Circuit for read-enabling a memory device synchronously with the reaching of the minimum functionality conditions of the memory cells and reading circuits, particularly for non-volatile memories

Abstract: A circuit for read-enabling a memory device with checking of the minimum functionality conditions of the memory cells and reading circuits, particularly for non-volatile memories, having the particularity of comprising: a structure for reproducing the operating conditions within the memory matrix (1, 2, 5) to determine the minimum duration of the step for pre-charging the bit lines involved in the reading operation, the structure being adapted to generate a pre-charge step interruption signal that depends on the reaching of the minimum functionality conditions; means (4) for generating a power-on-reset signal for enabling reading when the minimum functionality conditions for reading correctness are reached, said power-on-reset signal generating means (4) being adapted to drive control logic means (6) as well as reading control and stimulation means (7); the circuit furthermore comprising memory means (8) for the pre-charge step, which are driven by the structure for reproducing the operating conditions within the memory matrix (1, 2, 5), by the control logic means (6), and by the reading control and stimulation means (7), the memory means for the pre-charge step (8) causing, at the onset of the minimum functionality conditions, the end of the pre-charge step to perform a first reading of the memory cells in assured conditions.

Description: The present invention relates to a circuit for read-enabling a memory device synchronously with the reaching of the minimum functionality conditions of the memory cells and reading circuits, particularly for non-volatile memories. It is known that in non-volatile memories with so-called static reading it is possible to assuredly read a correct data item in the location addressed by the read address, even if initially the reading circuits have not reached their minimum functionality conditions. Even if reading is started at a voltage level that is not yet sufficient to ensure assuredly correct reading of the data item, its correctness is in fact achieved nonetheless after a certain period of time, when the supply voltages assumes the steady state,' i.e., when the supply voltage reaches at least the level of the threshold voltage of a generic non-volatile memory cell (approximately 2.1 V). However, static-type reading approaches show difficulties in management for solving the problems of noise generated by the switching of the outputs. Furthermore, they do not easily allow to reduce consumptions. In order to obviate this problem, solutions known as sampled and with dynamic (time-controlled) data sensing structures have been proposed. The sampled-type reading approach entails reading the memory cell after a certain preset time and in this case it is indispensable that the read data item be actually correct, since dynamic reading does not allow to modify the data item at a later time. Therefore, if reading is activated before said minimum functionality condition of a generic memory cell (voltage at least equal to the threshold voltage of a non-volatile cell) has been reached, the data item read in incorrect conditions remains incorrect until the next reading operation. This drawback is particularly felt in memory devices that are meant to start up a more complex system, such as for example a computer, when the system enables said memory from the very start. In this case, the first reading operation after device power-on and while the supply voltage level has not yet reached the steady state can have problems as regards the reliability of the read data. This occurs in all cases involving, for any reason, a transient situation of the power supply. EP-0 145 488 and EP-0 500 958 respectively disclose a circuit for controlling the precharge operation of a memory bit line and a power-on-reset circuit. The aim of the present invention is therefore to provide a circuit for read-enabling a memory device that synchronizes the first reading operation with the reaching of the minimum functionality conditions of the memory cells and reading circuits, particularly for non-volatile memories, that allows to perform a first reading operation in assured conditions, for the safe acquisition of a correct data item, even in case of reading performed with dynamic data sensing structures. Within the scope of this aim, an object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that allows to dynamically link the beginning of the reading operation to the actual reaching of the minimum reading conditions. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and of the reading circuits in which the reading event is dependent on the delay of the functionally most critical paths. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that allows to perform a first correct reading operation even if said operation is performed during a transient step of the supply voltage. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that allows to produce a first reading cycle at the end of a power-on-reset signal if the memory device is read-enabled from the very start. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits whose operation is correlated to the actual supply voltage values that are present (voltage V DD or boost voltage). Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that implements all the control functions of a reading process. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that checks that the functionality of every internal component of the memory device has actually been reached. Another object of the present invention is to provide a circuit for synchronizing the reading of a memory device with the reaching of the minimum functionality conditions of the memory cells and reading circuits that blocks reading throughout the period between the enabling of a reading process and the reaching of the minimum functionality conditions. Another object of the present invention is to provide a circuit that is highly reliable, relatively easy to manufacture, and at competitive costs. This aim, these objects, and others that will become apparent hereinafter are achieved by a memory system as defined in claim 1 and by the method of claim 7. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive of the device 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 a portion of the block diagram of figure 1; figure 3 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 4 is a detailed block diagram of a portion of figure 1; figure 5 is a detailed circuit diagram of figure 4; figure 6 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 7 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 8 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 9 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 10 is a detailed circuit diagram of a portion of the block diagram of figure 1; figure 11 is a chart of the timings of the various signals that are present in the circuit according to the invention when a boost voltage is not used; and figure 12 is a timing chart, similar to the chart shown in figure 11, but in the case in which the boost voltage is used. With reference to the above figures, and particularly initially to figure 1, the circuit according to the invention comprises: means 1 for reproducing the propagation along a generic word line of a non-volatile memory device, which are adapted to reproduce the propagation delay of the signal along a word line; means 2 for reproducing the selection/deselection and pre-charge conditions of a generic bit line, which are adapted to reproduce the same selection/deselection and pre-charge delays of a bit line; means 3 for detecting when a boost voltage reaches a preset threshold; means 4 for generating a so-called power-on-reset (POR) signal as a function of the threshold voltage of a non-volatile memory cell and of the minimum detectable conductivity condition of the memory cell; means 5 for determining whether the working point of a generic bit line has been reached, in order to produce a signal for interrupting the pre-charge step. The means 1, 2, and 5 constitute a structure for reproducing the operating conditions within the memory matrix to determine the duration of the pre-charge step; this duration is thus linked to the real conditions of the memory device and of the power supply. The means 1 for reproducing the propagation delay of a generic word line are described in detail in European patent application no. 96830160.6 (EP 0 798 729 A) in the name of the same Applicant as the present application, which is assumed included herein as reference. Likewise, the means 2 for reproducing the selection/deselection and pre-charge conditions of a generic bit line and the means 5 for determining the reaching of the working point for the consequent generation of a pre-charge interruption signal are described in detail in European patent application no. 96830197.8 (EP 0 801 393 A) in the name of the same Applicant as the present application, which is assumed included herein as reference. The means 3 for detecting when a boost voltage has reached a preset threshold are described in European patent application no. 95830360.4 (EP 0 757 352 A) in the name of the same Applicant as the present application, to which reference is explicitly made. Finally, the means 4 for generating the POR signal are described in European patent application no. 96830046.7 (EP 0 788 114 A) in the name of the same Applicant as the present application, to which reference is explicitly made. The means 1 for reproducing propagation along a generic word line are supplied at the voltage VPCx, which can vary between the supply voltage V DD and the boost voltage. Likewise, the means 2 for reproducing the selection/deselection and pre-charge conditions of a generic bit line and the means 5 for determining the reaching of the working point of a bit line are also supplied at the voltage VPCx. The means 1 for reproducing propagation along a generic word line receive in input a control signal PROP-EN and an address transition detection signal ATD for synchronizing their operation with a read address transition of the memory, and emit in output a signal WL-PROP that is sent in input to the means 5 for determining the reaching of the working point to determine the end of the pre-charge interval. The means 2 for reproducing the selection/deselection and pre-charge conditions of a generic bit line in turn receive in input a control signal PCH-EN in addition to the above-mentioned signal ATD, and emit in output the signal PC-ok, which is sent to the means 5 for determining the reaching of the working point, which in turn emit in output the pre-charge step interruption signal PCH-RES. The