Abstract:
Embodiments of a memory are disclosed that may allow for the detection of weak data storage cells. The memory may include data storage cells, a column multiplexer, a sense amplifier, and a load circuit. The load circuit may include one or more capacitive loads and may be operable to controllably select one or more of the capacitive loads to couple to the input of the sense amplifier.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    This invention is related to the field of memory implementation, and more particularly to techniques for data storage cell testing. 
         [0003]    2. Description of the Related Art 
         [0004]    Memories typically include a number of data storage cells composed of interconnected transistors fabricated on a semiconductor substrate. Such data storage cells may store a single data bit or multiple data bits and may be constructed according to a number of different circuit design styles. For example, the data storage cells may be implemented as a single transistor coupled to a capacitor to form a dynamic storage cell. Alternatively, cross-coupled inverters may be employed to form a static storage cell or a floating gate MOSFET may be used to create a non-volatile storage cell. 
         [0005]    During the semiconductor manufacturing process, variations in lithography, transistor dopant levels, etc., may result in different electrical characteristics between transistors that are intended to have identical characteristics. Additional variation in electrical characteristics may occur due to aging effects within transistors as the device is repeatedly operated. These differences in electrical characteristics between transistors can result in data storage cells that output different small signal voltages for the same stored data. In a memory array, there may be a large variation in the small signal output voltages across the data storage cells that comprise the memory array. 
         [0006]    Data from data storage cells that generate a smaller than average output signal due to the previously described variation may not be able to be read correctly, resulting in a misread. Data storage cells that fail to read properly may contribute to lower manufacturing yield and necessitate additional redundant data storage cells to maintain manufacturing yield goals. 
       SUMMARY 
       [0007]    Various embodiments of a memory circuit are disclosed. In an embodiment, the memory circuit may include a plurality of data storage cells, a column multiplexer, a sense amplifier, and a load circuit. The load circuit may couple load devices to the input of the sense amplifier. In some embodiments, the load circuit may be operable to controllably couple load devices to one of the inputs of a sense amplifier configured to amplify a differentially encoded signal. 
         [0008]    The load circuit may include a number of load devices and may be operable to controllably select one or more of the load devices to couple to the input of the sense amplifier. In some embodiments, the load devices may be of differing sizes. 
         [0009]    During operation, test data may be stored into a data storage cell. The data may be read from the data storage cell using the sense amplifier and compared to the original test data. The data may also be read from the data storage cell using the sense amplifier with load devices coupled to the input of the sense amplifier and compared to the original test data. The result of these comparisons may be used to determine the strength of the data storage cell. In some embodiments, information indicative of the strength of the data storage cell may be stored for later use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
           [0011]      FIG. 1  illustrates an embodiment of a data storage cell. 
           [0012]      FIG. 2  illustrates possible waveforms for the discharge of bit lines. 
           [0013]      FIG. 3  illustrates an embodiment of a memory sub-array. 
           [0014]      FIG. 4  illustrates a possible method of operating the embodiment illustrated in  FIG. 3 . 
           [0015]      FIG. 5  illustrates an embodiment of a load circuit. 
           [0016]      FIG. 6  illustrates an embodiment load circuit with multiple loads. 
           [0017]      FIG. 7  illustrates an embodiment of a memory. 
           [0018]      FIG. 8  illustrates a possible method of operating the embodiment illustrated in 
           [0019]      FIG. 7 . 
           [0020]      FIG. 9  illustrates a possible method of reading a memory and comparing the stored data to previously loaded test data 
           [0021]      FIG. 10  illustrates an embodiment of a computing system. 
       
    
    
       [0022]    While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the disclosure to the particular form illustrated, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to. 
