Reading Circuit of a Long time Constant Circuit Stage and Corresponding Reading Method

Abstract

A reading circuit for a charge-retention circuit stage is provided with a storage capacitor coupled between a first biasing terminal and a floating node, and a discharge element coupled between the floating node and a reference terminal. The reading circuit further has an operational amplifier having a first input terminal that is coupled to the floating node and receives a reading voltage, a second input terminal receives a reference voltage, and an output terminal on which it supplies an output voltage, the value of which is a function of the comparison between the reading voltage and the reference voltage and indicative of a residual charge in the storage capacitor. A shifting stage shifts the value of the reading voltage of the floating node, before the comparison is made between the reading voltage and the reference voltage for supplying the output voltage.

Claims

1. A long time constant (LTC) circuit comprising: a floating node configured to be coupled to a first capacitor having a first capacitance; a first biasing terminal configured to be coupled to the first capacitor; a second biasing terminal; a second capacitor coupled between the second biasing terminal and the floating node, the second capacitor having a second capacitance lower than the first capacitance; a discharge element coupled between the floating node and a ground terminal; a first switch coupled between the first biasing terminal and a reference terminal, the reference terminal coupled to the discharge element; a second switch coupled between the second biasing terminal and the reference terminal; and an amplifier comprising: a first supply terminal configured to receive a first positive voltage, a second supply terminal coupled to the ground terminal, a first input coupled to the floating node, and a second input configured to receive a reference voltage, wherein the LTC circuit is configured read a residual charge of the first capacitor using the amplifier by closing the first and second switches.

2. The LTC circuit of claim 1, wherein the second capacitance is between 0.01 pF and 50 pF.

3. The LTC circuit of claim 1, wherein the second capacitor includes a dielectric layer with a thickness between 70 and 100 .

4. The LTC circuit of claim 1, wherein the LTC circuit is configured to close the first and second switches when the control signal is asserted.

5. The LTC circuit of claim 4, wherein the amplifier is configured to receive an enable signal that is synchronized with a control signal.

6. The LTC circuit of claim 4, wherein the amplifier is configured to receive an enable signal that has a time delay with respect to the control signal.

7. The LTC circuit of claim 1, further comprising: a first resistor coupled between the first biasing terminal and the ground terminal; a second resistor coupled between the second biasing terminal and the ground terminal; and a third resistor directly connected between the reference terminal and the ground terminal.

8. The LTC circuit of claim 7, wherein resistance values of the first, second and third resistors are in the order of 10.sup.6, and wherein a resistance value of the discharge element is in the order of 10.sup.12, or higher.

9. The LTC circuit of claim 1, wherein the discharge element comprises a plurality of elementary discharge units coupled in series.

10. The LTC circuit of claim 9, wherein each elementary discharge unit comprises polysilicon.

11. The LTC circuit of claim 1, further comprising a shifting stage comprising: a first input configured to receive a first positive voltage; a second input configured to receive a first negative voltage; a third input configured to receive a shifting voltage; a first output coupled to the first biasing terminal; and a second output coupled to the second biasing terminal.

12. The LTC circuit of claim 11, further comprising a high-voltage generator configured to generate the first positive voltage and the first negative voltage.

13. The LTC circuit of claim 12, wherein the high-voltage generator comprises a charge-pump.

14. The LTC circuit of claim 1, further comprising a shifting stage configured to shift a reading voltage of the floating node from a negative value to a positive value before reading the residual charge of the first capacitor.

15. The LTC circuit of claim 1, further comprising a shifting stage configured to shift a reading voltage of the floating node by a shifting voltage before reading the residual charge of the first capacitor, wherein the shifting voltage satisfies the following expression:
V.sub.L0+V.sub.R<V.sub.x<V.sub.R wherein V.sub.L0 is a value assumed by the reading voltage at a start of reading the residual charge of the first capacitor, V.sub.R is a value of the shifting voltage, and V.sub.x is a value of the reference voltage.