means 3 for determining whether a boost voltage has reached a preset level receive in input a control signal BOOST-UP and emit in output a signal BST-DIS and a signal BST-DET, which are sent to logic control means 6 adapted to ensure the reaching of the minimum functionality conditions. The control logic means 6 emit in output the signal BST-ok. The means 4 for generating the POR signal emit in output the POR signal, which is sent in input to the control logic means 6 and to means 7 for controlling and stimulating the reading of the memory device. Said means 7 receive in input a signal CEn-ext for controlling the enabling of the device, in addition to the signal POR, and emit in output the signal CE for enabling the device, the signal ATD, and the inverted device enabling signal CEn. The output signals of the means 5 for determining the reaching of the working point, of the control logic means 6, and of the reading control and stimulation means 7 are sent in input to memory means 8 for the pre-charge step, which are set at the onset of the minimum functionality conditions to remain in the pre-charge step until the propagations of the signals on the lines (word lines and bit lines) have occurred completely. The memory means 8 for the pre-charge step emit the inverted pre-charge signal PCHn. The signals in input to the memory means 8 for the pre-charge step are the signals PCH-RES, CE, CEn, PCH-SET (for setting the pre-charge), and RES-EN (for control). The block diagram of figure 1 is shown in detail in the following figures 2 to 10; each figure shows in detail a single block of figure 1. Figure 2 is a detail view of the means 1 for reproducing propagation along a generic word line (which is the subject of a previous patent application, as mentioned); said means structurally and dimensionally reproduce a generic word line to provide in output a propagation end sensing signal. Said means are driven by a control signal WL-dis and by the signal ATD. In order to take into account the various possible operating voltages in the memory device (V DD or boost voltage), they have an elevator structure 12 that is supplied at the voltage VPCx and a plurality of RC structures that are adapted to reproduce the intrinsic delays of a generic word line managed with hierarchical decoders. Transistors 14 are provided to quickly reset to zero the propagation conditions along the line. The output signal WL-PROP can be taken in three different points of the reproduced word line: at the beginning 15, in an intermediate point 16, and at the end of the line 17. This allows to adapt the circuit for reproducing propagation along a generic word line to different speed requirements of a memory device. The output signal taken at the initial point of the line in fact corresponds to the fastest possible reading mode, whereas the signal taken at the end of the line corresponds to the slowest mode. Figure 3 is a detail view of the means 2 for reproducing the selection/deselection and pre-charge conditions of a generic bit line of a memory device (which, as mentioned earlier, is the subject of a patent application filed in the name of the same Applicant as the present application). The circuit shown in figure 3 comprises: means 9 for simulating the selection of a bit line, which are adapted to simulate the characteristics of the selection/deselection structures, and the loads connected thereto, of a generic bit line of a memory device; means 10 for simulating the pre-charge of a generic bit line; a biasing structure 11; and a dummy bit line 12 that fully reproduces a generic bit line of a memory device. Figure 6 is a detail view of the means 5 for determining the reaching of the working point of the bit and word lines, so as to produce the pre-charge interruption signal PCH-RES, thus determining a minimum pre-charge interval that is linked to the real delays of the propagations. The circuit shown in figure 6 (which is the subject of a patent application, as already mentioned) receives in input the signal PC-ok emitted by the means 2 for reproducing the selection/deselection and pre-charge conditions of a generic word line, together with the signal WL-PROP that is emitted by the means 1 for reproducing propagation along a generic word line, and emits in output the pre-charge interruption signal PCH-RES. As shown, the signal PCH-RES is therefore generated not only as a function of the signal ATD but also as a function of the signals WL-PROP and PC-ok. Furthermore, the power supply of the circuit is VPCx, i.e., the supply voltage varies between V DD and the boost voltage: in this manner, the generation of the signal PCH-RES is also linked to the real power supply conditions that are present in the memory device. Figure 4 is a detailed block diagram of the means 3 for detecting when a boost voltage reaches a threshold voltage and figure 5 is a detailed diagram of the circuit configuration (the circuit for generating the signal POR is described in a patent application mentioned above). Therefore, with reference to figures 4 and 5, said circuit is illustrated and described in summary form. The reference numeral 18 designates a voltage elevator and level adapter stage, which receives in input a control signal DET-UP and the boost voltage BOOST to be monitored. The outputs of the stage 18, DET-en and DET-dis, are sent respectively to a reference generating stage 20 (on the output whereof there