Reference word line and data propagation reproduction circuit, particularly for non-volatile memories provided with hierarchical decoders

Abstract: A reference word line and data propagation reproduction circuit, particularly for non-volatile memories provided with hierarchical decoders, characterized in that the memory is divided into at least two memory half-matrices that are arranged on different half-planes, and in that the circuit comprises, for each one of the at least two memory half-matrices, a reference unit (3 i ) for each one of the at least two memory half-matrices and an associated unit (4) for reproducing the propagation of the signals along the reference unit, the reference unit (3 i ) and the associated propagation reproduction unit (4) having a structure that is identical to each generic word line of the memory device, the reference and propagation reproduction units of one of the at least two memory half-matrices being activatable upon selection of a memory cell in the other one of the at least two memory half-matrices, in order to provide a reference that is synchronous and symmetrical with respect to the selection of the memory cell for reading it and so as to preset, according to the propagation reproduction unit (4), the conditions for starting correct and certain reading of the selected memory cell.

Description: The present invention relates to a reference word line and data propagation reproduction circuit, particularly for non-volatile memories provided with hierarchical decoders. More particularly, the invention also relates to the synchronization of said reference lines for a memory device provided with hierarchical decoders. In a memory device that comprises two half-matrices of memory cells, hierarchical decoders are advantageously used which drive portions of a word line and are stimulated by local decoding operations, all of which are stimulated by a single main line that branches out of a main decoder. Considering, for example, a generic memory with a 16-bit word length, each bit comprises a packet of bit lines (which belong to the same bit) and a reference bit line. In this manner, therefore, there are sixteen reference bit lines, one for each one of the sixteen outputs of a memory device, and this entails considerable space occupation. A multiplexer is usually used for bit line selection, whereas reference line selection requires a single transistor. The multiplexer introduces a significant capacitance, which is significantly different from the single capacitance introduced by the single transistor of each reference bit line. In this case, the dynamic comparison of the current of a selected memory cell with the current of a reference cell to determine the status of the first cell can be impaired by this difference in capacitance. Recovery of the asymmetry introduces delays. Furthermore, locating the bit line far from the matrix bit line that is actually selected for reading entails the drawback that the signals that are present on the reference and on the read line can be different, due to the different distance from the activation point, in view of the lack of symmetry of the reference line and of the bit line with respect to the activation point. US-A-4 811 290 whereon the preamble of claim 1 is based discloses a method for reading a memory cell wherein a sense amplifier (10) is operated by a control signal produced from a signal reflecting the signal propagation along the word line and then the signal is delayed by a wait period The aim of the present invention is therefore to provide a reference line and data propagation reproduction circuit, particularly for non -volatile memories provided with hierarchical decoders, that has a smaller number of reference lines than conventional embodiments. Within the scope of this aim, an object of the present invention is to provide a reference line and data propagation reproduction circuit that provides a correspondence between the propagation of a generic word line and a reference line. An object of the present invention is to provide a reference line and data propagation reproduction circuit that allows to avoid consumptions entailed by voltage boost lines. Another object of the present invention is to provide a reference line and data propagation reproduction circuit that presets the conditions for starting a correct reading step. Another object of the present invention is to provide a reference line and data propagation reproduction circuit that allows to harmonize the various power supply combinations that are present in the system. Another object of the present invention is to provide a reference line and data propagation reproduction circuit that is capable of synchronously setting the activation/deactivation time of the data propagation selection functions. Another object of the present invention is to provide a reference line and data propagation reproduction circuit that allows to rapidly restore the inactive condition of the reference so that it is ready for a new reading operation, even if it is an immediate one. Another object of the present invention is to provide a reference line and data propagation reproduction circuit that is highly reliable and relatively easy to produce at competitive costs. This aim and, these objects are achieved by a reference word line and data propagation reproduction circuit, particularly for non-volatile memories provided with hierarchical decoders, as defined in claim 1. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the circuit according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a block diagram of the circuit according to the invention; figure 2 is a detailed circuit diagram of the circuit according to the invention; figure 3 is a circuit diagram of a trigger for setting the trigger level in order to determine the end of precharging; figure 4 is a detailed view of the configuration of the bit lines of the memory device; figure 5 is a view of the activation means of the reference lines; and figure 6 is a view of the selection means of the reference line that belongs to a given left or right half- matrix. With reference to the above figures, the circuit according to the invention comprises a memory matrix that is divided into at least two half-matrices (not shown). Each half-matrix is divided into further matrix subsections for driving by means of hierarchical decoders. In the case shown in the figures, the memory device is divided into two half-matrices and each half-matrix is divided into four matrix subsections. Each half-matrix has a reference line that is provided with characteristics that are similar to a generic word line of the half-matrix. The reference lines (one for each half-matrix) are also divided into four segments that correspond to the segments into which a generic word line is divided. Figure 1 illustrates the general architecture of the circuit according to the invention, in which both reference lines are shown; it is clear that the circuit is perfectly symmetrical as regards a reference line located on one half-plane (half-plane of a half-matrix) and the reference line located on the opposite half-plane. Because of this, hereinafter the description will refer to a single reference line, since any remark must be understood to apply automatically to the other reference line as well. The reference numerals used will also be identical, with the only difference that the reference numerals of one of the two reference lines will be primed. In figure 1, the reference numeral 1 designates selection/deselection means that are driven by a signal UPsel (likewise, there is a signal DWsel for the selection/deselection means 1') and by a read address transition signal ATD for stimulating a line 2 that is adapted to activate the various segments of which the reference line is composed. Said segments are designated by 3 i (where i = 1... n, in this case i = 1... 4). Hereinafter, the reference line will be generically designated by the reference numeral 3. The selection/deselection means form, for all practical purposes, a decoder that allows to balance the capacitances of the decoder of the word lines of the memory half-matrices. The memory cells of the reference line are virgin, so as to constitute a reference for the matrix cells of a generic word line selected for reading. Said memory cells are connected to the corresponding bit lines of the half-matrix in which the reference line 3 is located. An additional propagation delay reproduction line 4, adapted to reproduce the propagation delay of a signal along a generic word line, has a structure that is similar to a segment 3 i of the reference line 3. A single segment is used because the delay of one segment 3 i is sufficient to determine the propagation delay of the entire line 3, since it is constituted by a known number of mutually identical segments 3 i . The memory cells of the propagation line 4 are instead disconnected from the corresponding bit lines of the half-matrix in which said reference line is located, since the conductivity of its cells is irrelevant. The propagation delay reproduction line 4 is activated by the same line 2 driven by the decoding means 1. The output of the propagation delay line 4 is received by means 5 for selecting the propagation delay generated by the line 4. Said means 5 send the propagation delay signal to means 6 for detecting the end of propagation along said reference line 3, which reproduces a generic word line of the memory matrix. The means 6 are conveniently provided for example by a trigger. The propagation delay selection means 5 are adapted to select an output point of said delay line 4, which can be an intermediate point or an end point according to the operating requirements. All remarks made also apply to the reference line 3' that lies on the half-plane that is opposite to the half-plane of the reference line 3. With reference to figure 2, the architecture shown in figure 1 is shown here explicitly in its circuital details. Each segment 3 i of the reference line 3 (and therefore of the reference line 3' as well) comprises a driver, which is advantageously provided by an elevator structure 7 i followed by an RC network 8 i . The RC network is composed of a plurality of RC circuits that are series-connected to each other. The RC network represents a delay that is similar to the one that exists in the generic word line. The hierarchical decoder allows to reduce the propagation of a generic word line with distributed capacitances. If all the capacitances of the line were series-connected, one would in fact have a highly capacitive line with propagation delays. The elevator structure 7 i drives the intermediate point of the line that comprises the plurality of RC structures, thus improving propagation. The memory cells of the reference line 3, which are virgin, are designated by 9 i and are shown, in figure 4, connected to the corresponding bit lines (designated by BL0, BL1, BL2, ..., BL(n-1)). Said memory cells are conveniently provided by floating-gate transistors. The virgin memory cells of the propagation delay