Patent Publication Number: US-11645004-B2

Title: Real-time update method for a differential memory, differential memory and electronic system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/087,070 (now U.S. Pat. No. 11,256,442), filed on Nov. 2, 2020, which is a continuation-in-part of U.S. patent application Ser. No. 16/225,557 (now U.S. Pat. No. 10,824,417), filed on Dec. 19, 2018, which claims the benefit of Italian Application No. 102018000000580, filed on Jan. 5, 2018, which applications are hereby incorporated herein by reference. 
     U.S. patent application Ser. No. 16/225,557 (now U.S. Pat. No. 10,824,417) was filed concurrently with U.S. patent application Ser. No. 16/225,492 (now U.S. Pat. No. 10,811,093), which claims the benefit of Italian Application No. 102018000000581, filed on Jan. 5, 2018, which applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to an electronic system and method, and, in particular embodiments, to a real-time update method for a differential memory, differential memory and electronic system. 
     BACKGROUND 
     As is known, electronic control units of vehicles include a memory designed to store a vehicle-control software. 
     Further known is the need to test a number of versions of the software and thus alternate execution of different versions until a final version is selected. 
     It is common practice to use a plurality of memories, each memory being designed to store each version of the software to be tested. Alternatively, it is possible to use a memory of a size such as to store each version of the software to be tested. This entails an increase in the costs. 
     The above problem is particularly felt in the case of use of memories of a differential type, where two memory cells of opposite state are associated to each information bit. Reading architectures of a differential type afford advantages in terms of reliability, in so far as the datum is stored in a redundant way; reading is carried out by comparing the data contained in the two memory cells of opposite state. 
     A class of memory of a differential type is the phase-change memory (PCM), where, in particular, reading is made by comparing respective currents that flow in the cells associated to a same information bit. 
     As is known, nonvolatile phase-change memories (PCMs) represent a generation of integrated memories where storage of information is obtained by exploiting phase-change materials, which have the property of being able to switch between phases that have electrical resistivities of considerably different value. In particular, these materials may switch between an amorphous phase, with high resistivity, and a crystalline or polycrystalline phase, with low resistivity. Consequently, in a phase-change memory cell it is possible to associate a different value of a datum stored in the cell to a respective phase of a memory element of the cell. 
     For instance, it is possible to use elements of group VI of the periodic table, such as tellurium (Te), selenium (Se), or antimony (Sb), which are referred to as “chalcogenides” or “chalcogenic materials” to form phase-change memory elements. In particular, an alloy made up of germanium (Ge), antimony (Sb), and tellurium (Te), known as GST (having chemical composition Ge2Sb2Te5) currently finds wide use in such memory elements. 
     Phase switching in a memory element may be obtained by locally increasing the temperature of the region of chalcogenic material, for example by causing passage of a programming electric current through resistive electrodes (generally known as “heaters”) arranged in contact with the region of chalcogenic material. The electric current, by the Joule effect, generates the temperature profile necessary for phase change. In particular, when the chalcogenic material is in the amorphous state, with high resistivity (the so-called RESET state), it is necessary to apply a first current pulse (the so-called SET pulse) of a duration and amplitude such as to enable the chalcogenic material to cool slowly. Subjected to this treatment, the chalcogenic material changes state and switches from the high-resistivity state to a low-resistivity crystalline state (the so-called SET state). Conversely, when the chalcogenic material is in the SET state, it is necessary to apply a second current pulse (the so-called RESET pulse) of large amplitude and short duration so as to cause the chalcogenic material to return into the high-resistivity amorphous state. 
     In PCM circuits of a known type, reading of the datum stored in a memory cell may be carried out by applying to the memory element of chalcogenic material a voltage sufficiently low as not to cause a sensible heating thereof, and then by reading the value of the current flowing in the memory cell. Given that the current is proportional to the conductivity of the chalcogenic material, it is possible to determine in which phase the material is, and thus arrive at the datum stored in the memory cell. 
     In a known way, PCMs of a differential type are typically designed according to a scheme with an array of memory cells arranged in rows (word lines) and columns (bit lines). Each memory cell is provided by a phase-change storage element and by a selector transistor, connected in series. A column decoder and a row decoder enable, on the basis of address logic signals received at input and more or less complex decoding schemes, selection of the memory cells, and in particular of the corresponding word lines and bit lines, each time addressed. 
     The column decoder comprises a plurality of analog selection switches (implemented with transistors), which receive on their respective control terminals the address signals. The selection switches are arranged according to a tree structure in hierarchical levels, and their number in each hierarchical level is linked to the arrangement and size of the memory array. The selection switches, once enabled, make it possible to bring the bit line selected to a definite value of voltage and/or current, according to the operations that are to be implemented. In particular, a current path is created between a programming stage or a reading stage and the bit line selected. The current path is defined by the series of a certain number of selection switches. 
     In a known way, sense amplifiers carry out reading of the data stored in the memory cells, comparing the current that flows in the memory cell selected (or an electrical quantity correlated thereto) with a reference current supplied by a reference-current generator (single-ended reading) and/or by one or more reference cells, used for the entire memory array. Single-ended reading is typically used during verification that programming of the cell has occurred or during testing. During normal use of the memory, reading of a double-ended (differential) type is instead adopted, where the sense amplifiers compare the current flowing in the memory cell selected with the current flowing in a respective cell associated to the memory cell selected in the differential architecture. 
     To carry out single-ended reading, one input of the sense amplifier receives the current of the memory cell that is to be read, whereas the other input of the sense amplifier receives the reference current supplied by the reference-current generator. 
     Reading architectures for PCMs of a differential type afford advantages in terms of reliability in so far as the datum is stored in a redundant way and further they do not require generation of a reference current, in so far as reading is carried out by simply comparing the respective currents that flow in the cells associated to a same bit. 
     In single-ended mode, it is possible to read and write independently two memory cells that are normally associated for carrying out differential reading. In effect, when it is operated in single-ended mode, each memory cell may have a value independent of the value of the other memory cells, and it is consequently possible to double the amount of data that may be stored in the PCM. 
     A drawback of the PCM devices used in single-ended mode lies in the fact that in single ended mode the degradation over time of the logic data stored may not be compensated and, in the absence of periodic refresh operations, the associated information is lost. Instead, when PCMs are used in double-ended mode, all the logic data undergo in time common-mode degradation and, consequently, via differential reading, it is in any case possible to arrive at the desired information content, cancelling out the offset due to common-mode degradation. 
     In order to prevent any loss of data in a memory cell, albeit continuing to use the single-ended mode, it is expedient to refresh the memory cell periodically. Refresh methods are known for architectures based upon fast-writing technology, such as the dynamic random-access memory (DRAM), where the time necessary for each writing operation is of the order of nanoseconds. In these methods, the datum to be refreshed is read and overwritten periodically before it gets lost. 
     Conversely, in PCM technologies of a known type, the duration of a writing operation is in the region of several microseconds, typically 10 microseconds. Consequently, it would not be possible to update entire memory banks if used for applications that require real-time access to the memory (in reading and/or writing). 
     SUMMARY 
     Some embodiments relate to a method for managing a differential memory, a differential memory, and a system that comprises the differential memory. Some embodiments relate to a method for real-time updating of a differential memory with continuous reading accessibility by a user. 
     In an embodiment, a method for real-time management of a differential memory includes: storing first logic data associated with a first informative content in an auxiliary memory module of the differential memory, the first logic data being stored in a first submodule of a main memory module; storing third logic data associated with a second informative content in a second submodule of the main memory module of the differential memory by overwriting second logic data associated with the first informative content, while maintaining the first logic data contained in the first submodule of the main memory module unaltered, where the second logic data are complementary to the first logic data; receiving a request for reading the first informative content; when the third logic data is being stored, reading the first logic data from the auxiliary memory module in a single-ended mode in response to the request for reading the first informative content; when the third logic data is not being stored, reading the first logic data from the first submodule in response to the request for reading the first informative content; receiving a request for reading the second informative content; and reading the third logic data in single-ended mode in response to the request for reading the second informative content. 
