Abstract:
A semiconductor memory storage device is disclosed. This memory device has a plurality of storage cells for storing data; a plurality of access devices for allowing access to the corresponding plurality of storage cells, the plurality of access devices being arranged in at least two groups, each of the at least two groups being controlled by an access control line; access control circuitry for controlling a voltage level supplied to a selected one of at least two of the access control lines during access to the storage cell, the access control circuitry comprising a capacitor and switching circuitry; and control circuitry responsive to a data access request to access a selected storage cell to: connect a selected one of the access control lines to a voltage level to allow access via one of the access devices to the selected storage cell; and to control the switching circuitry of the access control circuitry to connect the capacitor of the access control circuitry to the selected access control line and thereby change the voltage level supplied to the selected access control line.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The field of the invention relates to the field of data storage and in particular, to the storage and access of data in semiconductor memories. 
         [0003]    2. Description of the Prior Art 
         [0004]    With ever increasing demands to reduce both the size of devices and their power consumption, it is becoming increasingly challenging to design robust semiconductor memories such as SRAM. Each storage cell in an SRAM comprises a feedback loop for holding a data value. In order to write to the feedback loop and store a new value, the input must have a high enough voltage level to be able to switch the state stored by the feedback loop if required, while reading from the feedback loop should be performed without disturbing the values stored in any of the feedback loops. 
         [0005]    As dimensions scale down the variations in device properties due to random dopant fluctuations, line edge roughness etc. increase drastically. 
         [0006]    Thus, designing a robust SRAM where cells can be read (without read disturb) and written to across all operational voltage ranges turns out to be very difficult. Reducing the voltage at which the SRAM cells can be read and written to successfully is not easy and in particular as the voltage scales down it becomes increasingly difficult to write to the cells. 
         [0007]    One proposed way of addressing the write problem is disclosed in “Low power SRAM with Boost Driver Generating Pulsed Word Line Voltage for Sub-IV Operation” by Iijima et al. Journal of Computers, vol 3, No 5 May 2008. In this article an active body-biasing controlled boost transistor generates a boost to the word line voltage thereby facilitating writes by capacitive coupling only when accessed. A drawback of this scheme is that there is a significant area overhead resulting from having one extra transistor per word line and also this transistor adds an additional delay to the turning off of the pass gate. The pass gate allows access to the storage cell, and this extra delay results in some charge leakage and therefore decreases the efficiency of the scheme. 
         [0008]    In addition to the above drawbacks, such schemes as this only address the write-ability problems and not the problems associated with read disturb where an increased voltage on the word line makes it more likely that cells may be unintentionally corrupted. 
         [0009]    It would be desirable to be able to reduce both read and write failures particularly during low voltage operation of a semiconductor memory. 
       SUMMARY OF THE INVENTION 
       [0010]    A first aspect of the present invention provides a semiconductor memory storage device comprising: a plurality of storage cells for storing data; a plurality of access devices for allowing access to said corresponding plurality of storage cells, said plurality of access devices being arranged in at least two groups, each of said at least two groups being controlled by an access control line; access control circuitry for controlling a voltage level supplied to a selected one of at least two of said access control lines during access to said storage cell, said access control circuitry comprising a capacitor and switching circuitry; and control circuitry responsive to a data access request to access a selected storage cell to: connect a selected one of said access control lines to a voltage level to allow access via one of said access devices to said selected storage cell; and to control said switching circuitry of said access control circuitry to connect said capacitor of said access control circuitry to said selected access control line and thereby change said voltage level supplied to said selected access control line. 
         [0011]    The present invention recognises that accessing data within a storage cell via access devices is not always straightforward and that it may be advantageous to change the voltage level supplied to an access device of the storage cell to aid the data access. The present invention provides access control circuitry that has a capacitor and switching circuitry, the switching circuitry being used to connect the capacitor to the access control line that has been selected by the access request. This allows the access control circuitry to control more than one access control line by simply connecting the capacitor to the appropriate line. This means that rather than needing individual access control circuits for each access control line, one circuit can control multiple lines. This provides a significant saving on area. Furthermore, the use of a capacitor to change the voltage value provides the system with great flexibility allowing the voltage level to be amended as is required for the particular circumstance. 
