Redundancy management method and architecture, particularly for non-volatile memories

Abstract: A redundancy management method, particularly for non-volatile memories, having the particularity of comprising the steps of: -- enabling, as a consequence of the presence of a pulsed read address transition signal of a memory line, and throughout the duration of the pulsed signal, the memory matrix line reading path, and blocking the selection of redundancy lines of the memory; and -- at the end of the pulsed signal, as a consequence of the absence/presence of a redundancy line read signal, confirming/disabling the selection of the lines of the memory matrix and blocking/releasing the selection of the redundancy lines. A redundancy management architecture for a memory matrix, adapted to perform the above method, is also described.

Description: The present invention relates to a redundancy management method and architecture, particularly for non-volatile memories. Non-volatile memories are usually provided with redundancy structures to allow replacement of word lines or bit lines of the memory matrix, which can be defective and therefore not usable both during reading and during programming. It is known that in a memory constituted by a very large number of lines, some of said lines may be defective due to the presence of corrupted bits. Therefore, in order to ensure maximum reliability of the memory device, a certain number of redundancy lines is provided, these lines being programmed, during testing of the memory device, with the data that would be meant for the lines that are found to be defective. The memory matrix usually has an address bus that carries the address of the line to be read or programmed; said address bus is fed to a decoder that selects, by multiplexing, the selected line, the "address" whereof is indicated by the address bus. For the redundancy lines, there are redundancy propagation means (PRO RED) allowing access to said lines. The address contained in the address bus is then compared with the address that is present in a set of registers that precede the redundancy decoder: if the address of the line selected by the address bus matches one of the addresses stored in the set of registers, this means that the desired line is a line subjected to redundancy and therefore the data propagation path must not pass through the memory matrix, but the data must be taken from the corresponded redundancy lines. The registers that contain the addresses of the lines subjected to redundancy for comparison with the address sent over the address bus are programmable structures that are written when the memory device is tested, when the defective lines of the memory matrix are detected. With reference to figure 1, which shows a block diagram of a known redundancy management architecture, the reference numeral 1 designates an address bus that carries the addresses of the memory lines that must be selected for reading or programming. Said address bus 1 is fed to a register structure 2 on one side and to a pre-decoder 3 on the other side. As mentioned earlier, the register structure stores the address of the lines subjected to redundancy for comparison with the addresses carried by the address bus 1. The number of registers is equal to the number of redundancy lines. A redundancy bus 4 then branches out from the register structure 2 and constitutes an input respectively for redundancy sensing logic means 5 and for redundancy propagation means 6, the output whereof is a redundancy line selection bus 7. The output of the redundancy sensing logic means 5 is a signal MSL for enabling/disabling line selection of the memory matrix, which is fed to the pre-decoder 3. The output of the pre-decoder 3 is a decoder bus 8 that is fed to a main decoder 9, the output whereof is in turn a memory matrix line selection bus 10. Figure 2 is a more detailed block diagram of the known architecture shown in figure 1. This second figure shows separate structures for the management of the left and right memory half-matrices (not shown); the elements that are shown are therefore identical for the left portion and for the right portion, and the same reference numerals are assigned thereto for the sake of simplicity, said reference numerals also corresponding to the corresponding elements of figure 1. The redundancy management method inserted in a normal memory reading operation can be identified from figures 1 and 2. The address carried by the address bus 1 is compared with the redundancy addresses stored in the registers 2, and as a consequence of this comparison the redundancy bus 4 carries an address that is subjected to redundancy if a match has been found between the address carried by the address bus 1 and the address subjected to redundancy stored in the registers 2. Said redundancy bus 4, as shown, activates the redundancy propagation means 6 to select the desired redundancy line. At the same time, a signal for indicating the match or mismatch of the address stored in the registers 2 with the address carried by the address bus 1 is passed through logic