         [0023]    Various units, circuits, or other components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the unit/circuit/component can be configured to perform the task even when the unit/circuit/component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits. Similarly, various units/circuits/components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a unit/circuit/component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, paragraph six interpretation for that unit/circuit/component. More generally, the recitation of any element is expressly intended not to invoke 35 U.S.C. §112, paragraph six interpretation for that element unless the language “means for” or “step for” is specifically recited. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    During the manufacture of a semiconductor memory circuit, differences in lithography, implant levels, etc., may result in differences in electrical characteristics between data storage cells that are otherwise intended to be identical in characteristics and performance. In some cases, the variation of the electrical characteristics of a data storage cell may be sufficiently large that the data storage cell may not function (e.g., read or write) under normal operating conditions of the memory circuit, resulting in the data storage cell being identified as a failure and requiring replacement with a redundant data storage cell. Adding redundant data storage cells to the memory circuit to compensate for data storage cells with non-ideal electrical characteristics may result in additional chip area and power consumptions. The embodiments illustrated below may provide techniques to identify and compensate for data storage cells with non-ideal electrical characteristics. 
         [0025]      FIG. 1  illustrates a data storage cell according to one of several possible embodiments. In the illustrated embodiment, data storage cell  100  includes a true I/O  102  denoted as “bt,” a complement I/O  103  denoted as “bc,” and a selection input  101  denoted as “wl.” 
         [0026]    In the illustrated embodiment, bt  102  is coupled to selection transistor  104  and bc  101  is coupled to selection transistor  105 . Selection transistor  104  and selection transistor  105  are controlled by wl  101 . Selection transistor  104  is further coupled to pull-up transistor  108  and pull-down transistor  106  through node  110 , and selection transistor  105  is further coupled to pull-up transistor  109  and pull-down transistor  107  through node  111 . Pull-up transistor  108  and pull-down transistor  106  are controlled by node  111 , and pull-up transistor  109  and pull-down transistor  107  are controlled by node  110 . 
         [0027]    It is noted that although selection transistors, pull-up transistors, pull-down transistors, and pre-charge transistors may be illustrated as individual transistors, in other embodiments, any of these transistors may be implemented using multiple transistors or other suitable circuits. That is, in various embodiments, a “transistor” may correspond to an individual transistor or other switching element of any suitable type (e.g., a field-effect transistor (FET)), or to a collection of transistors. 
         [0028]    At the start of the storage operation true I/O  102  and complement I/O  103  may both be high and selection input  101  is low. It is noted that in this embodiment, low refers to a voltage at or near ground potential and high refers to a voltage sufficiently large to turn on n-channel metal oxide semiconductor field effect transistors (MOSFETs) and turn off p-channel MOSFETs. In other embodiments, other circuit configurations may be used and the voltages that constitute low and high may be different. During the storage, or write, operation, selection input  101  may be switched high which couples true I/O  102  to node  110  and complement I/O  103  to node  111 . To store a logical 1 into data storage cell  100 , complement I/O  103  may be switched to a low. Since selection transistor  105  is on, node  111  is also switched low. The low on node  111  activates pull-up transistor  108  which charges node  110  high. The high on node  110 , in turn, activates pull-down transistor  107 , which further reinforces the low on node  111  establishing regenerative feedback. Once the regenerative feedback between nodes  110  and  111  has been established, selection input  101  may be switched low turning off selection transistor  104  and selection transistor  105 , isolating node  110  from true I/O  102  and node  111  from complement I/O  103 . The method of storing a logical 0 may be similar. Selection input  101  may be switched high and true I/O  102  may be switched low. Selection transistor  104  couples the low on true I/O  102  to node  110 , which activates pull-up transistor  109 . The high on node  111  activates pull-down transistor  106 , reinforcing the low on node  110  and establishing the regenerative feedback. Data storage cells that store data via regenerative feedback are commonly referred to as static cells. 
         [0029]    In the illustrated embodiment, data storage cell  100  outputs its stored data as the difference in voltage between true I/O  102  and complement I/O  103 . (Data stored as the difference between two voltages may also be referred to herein as “differentially encoded”.) At the start of the output process, true I/O  102  and complement I/O  103  may both be high and selection input  101  may be low. Asserting selection input  101  activates selection transistor  104  and selection transistor  105 . If node  111  is low and node  110  is high, then a current will flow through selection transistor  105  and pull-down transistor  107  causing a reduction in voltage on complement I/O  103 . If node  110  is low and node  111  is high, then a current will flow through selection transistor  104  and pull-down transistor  106  causing a reduction in voltage on true I/O  102 . For either data state, the current that the data storage cell sinks from either the true I/O  102  or complement I/O  103  is referred to as the read current of the cell. 