         [0012]    It should be noted that the number of access control lines that a single access control circuit element can connect to depends on the size of the capacitor of the access control circuitry and the capacitance of the access control lines it connects to. In order for the capacitor to trigger a reasonably sized change in voltage its capacitance must be significantly larger than the capacitance of the access control lines that it connects to. Providing it with the ability to connect to several access control lines via switching devices provides an increased capacitance loading, and thus, there is an optimal number, where the required increase in size of the capacitor to service all of these lines is too large and it is preferable to use multiple access control circuitry elements servicing a lower number of the access control lines each. 
         [0013]    In some embodiments, said access control circuitry is configured to change said voltage level supplied to said selected access control line to either increase said voltage level or decrease said voltage level in dependence upon a control signal from said control circuitry. 
         [0014]    The use of a capacitor to change the voltage level supplied means that it can be used either to increase or decrease the voltage level depending on how the capacitor is precharged. This is advantageous as there may be circumstances where an increase in the voltage level will aid the data access while there are other circumstances where a decrease in the voltage level is required. Having a capacitor as the means of controlling the voltage change means that the same device can be used either to increase or decrease the voltage level which provides embodiments of the invention with increased flexibility without unduly increasing the area of the circuit. 
         [0015]    In some embodiments, said data access request comprises one of a read request to read data from said storage cell or a write request to write data to said storage cell, said control circuitry being configured to control said access control circuitry to reduce said supplied voltage level in response to said data access request being a read request and to increase said supplied voltage level in response to said data access request being a write request. 
         [0016]    It may, for example, be advantageous to reduce the voltage to the access control line when a read is being performed as the storage cell may become unstable if the access voltage is too high and it may then lose the data that it stores. However, when a write is being performed the cell needs to become unstable to overwrite the stored value and thus, an increase in the supplied voltage may be required. 
         [0017]    In other embodiments, said data access request comprises one of a read request to read data from said storage cell or a write request to write data to said storage cell, said control circuitry being configured to control said access control circuitry to increase said supplied voltage level in response to said data access request being a read request or a write request. 
         [0018]    Although, the data stored in a cell may be rendered more unstable by an increase in a voltage on the access control line, the design may be such that it is tolerant to this increase in voltage. In such a case, it may be advantageous to increase the voltage level on the access control line for both reads and writes as in the case of reads this will decrease the read time and thereby increase the performance of the device. 
         [0019]    In some embodiments, said switching circuitry is configured to connect said capacitor to a supply voltage of said semiconductor memory to charge said capacitor before connection of said capacitor to said selected access control line. 
         [0020]    In order to pre-charge the capacitor, it may be connected to the supply voltage of the semiconductor memory. This may be done before any data access requests are received and in this way, there is no timing penalty for charging this capacitor as it is done outside of the timing for the access requests. Furthermore, as the capacitor is isolated from the word line prior to connection thereto, charging in this way does not produce any loading on the word line. Additionally, charging the capacitor in this way can be advantageous as in the charged state when connected to the supply voltage it will act as a decoupling capacitor and help to compensate for fluctuations in the supply voltage. 
         [0021]    It is therefore advantageous in some embodiments to charge the capacitor early and to leave it charged until the charge is required to boost the access control lines. 
         [0022]    In some embodiments, said switching circuitry is configured to connect said capacitor to ground to discharge said capacitor in response to said control signal from said control circuitry indicating that said voltage level is to be reduced before connection of said capacitor to said selected access control line. 
         [0023]    If the capacitor is to be used to reduce the voltage level on the access control line then it is discharged prior to connection to this access control line. In this way, when connected to the capacitor the charge on the access control line will be shared and thus, some will dissipate to the capacitor to render the voltages on the access control line and capacitor equal and the voltage level will thereby be reduced. This will work provided the capacitor and the access control line are isolated from any voltage supply. 