16. A device comprising: a non-volatile memory; a long time constant (LTC) circuit comprising: a floating node configured to be coupled to a first capacitor having a first capacitance; a first biasing terminal configured to be coupled to the first capacitor; a second biasing terminal; a second capacitor coupled between the second biasing terminal and the floating node, the second capacitor having a second capacitance lower than the first capacitance; a discharge element coupled between the floating node and a ground terminal; a first switch coupled between the first biasing terminal and a reference terminal, the reference terminal coupled to the discharge element; a second switch coupled between the second biasing terminal and the reference terminal; and an amplifier comprising: a first supply terminal configured to receive a first positive voltage, a second supply terminal coupled to the ground terminal, a first input coupled to the floating node, and a second input configured to receive a reference voltage; and a control unit configured to: detect an attempt of an unauthorized access to the non-volatile memory, when an attempt of an unauthorized access is detected, place the device in a blocking state, read a residual charge of the first capacitor using the amplifier by closing the first and second switches, and cause the device to exit the blocking state based on the read residual charge.

17. The device of claim 16, wherein the control unit is further configured to read the residual charge of the first capacitor at regular intervals.

18. The device of claim 16, wherein the device is a cell phone, a smartphone, a camera, a controller for videogames, or a smartwatch.

19. A long time constant (LTC) circuit comprising: a floating node coupled to a first capacitor having a first capacitance; a first biasing terminal configured to be coupled to the first capacitor; a second biasing terminal; a second capacitor coupled between the second biasing terminal and the floating node, the second capacitor having a second capacitance lower than the first capacitance; and a discharge element coupled between the floating node and a ground terminal, wherein the LTC circuit is configured to: shift a value of a reading voltage of the floating node, and after shifting, compare the reading voltage of the floating node with a reference voltage in order to supply an output voltage that has a value that is a function of a result of the comparing and is indicative of a residual charge in the first capacitor during a discharge of the first capacitor.

20. The LTC circuit of claim 19, wherein the LTC circuit is further configured to discharge a charge stored in the first capacitor by leakage through a corresponding dielectric.

21. The LTC circuit of claim 1, wherein the first capacitance is between 1 pF and 100 pF, and the second capacitance is between 0.01 pF and 50 pF.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] For a better understanding of the present invention, a preferred embodiment thereof is now described, purely by way of non-limiting example and with reference to the attached drawings, wherein:

[0036] FIG. 1 shows the circuit diagram of an LTC stage of a known type;

[0037] FIG. 2 shows a reading circuit associated to the LTC stage of FIG. 1, being also of a known type;

[0038] FIG. 3 shows a reading circuit for an LTC stage, according to one embodiment of the present solution;

[0039] FIG. 4 shows the reading circuit of FIG. 3, in a read operating condition; and

[0040] FIG. 5 is a general block diagram of an electronic device that incorporates the LTC stage and the corresponding reading circuit, according to a further aspect of the present solution.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0041] FIG. 3 shows a reading circuit 19 for an LTC stage, once again designated by 1, which includes, as described previously (and as described in detail in the aforesaid US 2015/0043269 A1): the storage capacitor 2, coupled between the first biasing terminal 3a, set in use at the first biasing voltage V.sub.1, and the floating node 4; the transfer capacitor 5, coupled between the second biasing terminal 3b, set in use at the second biasing voltage V.sub.2, and the floating node 4; and the discharge element 6, coupled between the same floating node 4 and the reference terminal 7, and formed by a plurality of elementary discharge units 8, which are coupled together in series and define between them the internal nodes N.sub.i.

[0042] The reading circuit 19 comprises an operational amplifier 20 (in particular, an operational transconductance amplifierOTA), operating as a comparator, which has a first input terminal 20a, for example the negative input terminal, coupled to the floating node 4, a second input terminal 20b, in the example the positive input terminal, which receives a comparison reference voltage V.sub.x, of an appropriate value, and an output 20C, which supplies the comparison voltage V.sub.out, the value of which is indicative of the residual charge in the storage capacitor 2. The operational amplifier 20 further has an enabling input 20d, which receives a read-enable signal EN.