         [0030]    Ideally, the electrical characteristics of pull-down transistor  106  and pull-down transistor  107  would be identical, as would be the electrical characteristics of selection transistor  104  and selection transistor  105 . Furthermore, in an ideal circuit, it might be desirable that pull-down transistor  106  and pull-down transistor  107  in one data storage cell in a memory device have identical electrical characteristics to pull-down transistor  106  and pull-down transistor  107  in another data storage cell in the memory device. However, during the semiconductor manufacturing process, differences in lithography, fluctuations in dopant levels, etc., may result in these transistors having different electrical characteristics (e.g., saturation current). Aging effects induced by, e.g., hot-carrier injection may also change a transistor&#39;s electrical characteristics over time. Variation, due to both manufacturing and aging effects, in pull-down transistor  106 , pull-down transistor  107 , selection transistor  104  and selection transistor  105  from one data storage cell to another may result in variation in read currents and, therefore, variation in output voltages for the same stored data. 
         [0031]    In some cases, the variation in the electrical characteristics of the transistors may result in larger than average output voltages when the storage cell is read. Data storage cells that generate larger than average output voltages may be referred to as strong cells. In some cases, the variation in the electrical characteristic of the transistors may result in smaller than average output voltages when the storage cell is read. Data storage cells that generate smaller than average output voltages may be referred to as weak cells. If the value of the output voltage generated by a weak storage cell is sufficiently small, it may not be possible to properly determine the data stored in the data storage cell, because the output voltage may not be able to overcome imbalances and signal noise within a sense amplifier. 
         [0032]    It is noted that the number of transistors and the connectivity shown in  FIG. 1  are merely an illustrative example, and that in other embodiments, other numbers, types of transistors, and/or circuit configurations may be employed. It is also noted that in other data storage cell embodiments, other storage mechanisms may be employed. For example, a capacitor (as, e.g., in a dynamic random access memory (DRAM)), transistor implants (as, e.g., in a depletion programmable read-only memory (ROM)), or a floating gate structure (as, e.g., in a single-bit or multi-bit non-volatile or flash memory) may be used to store data in a data storage cell. 
         [0033]      FIG. 2  illustrates possible waveforms resulting from the operation of the embodiment of the data storage cell shown in  FIG. 1 . At time t 0    205 , the selection input  101  is asserted (waveform  201 ). Depending on the value of the stored data, either true I/O  102  or complement I/O  103  will begin to discharge (waveform  203 ). At time t 1    206 , the small signal differential between true I/O  102  and complement I/O  103  is amplified by a sense amplifier. The system including one or more data storage cells may be modeled as a capacitor and current source. The capacitor may represent the total capacitance present on either true I/O  102  or complement I/O  103 , which may include the junction capacitance of other data storage cells&#39; I/O ports and the capacitance of the interconnect between the data storage cells. The current source is the read current of the data storage cell. With this model, the voltage on the low-going I/O from time t 0  to time t 1  can be estimated using equation 1. 
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         [0034]    Over a limited range of time and voltages, the read current can be treated as a constant. This allows the equation to be simplified as shown in equation 2. For a constant load capacitance, the voltage change on the low-going I/O is proportional to the read current of the data storage cell. If the read current of the data storage cell is less than average, then the change in voltage on the low-going I/O will be less (waveform  204 ), resulting in a smaller differential voltage at the time the sense amplifier is activated. If the read current of the data storage cell is larger than average, then the change in voltage on the low-going I/O will be greater (waveform  202 ), resulting in a larger differential at the time the sense amplifier is activated. It is noted that the waveforms shown in  FIG. 2  are merely an illustrative example and that, in other embodiments, differing waveform behavior may be possible. 
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         [0035]      FIG. 3  illustrates an embodiment of a memory sub-array which includes a data output  310  denoted as “dout,” a pre-charge control input  307  denoted as “pchgb,” a sense amplifier enable input  306  denoted as “saen,” a true data selection input  311  denoted as “dselt,” and a complement data selection input  312  denoted as “dselc.” The illustrated embodiment also includes one or more column selection inputs  305  denoted as “cs” and one or more row selection inputs  304  denoted as “rs.” 
