Abstract:
The invention concerns an amplifier ( 1 ), capable of being controlled by an activation signal, for reading storage cells of a crossbar network comprising, for each column, a direct bit line (BLdi) and a reference bit line (BLri), the amplifier being common to two columns and producing an OR-Exclusive type combination of the states of the cells read in said two columns.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to memories made in the form of an array of memory cells in an integrated circuit. The present invention more specifically applies to DRAMs, that is, memories in which the reading of the content of a memory cell is performed by comparison with a precharge level. 
   2. Discussion of the Related Art 
     FIG. 1  partially and schematically shows a conventional example of a DRAM structure. 
   Storage elements or memory cells C(j, i) are arranged in an array. Each storage element is associated with a pair of bit lines BLdi and BLri in a column BLi (of rank i) shown vertically and with a word line (WLj) shown horizontally. The bit lines are common to columns of storage elements and the word lines are common to rows of storage elements. Each storage element C(j, i) of column i and of row j includes, between a bit line (for example, BLdi) and the ground, the series connection of a MOS transistor T and a capacitive element C. The gate of transistor T is connected to word line BLj. The bit line to which the storage element is not connected forms, for this cell, a reference or precharge line. Most often, each bit line alternately forms the precharge line of a storage element one row out of two. Each column BLi of the memory plane is associated with a sense amplifier SAi having the function of comparing the analog levels present on the bit lines in a read cycle of a storage element, for conversion into logic levels. Generally, the bit lines are precharged and an interval of a few hundreds of millivolts at most is measured by means of the comparator constitutive of the sense amplifier of the column, to differentiate a low level (0) from a high level (1). 
   The structure and operation of a conventional sense amplifier of DRAM cells as well as the selection and precharge elements associated therewith are known and will not be detailed. 
   In some applications, the logic signals provided by the sense amplifiers of a DRAM cell are combined by a wired state machine to provide a specific result. An example of such an application is the finding of image contours in graphical applications. Combinations of XOR or XNOR type are generally used to locate the contours of a digital image stored in an array of memory cells while respecting the pixel arrangement with respect to the display. 
   In such applications, the implementation of the logic combinations by means of wired state machines requires, for a same row, several successive combinations due to the individual reading of the memory cells in the array. The performed comparison is generally sequential, which takes time. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a novel sense amplifier for DRAM cells which enables performing a faster XOR-type logic combination. 
   The present invention also aims at providing a solution which does not increase the bulk of the input/output circuits of such a memory. 
   To achieve these and other objects, the present invention provides an amplifier, controllable by an activation signal, for reading memory cells of an array including, for each column, a direct bit line and a reference bit line, the amplifier being common to two columns and performing a combination of XOR type of the states of cells read from these two columns. 
   According to an embodiment of the present invention, the amplifier includes: 
   a first branch formed of a first transistor in series with a second transistor and a third transistor between a high voltage terminal and a reference point, the respective gates of the second and third transistors, preferably with N channels, being connected to a first bit line of a first column and to a first bit line of a second column, and the terminal of the first transistor, preferably with a P channel, opposite to the high voltage terminal defining a first output terminal; 
   a second branch formed of a fourth transistor, of a fifth transistor, and of a sixth transistor connected between the high voltage terminal and the reference point, the respective gates of the fifth and sixth transistors, preferably with N channels, being connected to the first bit line of the first column and to a second bit line of the second column, and the terminal of the fourth transistor, preferably with a P channel, opposite to the high voltage terminal defining a second complementary output terminal; and 
   a seventh and an eighth transistor connecting, respectively, the first output terminal to a first common point between the fifth and sixth transistors and the second output terminal to a second common point between the second and third transistors, the gates of the seventh and eighth transistors, 
   According to an embodiment of the present invention, the amplifier includes a ninth and a tenth transistor, preferably with P channels, respectively connecting the gate of the first transistor and that of the fourth transistor to the high voltage terminal. 
   According to an embodiment of the present invention, the gates of the ninth and tenth transistors receive said activation signal, an eleventh transistor, preferably with a P channel, the gate of which receives said activation signal connecting, preferably, the gates of the first and fourth transistors. 
   According to an embodiment of the present invention, said first and second common points are respectively connected, by twelfth and thirteenth transistors, preferably with P channels, to the high voltage terminal. 
   According to an embodiment of the present invention, the gates of the twelfth and thirteenth transistors receive said activation signal. 
   According to an embodiment of the present invention, the reference point is connected, by a fourteenth transistor, preferably with an N channel, to a reference voltage. 
   According to an embodiment of the present invention, the gate of the fourteenth transistor receives said activation signal. 