         [0024]    In some embodiments, said switching circuitry is configured to connect said capacitor to a selected voltage level to charge said capacitor by a predetermined amount before connection of said capacitor to said selected access control line, said selected voltage level being selected in response to a received tuning signal. 
         [0025]    In some embodiments, rather than connecting the capacitor to the supply voltage it may be connected to a selected voltage level that can have a tuned value. By tuning the voltage level in this way, the amount of charge on the capacitor can be controlled and thus, the amount of boost or the amount of reduction supplied to the voltage of the access control line is also controlled. The tuning signal may be a signal generated by the processing system that is currently accessing the memory and it may be based on the processing system&#39;s knowledge of operating conditions at the time and thus, its knowledge of the amount of reduction and/or boost in the access control signal that is currently required. 
         [0026]    In some embodiments, said control circuitry is configured to change a capacitance of said capacitor in response to a received tuning signal. 
         [0027]    Alternatively, the system can be tuned by changing the capacitance of the capacitor. The amount that the voltage level is reduced or boosted will depend on the amount of charge stored on the capacitor which may depend on the voltage level used to pre-charge it but will also depend on the size of the capacitor. Thus, in some embodiments the size of the capacitor can be changed in response to a received tuning signal. This may be done if the capacitor is formed of several elements that may be coupled together or isolated from each other. 
         [0028]    In some embodiments, said control circuitry is configured to determine when said access control line connected to said voltage level attains a predetermined value and in response to determining attainment of said predetermined value to control said switching device to isolate said capacitor from a voltage supply and to isolate said access control line from said selected voltage and to connect said capacitor to said access control line. 
         [0029]    In order to control the timing of the access there is control circuitry that determines when the access control line obtains a predetermined value and in response to this it isolates the capacitor from its voltage supply and isolates the access control line from the selected voltage and then connects the capacitor to the access control line. In response to this the voltage level changes and the access can be performed. In order for the access not to be unduly delayed this switching should be done as soon as it is determined that the required predetermined voltage level has been attained on the access control line as any delay will defeat the purpose of lowering the voltage for reads or increase the access time for writes. If the switching occurs before the value is attained, then the effect of boosting the voltage is defeated and in the case of the voltage being lowered it may weaken the read more than is intended. It should be noted that determining when the required voltage level is attained may be done with a voltage detector but in many embodiments it is done by detecting a voltage transition and providing a specific delay between the transition and the switching, the delay being selected such that the voltage level attained by the access control line during this delay is the required predetermined voltage level. 
         [0030]    In some embodiments, said semiconductor memory comprises a replica access control line replicating an access control line, said control circuitry being configured to connect said replica access control line to said voltage level when connecting said selected access control line to said voltage level, said control circuit being configured to detect a voltage transition on said replica access control line and to provide a predetermined delay between detecting said transition and isolating the access control line from the selected voltage. 
         [0031]    In order to determine when the predetermined value has been reached, a replica control line can be used which is also connected to the voltage level. The use of a replica control line means that rather than requiring a detection circuit to detect each access control line, a single detection circuit for detecting voltage transitions on the replica line can be used. This provides a significant saving on area. It should be noted that the detector simply detects a voltage transition and then provides a delay circuit to provide a suitable delay between this detected transition and the isolation of the word line from the voltage supply line, that is sufficient to cause the voltage level on the word line to have reached the desired value. 
         [0032]    In some embodiments, said control circuitry is configured to select said predetermined value in response to a received tuning signal. 
         [0033]    An alternative way of tuning the circuit is to select the predetermined value that the voltage level of the access control line attains before connecting the capacitor to it with the use of a tuning signal to indicate this value. In this way, the final level attained by the signal following the boost or reduction due to connection to the capacitor can be controlled. 
         [0034]    In some embodiments, said control circuitry is responsive to receipt of a control signal to inhibit said access control circuitry from changing a voltage level supplied to said selected access control line, such that in response to a data access request, said control circuitry: connects said selected access control line to said voltage level to allow access to said selected storage cell; and maintains said access control circuitry isolated from said selected access control line. 