         [0036]    In the illustrated embodiment, columns  301   a ,  301   b ,  301   c , and  301   d  are coupled to the inputs of column multiplexer  302  through bit lines  308 . The differentially encoded output of column multiplexer  302  is coupled to the differential inputs of sense amplifier  303  through nodes  309   a  and  309   b , and the output of sense amplifier  303  is coupled to dout  310 . Load circuit  305  is also coupled to the differential inputs of sense amplifier  303 . 
         [0037]    Each column  301  may include one or more of data storage cell  100 . For example, the individual bit lines bt  102  of each data storage cell  100  within a column  301  may be coupled together to form a true bit line  308  of column  301 . Likewise, the individual bit lines be  103  of each data storage cell  100  within column  301  may be coupled together to form a complement bit line  308  of column  301 . Individual word lines wl  101  of each data storage cell  100  within column  301  may be coupled to a respective one of rs  304  such that when a given rs  304  is asserted, the corresponding data storage cell  100  creates a differentially encoded output on the true bit line and complement bit line of column  301 , while the bit line outputs of the remaining data storage cells  100  within column  301  remain quiescent. In other embodiments, the data storage cells may be dynamic storage cells, single-bit or multi-bit non-volatile storage cells, or mask programmable read-only storage cells. It is noted that in some embodiments, the data storage cell may transmit data in a single-ended fashion. In such cases, only a single bit line per column is required. 
         [0038]    In some embodiments, column multiplexer  302  may contain one or more pass gates controllable by cs  305 . The input of each pass gate may be coupled to the either the true or complement bit line output from one of columns  301   a ,  301   b ,  301   c , and  301   d . The output of each pass gate coupled to a true bit line is coupled to the true output of column multiplexer  302  in a wired-OR fashion, and the output of each pass gate coupled to a complement bit line is coupled to the complement output of column multiplexer  302  in a wired-OR fashion. In other embodiments, column multiplexer  302  may contain one or more logic gates configured to perform the multiplexer selection function. 
         [0039]    Sense amplifier  303  may use analog amplification techniques in some embodiments. In other embodiments, sense amplifier  303  may employ a latch based amplification technique. Load circuit  305  may contain load devices and selection transistors as will be described in reference to  FIG. 5  and  FIG. 6 . 
         [0040]    In some embodiments, the illustrated sub-array  300  may operate as follows. Referring collectively to  FIG. 3  and the flowchart illustrated in  FIG. 4 , the operation may start by initializing the sub-array (block  401 ) by setting pchgb  307  low and setting rs  304 , cs  305 , and saen  306  to inactive states. Once sub-array  300  has been initialized, one of rs  304  may be asserted (block  402 ) selecting a data storage cell in each of columns  301   a ,  301   b ,  301   c , and  301   d . One of cs  305  may then be asserted (block  403 ), causing column multiplexer  302  to output data selected from one of bit lines  308 . 
         [0041]    The operation then depends on whether or not sub-array  300  is operating in test mode (block  404 ). When sub-array  300  is not operating in test mode, pchgb  307  may be set high (disabling pre-charge) and saen  306  may be asserted causing sense amplifier to amplify the difference between nodes  309   a  and  309   b  and couple the amplified result to dout  310  (block  407 ). Sub-array  300  may then be re-initialized by de-asserting saen  306 , and the asserted one of rs  304  and cs  305 , and setting pchgb  307  low (block  401 ). 
         [0042]    When sub-array  300  is operating in test mode, the operation then depends on the value of the test data previously loaded into the selected data storage cell (block  404 ). When a logical 1 was loaded then dselc  311  may be set low causing load circuit  305  to couple additional capacitive load onto the complement input of sense amplifier  303 . With the additional capacitive load, the equation governing the change of voltage on the complement input of sense amplifier  303  may be re-written as shown in Equation 3. 
