a transistor might comprise several parallel smaller transistors. 
   In one embodiment, the driver unit comprises a plurality of driver transistors coupled to the dummy bit line, with the total transistor area or size of the one or those driver transistors which are actively driving the bit line is selectable. By altering the total transistor area of the active driving transistors, the driving capability of the driver unit is adjusted. This may be accomplished by switching on one or more transistors of the transistor bank, while switching off the remaining transistors of the transistor bank. 
   In one embodiment, the driving capability of the driver unit is independently selectable for at least two different supply voltage operating modes of the memory circuit: a high supply voltage mode (e.g., 1.8 V nominal supply voltage) and a low supply voltage mode (e.g., 1.2 V nominal supply voltage). In this case the driving capability for the high supply voltage mode is higher than the driving capability for the low supply voltage mode, particularly the area of the actively driving transistors is higher for the high supply voltage mode compared to the low supply voltage mode. Advantageously, the voltage potential of at least one first pin selects the driving capability. 
   The above-mentioned technical teaching is based on the idea that for high reliability of the memory circuit the timing margins are typically increased when the memory circuit is operated in a low supply voltage mode (e.g., 1.2 V) compared to an operation in a high supply voltage mode (e.g., 1.8 V). In case the memory circuit is operated in a low supply voltage mode, the driving capability is set to low, which results in a higher delay time for discharging the dummy bit line. This increases the read margin Δsa and/or the write margin Δt of the memory circuit (cf.  FIG. 2  and  FIG. 3 ), if the sense amplifier activation signal saen and/or the write buffer activation signal irwb and the word line deactivation signal wl_resetn (cf.  FIG. 1 ) are derived from the signal on the dummy bit line, respectively. Generally, the increased read margin Δsa and/or the write margin Δt reduces the memory operating speed. Thus, the low supply voltage mode is equivalent to a low operating speed mode and the high supply voltage mode is equivalent to a high operating speed mode. 
   According to the above-mentioned preferred embodiment, the driver unit advantageously comprises at least two MOS-type (metal oxide semiconductor) driver transistors: A first MOS driver transistor actively drives the dummy bit line in the low supply voltage mode, whereas a second MOS driver transistor (which may comprise several parallel smaller MOS transistors) actively drives the dummy bit line in the high supply voltage mode. The transistor width of the second driver transistor is essentially i times higher than the transistor width of the first driver transistor, with i&gt;1. Advantageously, i is a natural number. In this case the second transistor advantageously consists of i parallel transistors which are identical to the first MOS driver transistor (transistor matching). 
   It should be noted that necessarily not only one MOS transistor is assigned to the high or low supply voltage operation. The scope of the invention also includes that two or more MOS transistors are assigned to high or low supply voltage mode. 
   Additionally, it should be noted that the first MOS transistor is not necessarily switched off in the high supply voltage mode. This is due to the fact that the impact of the first MOS transistor on the driving capability of the driver unit in the high supply voltage mode is negligible (due to its small transistor area). 
   In one embodiment, the driving capability of the driver unit is independently selectable for the memory read access mode and for the memory write access mode. In some cases, the optimal delay for switching a control signal during read access (e.g., the signal saen) for a fixed supply voltage is different from the optimal delay for switching a control signal during write access for the same fixed supply voltage (e.g., the signal irwb). Advantageously, the selection is done via a second pin. 
   In one embodiment, the driving capability is selectable with respect to access mode (read or write mode) and also with respect to supply voltage mode (high or low supply voltage mode). Four different self-timing paths for the generation of control signals may be implemented. Thus, the driver unit may include at least four MOS driver transistors, including, a first MOS driver transistor actively driving the dummy bit line in case of the memory read access mode and the low supply voltage mode, a second MOS driver transistor actively driving the dummy bit line in case of the memory read access mode and the high supply voltage mode, a third MOS driver transistor actively driving the dummy bit line in case of the memory write access mode and the low supply voltage mode, and a fourth MOS driver transistor actively driving the dummy bit line in case of the memory write access mode and the high supply voltage mode. 
   In one embodiment, the transistor width of the second driver transistor is essentially i times higher than the transistor width of the first driver transistor, with i&gt;1. The transistor width of the fourth driver transistor is essentially n times higher than the transistor width of the third driver transistor, with n&gt;1. In one embodiment, the factors i and/or n are natural numbers. In this case, the corresponding transistors may be matched to each other (see transistor matching above). 