[0043] In particular, the operational amplifier 20 in this case comprises a first supply input 20e, which receives a positive supply voltage V.sub.cc (>0), for example 3.5 V, and a second supply input 20f, which receives the ground voltage gnd. According to a particular aspect of the present solution, the operational amplifier 20 is configured for operation in just the positive-voltage range and does not receive any negative supply voltage.

[0044] Thus, also the comparison reference voltage V.sub.x has an appropriate positive value, which satisfies the relation: 0<V.sub.x<V.sub.cc.

[0045] The reading circuit 19 further comprises a first switch element 22, which is coupled between the first biasing terminal 3a and the reference terminal 7 and is driven by a read control signal S.sub.R (received, for example, from a control unit of the electronic device, not illustrated herein, in which the LTC stage 1 is used). A second switch element 23 is coupled between the second biasing terminal 3b and the reference terminal 7 and is driven by the same read control signal S.sub.R. a generator stage 24 is configured for generating, for example, starting from the positive supply voltage V.sub.cc, a shifting voltage V.sub.R, having an appropriate positive value (as described in detail hereinafter).

[0046] A voltage-switching stage 26 has a first voltage input 26a and a second voltage input 26b, which receive respectively the high positive voltage +HV and the high negative voltage HV from a high-voltage generator stage 27 (of a known type, for example of the charge-pump type), a third voltage input 26c, which is coupled to the generator stage 24 and receives the shifting voltage V.sub.R, a fourth voltage input 26d, which receives the ground voltage gnd, and also a control input 26e, which receives the read control signal S.sub.R. The voltage-switching stage 26 further has a first output 26f and a second output 26g, which are coupled, respectively, to the first and second biasing terminals 3a, 3b, to which it supplies appropriate biasing voltage values (V.sub.1 and V.sub.2) during the operating conditions of the reading circuit 19 (as described in detail hereinafter).

[0047] In particular, the enable signal EN is conveniently timed with respect to the control signal S.sub.R, for example being generated with switchings that are synchronized, or have an appropriate time delay, with respect to the switchings of the control signal S.sub.R.

[0048] The reading circuit 19 further comprises a number of resistors. A first discharge resistor 28 is coupled between the first biasing terminal 3a and a ground node N.sub.g set at ground voltage gnd. A second discharge resistor 29 is coupled between the second biasing terminal 3b and the ground node N.sub.g. A third discharge resistor 30 is coupled between the reference terminal 7 and the ground node N.sub.g.

[0049] In particular, the resistance value of the first, second, and third discharge resistors 28, 29, 30 is much lower than the resistance of the discharge element 6, for example lower by at least one order of magnitude, for example in the order of megaohms.

[0050] In use, during the operation of programming (set) for initialization of the charge in the storage capacitor 2, the voltage-switching stage 26 sends, for example, the first biasing voltage V.sub.1 to the high positive voltage +HV and the second biasing voltage V.sub.2 to the high negative voltage HV. Further, the read control signal S.sub.R determines opening of the first and second switch elements 22, 23.

[0051] During the operation of reset or erasure of the charge stored in the storage capacitor 2, the voltage-switching stage 26 brings, for example, the first biasing voltage V.sub.1 to the high negative voltage HV and the second biasing voltage V.sub.2 to the high positive voltage +HV. The read control signal S.sub.R once again determines opening of the first and second switch elements 22, 23.

[0052] In both operating steps of programming and erasure, the value of resistance of the first, second, and third discharge resistors 28, 29, 30 is sufficiently high as to prevent an undesired current consumption by the high-voltage generator stage 27 (and by the corresponding charge-pump circuits).