reproduction line 4 are instead not connected to the bit lines. The RC structures of the reference line 3 and of the propagation reproduction line 4 are conveniently connected to the ground by virtue of means for restoring the pre-activation conditions; said means are advantageously provided by transistors 39 i that are adapted to rapidly restore the pre-activation conditions of the reference line 3 and of the propagation reproduction line 4. The elevator structures 7 i are supplied by the supply voltage that is present in the memory device (voltage V DD or boost voltage). The propagation delay reproduction line 4 likewise has a structure that is identical to each segment 3 i of the reference line 3 and is therefore also provided with an elevator structure 7 i that is also supplied by the voltage that is actually present in the memory device. The output of the line 4 for reproducing the propagation delays of a generic word line is selected by the propagation delay means 5; therefore, if a delay taken in an intermediate point of the line 4 is selected, one obtains in output the line 10, whereas if a delay taken at the end of the line 4 is selected, then one obtains in output the line 11. . In detail, the selection/deselection means 1 comprise an elevator structure 12 that is common both to the selection/deselection means 1 and to the selection/deselection means 1'. Said elevator structure 12 is supplied by the supply voltage that is actually present in the memory device, and receives in input the signal UP/DW for selecting one reference line 3 i or the other one 3 i '. The output of the elevator structure 12 is constituted by the two signals UPsel and DWsel for selection of the two reference lines 3 i and 3 i '. The selection/deselection means 1, with which the signal UPsel is associated, are now described in detail: similar remarks apply to the selection/deselection means 1' with which the signal DWsel is associated. The signal UPsel in output from the elevator structure 12 is therefore sent in input, through an inverter 13, to a NOR gate 14. The other input of the NOR gate 14 is the read address transition detection signal ATD. The output of the NOR gate 14 is in turn sent, across an inverter 15, so as to drive the activation line 2 of the reference line 3 (therefore each segment 3 i of the reference line 3). The signal UPsel is also sent in input to an additional elevator structure 16, whose output is sent in input to the gate terminal of a P-channel transistor of a pair of pass transistors 17, whereas the signal UPsel is sent to the gate terminal of the N-channel transistor of the same pair of pass transistors 17. The two pass transistors 17 are interposed between the lines 10 and 11 that exit from the propagation delay reproduction line 4 and the means 6 for detecting the end of propagation along said reference lines, which reproduce the propagation along a generic word line of the memory matrix. The propagation end detection means 6 therefore receive in input a propagation indication signal PROP and emit in output a detection signal DETup. The detection means 5 are also supplied at the voltage that is actually present in the memory device (V DD or boost). With reference to figure 3, said detector means 6 are shown in their circuital details and are advantageously constituted by a trigger. The trigger comprises a first complementary MOS inverter and a second complementary MOS inverter. The first inverter comprises a P-channel transistor 18 and an N-channel transistor 19, which are common-drain connected, and the second inverter comprises a P-channel transistor 20 and an N-channel transistor 21 that are common-drain connected. The trigger is completed by a further P-channel transistor 22. The signal PROP is sent to the gate terminals of the transistors 18 and 19. The drain terminals of the transistors 18 and 19 are connected to the gate terminal of the transistor 22, which is in turn connected to the gate terminals of the transistors 20 and 21. The source terminals of the transistors 18, 22, and 20 are connected to the supply voltage (which can be V DD or a boost voltage) and the source terminals of the transistors 19 and 21 are connected to the aground. The drain terminal of the transistor 22 is connected to the gate terminals of transistors 18 and 19. The output of the trigger is received by the drain terminals of the transistors 20 and 21 and is the signal DETup. Figure 5 shows in detail the elevator structure 7 i , which comprises two P-channel transistors 23 i and 24 i whose source terminals are connected to the supply voltage (which can correspond to the boost voltage) and whose respective drain terminals are connected to the drain terminals of an N-channel transistor 25 i and of a N-channel transistor 26 i respectively. The gate terminals of the transistors 23 i and 24 i are connected respectively to the common-drain connection of the transistors 23 i -25 i and 24 i -26 i . The drain terminals and source terminals of the transistors 25 i and 26 i are connected to the ground. An inverter 27 i is interposed between the gate terminals of the transistors 25 i and 26 i . The activation line 2 is sent in input to the gate terminals of the transistors 25 i and 26 i . An additional inverter 28 i is series-connected to the output of the elevator structure 7 i , taken at the common-drain connection of the transistors 24 i and 26 i . The signal in output from the inverter 28 i is the signal for driving the line 3 i . This elevator structure is repeated for each segment 3 i into