     In another embodiment, a differential memory includes: a first submodule configured to store first logic data and a second submodule configured to store second logic data that is complementary to the first logic data; an auxiliary memory module; and a controller configured to: control a storage operation of the first logic data in the auxiliary memory module; control a storage operation of third logic data, associated with a second informative content, in the second submodule by overwriting the second logic data while maintaining the first logic data contained in the first submodule unaltered; receive a request for reading the first informative content; when the third logic data is being stored, read the first logic data from the auxiliary memory module in single-ended mode in response to the request for reading the first informative content; when the third logic data is not being stored, read the first logic data from the first submodule in response to the request for reading the first informative content; receive a request for reading the second informative content; and read the third logic data in single-ended mode in response to the request for reading the second informative content. 
     In yet another embodiment, a differential memory includes: a first main memory module including a first submodule configured to store first logic data and a second submodule configured to store second logic data that is complementary to the first logic data, the first and second submodules being the same size; an auxiliary memory module; and a controller configured to: read and rewrite the first logic data in the first submodule; control a storage operation of the first logic data in the auxiliary memory module; control a storage operation of third logic data, associated with a second informative content, in the second submodule by overwriting the second logic data while maintaining the first logic data contained in the first submodule unaltered; receive a request for reading the first informative content; when the third logic data is being stored, read the first logic data from the auxiliary memory module in single-ended mode in response to the request for reading the first informative content; when the third logic data is not being stored, read the first logic data from the first submodule in response to the request for reading the first informative content; receive a request for reading the second informative content; and read the third logic data in single-ended mode in response to the request for reading the second informative content. 
     In yet another embodiment, a method for managing a PCM differential memory that is arranged in two branches and configured to enable differential reading of a first informative content stored in complementary form in the two branches includes the steps of: storing a second informative content in one of the two branches, preserving the data regarding the first informative content in the other branch; using an auxiliary module for temporarily storing the first informative content, and enabling storage of the second informative content without interrupting access in reading to the first informative content; and reading in asymmetrical, or single-ended, mode the first or second informative content. 
     Some embodiments provide a management method of real-time access to a differential memory operated in single-ended mode, a differential memory that may be operated in single-ended mode, and an electronic system including the differential memory that will be able to overcome the drawbacks of the prior art. 
     In some embodiments, the differential memory is a phase-change memory. In some embodiments, the differential memory may be operated in single-ended mode, i.e., accessing twice the number of address spaces with respect to differential memories of conventional use. 
     According to an embodiment, a differential memory, such as a phase-change memory, includes a first main memory module, which stores first logic data to be refreshed, and an auxiliary memory module. In some embodiments, the differential memory is operated in single-ended mode. In some embodiments, the differential memory is operated by storing second logic data in the auxiliary memory module, copying the first logic data; refreshing the first logic data; and fetching, in response to a request for reading the first logic data, the second logic data during the step of refresh of the first logic data, and fetching the first logic data, otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
         FIG.  1    is a schematic illustration of a portion of a nonvolatile memory device according to an embodiment of the present invention; 
         FIG.  2    shows schematically, by a block diagram, a method for managing the nonvolatile-memory device of  FIG.  1   , according to an embodiment of the present invention; 
         FIG.  3    shows an electrical circuit corresponding to a portion of the nonvolatile-memory device of  FIG.  1   , included in the portion of  FIG.  1   , according to an embodiment of the present invention; 
         FIGS.  4 A- 4 E  show the circuit of  FIG.  3    in respective steps of the method of  FIG.  2   , where circuit connections of particular importance for implementation of the respective step of the method are highlighted by thick lines, according to an embodiment of the present invention; 
         FIG.  5    shows an electronic system that includes the nonvolatile-memory device of  FIG.  1   , according to an embodiment of the present invention; 
         FIG.  6    shows schematically, by a block diagram, a method for refresh of the nonvolatile-memory device of  FIG.  1   , according to an embodiment of the present invention; 
         FIG.  7    shows an electrical circuit corresponding to a portion of the nonvolatile-memory device of  FIG.  1   , included in the portion of  FIG.  1   , according to an embodiment of the present invention; 
         FIGS.  8 A- 8 E  show the circuit of  FIG.  7    in respective steps of the method of  FIG.  6   , where circuit connections of particular importance for implementation of the respective step of the method are highlighted by thick lines, according to an embodiment of the present invention; 
         FIG.  9    shows schematically, by a block diagram, a method for refresh of the nonvolatile-memory device of  FIG.  1   , according to an embodiment of the present invention; 
         FIG.  10    shows schematically, by a block diagram, a method for memory recovery after an interruption during a refresh of the nonvolatile-memory device of  FIG.  1   , according to an embodiment of the present invention; and 
         FIG.  11    shows an auxiliary memory module of  FIG.  1   , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Represented schematically in  FIG.  1    and designated as a whole by the reference number  1  is a portion of a memory device, in particular a nonvolatile memory. According to an embodiment of the present invention, the memory  1  is of a differential type, in particular of a PCM type. The nonvolatile-memory device  1  is shown limitedly to just the parts necessary for an understanding of the present disclosure. 
     In a known way, phase-change memories (PCMs) of a differential type are typically designed according to a scheme with an array of memory cells arranged in rows (word lines) and columns (bit lines). Each memory cell is provided by a phase-change storage element and by a selector transistor, connected in series. A column decoder and a row decoder enable selection, on the basis of address logic signals received at input and more or less complex decoding schemes, of the memory cells, and, in particular, the corresponding word lines and bit lines, each time addressed. 
     The column decoder comprises a plurality of analog selection switches (provided by transistors), which receive at their respective control terminals the address signals. The selection switches are arranged according to a tree structure in hierarchical levels, and their number at each hierarchical level is linked to the arrangement and size of the memory array. The selection switches, when enabled, make it possible to bring the bit line selected to a definite value of voltage and/or current, according to the operations that it is desired to implement. In particular, a current path is created between a programming stage or a reading stage and the bit line selected. This current path is defined by the series of a certain number of selection switches. In a known way, sense amplifiers carry out reading of the data stored in the memory cells, comparing the current that flows in the memory cell selected (or an electrical quantity correlated thereto) with a reference current supplied by a reference-current generator (so-called asymmetrical or single-ended reading) and/or by one or more reference cells, used for the entire memory array. In the prior art, single-ended reading is typically used during verification that programming of the cell has occurred or during testing. During normal use of the memory, reading of a double-ended type is instead used, in which the sense amplifiers compare the current flowing in the memory cell selected with the current flowing in a respective cell associated to the memory cell selected in the differential architecture. 
     To carry out single-ended reading, an input of the sense amplifier receives the current of the memory cell that is to be read, while the other input of the sense amplifier receives the reference current supplied by the reference-current generator. 
     In single-ended mode, it is possible to read and write independently two memory cells that are normally associated in order to carry out differential reading. In effect, when it is operated in single-ended mode, each memory cell may have a value independent of the value of the other memory cells, and it is consequently possible to double the amount of data that may be stored in the PCM. 
     In particular, the nonvolatile-memory device  1  comprises a memory array  2 , constituted by a plurality of “n” main memory modules  4   a - 4   n . Each main memory module  4   a - 4   n  comprises a first submodule  4   a ′- 4   n ′ and a second submodule  4   a ″- 4   n ″. In particular, each first submodule  4   a ′- 4   n ′ comprises a plurality of memory cells, and each second submodule  4   a ″- 4   n ″ comprises a plurality of respective memory cells. 