   According to an embodiment of the present invention, the amplifier further includes: 
   a fifteenth transistor, preferably with an N channel, between the first and second transistors of the first branch, the gate of the fifteenth transistor being connected to the second output terminal; and 
   a sixteenth transistor, preferably with an N channel, between the fourth and fifth transistors of the second branch, the gate of the sixteenth transistor being connected to the first output terminal. 
   According to an embodiment of the present invention, the amplifier is applied to determining the contour of a digital image. 
   The present invention also provides an integrated circuit DRAM. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
       FIG. 1 , previously described, shows a conventional example of a DRAM of the type to which the present invention applies; 
       FIG. 2  partially shows the architecture of a memory equipped with amplifier-operators according to the present invention; 
       FIG. 3  shows a first embodiment of a sense amplifier with an XOR-type operation according to the present invention; and 
       FIG. 4  shows a second embodiment of a sense amplifier with an XOR-type operation according to the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been designated with the same reference numbers in the different drawings. For clarity, only those components of an integrated circuit memory and of its input/output circuits which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the respective structures of the column and row selection devices have not been detailed and are no object of the present invention. 
   A feature of the present invention is to combine, within the sense amplifier, the comparison element enabling detection of the state stored in a cell of the array and an XOR-type combination. In other words, an amplifier according to the present invention performs a comparison of XOR type between the two analog inputs of the sense amplifier. 
     FIG. 2  schematically shows the architecture of the inputs/outputs of an array of memory cells according to the present invention at the level of the sense amplifiers. 
   A feature of the present invention is that each sense amplifier  1  is associated with two columns of the memory cell array, that is, with four bit lines. In the example of  FIG. 2 , a first amplifier-operator  1  (SAOP(i, i+1)) receives the direct and reference bit lines of columns i and i+1. A second amplifier-operator SAOP(i+2, i+3) receives the direct and reference bit lines of columns i+2 and i+3. 
   Each amplifier-operator of the present invention provides the logic result of the XOR-type comparison between the two bit lines as well as its complementary (XNOR). Accordingly, the amplifiers of  FIG. 2  respectively provide signals XOR(i, i+1), XNOR(i, i+1), and XOR(i+2, i+3), XNOR(i+2, i+3). 
     FIG. 3  shows a first embodiment of an amplifier-operator 1 of the present invention. By arbitrary convention, the bit lines to which reference will be made hereafter are those of columns i and i+1. 
   An amplifier-operator according to the present invention includes two respectively direct and complementary branches, each providing, on a terminal OUT and {overscore (OUT)}, the result of the XOR comparison and its complement. Terminals OUT and {overscore (OUT)} are each connected, by a P-channel MOS transistor, respectively P 1  and P 2 , to a high voltage Vdd. The gate of transistor P 1  is connected to voltage Vdd by a P-channel MOS transistor P 3  having its gate controlled by a selection signal SEL. The gate of transistor P 2  is connected to voltage Vdd by a P-channel MOS transistor P 4 , the gate of which is controlled by signal SEL. A P-channel MOS transistor P 5 , controlled by this same signal SEL, further connects the gates of transistors P 1  and P 2 . Further, the gate of transistor P 1  is connected to terminal {overscore (OUT)} and, by an N-channel MOS transistor N 1 , to a point  10  of interconnection between two N-channel MOS transistors N 3  and N 5  connecting terminal OUT to a reference node  15 . Node  15  is connected, by an N-channel MOS transistor N 6  controlled by signal SEL, to ground. The gate of transistor P 2  is connected to terminal OUT and, by an N-channel MOS transistor N 2 , to a node  20  of interconnection between two N-channel MOS transistors N 4  and N 6  connecting terminal {overscore (OUT)} to node  15 . Nodes  10  and  20  are further each connected, by a P-channel MOS transistor, respectively P 6  and P 7 , to high voltage Vdd. 
   The gates of transistors N 1  and N 2  are connected to the direct bit line of rank i, BLdi. The gates of transistors N 3  and N 4  are connected to the reference bit line of rank i, BLri. The gate of transistor N 5  is connected to the reference line of rank i+1, BLri+1. The gate of transistor N 6  is connected to the direct bit line of rank i+1, BLdi+1. The gates of transistors P 6 , P 7 , and N 7  receive selection signal SEL. 
   Since it involves a DRAM cell sense amplifier-operator, the reading is performed by comparing the voltage level of a bit line of each column with the reference line of this column. The logic operation is performed on the so-called analog signals, that is, on a small variation amplitude with respect to a precharge level, as in a conventional sense amplifier. 