which the reference line is divided. Figure 6 is a view of the elevator structure 12, which is similar to the elevator structure 7 i . Therefore, this structure is not described in detail to avoid redundancy in description. The correspondence between the elements of the elevator structure 7 i and the elements of the elevator structure 12 is determined as follows. The transistors 23 i , 24 i , 25 i , and 26 i of the structure 7 i functionally correspond to the transistors 29, 30, 31, and 32, respectively, of the elevator structure 12. As in the elevator structure 7 i , in the elevator structure 12 there is an inverter 33 (which corresponds to the inverter 27 i ). The inverter 28 i of the structure 7 i is instead not present in the structure 12. With reference to the above figures, the operation of the circuit according to the invention is as follows. When the pulsed read address transition signal ATD switches from high to low, if one of the selection signals of one of the two half-matrices, i.e., the signals UPsel and DWsel, is high, then if the signal UPsel is high, the NOR gate 14 emits a low signal. This low signal is inverted in the inverter 15, so as to drive the activation line 2, activating the segment 3 i of the reference line 3, where a virgin memory cell is present that positionally corresponds to the memory cell of a generic word line selected for reading. Selection of the virgin reference cell occurs in the matrix half-plane that is opposite to the one in which the memory cell to be read is selected. In this manner, reference activation is synchronous and symmetrical with respect to the selection of a memory cell, and the comparison of the memory cell with a corresponding reference cell 9 i that belongs to the reference line 3 allows to determine the status of the memory cell to be read. Simultaneously with the activation of the reference line 3, the associated propagation reproduction line 4 is activated so as to assuredly establish the end of the propagation of the signals along the reference line; this propagation corresponds, for the structure of the reference line 3 (which is identical to the structure of a generic word line of each half-matrix), to the propagation of a generic word line. The output of the propagation reproduction line 4 can be optionally selected in an intermediate point of the line, in an end point thereof, or in a point determined by the OR combination of the intermediate point and of the end point; in the first case, presetting for fast reading is performed, whereas in the second case presetting for slow reading occurs; the third case is used for test reading. The advantage of having a hierarchical decoder that drives line segments instead of the entire line at once lies in the fact that the partial activation of a generic word line allows to provide faster read times. The drivers of the various segments 3 i of the reference line 3 are advantageously provided by elevator structures to avoid useless tapping of the boost line. It is evident that the activation of the reference line 3 and of the propagation reproduction line 4 occurs synchronously with the end of the signal ATD and is therefore tied, in terms of time, to the selection of a memory cell in a generic word line. The deselections (resets) of the reference line 3 and of the propagation reproduction line 4 occur by means of the transistors 39 i and are fast. The positional match of the activation points of said lines allows to have equal propagations and signals, since equidistance from the activation points is provided. The signals UPsel and DWsel are generated by the selection/deselection means 1 and in particular by the elevator structure 12, which receives the signals UP/DW in input. The output of the propagation delay selection means 5 (adapted to select the intermediate point, the end point, or their OR, of the preference line 4) is sent to the propagation end detection means 6, which are constituted by a trigger which, by tripping, presets the activation of the read circuits, emitting the signal DETup in output. Reading activation is provided assuredly, since it is closely tied, in terms of time, to the propagation conditions of the signals along the word line selected for reading, and is furthermore tied to the supply level that occurs at that moment. The circuit according to the invention entails a correlation to the power supplies that are actually present in the memory device and by means of which the word lines are activated. Both in the case of a normal power supply V DD and in the case of a supply with a boost voltage, the elevator structures that are present in the circuit can be supplied with the supply voltage that is actually present in the memory device (V DD or boost). In practice it has been observed that the circuit according to the invention fully achieves the intended aim and objects, since it allows to provide two reference lines that are structurally identical to a generic word line and are arranged so that one is in one memory half-matrix and the other is in the opposite half-matrix. In this manner, the selection of a generic memory cell that belongs to a bit line in a memory half-matrix is matched by the synchronous selection of a virgin reference memory cell that positionally matches the bit line to which said selected memory cell belongs, but is arranged in the domain of the opposite half-matrix. The reference thus provided allows to determine the status of a memory cell involved in a reading operation. The circuit according to the invention also entails