     In a per se known manner and not shown in  FIG.  1   , the plurality of first memory cells stores respective logic data of a binary type. The plurality of second memory cells corresponds, as regards number and manufacturing characteristics, to the plurality of first memory cells and is configured to store respective logic data complementary to the logic data stored in the respective plurality of first memory cells so as to enable differential reading of the logic data. 
     In any case, each main memory module  4   a - 4   n  may be used, in addition to differential mode, in single-ended mode, such that the logic data stored in the plurality of memory cells of each second submodule  4   a ″- 4   n ″ are not complementary to the logic data stored in the plurality of memory cells of the respective first submodule  4   a ′- 4   n ′, but are further information data. In this way, it is possible to double the total capacity of the memory array  2 . 
     The nonvolatile-memory device  1  further comprises an auxiliary memory module  6 . In particular, the auxiliary memory module  6  structurally and operatively corresponds to each main memory module  4   a - 4   n , and is thus formed by a first submodule  6 ′ and a second submodule  6 ″. In particular, the first submodule  6 ′ comprises a plurality of memory cells, and the second submodule  6 ″ comprises a plurality of respective memory cells, which may be addressed and thus may be used in single-ended mode. 
     In addition, the nonvolatile-memory device  1  comprises a controller  8 , operatively coupled to the memory array  2  and to the auxiliary memory module  6 . The controller  8  is configured to implement a method of reading and writing logic data in the memory array  2  and in the auxiliary memory module  6 . 
     Furthermore, the nonvolatile-memory device  1  comprises a bus  10 . The controller  8 , the main memory modules  4   a - 4   n , and the auxiliary memory module  6  are each coupled to the bus  10  by respective communication lines  12   a - 12   e  so as to enable, as described in greater detail hereinafter, a transfer of data and control signals between the controller  8 , the main memory modules  4   a - 4   n , and the auxiliary memory module  6 . 
     Further, the nonvolatile-memory device  1  comprises a controller interface  14 . The controller interface  14  is coupled to the controller  8  so as to send to the controller  8 , via a communication line  13 , logic data to be written in the memory array  2 . 
     In addition, the nonvolatile-memory device  1  comprises a memory-array interface  16 . The memory-array interface  16  is coupled to the bus  10  via a respective communication line  12   f . A user of the nonvolatile-memory device  1  may gain access, for example in reading, to the data stored in the nonvolatile-memory device  1  via the memory-array interface  16 . The user is, for example, a microprocessor, a DSP, or a microcontroller. 
       FIG.  2    shows schematically, by a block diagram, a method for managing the memory array  2 . In particular, by way non-limiting example,  FIG.  2    regards a memory array  2  in which a first main memory module  4   a  and a second main memory module  4   b  are present. In other embodiments, the number of main memory modules could be different. 
     In an initial step  20 , the memory array  2  contains a first set of data A, stored in a differential way. In particular, the first set of data A includes a first datum A 1 , a second datum A 2 , and the respective complementary data A 1   n , A 2   n . For instance, each of the first and second data A 1 , A 2  and the respective complementary data A 1   n , A 2   n  is a single bit or a set of bits having a size, for example, of 32 MB. 
     In particular, the first main memory module  4   a  contains the first datum A 1  and the respective complementary datum A 1   n , and the second main memory module  4   b  comprises the second datum A 2  and the respective complementary datum A 2   n . In particular, the first datum A 1  is stored in the first submodule  4   a ′ of the first main memory module  4   a ; the second datum A 2  is stored in the first submodule  4   b ′ of the second main memory module  4   b ; the datum Ain complementary to the first datum A 1  is stored in the second submodule  4   a ″ of the first main memory module  4   a ; the datum A 2   n  complementary to the second datum A 2  is stored in the second submodule  4   b ″ of the second main memory module  4   b.    
     In the initial step  20 , the user of the nonvolatile-memory device  1  has access to the first set of data A, which may be read in a differential way from the first and second main memory modules  4   a ,  4   b.    
     The contents of the auxiliary memory block  6 , which is represented empty in  FIG.  2   , are irrelevant in the initial step  20  of the management method. The user of the nonvolatile-memory device  1  does not have access to the data stored in the auxiliary memory block  6 , which is for this reason illustrated with a dashed line. 
     After step  20 , control passes to step  22  where each of the first and second data A 1 , A 2  is written in the auxiliary memory module  6  by the controller  8 , as described in detail hereinafter. 
     During step  22 , the user of the nonvolatile-memory device  1  continues to have access to the first set of data A, which may be read in a differential way from the first and second main memory modules  4   a ,  4   b.    
     After step  22 , control passes to steps of writing a second set of data B in the first and second main memory modules  4   a ,  4   b.    
     In particular, the second set of data B includes a first datum Bin and a second datum B 2   n . For instance, in a way similar to what has been described with reference to the first set of data A, each of the first and second data B 1   n , B 2   n  is a single bit or a set of bits having a size, for example, of 32 MB. 
     In particular, after step  22  control passes to step  24 , where the datum B 1   n  is written in the second submodule  4   a ″ of the first main memory module  4   a.    
     During step  24 , the contents of the first main memory module  4   a  are not accessible in reading for the user in so far as the module is occupied in the operation of writing of the datum Bin. For this reason, it is illustrated with a dashed line. 
     In any case, during step  24 , the user of the nonvolatile-memory device  1  continues to have access to the informative content associated to the first set of data A in so far as it is possible to read the first datum A 1  in the auxiliary memory module  6 , and the second datum A 2  in the second main memory module  4   b.    
     After step  24  control passes to step  26 , where the datum B 2   n  is written in the second submodule  4   b ″ of the second main memory module  4   b.    
     During step  26 , the contents of the second main memory module  4   b  are not accessible in reading in so far as the module is occupied in the operation of writing of the datum B 2   n . For this reason, it is illustrated with a dashed line. 
     In any case, during step  26 , the user of the nonvolatile-memory device  1  continues to have access to the informative content associated to the first set of data A in so far as it is possible to read the first datum A 1  in the first main memory module  4   a  and the second datum A 2  in the auxiliary memory module  6 . 
     After step  26 , control passes to step  28 , where the second main memory module  4   b  is rendered again accessible to the user. During step  28 , the contents of the auxiliary memory module  6  are not accessible and are irrelevant, and for this reason the module is illustrated with a dashed line. 
     During step  28 , the user of the nonvolatile-memory device  1  continues to have access to the informative content associated to the first set of data A in so far as it is possible to read the first datum A 1  in the first main memory module  4   a  and the second data A 2  in the second main memory module  4   b.    
     During step  28 , the user of the nonvolatile-memory device  1  has further access to the informative content associated to the second set of data B in so far as it is possible to read the datum Bin in the first main memory module  4   a  and the datum B 2   n  in the second main memory module  4   b.    
     For instance, the informative content associated to the first set of data A is a first vehicle-control software. In this case, the nonvolatile-memory device  1  is embedded in an electronic control unit of the vehicle. In addition, the informative content associated to the second set of data B is a second vehicle-control software. 
     During step  28 , it is possible to carry out alternatively the first or second vehicle-control software. 
     For instance, during step  28  it is possible to evaluate the performance of the vehicle during use of the first control software, and alternately, of the second control software, until the most adequate control software is determined. 
     Once step  28  is through, control passes to step  30 , where the control software deemed adequate is stored in a differential way in the memory array  2 . For instance, in the case of  FIG.  2   , the control software deemed adequate is that associated to the second set of data B. 