gates, generally designated by the reference numeral 5 in figure 1 and shown in greater detail in figure 2 with the reference numerals 11, 12, 13, 14, 15, and 16. In order to allow the address subjected to redundancy to be recognized as actually matching the address carried by the address bus 1, the output signals from said logic gates must set to a low state the memory matrix line selection enabling/disabling signal MSL. A high signal MSL (=1) enables this selection, while a low signal MSL (=0) disables the selection, at the same time enabling the selection of the lines subjected to redundancy by means of the redundancy decoder 6. It is thus evident that a normal cycle for reading a memory provided with such a redundancy management architecture can delay data propagation, since the redundancy structures, if selected, block the normal selection of the memory matrix lines. Delays in data propagation, even if the line is actually a line subjected to redundancy, are given by the logic gates 11, 12, 13, 14, 15, and 16. Substantially two cases can occur. In a first case, a redundancy line is selected as a consequence of the sensing of an actual redundancy address (with the consequence that the signal MSL becomes low), whereas in the second case there is a return from a redundancy situation, with the consequent need to normally access the memory matrix, thus bringing the signal MSL to a high state to release the reading of the normal lines of the memory matrix. Among the two above-mentioned cases, the one that is most heavily penalized in terms of time is the second one, i.e., the selection of a normal memory line on return from a redundancy event. In this case, the previous selection of a line subjected to redundancy, by means of the signal MSL=0, in fact blocks any selection of a normal line of the memory matrix. To allow to perform a normal selection, it is therefore necessary to deselect the redundancy line (bringing the signal MSL to 1) and then activate the normal line of the memory device. In general, the drivers of the redundancy lines have the same size as the drivers of the normal lines of the memory and also have the same data propagation characteristics. This circuital solution entails that the delays introduced by the various selection/deselection logic gates are maintained and added and accordingly the respective data propagations shift in time. In particular, between two selections of a normal line of the matrix, the first whereof occurs upon return from a redundancy event, the one that is most heavily penalized in terms of time is the second selection, since it maintains an additional time that is due to the need to deselect the previously activated redundancy path (thus setting the signal MSL to 1). From what has been mentioned above, it is thus evident that the conventional redundancy management architecture introduces unwanted delays in memory matrix line selection. A principal aim of the present invention is therefore to provide a redundancy management architecture that reduces the data propagation times of the memory. Within the scope of this aim, an object of the present invention is to provide a redundancy management architecture that reduces the data propagation times of the memory, regardless of the previous situation of the memory, i.e., regardless of the presence or absence of a redundancy event prior to a subsequent normal memory matrix line selection event. Another object of the present invention is to provide a redundancy management architecture that allows to select a redundancy line with propagation times that are equal to, or shorter than, those defined in case of selection of a normal memory line. Another object of the present invention is to provide a redundancy management architecture that allows selection of a line independently of the events occurring at the time. Another object of the present invention is to provide a redundancy management method with shorter data propagation times than the conventional management architecture. Another object of the present invention is to provide an architecture and a method that are highly reliable, relatively easy to execute, and at competitive costs. This aim, these objects, and others that will become apparent hereinafter are achieved by a memory redundancy management method, particularly for non-volatile memories, characterized in that it comprises the steps of: enabling, as a consequence of the presence of a pulsed read address transition signal of a memory line, and throughout the duration of said pulsed read address transition signal, the memory matrix line reading path, and blocking the terminal selection of redundancy lines of the memory; and at the end of the pulsed read address transition signal, as a consequence of the absence of a redundancy line read signal, confirming the selection of the lines of the memory matrix and blocking the selection of the redundancy lines; and at the end of the pulsed read adress transition signal, as a consequence of the presence of a redundancy line read signal, disabling the selection of the lines of the memory matrix