         [0043]    Since the change in voltage is inversely proportional to the total capacitance, the change in voltage on the complement input of sense amplifier  303  may be reduced. Once the additional capacitive load has been coupled to the complement input of sense amplifier  303 , the amplification operation (block  407 ) and initialization operation (block  401 ) can proceed as described above. 
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         [0044]      FIG. 5  illustrates an embodiment of a load circuit for use with differentially encoded data. The illustrated embodiment includes a true data port  501  and a complement data port  502 , respectively denoted as “datat” and “datac,” as well as a pre-charge control input  511  denoted as “pchgb.” The embodiment further includes a true data selection input  503  and a complement data selection input  504 , respectively denoted as “dselt” and “dselc.” 
         [0045]    In the illustrated embodiment, datat  501  is coupled to selection transistor  505  and datac  502  is coupled to selection transistor  506 . Selection transistor  505  is controlled by dselt  503  and selection transistor  506  is controlled by dselc. Selection transistor  505  is further coupled to load device  507  and pre-charge transistor  509 . Selection transistor  506  is further coupled load device  508  and pre-charge transistor  510 . Pre-charge transistor  509  and pre-charge transistor  510  are controlled by pchgb  511 . In some embodiments, load devices  507  and  508  may be capacitors fabricated using a dedicated oxide layer (e.g., MOM capacitor) or using the insulating material between metal layers (e.g., MIM capacitor). In other embodiments, the load devices may be gate terminals of MOSFETs, or the input/output ports of de-selected data storage cells. It is noted that in alternative embodiments, other numbers and configurations of transistors and devices may be employed. 
         [0046]    During normal read operation, dselt  503  and dselc  504  may both be set high isolating the data inputs from the load devices  507  and  508 , and pchgb  511  may be set low activating pre-charge transistors  509  and  510 . During test read operation, dselt  503  and dselc  504  may be initialized high, and pchgb  511  may be set low activating precharge transistors  409  and  410 . When test data is to be read from a data storage cell, either dselt  503  or dselc  504  may be be set low depending on the anticipated value of the test data, and pchgb  511  may be set high, deactivating precharge transistors  509  and  510 . For example, if the test data to be read is a logical 1, then dselc may be be set low activating selection transistor  506 , coupling datac  502  to load device  508 . Dselt  503  may remain high, isolating datat  501  from load device  507 . 
         [0047]      FIG. 6  illustrates a variant of load circuit  500  that provides multiple load devices. In the illustrated embodiment, load circuit  600  includes a number of input and I/O ports that are similar to load circuit  500 : a true data I/O  601  and a complement data 
         [0048]    I/O  602 , respectively denoted as “datat” and “datac,” a true data selection input  603  and a complement data selection input  604 , respectively denoted as “dselt” and “dselc,” and a pre-charge control input  623  denoted as “pchgb.” In contrast to load circuit  500 , load circuit  600  includes a first load selection input  615  and a second load selection input  616  denoted, respectively denoted as “lsel 1 ” and “lsel 2 .” 
         [0049]    As shown in  FIG. 6 , datat  601  is coupled to selection transistor  605  and datac  602  is coupled to selection transistor  606 . Selection transistor  605  is controlled by dselt  603  and selection transistor  606  is controlled by dselc  604 . Selection transistor  605  is further coupled to pre-charge transistor  621  and selection transistors  607  and  609 . 
         [0050]    Selection transistor  606  is further coupled to pre-charge transistor  621  and selection transistors  610  and  608 . Pre-charge transistors  621  and  622  are controlled by pchgb  623 . Selection transistors  607  and  610  are controlled by lsel 1   615 , and selection transistors  609  and  608  are controlled by lse 12   616 . Selection transistor  607  is further coupled to pre-charge transistor  617  and load device  611 , and selection transistor  609  is further coupled to pre-charge transistor  618  and load device  613 . Selection transistor  610  is further coupled to pre-charge transistor  619  and load device  614 , and selection transistor  608  is further coupled to pre-charge transistor  620  and load device  612 . Pre-charge transistors  617 ,  618 ,  619 , and  620  are controlled by pchgb  623 . It is noted that in other embodiments, the number and configuration of transistors and devices may be different. 