     Consequently, after step  28 , a datum B 1  complementary to the datum Bin is written in the first submodule  4   a ′ of the first main memory module  4   a , and a datum B 2  complementary to the datum Ben is written in the first submodule  4   b ′ of the second main memory module  4   b , to arrive at the configuration illustrated in step  30 . The auxiliary memory module  6  is shown with a dashed line, in so far as, at the end of the steps of the method described previously, the auxiliary memory module  6  is not used and its informative content is irrelevant. 
     The steps  22 - 26  of the management method are carried out for all “n” main memory modules  4   a - 4   n , as illustrated previously for the case of the two main memory modules  4   a ,  4   b.    
       FIG.  3    shows in greater detail a portion of the nonvolatile-memory device  1  of  FIG.  1   , comprising the main memory module  4   a , the auxiliary memory module  6 , the controller  8 , and the memory-array interface  16 , according to an embodiment of the present invention. In particular, the nonvolatile-memory device  1  further comprises a plurality of multiplexers designed to render transparent management of the first and second sets of data A, B for the user of the nonvolatile-memory device  1 . 
     The controller  8  is further configured to generate a first control signal BUSY, a second control signal BUSY_SP, and a third control signal S COMM  for controlling the plurality of multiplexers described more fully hereinafter. 
     The controller  8  is further configured to receive data contained in the main memory module  4   a  and in the auxiliary memory module  6  via an input signal S DATA_IN . In particular, the signal S DATA_IN  is one of the signals S MAIN_O  (which carries the data contained in the main memory module  4   a ) and S SPARE_O  (which carries the data contained in the auxiliary memory module  6 ) selected via a multiplexer  32  controlled by the first control signal BUSY. For this purpose, the multiplexer  32  is coupled to the main memory module  4   a  and to the auxiliary memory module  6  for receiving, respectively, the signal S MAIN_O  and the signal S SPARE_O . For instance, the signal S SPARE_O  is read after an operation of writing in the auxiliary memory module  6  in order to verify that the writing operation has been carried out correctly. Otherwise, according to known procedures, subsequent writing operations of the same datum are carried out until the datum has been written correctly. 
     In particular, the multiplexer  32  is coupled to the auxiliary memory module  6  via interposition of a first buffer  33  so as to reduce the propagation times associated to the auxiliary memory module  6 . 
     The controller  8  is further configured to send to the main memory module  4   a  an output signal S DATA_OUT ′ that carries data that may be stored in the main memory module  4   a , and to send to the auxiliary memory module  6  an output signal S DATA_OUT ″ that carries data that may be stored in the auxiliary memory module  6 . 
     The controller  8  is further configured to generate a signal S A ′, which contains an address of one of the memory cells of the main memory module  4   a . The signal S A ′ is supplied to an input of a multiplexer  34  controlled by the first control signal BUSY. The multiplexer  34  further receives a signal S INT_O  from the memory-array interface  16 , which contains an address for carrying out an operation of reading or writing on one of the memory cells of the main memory module  4   a . The first control signal BUSY enables selection of one of the input signals S A ′, S INT_O  in order to generate an output signal S ADDR ′ to the multiplexer  34 . 
     The signal S ADDR ′ is supplied to the main memory module  4   a  for addressing one of the memory cells of the main memory module  4   a.    
     According to one aspect of the present invention, an address-management block  44  is arranged between the multiplexer  34  and the main memory module  4   a . The controller  8  is further configured to generate a control signal SWAP, designed to control the address-management block  44 . Consequently, the address-management block  44  receives at input the signal S ADDR ′ and the control signal SWAP. In particular, if the control signal SWAP has logic value “0,” the signal S ADDR ′ is directly supplied to the main memory module  4   a . Otherwise, if the control signal SWAP has logic value “1,” the signal S ADDR ′ is modified so as to address the memory cell complementary to the memory cell originally addresses. Typically, in differential memories, the address of a memory cell and that of its complementary are separated by a fixed offset equal to the size of the memory cell. In this case, the address-management block  44  is configured to add the offset to the address indicated by the signal S ADDR ′ in the case where the control signal SWAP has logic value “1.” 
     Consequently, the signal S MAIN_O  at output from the main memory module  4   a  contains data corresponding to the data contained in the memory cell, addressed by the signal S ADDR ′, of the main memory module  4   a  if the control signal SWAP has logic value “0,” or alternatively the data contained in the complementary memory cell if the control signal SWAP has logic value “1.” In this way, it is possible to access, during step  28  of the management method, both the informative content associated to the set of data A and to that associated to the set of data B. In addition, it is possible to execute each of the control software corresponding to the set of data A and to the set of data B with reference to a same memory-address region, namely, the one for which each of the control software has been compiled and linked even though the set of data B physically resides in a different memory-address region. 
     The aforesaid signal S MAIN_O  is further supplied to a multiplexer  36  controlled by the third control signal S COMM . The multiplexer  38  further receives at input the signal S SPARE_O  from the auxiliary memory module  6  via interposition of the first buffer  33 . The multiplexer  38  selects one of the signals S MAIN_O , S SPARE_O  in order to generate a signal S INT_I  at input to the memory-array interface  16 . The signal S INT_I  consequently contains data sent in response to reading requests from the user. These data come from the main memory module  4   a  or from the auxiliary memory module  6 . 
     The controller  8  is further configured to generate a signal S A ″, which contains an address of one of the memory cells of the auxiliary memory module  6 . The signal S A ″ is supplied to an input of a multiplexer  38  controlled by the second control signal BUSY_SP. The multiplexer  38  further receives at input the signal S ADDR ′ via interposition of a second buffer  39  so as to reduce the propagation times associated to the auxiliary memory module  6 . The second control signal BUSY_SP enables selection of one of the input signals S A ″, S ADDR ′ for generating a signal S ADDR ″ at output from the multiplexer  38 . In particular, the output signal S ADDR ″ corresponds to the input signal S A ″ during copying of the data of the main memory module  4   a  into the auxiliary memory module  6  (step  22  of the management method of  FIG.  2   ), and to the input signal S ADDR ′=S INT_O  in the case where the user requests reading of the informative content of the memory cell  4   a  during writing of the datum Bin in the main memory module  4   a  (step  24  of the management method of  FIG.  2   ). 
     In a per se known manner, the controller  8  and the main memory module  4   a  receive at input respective clock signals generated by clock generators not shown in  FIG.  3   . Further, a multiplexer (not shown in the figure either) may be arranged between the main memory module  4   a  and the aforesaid clock generators for synchronizing the main memory module  4   a  to the controller  8 . 
     It is evident that the second main memory module  4   b  is structurally equivalent to the first main memory module  4   a  of  FIG.  3   , and is operatively coupled to the controller  8 , to the auxiliary memory module  6 , and to the memory-array interface  16  via respective multiplexers. 
     In addition, the second main memory module  4   b  may be associated to a respective clock signal, for example different from the clock signal of the first main memory module  4   a.    
     In order to describe in greater detail some steps of the management method  FIG.  2   , each of  FIGS.  4 A- 4 E  shows the portion of the nonvolatile-memory device  1  of  FIG.  3    during a different step of the method, according to an embodiment of the present invention. In each of  FIGS.  4 A- 4 E , communication lines corresponding to signals that are active in the corresponding step of the method are highlighted with the use of thick lines, where by “active signals” are meant signals that carry data during this step and signals that carry addresses of the data. 
     In particular,  FIGS.  4 A and  4 B  regard step  22  of the method, where the first datum A 1  is written in the auxiliary memory module  6 . 
     In even greater detail,  FIG.  4 A  regards a first sub-step of step  22 , where the controller  8  reads the datum A 1  from the main memory module  4   a . In the course of the sub-step of  FIG.  4 A , the controller  8  generates the first control signal BUSY so as to select the signals S MAIN_O  and S A ′, which are thus supplied at output by the respective multiplexers  32 ,  34 , respectively, via the signals S DATA_IN  and S ADDR ′. In this way, the controller  8  receives at input the signal S DATA_IN , corresponding to the datum A 1 , contained in the memory cell addressed by the signal S A ′ and contained in the main memory module  4   a.    