is provided a reference voltage) and to a limiter stage 19, which also receives in input said reference voltage. The reference voltage is also sent in input to a threshold reaching sensing stage 21, of the lock-in type, which has a second input that receives the boost voltage BOOST to be monitored and an output that provides a logic signal that is sent in input to a lock-in stage 22, which has an output BOOST-det that can be used by other circuits within the memory device. Finally, each one of the stages 21 and 22 has a respective reset input whereon it receives the signal PREset. In detail, the adapter stage 18 has the purpose of adapting the levels of the logic signals in input to the levels used in the threshold detector device and provides in output the signals DET-en and DET-dis, which can be switched between zero and the boost voltage BOOST (high state). Accordingly, when DET-en and DET-dis are in the high state, they are locked in and follow the dynamics of the boost voltage BOOST. The reference generating stage 20 has the purpose of generating in output the reference voltage, the value whereof is not constant but varies as the boost voltage BOOST varies. In particular, in the presence of the enabling provided by the signal DET-en, the reference voltage is initially locked in to the boost voltage and follows its development exactly. As soon as the boost voltage to be monitored reaches a first threshold, the reference voltage continues to rise, but more slowly than the boost voltage, producing a nonzero voltage difference between the terminals of the sensing stage 21. Finally, after the voltage BOOST to be monitored has reached its desired steady-state value, the reference voltage becomes zero. The limiter stage 19 intervenes only if the boost voltage to be monitored exceeds the maximum expected threshold value. This action has two purposes: on the one hand, to cause the switching of the sensing stage 21 in the described case (i.e., when the boost voltage exceeds the desired steady-state value) when mismatching problems arise between the reference generator 20 and the sensing stage 21 or when, for different reasons, the generator stage 20 behaves abnormally, and on the other hand to protect the generator stage 19 itself from excessively high voltages that might damage it. The sensing stage 21 has the purpose of monitoring the voltage difference on its inputs and of generating in output the signal BOOST-det, the logical value whereof indicates whether the voltage BOOST to be monitored has exceeded the preset threshold or not. The means 4 for generating the signal POR are shown in detail in figure 9 (these means, too, are the subject of a previous patent application). Power-on-reset circuits are widely used in logic devices to ensure correct positioning or initialization of the circuits at power-on. In particular, in non-volatile memories, power-on-reset circuits are used to initialize the registers containing the redundancy addresses. Furthermore, in latest-generation memory devices, the reading of the data stored in a non-volatile manner is internally time-controlled and requires correct start-up at device power-on time; this is indeed ensured by the use of power-on-reset circuits. The power-on-reset circuit 4 behaves directly as a function of the behavior of a generic memory cell that is fully identical to the matrix memory cells. The circuit shown in figure 9 is composed of a bistable circuit that is provided by two inverting stages that are cross-coupled in a so-called back-to-back configuration and is constituted by the transistors 23, 24, 25, and 26. The unbalancing of the circuit is intrinsically ensured at power-on by providing the two inverting stages with devices having different power-on threshold characteristics. Accordingly, at power-on, the bistable circuit sets itself in an unbalanced state (with the nodes C and D high and low, respectively) (generating a POR pulse) that is clearly defined and is maintained until the level of the voltage on the power supply node reaches a level that determines assured conductivity of the non-volatile elements. Furthermore, said non-volatile elements must have a conductivity that is equal to, or greater than, the detectable value. This value is determined by the dimensions given to the load (transistor 23) and assuredly allows correct reading of the data item contained in a non-volatile cell. This is achieved by a circuit which contains a non-volatile memory element that exactly reproduces the behavior of a generic matrix cell and by the dimensions given to the load 23. The tripping of the bistable circuit, which consists in the reversing of the flip-flop, is determined by the non-volatile memory element of the circuit, which is functionally coupled to the intrinsically high node of the bistable circuit at power-on time. The capacitors c p and c g facilitate initial presetting so as to produce the POR pulse. The node C, as a function of the intrinsic electrical characteristics linked to the production process, passes from a condition of substantially no conductivity to a conducting condition when its threshold is exceeded. The gradually increasing conductivity of the non-volatile memory element as the voltage rises on its drain node, which starting from the power-on of the device is coupled to the power supply node, causes the switching of the bistable circuit, causing the end of the POR pulse