         [0035]    Owing to the construction of the control circuitry, it is possible to inhibit it from changing the voltage levels supplied to the access control line if required. Thus, in response to receipt of a control signal indicating that no change in voltage is required, the control circuitry does not connect the capacitor to the access control line, but leaves it isolated from it. A signal indicating that no change in voltage is required may be generated by the processing system using the memory when it realizes that the memory is operating within conditions where the access control line does not need any boost or any reduction in voltage levels to operate correctly. As any variations in the voltage level on the access control line generated by the capacitor will add to the power consumed by the device it is advantageous if when these changes in voltage level are not required, they are inhibited. In this way the performance of the memory can be improved. As the access control circuitry has a capacitor that is generally isolated from the access control line and is only connected during the access it is a simple matter to inhibit its operation. 
         [0036]    In some embodiments, said control circuitry is responsive to a receipt of a control signal to inhibit said access control circuitry from changing a voltage level supplied to said selected access control line to control said switching circuitry to connect said capacitor to a supply voltage of said semiconductor memory. 
         [0037]    Even when inhibited from changing the voltage level on the access control lines, the access control circuitry can have a positive influence on the memory by charging the capacitor and coupling it to the supply voltage so that it acts as a decoupling capacitor and helps maintain the stability of the supply voltage without adding any loading to the word line. 
         [0038]    Although the semiconductor memory can have a number of forms, in some embodiments it comprises an SRAM memory, said groups of access devices and corresponding storage cells comprising a row of said SRAM memory and said access control line comprising a word line. 
         [0039]    The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  shows a memory according to an embodiment of the present invention; 
           [0041]      FIG. 2  schematically shows access control circuitry according to an embodiment of the present invention; 
           [0042]      FIG. 3A  shows access control circuitry and word line drivers according to an embodiment of the present invention; 
           [0043]      FIG. 3B  shows the transmission of charge during operation of the circuit of  FIG. 3A ; 
           [0044]      FIG. 4  shows the variation in the read disturb margin and the write margin with respect to the percentage of word line voltage overdrive; 
           [0045]      FIG. 5  shows access control circuitry where the voltage level that the capacitor is charged to is tunable; 
           [0046]      FIG. 6  shows access control circuitry where the value of the capacitance of the capacitor is tunable; 
           [0047]      FIG. 7  shows a timing diagram illustrating how the voltage on the word line varies with the control signals controlling the switching devices; 
           [0048]      FIG. 8  shows access control circuitry and word lines that are tunable according to an embodiment of the present invention; 
           [0049]      FIG. 9  shows multiple word lines and access control circuitry controlled by a detected voltage on a dummy word line according to an embodiment of the present invention; 
           [0050]      FIG. 10  shows a timing diagram of the circuit of  FIG. 9 ; 
           [0051]      FIG. 11  shows overdrive efficiency as a percentage of the capacitor size; and 
           [0052]      FIG. 12  shows a flow diagram illustrating steps in a method according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]      FIG. 1  shows a memory  5  according to an embodiment of the present invention. Memory  5  comprises a plurality of data storage cells  10  arranged in an array, each cell having access devices  12  controlling access to these storage cells. In this embodiment the memory is arranged as an array and requests to access data, access a selected row of the array via word lines WL. 
         [0054]    In order to access a data storage cell  10  specified by a data access request a driver  20  on the relevant word line will be selected and it will power the word line to raise the voltage such that the access device  12  for cell  10  is turned on and access to the cell  10  via this device is provided. In this embodiment the memory is a SRAM memory with word lines and bit lines and complementary bit lines. 
         [0055]    Memory  5  also comprises access control circuitry  30  which acts to change the voltage level supplied to the word line in response to a data access request in order to facilitate access to the storage cells. 
         [0056]    In the case of a request to write to a storage cell, then the request must be sufficient to switch the state stored in the cell if required and this can be difficult, particularly at low operating voltage levels. In order to ensure that the write is able to function correctly, access control circuitry  30  provides a boost to the voltage level on the word line signal in response to a write request and this boost enables the state in the access storage cell to switch. 