     In the course of the sub-step of  FIG.  4 A , the control signal SWAP has logic value “0,” so that the main memory module  4   a  will receive the unaltered signal S ADDR ′, which thus corresponds to the signal S A ′. 
       FIG.  4 B  regards a second sub-step of step  22  of the method, after the first sub-step of  FIG.  4 A , where the controller  8  writes in the auxiliary memory module  6  the datum A 1  read in the course of the sub-step of  FIG.  4 A . 
     In the course of the sub-step of  FIG.  4 B , the controller  8  generates the second control signal BUSY_SP so as to select the signal S A ″, which is then supplied at output by the multiplexer  38  via the signal S ADDR ″. Furthermore, the controller  8  sends to the auxiliary memory module  6  the output signal S DATA_OUT ″, which contains the datum A 1  to be written in the memory cell of the auxiliary memory module  6  addressed by the signal S ADDR ″=S A ″. The signal S A ″ supplied by the controller  8  in the course of the sub-step of  FIG.  4 B  contains the same address contained in the signal S A ′ supplied by the controller  8  in the course of the sub-step of  FIG.  4 A . 
     Consequently, at the end of the sub-step of  FIG.  4 B , the datum A 1  is stored in the auxiliary memory module  6  at the same address as the one to which it is associated in the main memory module  4   a.    
     It is evident that the second datum A 2  is read from the second main memory module  4   b  and written in the auxiliary memory module  6  as has been described with reference to the first datum A 1  and to the first main memory module  4   a  and illustrated in  FIGS.  4 A and  4 B . 
       FIGS.  4 C and  4 D  regard step  24  of the method, where the datum Bin is written in the first main memory module  4   a , and requests of reading of the datum A 1  by the user are re-addressed to the auxiliary memory module  6 . 
     In particular,  FIG.  4 C  illustrates an operation of reading by the user of an informative content of the main memory module  4   a , re-addressed to the auxiliary memory module  6 . 
     During step  24 , the controller  8  generates the second control signal BUSY_SP so as to select the signals S INT_O , S ADDR ′, which are thus supplied at output by the respective multiplexers  34 ,  38 . In addition, the controller  8  generates the third control signal S COMM  so as to select the signal S SPARE_O , supplied at output by the multiplexer  36 . In this way, whenever during step  24  the user asks to read the datum A 1 , associated to the memory cell of the main memory module  4   a  addressed by the signal S INT_O , the datum contained in the memory cell of the auxiliary memory module  6  addressed by the signal S ADDR ″=S ADDR ′=S INT_O  is read. This datum, supplied at output by the auxiliary memory module  6  via the signal S SPARE_O , is then sent to the memory-array interface  16  via the signal S INT_1 . 
       FIG.  4 D  illustrates a sub-step of writing of the datum Bin in the second submodule  4   a ″ of the main memory module  4   a , carried out during step  24 , after the sub-step of  FIG.  4 C . 
     The datum B 1   n  is, for example, supplied to the controller  8  by the controller interface  14  of  FIG.  1   . 
     During the sub-step of  FIG.  4 D , the controller  8  generates the first control signal BUSY so as to select the signal S A ′, which is then supplied at output by the multiplexer  34  so as to indicate the address of writing of the datum Bin. For instance, writing address of the datum Bin may point to the second submodule  4   a ″ of the first main memory module  4   a . In this case, the logic value of the control signal SWAP is “0.” Alternatively, the writing address of the datum Bin may point to the first submodule  4   a ′ of the first main memory module  4   a . In this case, the logic value of the control signal SWAP is “i” and consequently, after applying the fixed offset to the signal S ADDR ′ at output from the address-management block  44 , the datum Bin is in any case written in the second submodule  4   a″.    
     It is evident that step  26  of the method, where the datum B 2   n  is written in the second main memory module  4   b  and requests of reading of the datum A 2  by the user are re-addressed to the auxiliary memory module  6 , is carried out in a way similar to what has been illustrated in  FIGS.  4 C and  4 D  with reference to the datum B 1   n , to the datum A 1 , and to the first main memory module  4   a.    
       FIG.  4 E  regards step  28  of the management method, where the user may access the set of data A (for example, the first vehicle-control software) or alternatively the set of data B (in this example, the second vehicle-control software). In particular,  FIG.  4 E  illustrates an operation of reading by the user of an informative content of the first main memory module  4   a.    
     During step  28 , the controller  8  controls the multiplexers  34  and  36  so as to select and supply at output, respectively, the signals S INT_O  and S MAIN_O . In this way, in the case where during step  28  the user requests reading of the informative content of the main memory module  4   a  associated to the memory cell addressed by the signal S INT_O , the datum A 1  is read if the control signal SWAP has logic value “0,” and the datum Bin is read if the control signal SWAP has logic value “1.” Consequently, in the case where the user has to use the first control software, the control signal SWAP has logic value “0.” Otherwise, the control signal SWAP has logic value “1.” In any case, the datum read, supplied at output by the first main memory module  4   a  via the signal S MAIN_O , is sent to the memory-array interface  16  via the signal S INT_I . 
       FIG.  5    illustrates a portion of an electronic system  50  according to an embodiment of the present invention. The electronic system  50  may be used in electronic devices, such as: an electronic control unit of a vehicle; a PDA (Personal Digital Assistant); a portable or fixed computer, possibly with capacity of wireless data transfer; a mobile phone; a tablet; a smartphone; a digital audio player; a photographic or video camera; or further devices that are able to process, store, transmit, and receive information. 
     In detail, the electronic system  50  comprises a controller  51  (for example, provided with a microprocessor, a DSP, or a microcontroller) and the nonvolatile-memory device  1  described previously. Optionally, the electronic system  50  comprises one or more from among an input/output device  52  (for example, provided with a keypad and a display) for input and display data, a wireless interface  54 , for example, an antenna for transmitting and receiving data through a radio-frequency wireless-communication network, and a RAM  55 , all coupled through a bus  56 . A battery  57  may be used as electrical supply source in the electronic system  50 , which may further be provided with a photographic or video camera  58 . 
     According to a further embodiment (not illustrated in the figures), the nonvolatile-memory device  1  may be embedded in the controller  51 . 
     In any case, embodiments of the present invention find application in an operating condition where the electronic system  50  is controlled by the first control software, and the second control software is written in the nonvolatile-memory device  1 , without interrupting execution of the first control software. 
     For instance, writing of the second control software is carried out using over-the-air methodology. 
     From an examination of the characteristics of embodiments of the present invention described and illustrated herein, the advantages that it affords are evident. 
     In particular, in some embodiments, the user has an uninterrupted access in reading to the informative content of the set of data A during writing of the set of data B, and further, at the end of writing of the set of data B, may access alternatively the set of data A or the set of data B without any need to provide an additional memory module dedicated to storage of the set of data B. Consequently, it is possible to store twice the amount of the data as compared to differential memories of a conventional type and having the same amount of main memory modules. 
     Furthermore, it is possible to execute a software corresponding to the set of data B compiled and linked as if it physically resided in the memory region occupied by the set of data A. In this way, there is no need to specify in an image of the software, contained in the set of data B, its physical location in the memory. In particular, in a context where a number of versions of the software are tested, typically the sequence of updates of the versions of the software is not foreseeable, thus rendering not possible specification of the physical location of the respective images in the memory. 
     Additionally, it is clear that modifications and variations may be made to the disclosure described and illustrated herein, without thereby departing from the scope of the present invention. 