produced by the circuit. The means 7 for controlling and stimulating the reading of the memory device are shown in detail in figure 7. Said means 7 receive in input a signal EXT-CEN, which is sent to a circuit 27 (not shown in detail) for generating a memory device enabling control signal, which produces the signal CEn-EXT. The output of the circuit 27 is sent in input to a pair of pass transistors 28 that is formed by a P-type transistor and by an N-type transistor. The signal POR, generated by the means 4 for generating the signal POR, is sent to the gate terminal of the P-type transistor, whereas the signal PORn (inverted POR) is sent to the gate terminal of the N-type transistor. The signal PORn is also sent in input to the gate terminal of a P-channel transistor 29, which is interposed between the supply voltage (with its source terminal) and the pair of pass transistors 28 (with its drain terminal). A first inverter 30 is connected to the drain terminal of the transistor 29, and its output is the memory device enable signal CE and, after passing through a second inverter 31, the signal CEn. The signals CE and CEn are sent in input to a circuit 32 for generating the read address transition detection signal ATD. Figure 10 is a detail view of the control logic means 6 adapted to generate in output the signal BST-ok and the signal RES-EN. The signal POR is sent in input to a first inverter 33, the output whereof is sent to a NAND gate 34 having two inputs. The second input of the NAND gate 34 is constituted by the output of an OR gate 35 that receives in input the signals BST-det and BST-dis. The output of the NAND gate 34, after passing through a second inverter 36, is the signal BST-ok. In turn, the signal BST-ok, which is inverted by means of an inverter 37, is sent in input to a NOR gate 38, the second input whereof is a timing disable signal TIM-dis. The output of the NOR gate 38 is the signal RES-EN. Finally, figure 8 illustrates the memory means 8 for the pre-charge step, which comprise a circuit of the flip-flop type constituted by two P-type transistors 39-40 and by two N-type transistors 41-42 that are common-drain connected between the supply voltage and the ground; the gate terminals of the transistors 39-41 and 40-42 are respectively connected to each other. The node B of the flip-flop receives in input, by means of transistors 43 and 49 of the N type, the signals ATD and CE respectively. The output node A of the flip-flop is connected to an N-channel type transistor 44, which receives the signal CEn at its gate terminal, and to a pair of N-channel transistors 45 and 46 that respectively receive in input, at their gate terminals, the signal PCH-RES, emitted by the means 5 for generating the pre-charge interval interruption signal, and the signal RES-EN, emitted by the control logic means 6. The output of the circuit of figure 8, inverted in an inverter 47, is the inverted pre-charge signal PCHn. A capacitor 48 is connected between the output of the inverter 47 and the ground. Figures 11 and 12 will be described with reference to the operation of the circuit according to the invention. With reference to the above figures, operation of the circuit according to the invention is as follows. The means 4 for generating the signal POR produce said signal when the memory device switches on, and said POR signal remains until the supply voltage reaches a value that allows assured reading of the information contained in the non-volatile elements (cells) of the memory device. During this transient period, the elements 28 and 29 ensure the disabling of all reading operations (CE=0). At the end of the POR signal, the means 7 for stimulating and controlling reading produce the read address transition detection signal ATD if, and only if, EXT-CEN=0 (from the very start). Otherwise, this occurs only on the high-low switching front of said input. With reference to figure 7, when the signal POR is high, i.e., PORn is low, the transistor 29 is in fact on and the pair of pass transistors 28 is off; accordingly, the signal that enters the inverter 30 is high and is thus inverted to the low level. In this manner, the signal CE for enabling the memory device is low and the signal ATD is not generated. On the contrary, at the end of the POR signal (low POR), the signal CE is high (CEn is therefore low), and the signal ATD is generated. The signal ATD thus generated is sent to the means 1 for reproducing propagations along a generic word line, which emit a signal WL-PROP that can be taken at one of the points 15, 16, and 17 of figure 2. The signal ATD is also sent in input to the means 2 for reproducing the selection/deselection and pre-charge delays of a generic bit line, which emit in output a pre-charge enable signal PC-ok; said signal is sent in input, together with the signal WL-PROP and the signal ATD, to the means 5 for determining the reaching of the working point of said bit line, to generate the signal PCH-RES for interrupting the pre-charge step. In this manner, the duration of the pre-charge step is linked to the minimum functionality conditions required for the lines, taking into account the delays of the selection/deselection and propagation circuits, as well as the actual power supply conditions that are present in the memory device (V DD or boost voltage), in order to determine the minimum duration of the pre-charge step. In general, the presence/absence