   When signal SEL is inactive (low), both outputs OUT and {overscore (OUT)} are precharged to the high voltage level (Vdd). Indeed, a low signal SEL turns transistors P 3 , P 4 , and P 5  on. Voltage level Vdd is thus transferred to the base of transistors P 1  and P 2  and onto terminals OUT and {overscore (OUT)}. Transistors P 1  and P 2  are accordingly off and outputs OUT and {overscore (OUT)} are pulled to the high level. Under the effect of control signal SEL at the low level, transistors P 6  and P 7  are also on. As a result, transistors N 1 , N 2 , N 3 , and N 4  are not responsive to the signals present on lines BLdi and BLri, their respective drains and sources being at the same voltage Vdd (neglecting the voltage drops in the on transistors). Further, since transistor N 7  is turned off by signal SEL in the low state, the level of nodes  10  and  20  is maintained whatever the state of transistors N 5  and N 6 . This state of amplifier-operator  1  corresponds to a quiescent state where no reading is performed. 
   To perform a reading from a dynamic array, one of the bit lines of each pair associated with each column is first conventionally precharged to level Vdd/2. This precharge is generally performed by means of MOS transistors connecting the direct and reference bit lines to the high level. The precharge comprises introducing level Vdd/2 into a reference cell associated with the reference bit line and controlled by a specific word line (not shown). The precharge thus comprises biasing to Vdd/2 a reference line of each pair of bit lines, this reference line corresponding to a line which, for the row to be read from, is connected to no memory cell transistor. 
   Performing a reading comprises allowing transfer of the levels present on the direct and reference bit lines to the amplifier-operator. For this purpose, separation or isolation transistors, not shown, interposed between the memory cell array and the sense amplifiers-operators on each bit line are conventionally used. Accordingly, what has been illustrated in  FIG. 2  as being bit lines BLri, BLdi, BLdi+1, and BLri+1 actually corresponds to the analog inputs of the amplifier-operator, which are connected to the array columns via isolation transistors. 
   To read and compare the content of two memory cells of a same row by means of an amplifier-operator according to the present invention, the levels present on the direct and reference bit lines are transferred onto the respective gates of transistors N 1 , N 2 , N 3 , N 4 , N 5 , and N 6 . The amplifier-operator is then selected by switching signal SEL to the high state. This turns transistors P 3 , P 4 , P 5 , P 6 , and P 7  off and turns transistor N 7  on. The turning-off of transistors P 3  to P 5  frees the gates of inverter-assembled transistors P 1  and P 2  to enable reading. The turning-off of transistors P 6  and P 7  frees nodes  10  and  20  to enable comparison. The turning-on of transistor N 7  brings node  15  to ground to set the low state. 
   The content of the read cell (for example of row j) of column i is assumed to contain a “1”. This means that, upon its selection for reading, after addressing by word line WLj, bit line BLdi is brought to a voltage greater than the precharge voltage (for example, Vdd/2). Reference line BLri remains, as for itself, at level Vdd/2. This results in a turning-on of transistors N 1  and N 2  with a stronger conduction than transistors N 3  and N 4 . The output state is, according to the present invention, conditioned by the states of transistors N 5  and N 6 , themselves conditioned by the reading from the cell of column i+1. 
   Column i+1 is assumed to also contain, in the read cell, a high state. Line BLri then is at a level greater than the precharge level of line BLri+1. Transistor N 5  thus is more conductive than transistor N 6 . This conduction imbalance causes node  10  to be at a voltage smaller than that of node  20 . Accordingly, transistor N 1  lowers the voltage of terminal {overscore (OUT)} faster than transistor N 2  lowers the voltage of terminal OUT. Transistor P 1  thus turns on, which forces the voltage of terminal OUT to the high state. This confirms the turning-off of transistor P 2 . A high direct output (OUT) and a low indirect output ({overscore (OUT)}) are thus obtained. This is in accordance with the result of an XOR function having its two inputs in the high state. 
   If the content of the cell of column i+1 is low, its selection causes a voltage decrease with respect to the precharge level. As a result, transistor N 5  is less conductive than transistor N 6 , which has remained at the precharge level. The conduction imbalance causes a faster lowering of the voltage at node  20  and, accordingly, a stronger conduction of transistor N 2 . The consequence is that the voltage of terminal OUT decreases and that transistor P 2  is turned on. This on state of transistor P 2  confirms a high state on terminal {overscore (OUT)} and confirms the off state of transistor P 1 , and thus the low state on terminal OUT. This state is coherent with the XOR function when the two inputs have different states. 