     For instance, the method of management of the nonvolatile-memory device  1  envisages operations of refreshing of the data contained in the memory array  2 , for example between step  26  and step  28  and/or between step  20  and step  22  and/or in any other moment, according to the need. In this case, access in reading to the informative content associated to the datum during refresh is guaranteed by a method implemented by the controller  8  that envisages the steps of: copying the datum to be refreshed in the auxiliary memory module  6 ; carrying out the aforementioned refresh of the datum in the memory array  2 ; and reading the informative content from the auxiliary memory module  6 , during the step of refresh, and from the memory array  2 , otherwise. 
     Consequently, refresh is carried out in a way transparent for the user, which has an uninterrupted access in reading and writing to the informative content of the entire nonvolatile-memory device, enabling use thereof for real-time applications. 
       FIG.  6    illustrates schematically, by a block diagram, a method of refresh of the memory array  2 , according to an embodiment of the present invention. The method of  FIG.  6    may be performed, e.g., between steps  20  and  22 , and/or between steps  28  and  30 , and/or after step  30 . 
     The refresh method first of all comprises the step  70  of selection of a main memory module  4   a - 4   n  to be refreshed. By way of example, in the course of step  70  the main memory module  4   a  is selected. 
     Step  70  is followed by step  72 , where the controller  8  reads the data contained in the main memory module  4   a  selected in step  70 . In particular, step  72  is performed in a way transparent for the user of the nonvolatile-memory device  1 , which may request reading operations by addressing any cell of any main memory module  4   a - 4   n . In the case where the user requests access to the main memory module  4   a  simultaneously to step  72 , there occurs a temporary conflict that has a negligible impact upon the reading times. In particular, when the user has access in reading to the main memory module  4   a , a control signal READY assumes the logic value “0”; at the end of reading, the control signal READY assumes the logic value “1.” The controller interface  14  has access to the control signal READY. If in the course of step  72  the value of the control signal READY is “0” (i.e., reading by the user is in progress), the controller interface  14  postpones the reading operation to the end of the reading operation of the user, and sets the control signal READY to the value “0” up until the end of its own reading operation. If, in the course of step  72 , the value of the control signal READY is “1” (i.e., a reading access to the main memory module  4   a  by the user is not in progress), the controller interface  14  sets the control signal READY at the value “o” up until the end of its own reading operation so that the user may not gain access during the reading operation and thus conflicts are prevented. In any case, at the end of its own reading operation, the controller interface  14  sets the control signal READY to the logic value “1”. 
     Step  72  is followed by step  74 , where the controller  8  writes in the auxiliary memory module  6  the data read in step  72 . In particular, the entire contents of the main memory module  4   a  are copied into the auxiliary memory module  6 . For this purpose, it could be necessary to reiterate in a number of cycles the succession of steps  72  and  74  up to copying of the entire contents. Furthermore, in order to preserve the consistency of the data contained in the memory array  2 , the controller  8  prevents in this step execution of operations of writing in the memory array  2 . 
     Step  74  is followed by step  76 , where requests by the user for reading the information content of the main memory module  4   a  are re-addressed to the auxiliary memory module  6 . 
     Step  76  is followed by step  78 , where the content of the main memory module  4   a  is physically refreshed, i.e., its content is read and overwritten. According to an aspect of the present invention, during step  78  the user may access the auxiliary memory module  6  in order to read the content thereof, which, as has been said, corresponds to the information content of the main memory module  4   a.    
     In this way, the user has an uninterrupted access to the information content of the entire memory array  2 , enabling use thereof for real-time applications. 
     Step  78  is followed by step  80 , where requests for reading the contents of the main memory module  4   a , previously re-addressed to the auxiliary memory module  6  in step  76 , are again addressed to the main memory module  4   a.    
     Following upon step  80 , it is possible to return to step  70  in order to select a new main memory module  4   a - 4   n  to be refreshed. For instance, the controller  8  may select periodically and in a sequential way each main memory module  4   a - 4   n  according to pre-set time intervals. In particular, these time intervals have a duration compatible with the retention time of each memory cell of the nonvolatile-memory device  1  so as to enable an uninterrupted use of the single-ended mode of the memory array  2 . In particular, the retention time of a memory cell is the duration that elapses between writing of a logic datum in the cell and the instant when the datum is no longer readable in so far as it is excessively degraded. 
       FIG.  7    shows in greater detail a portion of the nonvolatile-memory device  1  of  FIG.  1   , comprising the main memory module  4   a , the auxiliary memory module  6 , the controller  8 , and the memory-array interface  16 . In particular, the nonvolatile-memory device  1  further comprises a plurality of multiplexers designed to render transparent the refresh operation for the user of the nonvolatile-memory device  1 . 
     The controller  8  is further configured to generate a first control signal BUSY, a second control signal BUSY_SP, and a third control signal S COMM  for controlling the plurality of multiplexers described more fully hereinafter. 
     The controller  8  is further configured to receive data contained in the main memory module  4   a  and in the auxiliary memory module  6  via an input signal S DATA_IN . In particular, the signal S DATA_IN  is one of the signals S MAIN_O  (which carries the data contained in the main memory module  4   a ) and S SPARE_O  (which carries the data contained in the auxiliary memory module  6 ) that are selected via a multiplexer  32  controlled by the first control signal BUSY. For this purpose, the multiplexer  32  is coupled to the main memory module  4   a  and to the auxiliary memory module  6  for receiving, respectively, the signal S MAIN_O  and the signal S SPARE_O . For instance, the signal S SPARE_O  is read following upon an operation of writing of the auxiliary memory module  6 , in order to verify that the writing operation has been carried out correctly. Otherwise, according to known procedures, subsequent writing operations are carried out. 
     In particular, the multiplexer  32  is coupled to the auxiliary memory module  6  via interposition of a first buffer  33  so as to reduce the propagation times associated to the auxiliary memory module  6 . 
     The controller  8  is further configured to: send to the main memory module  4   a  an output signal S DATA_OUT ′ that carries data read from the main memory module  4   a  so as to implement step  78  of the method of  FIG.  6   ; and send to the auxiliary memory module  6  an output signal S DATA_OUT ″, which carries data read from the main memory module  4   a  so as to implement step  74  of the method of  FIG.  6   . 
     The controller  8  is further configured to generate a signal S A ′, which contains an address of one of the memory cells of the main memory module  4   a . The signal S A ′ is supplied to an input of a multiplexer  34  controlled by the first control signal BUSY. The multiplexer  34  further receives a signal S INT_O  from the memory-array interface  16 , which contains an address for carrying out a writing operation of a conventional type of one of the memory cells of the main memory module  4   a . The first control signal BUSY enables selection of one of the input signals S A ′, S INT_O  in order to generate a signal S ADDR ′ at output from the multiplexer  34 , according to whether it is executing the refresh method of  FIG.  6    or a writing operation of a conventional type. 
     The signal S ADDR ′ is supplied to the main memory module  4   a  for addressing one of the memory cells of the main memory module  4   a  so as to implement one of steps  74  and  78  of the refresh method of  FIG.  6   . 
     Consequently, the signal S MAIN_O  at output from the main memory module  4   a  contains data corresponding to the data contained in the memory cell of the main memory module  4   a  addressed by the signal S ADDR ′. 
     The signal S MAIN_O  is further supplied to a multiplexer  36  controlled by the third control signal S COMM . The multiplexer  38  further receives at input the signal S SPARE_O  from the auxiliary memory module  6  via interposition of the first buffer  33 . The multiplexer  38  selects one of the signals S MAIN_O , S SPARE_O  to generate a signal S INT_I  at input to the memory-array interface  16 . The signal S INT_I  consequently contains data sent in response to reading requests from the user. These data come from the main memory module  4   a  or, during a refresh thereof, from the auxiliary memory module  6 . 