providing a line for reproducing the propagation of the signal both in said reference line and in a generic word line in which the memory cell to be read is selected. The possibility of determining the end of propagation allows to establish the moment of activation of the read circuits as a consequence of the certain attainment of the conditions for correct reading, such as indeed the propagation of the word lines. The correspondence between the activations (of the word line and of the reference line) in the two half-matrices and the propagation line is due to the equidistance from the activation points, since the reference is activated so as to positionally match the selected memory cell. Furthermore, the provision of just two reference lines, one for each half-matrix, drastically reduces the number of necessary reference lines (sixteen in a similar memory device provided according to a conventional architecture). The provision of a reference line that is similar to a generic word line allows to reproduce the capacitive loads of the generic word line, allowing actual synchronization of the reference with the word line affected by the reading operation. Finally, the circuit according to the invention provides for harmonization with the various operating voltages that are present in the memory device, providing a lock on the actual voltage that is used (normal or boost voltage), and the circuital solutions allow to eliminate consumptions, which are negative for boost lines. The circuit thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, it is possible to divide the memory matrix into a number of memory half-matrices that is different from two. In this case, too, for each half-matrix there must be a reference line 3 and an associated propagation reproduction line 4. In turn, each half-matrix can be divided into any number of half-matrix subsections, which are matched by an equal number of segments of the reference line 3. Finally, all the details may furthermore be replaced with other technically equivalent elements. In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to the requirements and the state of the art. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A reference word line (2) and a data propagation reproduction circuit, (3, 4) in a memory device particularly for non-volatile memories provided with hierarchical decoders, wherein said memory is divided into at least two memory half-matrices that are arranged on different half-planes and comprise word lines (2, 21), and wherein said data propagation reproduction circuit (3, 4) comprises, for each one of said at least two memory half-matrices, a reference unit (3) for each one of said at least two memory half-matrices, said reference unit (3) having the same structure as said reference word line (2), and an associated propagation reproduction unit (4), having the same structure of segment (3i) of the reference word line (2) for reproducing the propagation of the signals along a word line (2'), means (8i) for generating an output signal (10) that reproduces the signal propagation of said word line (2'), said associated propagation reproduction unit (4) generating, from said output signal (10), a delayed output signal (11), the reference word lines and the propagation reproduction units (3, 4) of one of said at least two memory half-matrices being activatable upon selection of a memory cell in the other one of said at least two memory half-matrices, in order to provide a references (DETup) that is synchronous and symmetrical with respect to the selection of said memory cell for reading it and so as to preset, according to the associated propagation reproduction unit (4), the conditions for starting correct and certain reading of said selected memory cell, characterized in that said means (8i) for generating said output signal (10) that reproduces the signal propagation of said word line is said reference unit (3). The memory according to claim 1, characterized in that said reference unit and said signal propagation reproduction unit are arranged in the half-plane of each one of said at least two memory half-matrices. The memory according to claim 1, characterized in that said reference unit and said propagation reproduction unit comprise, respectively, a reference word line and a propagation reproduction line, said lines being structurally identical to a generic word line of said memory device. The memory according to claim 1, characterized in that said reference unit comprises a plurality of virgin memory cells, said memory cells being connected to the corresponding bit lines of the half-matrix in which said reference unit is located. The memory according to claim 1, characterized in that said propagation reproduction unit comprises a plurality of virgin memory cells, said memory cells being disconnected from the bit lines of the memory half-matrix in which said propagation reproduction unit is located, said propagation reproduction unit being adapted to synchronously reproduce the propagation of said reference unit and of the associated word line as a consequence of a selection of a memory cell for reading said memory cell. The memory according to claim 1, characterized in that each one of said memory half-matrices is divided into a plurality of memory half-matrix subsections, for driving by means of hierarchical decoders. The memory according to claim 6, characterized in that said reference unit is divided into a plurality of reference sub-units, the number of said reference sub-units being equal to the number