         [0057]    In this embodiment, access control circuit  30  is schematically shown as providing voltage level control to two word lines. The word line containing the cell to be accessed is selected via the select signal on the switching transistor  35 . Thus, a single access control circuit  30  can be used to provide voltage level control for a plurality of word lines which makes embodiments of this invention very area efficient. 
         [0058]    Although in this embodiment access control circuit  30  is only shown as providing voltage control to two word lines, in some embodiments it can provide it to many more. The factor which controls the number of word lines to which a single access control circuit can provide voltage control, is the size of the capacitor  40  within the access control circuit as compared to the size of the capacitance of the word lines themselves. This will be explained in more detail with respect to later figures. 
         [0059]    In addition to providing a boost to the voltage level on the word line, the access control circuitry  30  can also be used to reduce the voltage level on the word line if this is required. Whether access control circuitry  30  provides a boost or a reduction in voltage level is controlled by control circuitry  50 . It may be that in some circumstances such as when performing a read, the voltage level is reduced during the data access. During a read it is important that the state of the storage cells  10  on the accessed word line are not corrupted by the access signal, and thus, an access signal with a lower voltage level may be preferable. 
         [0060]    There may also be circumstances where the memory is operating under conditions where accesses can be made correctly without any need to boost or reduce the voltage levels. In such cases, control signal  50  inhibits access control circuitry  30  from connecting the capacitor  40  to the word lines during data accesses such that the access control circuitry  30  does not affect the voltage levels during these data accesses. 
         [0061]    The memory  5  of  FIG. 1  also comprises a tuning signal input  60  which receives a tuning signal from a processor that accesses the memory. This tuning signal provides information to control circuit  50  regarding whether the voltage level needs increasing, reducing or should not be altered. It may also provide information on the amount that the voltage level should be changed by. 
         [0062]      FIG. 2  shows in schematic form access control circuitry  30 . Access control circuitry  30  comprises capacitor  40  and a plurality of switches  42 ,  44 ,  46  and  48 . In pre-charge mode switches  42  and  48  are switched on and charge flows from supply line VDD to capacitor  40  and charges it. When the world line WL fires switches  42  and  48  are turned off and switches  44  and  46  are turned on. The charge held on capacitor  40  is then coupled to the word line via switch  44  and this provides a boost to the voltage level on the word line. 
         [0063]    Where a reduction in voltage level on the word line WL is required capacitor  40  is discharged via switch  48  before the data access occurs. Then, in response to the word line firing the discharged capacitor  40  is connected via switch  44  to the word line and charge sharing between the word line and the capacitor  40  occurs and the voltage level on the word line is reduced. 
         [0064]      FIG. 3A  shows an example embodiment of access control circuitry such as that shown in  FIG. 2 . In this embodiment the switching circuitry is shown in more detail and the word line and word line driver  20  are also shown. Thus, the access control circuit  30  and the word line driver  20  are controlled by two signals PHI 1  and PHI 2 . These signals are phase signals that are used to manage the charging of the capacitor and the charge sharing in order to achieve the desired voltage change on the word line to allow increased gate overdrive or reduced gate overdrive during a write or a read operation. 
         [0065]    The circuit operates as follows, when the memory is idle the signals PHI 2  and PHI 2 B are active, asserted and signals PHI 1  and PHI 1 B are inactive, de-asserted. This allows the capacitor  40  to charge through the pass transistors M 1 , M 2  (which correspond to switch  42  of  FIG. 2 ) through nodes N 1 , N 2  and through pass transistors M 5 , M 6  (which correspond to switch  48  of  FIG. 2 ) so that the voltage at the capacitor  40  is held at a particular voltage illustrated as V 3  and which is generally the supply voltage VDD. 