     The controller  8  is further configured to generate a signal S A ″, containing an address of one of the memory cells of the auxiliary memory module  6 . The signal S A ″ is supplied to an input of a multiplexer  38  controlled by the second control signal BUSY_SP. The multiplexer  38  further receives at input the signal S ADDR ′ via interposition of a second buffer  39  so as to reduce the propagation times associated to the auxiliary memory module  6 . The second control signal BUSY_SP enables selection of one of the input signals S A ″, S ADDR ′ in order to generate a signal S ADDR ″ at output from the multiplexer  38 . In particular, the output signal S ADDR ″ corresponds to the input signal S A ″ in the course of copying of the data of the main memory module  4   a  into the auxiliary memory module  6  (step  74  of the refresh method of  FIG.  6   ), and to the input signal S ADDR ′=S INT_O  in the case where the user requests reading of the information content of the memory cell  4   a  in the course of refresh of the main memory module  4   a  (step  78  of the refresh method of  FIG.  6   ). 
     The controller  8  is further configured to supply a multiplexer  40  with a clock signal CLK generated by a clock generator (not shown in  FIG.  7   ). The multiplexer  40  further receives at input a clock signal CLKA (generated by another clock generator not shown in  FIG.  7   ), which represents the standard operating clock of the main memory module  4   a . The multiplexer  40  is controlled by the first control signal BUSY so as to generate an output signal used as clock of the main memory module  4   a  by selecting the signal CLK at output from the controller  8 , during refresh of the main memory module  4   a , and the signal CLKA, otherwise. 
     The clock signal CLK at output from the controller  8  is further supplied to a multiplexer  42  controlled by the second control signal BUSY_SP. The multiplexer  42  further receives at input the clock signal CLKA. The multiplexer  42  generates an output signal used as clock of the auxiliary module  6  by selecting the signal CLK, during copying of the data of the main memory module  4   a  into the auxiliary memory module  6  (step  74  of the refresh method of  FIG.  6   ), and the signal CLKA, in the case where the user requests reading of the information content of the memory cell  4   a  during refresh of the main memory module  4   a  (step  78  of the refresh method of  FIG.  6   ). 
     It is evident that the other main memory modules  4   b - 4   n  are structurally equivalent to the main memory module  4   a  of  FIG.  7   , and are operatively coupled to the controller  8 , to the auxiliary memory module  6 , and to the memory-array interface  16  via respective multiplexers. Furthermore, each main memory module  4   a - 4   n  may be associated to a respective clock signal, for example different from the clock signal of the other main memory modules. In other words, each main memory module  4   a - 4   n  may operate asynchronously with respect to the others, and send respective address signals and clock signals at input to the multiplexers  38  and  42 , respectively. 
     In particular, each clock signal may have a frequency of its own different from the frequency of the other clock signals. Consequently, the auxiliary module  6  may be controlled for operating according to the synchronism of each clock signal and thus to the respective frequencies so as to replicate the operation of the main memory module during refresh. 
     As shown in  FIG.  7   , in some embodiments, the auxiliary memory module  6  used to perform the method of  FIG.  6    is the same as the auxiliary memory module used to perform the method of  FIG.  2   . Some embodiments may perform the method of  FIG.  6    using an auxiliary memory that is different from the auxiliary memory used to perform the method of  FIG.  2   . In such embodiments, the method of  FIG.  6    may be performed, e.g., between steps  20  and  22 , and/or between steps  22  and  24 , and/or between steps  24  and  26 , and/or between steps  26  and  28 , and/or between steps  28  and  30 , and/or after step  30 . 
     In order to describe in greater detail some steps of the refresh method of  FIG.  6   , each of  FIGS.  8 A- 8 E  shows the portion of the nonvolatile-memory device  1  of  FIG.  7    during a different step of the method, according to an embodiment of the present invention. In each of  FIGS.  8 A- 8 E , communication lines corresponding to signals that are active in the corresponding step of the method are highlighted with the use of thick lines, where by “active signals” are meant signals that carry data during the step, signals that carry addresses to the data, and clock signals designed to time the data transport. 
     In particular,  FIG.  8 A  regards step  72  of the method, where the controller  8  reads the data contained in the main memory module  4   a.    
     During Step  72 , the controller  8  generates the first control signal BUSY so as to select the signals S MAIN_O , S A ′, and CLK, which are thus supplied at output by the respective multiplexers  32 ,  34 ,  40 . In this way, the controller  8  receives at input the signal S DATA_IN , corresponding to the datum contained in the memory cell that is addressed by the signal S A ′ and is contained in the main memory module  4   a . The controller  8  is configured to read in this way all the data contained in the main memory module  4   a  varying the address carried by the signal S A ′. 
       FIG.  8 B  regards step  74  of the method, where the controller  8  writes in the auxiliary memory module  6  the data read during step  72 . 
     During step  74 , the controller  8  generates the second control signal BUSY_SP so as to select the signals S A ″ and CLK, which are thus supplied at output by the respective multiplexers  38 ,  42 . In addition, the controller  8  sends to the auxiliary memory module  6  the output signal S DATA_OUT ″, containing data to be written in the memory cell of the auxiliary memory module  6  addressed by the signal S ADDR ″=S A ″. The controller  8  is configured to write in this way all the data read previously (step  72 ) from the main memory module  4   a , varying the address carried by the signal S A ″. 
       FIG.  8 C  regards step  76  of the method, and represents an operation of reading by the user of an information content of the main memory module  4   a , re-addressed to the auxiliary memory module  6 . 
     In the time interval that elapses from execution of step  76  to start of step  80 , the controller  8  generates the second control signal BUSY_SP so as to select the signals S INT_O , S ADDR ′ and CLK A , which are thus supplied at output by the respective multiplexers  34 ,  38 ,  42 . Further, the controller  8  generates the third control signal S COMM  so as to select the signal S SPARE_O , supplied at output by the multiplexer  36 . In this way, whenever in the time interval the user requests reading of the information content of the main memory module  4   a  associated to the memory cell addressed by the signal S INT_O , the datum contained in the memory cell of the auxiliary memory module  6  addressed by the signal S ADDR ″=S ADDR ′=S INT_O  is read. The datum, supplied at output by the auxiliary memory module  6  via the signal S SPARE_O , is thus sent to the memory-array interface  16  via the signal S INT_1 . 
       FIG.  8 D  regards step  78  of the method, where the content of the main memory module  4   a  is refreshed. 
     In the course of step  78 , in a first sub-step (not illustrated graphically in  FIG.  8 D ), the controller  8  reads the datum of the memory cell of the main memory module  4   a  addressed by the signal S ADDR ′=S A ′. For this purpose, as already shown in  FIG.  8 A  for step  72 , the controller  8  generates the first control signal BUSY so as to select the signals S MAIN_O , S A ′, and CLK, which are thus supplied at output by the respective multiplexers  32 ,  34 ,  40 . 
     Following upon reception by the controller  8  of the datum read and carried by the signal S DATA_IN , a second sub-step of step  78 , shown in  FIG.  8 D , is carried out, where the controller  8  continues to generate the first control signal BUSY as in the first sub-step, and further sends to the main memory module  4   a  the output signal S DATA_OUT ′, containing the datum that is read in the first sub-step and is to be physically refreshed via overwriting. 
     The controller  8  is configured to repeat the first and second sub-steps for each datum contained in the main memory module  4   a . In particular, it is possible to carry out in a first time interval the first sub-step for each datum and in a second time interval the second sub-step. Alternatively, it is possible to carry out the second sub-step for one datum immediately after the first sub-step for the same datum, and then pass to the next datum. 
       FIG.  8 E  regards step  80  of the method and illustrates an operation of reading by the user of an information content of the main memory module  4   a.    