   Now assume that the cell of column i contains a low state. Its selection then causes a faster conduction of transistors N 3  and N 4  precharged to level Vdd/2 with respect to transistors N 1  and N 2 , the gates of which receive a signal of lower level. 
   If the cell of rank i+1 contains a state 1, transistor N 5  is more strongly conductive. Node  10  lowers faster than node  20  and transistor N 3  turns transistor P 2  on and lowers output node OUT. As previously, the on state of transistor P 2  confirms the off state of transistor P 1  and the high output state {overscore (OUT)}. This state is coherent with the XOR function having its two inputs in the low state. 
   Finally, if the cell of rank i+1 contains a 1, transistor N 6  lowers the voltage of node  20  faster and, accordingly, terminal {overscore (OUT)} falls more rapidly, which turns transistor P 1  on and confirms the off state of transistor P 2 . A state corresponding to the XOR function of two inputs of different states is here reached. 
   As an alternative, and as known in conventional sense amplifiers, the precharge level may be greater than Vdd/2 (between Vdd/2 and Vdd). 
   It should be noted that an amplifier-operator of the present invention provides both the result of an XOR operation and of its complementary (XNOR). 
   An advantage of the present invention is that the output level is obtained very rapidly with respect to a conventional circuit. Indeed, by combining the reading and the logic function, it is no longer necessary to wait for the output states of the sense amplifiers to stabilize, the comparison being performed directly on the analog states. It should however be noted that the output states provided do correspond to logic states. The present inventors have acknowledged, in tests, that the result is available a few hundreds of picoseconds after the switching of the amplifier-operator selection signal. 
   Another advantage of the present invention is that it reduces the number of transistors necessary to perform the amplifying and the logic comparison with respect to conventional circuits. Indeed, in a conventional read amplifier, at least a dozen transistors are used. For a logic gate of XOR type, the minimum number of transistors is six. Since the present invention processes two columns at once, the number of transistors of the amplifier-operator, which is 14 in the example of  FIG. 2 , should be compared to the some forty transistors necessary in the conventional case. 
   Another advantage of the present invention is that the rapidity with which the output states are obtained helps reduce the input and output noise of the amplifier-operator. 
   Another advantage of the present invention is that the rapidity with which the output states are obtained helps minimizing the input and output noise of the amplifier-operator. 
     FIG. 4  shows a second embodiment of an amplifier-operator  1 ′ according to the present invention. As compared to the circuit of  FIG. 3 , the difference is the addition of two inverter-assembled N-channel MOS transistors N 9  and N 10  between transistors P 1  and N 3  and between transistors P 2  and N 4 . The gate of transistor N 9  is connected to terminal {overscore (OUT)} and the gate of transistor N 10  is connected to terminal OUT. In  FIG. 4 , input signals BLdi, BLri, BLdi+1, and BLri+1, have been designated as A, Ā, B, and {overscore (B)}. 
   The function of transistors N 9  and N 10  is to block the state read from the cell to lock the output states. 
   The operation of the circuit of  FIG. 4  can be deduced from that discussed hereabove in relation with FIG.  3 . 
   An advantage of the circuit of  FIG. 4  is that the power consumption of the circuit consumption is eliminated as soon as the output signal is completely established. 
   To perform an image contour detection, each bit line (each pair of bit lines) may for example be associated with two neighboring amplifiers to compare the column state with respect to the two neighboring columns. XOR-type comparisons are thus successively performed between two neighboring columns. Each column (except for the first and the last one) is thus examined (read from and combined) twice. The number of circuits  1  or  1 ′ of the present invention thus is n−1, where n represents the number of memory plane columns to be examined. In a single read cycle, the separative lines of an image, for example black and white, with the black corresponding to state 1 and the white corresponding to state 0, are determined for a given row. 
   The above discussion is simplified to illustrate an application of the present invention. This simplified example shows the advantages of the present invention with respect to a separate use of amplifiers and XOR-type gates used sequentially. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the connections of the different bit lines to the amplifiers-operators and the exploitation of the output signals of these amplifiers-operators will depend on the application. Further, the voltage levels (in particular, the used precharge levels) may be modified. 
   Further, the present invention applies to any read mode conventionally used for DRAMs. For example, the present invention also applies to the case where the reference bit line of each column is dedicated to the reference function, as well as to the case where the reference line of a column actually is the direct line of a neighboring column. Further, the dimensions of the different transistors are within the abilities of those skilled in the art based on the functional indications given hereabove and on the envisaged application. 
   Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within and scope of the invention. Accordingly, the foregoing description is by way of example only and is not as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.