         [0051]    During normal read operation, dselt  603 , dselc  604 , lsel 1   615 , and lsel 2   616  may be set high, isolating datat in  601  and datac in  602  from the load devices  611 ,  612 ,  613  and  614 . Pchgb  623  may be set low activating precharge transistors  617 ,  618 ,  619 ,  620 ,  621 , and  622 . 
         [0052]    During test read operation, dselt  603 , dselc  604 , lsel 1   615 , and lsel 2   616  may be initialized high. Additionally, pchgb  623  may be set low activating precharge transistors  617 ,  618 ,  619 ,  620 ,  621 , and  622 . When test data is to be read from a data storage cell, pchgb  623  may be set high, and either dselt  603  or dselc  604  may be set low depending on the anticipated value of the test data, and either lsel 1   615  or lsel 2   616  may be set low depending on the desired amount of additional load. For example, if the test data to be read is a logical 1, then dselc  604  may be set low activating selection transistor  606 . If lsel 1   615  is set low, selection transistor  610  will become active, coupling datac  602  to load device  614 . Dselt  603  will remain high, isolating datat  601  from load devices  611  and  613 . In some embodiments, dselt  603  and dselc  604  may be set low simultaneously, and lsel 1   615  and lsel 2   616  may be set low simultaneously. 
         [0053]      FIG. 7  illustrates a memory according to one of several possible embodiments. In the illustrated embodiment, memory  700  includes data I/O ports  709  denoted “dio,” an address bus input  712  denoted “add,” mode selection inputs  711  denoted “mode,” and a clock input  710  denoted “clk.” 
         [0054]    In the illustrated embodiment, memory  700  includes sub-arrays  701   a ,  701   b , and  701   c , timing and control unit  702 , address decoder  703 , and address comparator  704 . Sub-arrays  701   a ,  701   b , and  701   c  may incorporate some or all of the features described above with respect to sub-arrays  300 . Timing and control unit  702  is coupled to provide a decoder enable signal  708  to address decoder  703  and address comparator  704 , and control signals  705  to sub-arrays  701   a ,  701   b , and  701   c . In some embodiments, control signals  705  may include a pre-charge signal, a sense amplifier enable signal, a true data selection signal, a complement data selection, a first load selection signal, and a second load selection signal that may operate as described above with respect to sub-array  300  and load circuits  500  and  600 . 
         [0055]    Address decoder  703  is coupled to provide row selects  706  and column selects  707  to sub-arrays  701   a ,  701   b , and  701   c , in response to the assertion of decoder enable signal  705  and the address value on address bus  712 . Address comparator  704  is coupled to provide read-miss indication signal  708  to timing and control unit  702  based upon a comparison of the address value on add  712  to a collection of address values previously determined to select weak data storage cells in sub-arrays  701   a ,  701   b , and  701   c . In some embodiments, address comparator  704  may include a storage unit  714  configured to store address values that select weak data storage cells. 
         [0056]      FIG. 8 . Illustrates a possible method of operating memory  700  to test for weak data storage cells. Referring collectively to  FIG. 7  and the flowchart illustrated in  FIG. 8 , the operation starts in block  801 . The value presented to add  712  may be set to zero (block  802 ). The operation then depends on the value presented to add  712 . When the value presented to add  712  exceeds the maximum address of memory  700 , the test ends (block  807 ). When the value presented to add  712  is less than the maximum address of memory  700 , mode  711  may be set for a write operation, test data may be presented to dio  709 , and clk  710  may be asserted, writing the test data into the data storage cells selected by the value presented to add  712  (block  804 ). 
         [0057]    Once the test data has been loaded, memory  700  may be re-initialized. Mode  711  may be set for read and test operation and clk  710  may be asserted initiating the read and comparison operation as will be described in reference to  FIG. 9  (block  805 ). When the read and comparison operation has completed, memory  700  may be re-initialized and the value on add  712  may be incremented (block  806 ) and the value checked against the maximum address for memory  700  (block  803 ). It is noted that operations shown in  FIG. 8  are merely an illustrative example and that in actual circuit operation, other operations and order of operations may be possible. 