         [0066]    The operation of the circuit of  FIG. 3A  is shown in  FIG. 3B . When PHI 2  and PHI 2 B are asserted and PHI 1  and PHI 1 B are de-asserted, in the pre-charging phase then charge flows to the capacitor via arrow  72 . When the word line fires and has attained a voltage level equal to V 2  which may be set to the supply voltage VDD or may be set lower if the circuit is to be tuned using this voltage and it is desired to reduce the voltage boost, PHI 1  is de-asserted and PHI 2  is asserted. There should be no overlap between the de-assertion of PHI 1  and the assertion of PHI 2  to avoid losing charge that has developed on capacitor  40  during the switching. When PHI 1  is asserted charge flows along arrow  74  to word line WL and the voltage on the word line is raised to a voltage that depends on the capacitance CSW of the capacitor  40  and the capacitance CL of the node on the word line. The rise in the voltage will be equal to VDD (1+CSW)/(CSW+CL) if the capacitor has charged to VDD. Thus, the amount that the voltage is raised by depends on the capacitance of capacitor  40  and the capacitance of the word line and also the voltage that the word line is raised to before PHI 1  is de-asserted. In this case the rise in voltage would be equal to VDD+V 2 (CSW/(CSW+CL)). 
         [0067]    Where control circuitry is controlling several word lines then the capacitance CL of the word line will increase and this limits the number of word lines that access control circuitry  30  can control as when the capacitance CL seen at the node reaches a value that is significantly greater than the capacitance CSW of the capacitor  40  then the voltage increase that capacitor  40  provokes will be small. 
         [0068]    The assertion of PHI 1  is also used to cut off the word line node from the supply voltage by turning off the word line driver  20  via switch  22 . This ensures that charge sharing between the capacitance CL of word line WL and the capacitance CSW of capacitor  40  occurs. 
         [0069]    It should be noted that the rise in voltage is equal to VDD (1+CSW)/(CSW+CL) where the word line and the capacitor are charged to VDD. 
         [0070]    The same principle can be applied to reduce the voltage on the word line WL to allow for reduced gate overdrive. Thus, this single circuit  30  can not only control a voltage increase on a plurality of word lines, it can also control a voltage reduction if that is desired. In order to reduce the voltage, the sharing can be set up such that in the first phase (PHI 2  asserted) capacitor  40  is discharged to a lower voltage. This may be zero volts or it may be set up to have a predetermined low voltage as will be seen with respect to later figures. When the voltage of the word line is raised, PHI 1  is asserted and switch  22  is turned off to disconnect the word line from the supply voltage and charge sharing occurs such that the voltage level on the word line settles to VDD (CL/(CL+CSW)). Thus, in this way the capacitor  40  can be used to reduce the voltage level on the word line as well as increasing it by controlling the amount of charge the capacitor holds prior to the data access request being processed. 
         [0071]    One thing to bear in mind is that an increase in the voltage on the word line during data access increases the write margin but also increases the read disturb margin. In other words, there is more likelihood that the storage cells in the word line that is accessed and that are not being written to will be disturbed if the voltage level on the word line is increased during a write. 
         [0072]      FIG. 4  shows a graph illustrating the change in write margin and read disturb margin with respect to the increase in voltage on the word line. Thus, it can be seen that increasing the word line voltage increases the write margin but also affects the read disturb margin and there is therefore a limit to the amount that the level of voltage on the word line should be increased and control of this value is important in ensuring that the memory operates correctly. It should be noted that during a read as the value stored in the cell is not overwritten a reduction in the voltage on the word line during a read access can be implemented to lower the probability of the value stored being disturbed. However, it should be noted that this has the drawback of reducing the speed of the memory access. Therefore, if the cells have been designed such that they are stable in response to a write access which has had its voltage level increased by a known amount, it may be advantageous to increase the voltage level by this amount during the read as well as this will increase the speed of the data access and improve the memory performance. 
         [0073]    As can be seen there are various factors such as the operating voltage of the memory and its components, that affect how much it is desirable to increase or reduce the voltage level on the word line by. It is therefore advantageous if this level can be set. Thus, providing the circuit with a tuning ability such that the change in voltage level can be tuned in response to a tuning control signal is advantageous. There are different ways in which the change in voltage level can be tuned and these involve either selecting a particular size of capacitance, charging the capacitance to a particular voltage or changing the voltage level that the word line has attained before implementing the boost or reduction mechanism. 