of the voltage determines a different pre-charge interval (a shorter one in the first case), because the propagation conditions are different. Said minimum duration in any case allows to perform assured reading of the data of the memory, indeed because the delays that are intrinsic to said memory have been considered and therefore a so-called "dynamic" reading, i.e., a reading that is correlated to the actual operating conditions of the devices, is performed. The signal ATD is also sent in input to the memory means 8 for the bit line pre-charge step. If the boost voltage is present, the means 3 for detecting when the boost voltage reaches a preset threshold intervene so as to link the reading timing to the reaching of the correct level of said boost voltage as well. The means 3 emit in output the signal BST-dis for checking the absence/presence of the boost voltage and the signal BST-det for detecting the steady-state level of the boost voltage. Said signals are sent in input, together with the POR signal, to the control logic means 6, which enable the reading of the memory device. The control logic means 6 described in figure 10 thus receive in input the POR signal. When the POR signal is high, the signal BST-ok in output from the means 6 is always low, regardless of the conditions of the signals BST-dis and BST-det, indicating that it is not yet time to perform reading. When the POR signal becomes low, if BST-dis=1, then BOOST-ok is produced almost immediately, whereas if BST-dis=0, then until BST-det is also 0, RES-EN is also 0. Therefore, one waits until the signal BST-det is high, enabling the generation of the signal RES-EN (high), by means of the NOR gate 37, when the control signal TIM-dis is low (i.e., reading is really intended). Otherwise, if TIM-dis is high, all reading is disabled. The signal RES-EN, when high, resets the memory means 8 for the pre-charge step, ending the pre-charge step. Otherwise, if the signal BST-dis and the signal BST-det are both at zero, then the output of the OR gate 35 is low and the output of the NAND gate 34 (when POR is low) is high, the signal BST-ok is low, and the signal RES-EN is not generated. The memory means 8 for the pre-charge step therefore receive in input the signals PCH-RES, RES-EN, CEn, ATD, and CE, to generate the signal PCHn that is adapted to determine the end of the pre-charge step in correlation with the minimum functionality conditions of the various circuits of the memory, represented by the control signals that said means 8 receive in input. In order to start a first correct reading of the memory device, the signal CE must be high (CEn must be low) and the signal of ATD must accordingly also be high. However, when the POR signal is active, the signal CE is forced to a low value (CEn is forced to a high value); when the POR signal ends (POR is low), and assuming that EXT-CEn is active from the very start (EXT-CEn=0), then a pulse of ATD is produced which: presets the pre-charge step; blocks all word line propagation; prevents any dummy pre-charge selection; presets any start-up condition. At the end of the pulse of ATD, the following operations are started: propagation of the signal WL-PROP; selection of the dummy bit line; the wait for BST-ok to become high by means of the signal BST-det. Only when BST-det=1, PC-ok=1, and WL-PROP=1 the pulse PCH-RES is produced, which ends the pre-charge step. When the signal PCH-RES is high and the signal RES-EN is also high, the flip-flop of the memory means 8 is reset and the output signal PCHn is high to therefore interrupt the pre-charge step and capture the data for reading. The signal RES-EN is generated by the control logic means 6 in relation to the signal BST-ok, when this last signal is high. Of course, if the boost voltage is not present, the signal RES-EN is also not present and therefore the flip-flop of the memory means 8 is reset to produce the high signal PCHn only when the signal PCH-RES is high. In this case, the only control for interrupting the pre-charge step is caused by the signal PCH-RES, which is generated simultaneously with the presence of the high signals WL-PROP and PC-ok, i.e., when propagation on the word line and selection/deselection of the bit line involved in the reading operation have occurred. In summary, therefore, when all the conditions dictated by the above-mentioned control signals have occurred, i.e., when the various devices have reached their minimum functionality level (first of all, the memory cell has reached its minimum conductivity value), then the memory means 8 for the pre-charge step determine that the pre-charge step can be interrupted to perform reading. It is evident that the determination of the pre-charge step interruption time and its actual execution are not performed with a preset delay, which might entail risks for reading, since it is not closely correlated to the conditions of the devices, but by defining a structure for controlling the activities and conditions of the various circuits of the memory, so as to link the read timing to the reaching of minimum operating conditions of the various devices, said minimum conditions in any case allowing to safely and assuredly read the data that are present in the memory. Figures 11 and 12 are useful in showing the developments of the above-described signals. In particular, the figures show that as the supply voltage V DD rises, if no boost