   It should be noted that in one embodiment, the first and/or the third MOS transistor may not be switched off in high supply voltage mode. In addition, the number of MOS driver transistors is not necessarily limited to four. 
   In one embodiment, the memory access means comprises a sense amplifier for memory read access, for example, a dynamic sense amplifier. The sense amplifier (e.g., sense amplifier  8  in  FIG. 1 ) is selectively coupled to one of the plurality of bit lines and receiving data transmitted via the coupled bit line from one of the memory cells. The sense amplifier is activated by a first control signal, e.g., the signal saen in  FIG. 1 , which is delayed by the delay mean, i.e., generated in dependence of the signal on the dummy bit line. In addition, the memory access means may comprise a write buffer (e.g., write buffer  9  in  FIG. 1 ) for memory write access, which is selectively coupled to one of the plurality of bit lines and transmitting data via the coupled bit line to one of the memory cells. The write buffer is activated and deactivated by a second said control signal, e.g., the signal irwb in  FIG. 1 , which is delayed by the delay mean, i.e., generated in dependence of the signal on the dummy bit line. 
   In one embodiment, the memory access means may also comprise a word line selection mean, which selects one row of the plurality of memory cells for memory access. The selection of the word line selection means is deactivated by a third control signal, e.g., the signal wl_resetn in  FIG. 1 , which is delayed by the delay mean, i.e., generated in dependence of the signal on the dummy bit line. 
   In one embodiment, the inventive memory circuit is implemented as an SRAM circuit. Nevertheless, the scope of invention may also include other memory circuits, e.g., DRAM (dynamic random access memory) circuits, ROM (read-only memory) circuits or PROM (programmable read-only memory) circuits, particularly flash-type EEPROM (electrical erasable and programmable read-only memory) circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  shows an internal block diagram of a state-of-the-art SRAM circuit; 
       FIG. 2  shows the internal signal timing during a memory read access in a state-of-the-art or in an inventive SRAM circuit; 
       FIG. 3  shows the internal signal timing during a memory write access in a state-of-the-art or in an inventive SRAM circuit; and 
       FIG. 4  shows a memory circuit according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 4  illustrates a memory circuit according to one embodiment of the invention. 
   In one embodiment, the SRAM circuit depicted in  FIG. 4  comprises a control circuit block  30  for generating the three control signals wl_resetn, irwb and saen. The circuit block  30  comprises a dummy bit line, a plurality  33  of dummy memory cells  9 ′ connected to the dummy bit line  32  and a driver unit  31 , which recharges the dummy bit line  32 . The output of the dummy bit line  32  is connected to a logical circuit  41 , which generates the three control signals wl_resetn, irwb and saen in dependence of the signal on the dummy bit line  32 . 
   In one embodiment, the control circuit block  30  is driven by a dummy word line signal  34 . The corresponding transmission line of the signal  34  may be designed similar or identical to one of the word lines wl i  to generate a similar line capacitance and therewith a similar propagation delay. The dummy word line signal  34  is synchronized to the signal which generates the word line decoding. The dummy word line signal  34  is received by the driver unit  31 , particularly by the gates of the four NMOS driver transistors  36   a - 39   a , with their drains connected to the dummy bit line  32 , and in parallel by a precharge PMOS transistor  35 . 
   Each NMOS driver transistor  36   a - 39   a  may be assigned to one of four possible memory circuit states, which are selected via the pins slowb (for high supply voltage mode: slowb=high; for low supply voltage mode: slowb=low) and rwb (for write access mode: rwb=low; for read access mode: rwb=high):
         1. The NMOS driver transistor  36   a  (selected if slowb=high and rwb=low) is assigned to write access mode and high supply voltage mode.   2. The NMOS driver transistor  37   a  (selected if slowb=low and rwb=low) is assigned to write access mode and low supply voltage mode.   3. The NMOS driver transistor  38   a  (selected if slowb=high and rwb=high) is assigned to read access mode and high supply voltage mode.   4. The NMOS driver transistor  39   a  (selected if slowb=low and rwb=high) is assigned to read access mode and low supply voltage mode.       

   In one embodiment, the transistor width of the transistor  36   a  for write access and high supply voltage mode is n times higher than the transistor width w 1  of the transistor  37   a  for write access and low supply voltage mode. Additionally, the transistor width of the transistor  38   a  for read access and high supply voltage mode is i times higher than the transistor width w 3  of the transistor  39   a  for read access and low supply voltage mode. The larger the width of the selected driver transistor  36   a - 39   a  is, the faster the dummy bit line  32  is discharged. For the high supply voltage mode or for high-speed operation the larger driver transistors  36   a  or  38   a  are selected, whereas for the low supply voltage mode or for low speed operation the smaller driver transistors  37   a  or  39   a  are selected. 