         [0074]      FIG. 5  shows in schematic form access circuitry  30  similar to that of  FIG. 2  but with the ability to tune the voltage level that capacitor  40  is charged to, such that the amount of charge that the capacitor  40  holds can be varied and thus, the boost or reduction to the word line voltage supplied by this capacitor  40  can be controlled. In this embodiment, the tuning circuitry comprises variable resistor circuits  80  and  82  which are used to control the voltage seen by capacitor  40 . In all other respects, the circuit operates in the same way as the circuit of  FIG. 2 , it is simply that the amount of charge held by capacitor  40  is controllable and thus, in response to the tuning signal the increase or reduction in voltage that this circuit supplies to the word line can be varied. 
         [0075]      FIG. 6  shows an alternative embodiment where the tuning is provided by a variable capacitor such that the capacitance of this capacitor can be altered in response to a tuning signal and this changes the amount that the voltage level is boosted or reduced by. 
         [0076]    An alternative way of controlling the amount of change in voltage that circuitry  30  supplies to the word line is to control when the capacitor  40  is connected to the word line and when the word line is disconnected from its supply voltage. If the word line is disconnected from its supply voltage before it has attained the voltage level of the supply voltage then clearly the boosted value will be lower than if the boost was supplied once the word line had attained the value.  FIG. 7  shows a timing diagram showing the control signals PHI 1  and PHI 2  and the value of voltage on the word line. This diagram shows that when PHI 1  is asserted too early and a voltage boost is being supplied, the effect of this boost is defeated. Furthermore, if it is asserted too early and a reduction in voltage is required, then this might weaken the read more than needed. On the other hand, asserting it with a fixed time delay might cause the effect to occur late resulting in an extended word line pulse for write or defeating the purpose of lowering the voltage during read and having a detrimental effect on the memory performance. 
         [0077]    In order to determine an appropriate time to assert the PHI 1  signal it is advantageous to detect the voltage level reached on the node WL and when it has attained the required value then PHI 1  can be asserted. 
         [0078]    Such a voltage detector can be used to ensure correct timing of the PHI 1  signal. It can also be used when tuning the circuit by varying the voltage level that the word line has attained in order to vary the final boosted or reduced value to be the level required. 
         [0079]      FIG. 8  shows a schematic diagram illustrating access control circuitry  30  controlled by voltage detector  90 . In this embodiment a dummy word line  95  is formed that replicates the word lines within the memory  5 . This dummy word line is controlled in the same way as the actual word line and thus, it is connected to the supply voltage by the same control signal and the voltage level rises in the same way as the voltage level on the word line. Voltage detector  90  detects the rise in voltage level on the dummy word line  95  and on detecting it reaching a desired value it sends a control signal to control circuit  50  which controls the switching circuits within access control circuitry  30  and word line driver  20 . 
         [0080]    The desired value stored in voltage detector  90  can be set in response to a tuning signal received from the processor that is accessing the memory and in this way the boosted or reduced level of the voltage can be controlled. The timing of the signals is also controlled by this tuning signal. 
         [0081]    A dummy word line is used to mimic the behaviour of a real word line as in this way only one voltage detector  90  is required to detect transitions on this dummy word line, rather than multiple detectors being required to detect the voltage rise on multiple word lines. The detection of this transition is used to control the timing of the system. 
         [0082]    A transition of the voltage on the dummy word line can be detected and as the transition on the selected word line will either occur at the same time or a known time earlier or later, it can be estimated when the word line will have reached the supply voltage and the switching should occur. Thus, the switching can be controlled to occur a certain time after the dummy word line transitions. This control in the timing can also be used to tune the system as is described in below. 
         [0083]    If the characteristics of the rise in voltage of the word line are known then by changing the delay between when the dummy word line either transitions or attains a predetermined value and when the signal is sent to control circuit  50  to control the switching, the level that the word line voltage has attained when the switching occurs can be controlled and thus, the final value of the voltage level attained can be tuned. This delay can be controlled by sending the output of the voltage detector via a tunable delay chain to the control circuit. Tuning of the circuit in this way can be advantageous in a compiler environment for example, where the wordline load is variable and thus, the required overdrive also varies. 