     In a time interval that elapses from execution of step  80  up to execution of step  76  of a subsequent iteration of the refresh method of  FIG.  6   , the controller  8  controls the multiplexers  34 ,  36 ,  40  so as to select and supply at output the signals S INT_O , S MAIN_O  and CLK A , respectively. In this way, whenever in the time interval the user requests reading of the information content of the main memory module  4   a  associated to the memory cell addressed by the signal S INT_O , the datum contained in the memory cell of the main memory module  4   a  addressed by the signal S ADDR ′=S INT_O  is read. This datum, supplied at output by the main memory module  4   a  via the signal S MAIN_O  is thus sent to the memory-array interface  16  via the signal S INT_I . 
     In some embodiments, the implementation of the refresh method advantageously enables uninterrupted use of a PCM of a differential type in single-ended mode, and consequently storage of different data in memory cells normally coupled for reading a single datum in differential mode. 
     In this way, in some embodiments, it is possible to increase the total capacity of a PCM designed for operating in differential mode, without appreciably increasing the size and costs thereof. 
     Furthermore, in some embodiments, the refresh method is carried out in a way transparent for the user, which has an uninterrupted access in reading and writing to the information content of the entire nonvolatile-memory device, enabling use thereof for real-time applications. 
     Additionally, it is clear that modifications and variations may be made to the disclosure described and illustrated herein, without thereby departing from the scope of the present invention. For instance, the refresh method may be applied to any memory of a differential type, not necessarily based upon phase-change technology. 
     In some embodiments, the memory array implements a recovery method in case the refresh method (e.g., the method of  FIG.  6   ) is aborted, e.g., because of an asynchronous event (e.g., reboot, reset, power off, etc.) abruptly interrupting the refresh sequence. For example,  FIG.  9    shows schematically, by a block diagram, a method for refreshing the memory array  2 , according to an embodiment of the present invention.  FIG.  10    shows schematically, by a block diagram, a method for memory recovery of the memory array  2 , according to an embodiment of the present invention.  FIGS.  9  and  10    may be understood together. 
     In the embodiment illustrated in  FIG.  9   , steps  70 ,  72 ,  74 ,  76 ,  78 , and  80 , are performed as described with respect to  FIG.  6   . 
     During step  70 , pointers A, B, and C are cleared (not set). In some embodiments, pointers A, B, and C are located in auxiliary memory module  6 . In some embodiments, pointers A, B, and C may be stored in a different memory location, such as a different auxiliary memory, for example. 
     During step  90 , which is performed after step  72  and before step  74 , pointer A is set. During step  92 , which is performed after step  74  and before step  77 , pointer B is set. During step  94 , which is performed after step  78  and before step  80 , pointer C is set. In some embodiments, after step  80 , pointers A, B, and C, are cleared. 
     As can be seen from  FIG.  9   , if an interruption of the memory refresh method occurs, it is possible to determine the status of the memory refresh at the time of interruption based on pointers A, B, C. For example, if after a memory refresh interruption, pointers A, B, and C, are cleared, the memory refresh was interrupted either before step  90  or after step  80 . If instead pointer A is set and pointers B, and C, are cleared, the memory refresh was interrupted between step  90  and step  92  (e.g., during step  74 ). If instead pointers A and B are set and pointer C is cleared, the memory refresh was interrupted between step  92  and step  94  (e.g., during steps  76  or  78 ). If instead pointers A, B and C are set, the memory refresh was interrupted after step  94  (e.g., during step  80 ). 
     In some embodiments, determining at which point the memory refresh method is interrupted is advantageous because it allows for configuring the memory array  2  such that no data is lost. For example, as shown in  FIG.  10   , in some embodiments, the pointer status is determined during step  100 . Step  100  may occur, e.g., immediately after boot (after power-up), or during boot (during power-up) of the memory. In some embodiments, step  100  may be performed after a memory refresh interruption flag is asserted. 
     If during step  100  it is determined that pointers A, B, and C are all set or all cleared, then the interruption, if any, occurred after the memory module  4   a  was refreshed (step  80 ). Thus, read request are addressed to the main memory module  4   a  and memory array  2  continues operating normally (e.g., the next memory module may be refreshed). 
     If during step  100  it is determined that pointer A is set and pointers B, and C are cleared, then the interruption occurred before refreshing memory module  4   a  (step  80 ) and before writing the auxiliary memory module  6  (step  74 ). Thus, read request are addressed to the main memory module  4   a  and memory module  4   a  is selected to be refreshed (step  70 ). In some embodiments, a notification is sent to a user (e.g., a microprocessor, a DSP, or a microcontroller) that the memory refresh was not completed. 
     If during step  100  it is determined that pointers A and B are set and pointer C is cleared, then the interruption occurred before refreshing memory module  4   a  (step  80 ) but after writing the auxiliary memory module  6  (step  74 ). Thus, read request are addressed to the auxiliary memory module  6  (step  76 ) and step  78  is performed. In some embodiments, a notification is sent to the user that the memory refresh was not completed. 
     By performing a memory recovery method, such as shown in  FIG.  10   , some embodiments advantageously allow for avoiding data corruption in the event of an asynchronous interruption of a memory refresh. 
     In some embodiments, the refresh pointers (e.g., A, B, or C) are stored inside auxiliary memory module  6 . For example,  FIG.  11    shows auxiliary memory module  6 , according to an embodiment of the present invention. As shown in  FIG.  11   , auxiliary memory module  6  includes memory portion  112  and memory portion  114 . Memory portion  112  may be used, e.g., for performing the method of  FIG.  2    and/or the method of  FIG.  6   . Memory portion  114  may be used for storing the refresh pointers, e.g., used during steps  90 ,  92 , and  94 . In some embodiments, memory portion  114  comprises a plurality of registers, as shown in  FIG.  11   . 
     In some embodiments, the memory locations where the refresh pointers are stored are registers having a plurality of bits, e.g., 8, 16, 32, etc. In some embodiments, error correction code (ECC) is used to store the refresh pointers. For example, pointer A may be stored in a first 8-bit register, pointer B may be stored in a second 8-Bit register, and pointer C may be stored in a third 8-bit register. 
     In some embodiments, each of the refresh pointers may store a first value (e.g., N) to indicate that the pointer is cleared, or a second value (e.g., N+1) to indicate that the pointer is set. Values different than the first and second values are invalid. In this way, some embodiments, may determine that: 
     pointer A is cleared when it has the first value and is set when it has the second value; 
     pointer B is cleared when it has the first value and is set when it has the second value; and 
     pointer C is cleared when it has the first value and is set when it has the second value. 
     In some embodiments, the first and second values are selected such that there is no ambiguity, in case the memory refresh is interrupted during a writing of the refresh pointer into memory portion  114 , as to whether the pointer has the first value, the second value, or is invalid. For example, in some embodiments, the first value is equal to 0x55 (b′01010101), and the second value is equal to 0xAA (b′10101010). Other values are also possible. 
     In some embodiments, a pointer having an invalid value is indicative that the refresh method was aborted during update of the pointer. In some embodiments, thus, an invalid value is interpreted as having the second value (since, e.g., if the memory refresh was aborted during the refresh pointer update, then the previous step was completed successfully). In some embodiments, a pointer having an invalid value is replaced with the second value during boot and before performing step  100 . 
     In some embodiments, the first and second values may also include information about which memory module is being refreshed. 
     Advantages of some embodiments include allowing for the use of a differential memory (e.g., PCM) in single-ended mode to increase the memory capacity so that, e.g., two different vehicle-control software can be simultaneously stored in the memory so that both software can be evaluated. In some embodiments, the memory retention is extended by the refresh method without comprising real-time operation (since read access is not interrupted during the refresh method). In some embodiments, a recovery method allowed for avoiding corrupted data that may be caused by an abrupt asynchronous event, such as a reboot. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.