voltage is present (figure 11), the developments of the signals are determined by the solid lines, whereas if the boost voltage is present (figure 12), the developments of the signals are shifted (i.e., the dashed lines now plot the development when the boost voltage is not present), since the read timing waits for the trip point, designated by TRIP-POINT, to indicate the reaching of a preset minimum threshold on the part of the boost voltage. These two charts therefore show the flexibility of the reading timing as a function of the level of the supply voltage. The circuit according to the invention is therefore extremely flexible, since it can be used with the same structure even if the technology used in the production of the memory devices changes, as well as if the supply voltage and the temperature at which the memory device operates change. The possibility, provided by the circuit according to the invention, of not having to set an arbitrary delay to be observed before performing reading ensures a read timing that is assuredly faster, in most cases, than the timing that can be provided otherwise, and at the same time allows a correctness of the read data that cannot be obtained with sampled-type readings having a predefined delay. The set delay may in fact even be valid in most operating conditions, but it is evident that this does not in any way ensure correct reading, unless one initially defines a delay that is so long as to not be subjected to errors. It is evident that this clashes with the need to provide reading in the fastest possible manner as well as correctly. The circuit according to the invention therefore provides, in practice, a sort of "dynamic delay" that flexibly adapts to the operating conditions of the memory device and to its structure. In practice, it has been observed that the circuit according to the invention fully achieves the intended aim, since it allows to link the reading timing to the reaching of the minimum functionality conditions of a generic memory cell of the memory device, so as to allow a first reading of the memory device, after activation of the power supply, only in assured data reading conditions, eliminating the drawbacks due to the transient step of the supply voltage. Indeed, even if the supply voltage has not yet reached its steady-state level, the data of the memory device are always read in assured reading conditions. The circuit according to the invention allows to link the first reading operation to the power supply conditions that are actually present in the memory device (voltage V DD or boost voltage) and to the selection/deselection, propagation, and pre-charge delays of a generic bit line and of the word line of the memory device. The circuit according to the invention therefore allows to overcome all the problems linked to the power-on of a device the operation whereof is controlled by a memory, since the first reading of the data is always performed correctly because it is correlated with the actual operating conditions of the memory device. 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, the means 5 for determining the reaching of the working point on the part of the bit line to generate the signal PCH-RES for interrupting the pre-charge step may not provide for lock-in with the propagation of the signal along the generic bit line but only along the generic word line. Therefore, in this case the signal PC-ok is not generated and the portion of circuit to which said signal is sent in input is not provided. The previously described embodiment of the invention provides for the correlation between the beginning of a reading cycle for a non-volatile memory and the reaching of the minimum functionality characteristics of the functionally most critical memory element, i.e., the non- volatile cell. As technology evolves, the functionally most critical element might no longer be the non-volatile cell; in this case, the correlation of the timing must be performed with said new most critical element and all the above remarks still hold. Finally, all the details may 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 memory system comprising a memory device, in particular a nonvolatile memory device, and a circuit for read-enabling the memory device with checking of first and second minimum functionality conditions of memory cells and reading circuits contained in said memory device, said circuit comprising: a structure (1, 2, 5) for reproducing operating conditions within the memory matrix to determine the minimum duration of a step for pre-charging bit lines involved in the reading operation, said structure (1, 2, 5) being adapted to generate a pre-charge step interruption signal (PCH-RES) that depends on the reaching of the first minimum functionality conditions defined by reproduction of a propagation delay along a word line, a bit line selection/deselection delay and a bit line precharge time period; means (4) for generating a power-on-reset signal (POR) for enabling reading when the second minimum functionality conditions for reading correctness are reached, i.e. when the power supply exceeds the threshold voltage of a dummy memory cell which thus begins to conduct, said power-on-reset signal generating means being adapted to drive control logic means (6) as well as reading control and stimulation means (7); the circuit further comprising memory means (8) for the pre-charge step, said