   For the selective activation of one of the four NMOS driver transistors  36   a - 39   a  the source of each NMOS driver transistor  36   a - 39   a  is connected to the drain of a corresponding NMOS select transistor  36   b - 39   b . The ratio of the widths of each two select transistors  36   b - 39   b  is identical to the ratio of widths of the corresponding driver transistors  36   a - 39   a . Depending on the gate potential of the corresponding select transistor  36   b - 39   b , the select transistor  36   b - 39   b  is switched on or off. In case the select transistor  36   b - 39   b  is off, the corresponding driver transistor  36   a - 39   a  is also switched off (independent from the signal  34 ). The gates of the select transistors  36   b - 39   b  are connected via the logical circuit  40  to the pins slowb and rwb. The logical circuit may be designed such that in dependence of the potential slowb and rwb only one of the four select transistors  36   b - 39   b  and therefore only one of the four driver transistors  36   a - 39   a  is selected. 
   If the signal  34  is low, a precharge PMOS transistor  35  is active, which precharges the dummy bit line  32  connected to the drain of the PMOS transistor  35  to the supply voltage V dd . If the signal  34  switches from low to high, the selected (via the pins slowb and rwb) NMOS driver transistor  36   a - 39   a  is activated. The activated driver transistor  36   a - 39   a  discharges the dummy bit line  32  resulting in a logically low potential. The time for discharging the dummy bit line  32  from high to low depends on the width of the selected driver transistor  36   a - 39   a , the total capacitance of the dummy bit line  32  and the load of the dummy bit line, mainly the plurality  33  of dummy memory cells  9 ′. By the usage of four different transistor widths, the delay time for discharging differs from read operation to write operation and differs from high supply voltage mode to low supply voltage mode. 
   The potential of the signal on the dummy bit line  32  and the potential of the pin rwb determines the potential of the control signals irwb, saen and wl_resetn by means of the logical circuit  41 . 
   In one embodiment, the NAND-gates and inverters of the logical circuit  41  are arranged in such a manner that in case the dummy bit lines  32  is discharged, i.e., the potential of the dummy bit line  32  switches from high to low, the control signal saen for the sense amplifier  8  switches from low to high if rwb is high (read access mode), the control signal irwb for the write buffer  9  switches from high to low if rwb is low (write access mode) and the control signal wl_resetn switches from high to low (independently form the signal on the pin rwb, i.e., for read and write access mode),and vice versa. 
   Since the number of gates in the signal propagation path differs for the signals saen and wl_resetn (read mode access) or the signals irwb and wl_resetn (write mode access) the edges do not necessarily coincide. 
   In one embodiment, if the dummy bit line  32  is discharged during read access, the signal saen switches from low to high (at the time instant t a  in  FIG. 2 ), which activates the sense amplifier  8 . Additionally, the selected word line wl i  is deactivated, since the signal wl_reset switches from high to low. The selection of the driver transistor  36   a - 39   a  determines the time for discharging the dummy bit line  32  and therewith determines the read margin Δsa (cf.  FIG. 2 ). The smaller the width of the selected driver transistor  36   a - 39   a , the slower the dummy bit line  32  is discharged and the larger the read margin Δsa is. 
   During write access, the write buffer  9  is already activated (cf.  FIG. 3 ) prior to the discharge of the dummy bit line  32 . When the dummy bit line  32  is discharged, the signal irwb switches from high to low (cf.  FIG. 3 ) stopping the operation of the write buffer  9 . In addition, the selected word line wl i  is deactivated since the signal wl_reset switches from high to low. The selection of the driver transistor  36   a - 39   a  determines the time for discharging the dummy bit line  32  and therewith determines the write margin Δt (cf.  FIG. 3 ). The smaller the width of the selected driver transistor  36   a - 39   a , the slower the dummy bit line  32  is discharged and the larger the write margin Δt is. 
   In general, if the read margin Δsa and/or the write margin Δt are increased, the operating speed of the circuit is decreased. 
   In conclusion it should be noted that the inventive concept of an adjustable delay in view of two or more different supported supply voltage modes might also be employed in view of two or more different selectable operating speed modes of the memory circuit, with each speed mode being operated at a different supply voltage. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.