of said memory half-matrix subsections. The memory according to claim 7, characterized in that said propagation reproduction unit is structurally identical to each one of said reference sub-units, a single propagation reproduction unit being associated with said plurality of reference sub-units. The memory according to claim 1, characterized in that it comprises, for each one of said at least two memory half-matrices, means for the selection/deselection of said reference and propagation reproduction units, said selection/deselection means being driven by a read address transition signal of the memory device and by a signal for the selection of one of said at least two memory half-matrices in which the memory cell to be read is selected, and being suitable to simultaneously activate/deactivate, by means of a common line, said reference and propagation reproduction units. The memory according to claim 1, characterized in that it comprises propagation selection means that are adapted to select the output of said propagation reproduction unit at an intermediate point or at an end point of said propagation reproduction unit, in order to activate the read circuits. The memory according to claim 10, characterized in that it comprises means for detecting the end of propagation, said means being adapted to trigger said read circuits and receiving, in input, the signal that leaves said propagation reproduction unit, said propagation end detection means being correlated to the supply voltage levels that are actually present in said memory device. The memory according to claim 1, characterized in that said reference unit comprises an elevator structure that is adapted to drive an RC structure, said elevator structure being supplied at the supply voltage of the memory device. The memory according to claim 12, characterized in that it comprises means for the quick deselection of said reference unit, said quick deselection means being adapted to connect said RC structure to the ground to restore the pre-activation conditions. The memory according to claim 1, characterized in that said propagation reproduction unit comprises an elevator structure that is adapted to drive an RC structure, said elevator structure being supplied at the supply voltage of the memory device. The memory according to claim 14, characterized in that it comprises means for the quick deselection of said propagation reproduction unit, said quick deselection means being adapted to connect said RC structure to the ground to restore the pre-activation conditions. The memory according to claim 9, characterized in that it comprises means that are adapted to produce said signals for the selection of said at least two memory half-matrices, said signals being sent to said means for the selection/deselection of said reference and propagation reproduction units. The memory according to claim 16, characterized in that said means adapted to produce said signals for the selection of said at least two memory half-matrices comprise an elevator structure, said elevator structure being supplied at the supply voltage of the memory device. The memory according to claim 10, characterized in that said means for the selection/deselection of said reference and propagation reproduction units comprise an elevator structure, a NOR gate, and two inverters, said elevator structure being supplied at the supply voltage of the memory device. The memory according to claim 11, characterized in that said means (6) for detecting the end of propagation comprise an elevator structure that is supplied at the supply voltage of the memory device. A method for reading a memory device, particularly of the non-volatile type, comprising the steps of: selecting a memory cell of a word line (21) in one of at least two memory half-matrices into which said memory device is divided; simultaneously selecting, in a reference unit (3) having the same structure as said word line (2'), a virgin reference cell that is connected to a bit line that positionally matches the selection of said memory cell, said reference unit being located in the domain of the other one of said at least two memory half-matrices; generating an output signal (10) reproducing the signal propagation on said word line; activating, simultaneously with said reference cell, a propagation reproduction unit (4) that generates, from said output signal (10), a delayed output signal (11); detecting said delayed output signal to determine the moment of activation of circuits for reading said memory cell; characterized in that said output signal (10) that reproduces the propagation on said word line is taken from the reference unit (3). The method according to claim 20, characterized in that said step of activating said propagation reproduction unit is provided in synchronization with timing signals of the memory device, said activation being provided synchronously with the end of a pulsed read address transition signal. The method according to claim 20, characterized in that said step of selecting a virgin reference cell in said reference unit is provided synchronously with the selection of said memory cell in the half-matrix that is opposite to the one in which said memory cell is selected, said selection being performed synchronously with the end of a pulsed read address transition signal.

Inventors: BARCELLA ANTONIO, FONTANA MARCO, PASCUCCI LUIGI, ROLANDI PAOLO, BARCELLA, ANTONIO, FONTANA, MARCO, PASCUCCI, LUIGI, ROLANDI, PAOLO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C7
IPC codes: G11C16, G11C7