and releasing the selection of the redundancy lines. This aim and these objects are furthermore achieved by a memory redundancy management architecture, particularly for non-volatile memories, comprising: a memory matrix line decoder; redundancy propagation means; an address bus that is connected to said memory matrix Line decoder for the selection of a line of a memory matrix and to a register structure that is adapted to store redundancy line addresses, for the comparison of a current address of said address bus with said stored redundancy addresses; and logic redundancy detection means; characterized in that it furthermore comprises: first logic means that receive, in input, a redundancy detection signal produced by said redundancy detection means and a pulsed read address transition signal; and second logic means that receive, in input, said pulsed read address transition signal and a redundancy signal, and are adapted to block the selection of the redundancy lines, throughout the duration of said pulsed read address transition signal; said first logic means simultaneously enabling the selection of said redundancy lines. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the architecture according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a block diagram of a conventional redundancy management architecture; figure 2 is a more detailed block diagram of the architecture shown in figure 1; figure 3 is a chart of the data propagation timings of the memory in the case of the conventional architecture shown in figures 1 and 2; figure 4 is a block diagram of a redundancy management architecture according to the invention; figure 5 is a chart of the data propagation timings of the memory in the case of the architecture according to the invention, shown in figure 4; and figures 6a and 6b respectively show a first embodiment of the driving of the matrix lines, of a known type, and of the redundancy lines, according to the present invention; and figures 7a and 7b respectively show a second embodiment of the driving of the matrix lines, of a known type, and of the redundancy lines according to the present invention. In the following figures, the same reference numerals as in the above described figures 1 and 2 will be used for identical elements. With reference to figure 4, the architecture according to the invention comprises an address bus 1 that carries information (the addresses) to a register structure 2, which stores the addresses of the redundancy lines, and to a pre-decoder 3. A decoder bus 8 branches out of the pre-decoder 3 and leads to a main decoder 9. The output of the main decoder is a bus 10 for selecting the lines of the memory matrix. The output of the register structure 2 is a redundancy bus 4 that carries redundancy information (i.e., signals indicating the match or mismatch of the addresses of the address bus 1 with respect to the addresses stored in the register structure 2) to logic redundancy detection means 5 and to redundancy propagation means 6 adapted to perform the propagation of the redundancy lines by means of the output bus 7. The output of the logic means 5 is sent to additional logic means 17 that receive in input the signal ATD, i.e., the address transition detection signal, which is therefore the read request signal. Said logic means 17 are advantageously constituted by an OR gate. The output of the logic means 17 constitutes the signal MSL for enabling/disabling the reading of a normal line of the memory matrix; said signal is fed to the main decoder 9 to enable/disable access to the memory matrix lines. The redundancy bus 4 is furthermore sent as input to the redundancy propagation means 6 after passing through logic means 18 that also receive in input the inverted signal ATD. The signal ATD is inverted by means of an inverter 19. Figure 3 is a chart of the timings of the conventional redundancy management architecture: this chart will be explained in detail hereinafter by comparison with the timings chart of the architecture according to the invention, shown in figure 5. Figure 6a is a view of a first conventional embodiment of the matrix line driver, whereas figure 6b is a view of a first embodiment of the redundancy line driver according to the invention. Likewise, figure 7a is a view of a second embodiment of the conventional matrix line driver, whereas figure 7b is a view of a second embodiment of the redundancy line driver according to the invention. In the first embodiment, matrix line contact formation occurs in the center of said line, whereas in the second embodiment it occurs at one of its ends. In the solutions according to the invention, the drivers 21 of the redundancy lines 24 i are larger, in terms of size, than the corresponding drivers 20 of the normal lines 25 i of the memory matrix in order to make the rising transitions faster. Furthermore, each redundancy propagation line is provided so as to have a delay that is due to the presence of pairs of resistive and capacitive means, respectively 22 i and 23 i . Therefore, the propagation delay of the generic redundancy line 24 