         [0084]    At the controlled switching point signal PHI 2  is de-asserted and signal PHI 1  is asserted and the capacitor  40  is then connected to the word line and this either reduces or raises its voltage level depending on the charge stored on the capacitor  40 . 
         [0085]    It should be noted that in addition to tuning the circuit, control circuitry  50  for example can be used to inhibit switches  44  and  46  from opening and to open switches  42  and  48  when it detects a tuning signal that indicates that the memory is operating in a region where it needs no boosting or reduction of the voltage on its word line to successfully access the memory. In such a case, capacitor  40  acts as a decoupling capacitor being charged and connected to supply voltage VDD and this helps to compensate for any voltage variations in the supply voltage. Furthermore, as switch  44  is open no load is provided on the word line by capacitor  40 . Thus, owing to the arrangement of this system it can be simply disconnected from the word line and will function as a decoupling capacitor thereby improving the performance of the system. It is advantageous to stop the reduction or increase in voltage level if it is not required as each time this voltage level is altered by capacitor  40  some charge is lost and the power consumption of the memory therefore increases. 
         [0086]    It should also be noted that switch  22  can also be used as a power switch. 
         [0087]      FIG. 9  schematically shows multiple word lines and their connection to capacitor  40 . It also shows an input from a dummy word line which controls the delay at which the switches switch. Thus, there are multiple word lines WL 0  . . . WLn which are selected in response to an address signal received at word line drivers  20 . Switch  22  switches the selected word line driver such that VDD is supplied to the word line from the supply voltage line VDD. There are additional switches  35  that are used to connect the selected word line to capacitor  40  following the word line attaining the desired voltage level. There is also a switch  45  used to charge the capacitor  40  to the desired voltage level, this switch being switched off prior to switches  35  being switched on. 
         [0088]      FIG. 10  shows a timing sequence in the voltage overdrive example of  FIG. 9 . As can be seen in response to the clock signal the selected word line is charged. The dummy word line is also charged and after the calculated delay a control signal is sent to switch  22  that applies the voltage via word line driver  20  to the selected word line. The signal on this word line is then boosted in response to switch  35  switching and the charge from capacitor  40  flowing onto the word line. 
         [0089]      FIG. 11  shows a graph illustrating how the efficiency of the switching of the memory increases with the boost to the voltage. This figure shows the overdrive efficiency related to the percentage of switch capacitor value compared to the total capacitance of the word line and switch capacitor. This varies with the delay in the switch connecting the capacitor to the word line to apply the boost or overdrive decreasing for a 16 word line driver. As can be seen this method is very efficient for a small amount of overdrive and furthermore, in this embodiment the switch capacitor is shared amongst  16  word line drivers. 
         [0090]      FIG. 12  shows a flow diagram illustrating a method according to an embodiment of the present invention. This method involves boosting the word line to improve the success of write requests. Initially the capacitor in the access control circuitry is pre-charged. This capacitor can hold its charge and act as a decoupling capacitor while the memory is inactive. Once a data access request is received then the selected word line is fired. It is then determined if the memory is operating in a region that requires a voltage change to improve write success. If it is then the word line is isolated from the supply voltage and the capacitor is isolated from the supply voltage. The capacitor is then connected to the word line to provide a boost in voltage and the data is accessed. If the memory is not operating in a region requiring a voltage boost then the data is simply accessed and the capacitor is not connected to the word line and remains charged and acting as a decoupling capacitor for the supply voltage. 
         [0091]    This method shows the steps that occur when the word line voltage is boosted to improve a write. As noted earlier the capacitor and access control device can also function to reduce the voltage level on the word line in response to a read. In such a case the capacitor is discharged initially and then the same steps occur but rather than boosting a voltage the discharge capacitor will act to reduce the voltage level on the word line thereby decreasing the probability of a read disturb occurring. 
         [0092]    Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.