memory means (8) being driven by said structure (1, 2, 5) for reproducing the operating conditions within the memory matrix, by said control logic means (6), and by said reading control and stimulation means; (7); said memory means (8) for the pre-charge step causing, at the onset of the first and second minimum functionality conditions, the end of the pre-charge step to perform a first reading of the memory cells in assured conditions. The system according to claim 1, characterized in that it comprises means (3) for determining the reaching of a preset threshold level on the part of a boost voltage that is present in the memory device, said means for determining the reaching of the threshold level being connected to said control logic means (6) to have said control logic means generate a read enable signal (BST-ok) as a consequence of the reaching of the minimum functionality conditions of the memory cell involved in the reading operation. The system according to claim 1, characterized in that said structure for reproducing the operating conditions inside the memory matrix comprises: means (1) for reproducing the propagations along a generic word line; means (2) for reproducing the selection/deselection and pre- charge delays of a generic bit line; and means (5) for determining the reaching of the working point of the bit line involved in the reading operation; said means for reproducing the propagations along a generic word line and said means for reproducing the selection/deselection and pre-charge delays of a generic bit line driving said means for determining the reaching of the working point, in order to generate a pre-charge step interruption signal that is sent to said memory means for the pre-charge step. The system according to claim 1, characterized in that said reading control and stimulation means (7) generate a read address transition detection signal (ATD), said signal being sent in input to said structure for reproducing the operating conditions inside the memory matrix, to said control logic means, and to said memory means for the pre-charge step, said read address transition signal being generated when said power-on-reset signal (POR) is reset to zero. The system according to claim 4, characterized in that said means (4) for generating the power-on-reset signal (POR) comprise at least one non-volatile element that is identical, in terms of characteristics and dimensions, to a generic matrix cell, in order to allow recognition of a minimum activation threshold of said non-volatile element and of its conductivity in relation to the tripping condition of said POR generation means, said power-on-reset signal (POR) being reset to zero when said minimum functionality conditions of the memory device are reached, in order to generate said read address transition signal (ATD). The system according to claim 3, characterized in that said means (4) for reproducing propagations along a generic word line and said means (5) for determining the reaching of the working point on the part of a bit line that is involved in the reading operation are supplied at the actual supply voltage of said memory device, said voltage varying from a low-level voltage to a boost voltage. A method for performing a first reading in assured conditions for a memory device, particularly of the non-volatile type, characterized in that it comprises the steps of: generating a power-on-reset signal (POR) when the supply voltage exceeds the threshold voltage of a dummy memory cell which thus begins to conduct, the memory cell being identical to a generic memory cell of said memory device, said signal indicating the onset of the minimum functionality conditions of said memory cell and deactivating all reading activity throughout its duration; generating, at the end of said power-on-reset signal, a read address transition signal to stimulate the reading of said memory device; reproducing the propagation delays of the selection/deselection and pre-charge signals for a generic word line and for a generic bit line of the memory device, and generating a signal for determining the reaching of a working point of the bit line involved in the reading operation; initializing the memory matrix line pre-charge activities; and generating, at the onset of the minimum functionality conditions, a pre-charge step end signal, for the subsequent first reading, in assured conditions, of said memory device. The method according to claim 7, characterized in that it comprises the additional step of generating a signal for indicating the exceeding of a preset minimum threshold on the part of a boost voltage, said signal determining the onset of the minimum power supply conditions provided for the memory device. The method according to claim 8, characterized in that said signal for indicating the exceeding of a preset minimum threshold on the part of a boost voltage (BOOST) allows to generate a signal for enabling the reading timing of said memory device. The method according to claim 7, characterized in that said step of generating the signal that determines the reaching of the working point on the part of said bit line is provided in correlation with the actual power supply conditions of the memory device, said actual power supply conditions varying from a low supply voltage to a boost voltage (BOOST).

Inventors: PASCUCCI LUIGI, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C7
IPC codes: G11C7