i would be given by the sum of the delays caused by the individual pairs 22 i and 23 i , as occurs for the generic normal line 25 i of the matrix. Therefore, in order to reduce the total time constant RC (obtained by multiplying the value of the resistance by the value of the capacitance), one thus resorts to shorting the redundancy line so that the total delay is halved. This is possible because the number of redundancy lines is considerably reduced with respect to the number of memory matrix lines and therefore the space occupied by the shorting of the lines is easily available within the configuration of a memory device. This solution is applied, as shown in figures 6b and 7b, both in the case of the formation of central contacts for the redundancy line, and in the case of lateral contact formation. In the first case, the shorting is provided by connecting the central point of the line to both ends, whereas in the second case it is provided by connecting the ends together. With reference to figures 4, 5, 6b, 7b, and 3, the operation of the architecture according to the invention is as follows. At each pulse of ATD, i.e., at each address transition and therefore at each read request, the main decoder 9 is free to start the native memory matrix line selection transient without initially taking into account the presence or absence of redundancy. This is achieved by means of the signal ATD, which varies in pulses. Therefore, when ATD is high, the redundancy bus 4 is blocked. In fact, when ATD is high, the signal that enters the AND logic gate 18 is the inverted ATD signal, and therefore is the low ATD signal (=0). Therefore, in any case the output signal from the AND logic gate 18 will be low, regardless of the condition of the redundancy bus 4, i.e., both when a high signal indicating the presence of a line subjected to redundancy is present on said bus, and when a low signal indicating no redundancy is present on said bus. At the same time, the high ATD signal is provided to the OR logic gate 17, freeing the path towards the main decoder 9, i.e., making the signal MSL go high. In fact, regardless of the condition of the output signal from the logic means 5 (therefore both in the presence and in the absence of redundancy), the signal MSL is always high when ATD is high. Then, at the end of the ATD pulse (ATD=O), in case of redundancy the redundancy bus 4 carries a high signal indicating the presence of redundancy, and the logic means 18 emit a signal for enabling access to the redundancy decoder so that the redundancy path is activated for selecting the desired redundancy line. In case of no redundancy, instead, the redundancy bus 4 carries a low signal, so that the signal leaving the logic means 18 is a signal for blocking the redundancy path, whereas the signal MSL leaving the logic means 17 is high, so as to allow access to the main decoder 9. It is thus evident that the redundancy management protocol is configured so as to in any case enable access to the main decoder 9 regardless of whether there is or not a redundancy situation (an address of a line subjected to redundancy on the address bus 1). Therefore, the idle time due to the need to deselect a redundancy line on returning from a redundancy event is eliminated, since the path for the selection of a normal line of the memory matrix is always activated first. The path for selecting the redundancy line is also activated in parallel, but it is kept blocked throughout the duration of the pulse of ATD. When the signal ATD returns low, if the redundancy bus 4 carries the high signal, which indicates a match between the address of the line subjected to redundancy and the address carried by the address bus 1, the redundancy path is enabled with the selection of the redundancy line by means of the redundancy bus 7. In any case, therefore, the signal MSL attempts to enable reading of a normal line of the memory matrix, but promptly blocks this reading when the signal ATD goes from high to low and simultaneously the redundancy bus carries a high signal indicating redundancy. With reference to figures 6b and 7b, said figures show the i-th redundancy propagation means 6 that drive the i-th redundancy line, the propagation time whereof is reduced, with respect to a normal i-th line (shown in figures 6a and 7a) of the matrix, by shorting together the ends of the line and the intermediate point in the case of figure 6b and by shorting together only the ends in the case of figure 7b. In this manner, in both cases, the resistances of the i-th redundancy line are in parallel to each other as a consequence of the shorting. The driver 21 of the i-th redundancy line is larger than the corresponding driver 20 of the i-th matrix line, and this entails a higher driving speed of the redundancy line with respect to the i-th normal line of the memory matrix. Figure 3 is a chart of the timings of the read and redundancy signals in the case of a conventional architecture, whereas figure 5 is the same chart in the case of the architecture according to the invention. In figure 3, when the redundancy line