         [0058]    A possible method of operating memory  700  to read and compare previously loaded test data is illustrated in  FIG. 9 . Referring collectively to  FIG.7  and the flow chart illustrated in  FIG. 9 , the operation may begin by de-asserting clk  710  to initialize memory  700  (block  901 ). Mode  711  may be set for normal read operation and clk  710  may be asserted which causes timing and control unit  702  to assert decoder enable signal  708 . Address decoder  703  decodes the address presented to add  712  (block  903 ) in response to the assertion of decoder enable signal  708 , and asserts one of row selects  706  and one of column selects  707  (block  903 ) selecting a data storage cell in each of sub-arrays  701   a ,  701   b , and  701   c . Timing and control unit  702  may then assert the appropriate signal in control signals  705  to activate the sense amplifiers in sub-arrays  701   a ,  701   b , and  701   c , causing them to amplify the data from the selected data storage cells and output the amplified data to dio  709  (block  904 ). 
         [0059]    The operation then depends on value of data output on dio  709  (block  905 ). When the data output on dio  709  does not match the originally loaded test data, the selected data storage cells may contain one or more hard failures (block  906 ). In this test flow, no further action is taken and the test of data storage cells at the given address is complete (block  915 ). When the data output on dio  709  matches the originally loaded test data, further testing may be necessary and clk  710  is de-asserted, re-initializing memory  700  (block  907 ). Mode  711  may be set for test read operation and clk  710  may be asserted. In response to the assertion of clk  710 , timing and control unit  702  asserts decoders enable  708 , causing decoder  703  to decode the address presented to add  712  (block  908 ). Address decoder  703  then asserts one of row selects  706  and one of column selects  707 , selecting a data storage cell in each of the sub-arrays  701   a ,  701   b , and  701   c  (block  909 ). Timing and control unit  702  may then assert the necessary control signals  705  to activate the load circuits in sub-arrays  701   a ,  701   b , and  701   c  (block  910 ). Dependent upon original test data, the load circuits may couple the load devices to either the true input or the complement input of the sense amplifiers. Timing and control unit  702  may then assert the necessary control signals  705  to activate the sense amplifiers, causing the sense amplifiers to amplify the data from the selected data storage cells and output the amplified data to dio  709 . 
         [0060]    The operation then depends on the value of the data output on dio  709  (block  912 ). When the data output on dio  709  matches the originally loaded test data, the selected data storage cells have sufficient read current to overcome the additional load provided by the load circuits. The address that selected these data storage cells may be noted as containing cells of normal strength (block  914 ). The test operation at the given address is the complete (block  915 ). When the data output on dio  709  does not match the originally loaded test data, one or more of the selected data storage cells do not have sufficient read current to overcome the additional load provided by the load circuits. The address that selected these data storage cells may be noted as containing weak data storage cells (block  913 ). The test operation at the given address may then be complete (block  915 ). In some embodiments, the address may be loaded into storage unit  714  such that when the given address is encountered in subsequent read access to memory  700 , address comparator  704  asserts misread indication signal  713 . It is noted that during actual circuit operation, some or all of the operations illustrated in  FIG. 9  may occur in a different order, or may occur concurrently rather than sequentially. 
         [0061]    Turning now to  FIG. 10 , a block diagram of a system is illustrated. In the illustrated embodiment, the system  1000  includes an instance of a random access memory (RAM)  1002  and a read-only memory (ROM)  1003  each of which each may include one or more sub-arrays that may incorporate some or all of the features described above with respect to sub-array  300 . 
         [0062]    The illustrated embodiment also includes a CPU  1001  which may include one or more local storage units  1009 . For example, CPU  1001  may include a Cache Data RAM, a Tag RAM, one or more register files, and one or more FIFOs. Each one of the local storage units  1009  may include one or more load circuits that may incorporate some or all of the features described above with respect to load circuits  500  and  600 . In some embodiments, CPU  1001  may include a test unit  1010  configured to operate the load circuits. Additionally, the illustrated embodiment includes an I/O adapter  1005 , a display adapter  1004 , a user interface adapter  1006 , and a communication adapter  1007 . 
         [0063]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.