RED goes from high to low as a consequence of its selection, and therefore simultaneously the signal MSL goes low, blocking the selection path of a native memory line, the word line referenced by WL, which should be read in a normal case as a consequence of the pulse of ATD, is stopped and therefore attempts to rise and then falls when the redundancy line RED is high. The subsequent read cycle, timed by the pulse of ATD, should imply a normal reading of a normal line WL of the memory, and this is indicated by the dashed part of WL itself. Actually, however, it is necessary to wait for a time that is determined by the time required by the redundancy line to be deselected and by the signal MSL to return high and thus enable the reading of a line WL. This delay is indicated by "a" in the figure. Furthermore, the rising and falling transitions of a redundancy line, given respectively by the intervals t 1 -t 2 and t 3 -t 4 , are slow, since the propagation path of a redundancy line is greater than the path of a normal line WL, because the activation path is longer. As regards instead the architecture according to the invention, with reference to the above-described situations and with reference now to figure 5, the following remarks should be made. When the signal ATD goes from low to high and the signal MSL goes from high to low and therefore the redundancy path is enabled when the redundancy line is high, there is a rising transition of said line that is faster than a similar transition performed in the case of figure 3. In fact, as can be deduced from the charts of figures 3 and 5, the transition t 1 '-t 2 ' of the redundancy line RED (figure 5) has a steeper slope than the similar transition t 1 -t 2 shown in figure 3. This, as shown, is due to the presence of a larger driver for the redundancy line and to the shorting of the resistors of said line. At this point, the redundancy line is high and the signal MSL is correspondingly low. Upon the subsequent transition of the signal ATD, and throughout its duration, the signal MSL is set to the high level, thus enabling the selection of a normal line of the memory matrix. Throughout the duration of ATD, therefore, the corresponding redundancy line is low, since the redundancy path is not activated. Therefore, when MSL is high there is an attempt to read a normal line WL of the memory that goes from low to high. At the end of the ATD, if the redundancy line is actually selected (high), the line WL returns low, and is thus "suppressed", while the line RED subjected to redundancy takes over. At the end of the reading of the line subjected to redundancy, and therefore at a subsequent pulse of ATD, a condition of return from a redundancy situation occurs. The falling times of the redundancy line RED are designated by t 1 '-t 2 ' and are shorter than the corresponding times t 1 -t 2 of the known case due to the same reason mentioned above. Assuming that the subsequent reading operation is a normal reading of a line WL, said reading begins at the end of the fall of the redundancy line RED, i.e., at t 4 '. In this case, the above-mentioned delay "a" is avoided, because it is not necessary to wait for the deselection of the redundancy line RED, since the signal for enabling the normal reading MSL is already high at this time, thus freeing the path to the main decoder 9. The delay introduced by halting the redundancy line RED throughout ATD is widely compensated by the higher propagation rate of the redundancy lines according to the present invention and by the lack of delay with which normal reading is reselected when returning from a redundancy situation. Therefore, each selection of the redundancy lines, although it begins only at the end of the pulse of ATD, occurs with propagation times that are shorter than (or at the most equal to) those of a generic normal line of the memory matrix. The non-final preselection of a normal line in the initial step of the addressing entails no penalization for the system, since it is promptly blocked before it can reach levels that are significant for real reading and in any case long before full propagation. With the architecture according to the invention, a redundancy management protocol that does not slow down reading of the memory device has thus been provided. This protocol can be identified by the following steps: as a consequence of a pulse of the signal ATD, placing the signal MSL in the high state, independently of a previous event of the memory, to enable the reading path of a normal line of the memory matrix; keeping said signal MSL high throughout the duration of the pulse of ATD, thus blocking at the same time the selection of a line subjected to redundancy; at the end of the pulse of ATD, if redundancy is detected, enabling the redundancy path, blocking the native selection path and halting the initiated selection of a normal line; if no redundancy is detected, continuing the native selection that has already started. In practice it has been observed that the architecture and the method according to the invention fully achieve the intended aim, since they allow to reduce the propagation times for the selection of the lines of the memory matrix, making said selection times independent of the previous events. Therefore, in case of return from a redundancy situation, it is no longer necessary to wait for the deselection of the redundancy line before being able to select a normal memory line. Furthermore, each selection of the redundancy line, despite being performed at the end of each pulse of ATD, occurs with propagation times that are shorter than, or equal to, those for the immediate selection of a generic normal line of the memory matrix. To conclude, an architecture for the selection of redundancy and normal lines has been provided which is independent of the events occurring at the time and has, at the same time, a minimal effect with respect to the propagation times. The architecture according to the present invention can also be used in case of packet redundancy management, i.e., for column redundancy performed by replacing an entire set of columns of the memory matrix with a set of redundancy columns. The architecture thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, the pre-decoder 3 can be omitted and in this case the address bus 1 is directly connected to the main decoder 9. 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: Memory redundancy management method, particularly for non-volatile memories, characterized in that it comprises the steps of: enabling, as a consequence of the presence of a pulsed read address transition signal of a memory line, and throughout the duration of said pulsed read address transition signal, the memory matrix line reading path, and blocking the terminal selection of redundancy lines of the memory; and at the end of the pulsed read address transition signal, as a consequence of the absence of a redundancy line read signal, confirming the selection of the lines of the memory matrix and blocking the selection of the redundancy lines; and at the end of the pulsed read address transition signal, as a consequence of the presence of a redundancy line read signal, disabling the selection of the lines of the memory matrix and releasing the selection of the redundancy lines. Method according to claim 1, characterized in that said step for the selection of said lines of the memory matrix is independent of the type of previous reading performed on the memory matrix. Method according to claim 1, characterized in that the selection/deselection of said redundancy lines occurs with transitions that are faster than the selection/deselection of said lines of the memory matrix. Memory redundancy management architecture, particularly for non-volatile memories, comprising: a memory matrix line decoder (9); redundancy propagation means (6); an address bus (1) that is connected to said memory matrix line decoder (9) for the selection of a line of a memory matrix and to a register structure (2) that is adapted to store redundancy line addresses, for the comparison of a current address of said address bus (1) with said stored redundancy addresses; and logic redundancy detection means (5); characterized in that it furthermore comprises: first logic means (17) that receive, in input, a redundancy detection signal produced by said redundancy detection means (5) and a pulsed read address transition signal; and second logic means (18, 19) that receive, in input, said pulsed read address transition signal and a redundancy signal, and are adapted to block the selection of the redundancy lines, throughout the duration of said pulsed read address transition signal; said first logic means (17) simultaneously enabling the selection of said redundancy lines. Architecture according to claim 4, characterized in that said redundancy propagation means (5) comprise means for driving the propagation of said redundancy lines that are larger than the corresponding means for driving said memory matrix lines and are present in said memory matrix line decoding means (9). Architecture according to claim 4, characterized in that the intermediate point of each redundancy line is connected to each one of the ends of said redundancy line so as to short the resistive elements that are present in said redundancy line. Architecture according to claim 4, characterized in that the ends of each one of said redundancy lines are connected with one another so as to short the resistive elements that are present in said redundancy line. Architecture according to claim 4, characterized in that said first logic means (17) comprise an OR gate (17). Architecture according to claim 4, characterized in that said second logic means (18, 19) comprises an AND gate (18), the pulsed read address transition signal in input to said AND gate (18) being inverted beforehand by means of an inverter (19). Architecture according to claim 4, characterized in that it comprises pre-decoding means (3) that receive in input said address bus (1) and communicate with said memory matrix line decoder (9).

Inventors: PASCUCCI LUIGI, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G06F11
IPC codes: G11C29