[0053] Next, during discharge of the charge stored in the storage capacitor 2 through the discharge element 6, the voltage-switching stage 26 once again determines opening of the first and second switch elements 22, 23.

[0054] According to a particular aspect of the present solution, as illustrated in FIG. 4, the operation of reading of the residual charge present in the storage capacitor 2, by detecting the reading voltage V.sub.L on the floating node 4, envisages that the read control signal S.sub.R determines closing of the first and second switch elements 22, 23 (thus shorting the first and second biasing terminals 3a, 3b), and further that the voltage-switching stage 26 brings both the first biasing voltage V.sub.1 and the second biasing voltage V.sub.2 to the shifting voltage V.sub.R. It should be noted that start of the reading step is in this case determined by switching of the read control signal S.sub.R.

[0055] Consequently, the reading voltage V.sub.L increases instantaneously by a value equal to the shifting voltage V.sub.R, assuming an incremented value: V.sub.L+V.sub.R.

[0056] In particular, the value of the shifting voltage V.sub.R is chosen so that, given the initial voltage value V.sub.L0 assumed at the end of the programming step, for example, negative and equal to 1.5 V, the following relation is satisfied:

V.sub.L0+V.sub.R>0

[0057] For example, the value of the shifting voltage V.sub.R is 2.5 V, and the incremented value is initially 1 V (as illustrated in the aforesaid FIG. 4).

[0058] The generator stage 24 and the voltage-switching stage 26 thus operate jointly as a stage for shifting the reading voltage V.sub.L of the floating node 4, to bring the same reading voltage V.sub.L to positive values (and within the operating voltage range accepted by the operational amplifier 20) before carrying out the comparison with the comparison reference voltage V.sub.x and thus provide the indication of the residual charge in the storage capacitor 2.

[0059] Moreover, the value of the comparison reference voltage V.sub.x is chosen so that the following relation (valid in the case where the initial voltage V.sub.L0 has a negative value) is satisfied:

V.sub.L0+V.sub.R<V.sub.x<V.sub.R

[0060] It should be noted that, advantageously, the operational amplifier 20 in this way works only with positive voltages at the input terminals 20a, 20b.

[0061] During discharge of the storage capacitor 2, on the hypothesis of a negative charge having been stored in the same storage capacitor 2, the reading voltage V.sub.L goes from the initial value V.sub.L0 to ground. Consequently, the incremented value V.sub.L+V.sub.R evolves from the initial value V.sub.L0+V.sub.R to the value of the shifting voltage V.sub.R.

[0062] When this incremented value crosses the value of the comparison reference voltage V.sub.x, the output of the operational amplifier 20 switches, or triggers, and the comparison voltage V.sub.out assumes a value (for example, a high value) indicating the end of the discharge step.

[0063] In particular, the reading operation is enabled by the read-enable signal EN received by the operational amplifier 20 at the enabling input 20d.

[0064] It should be noted that the value of the comparison reference voltage V.sub.x, which thus represents the triggering threshold of the operational amplifier 20, may thus be selected in an appropriate way to regulate the desired duration of the discharge step of the storage capacitor 2.

[0065] The advantages of the solution proposed emerge clearly from the foregoing description.

[0066] In any case, it is underlined once again that the reading circuit 19 allows solving of the problems highlighted previously, in so far as it enables use of an operational amplifier 20 operating with just positive voltages, thus preventing the need for purposely provided circuits for generation of negative references, and further preventing the associated reading delays; and it reduces the spread of the value of the discharge time constant, in particular during powering-off, thanks to introduction of effective discharge resistive paths towards the ground terminal.

[0067] In this regard, it should be noted that, in the power-off condition, advantageously the presence of the first and second discharge resistors 28, 29 ensures the presence of an effective discharge path from the biasing terminals 3a, 3b to ground. In particular, this discharge path prevents formation of alternative discharge leakage paths that might vary the value of the discharge time constant RC.

[0068] Instead, the resistance value of the discharge resistors 28, 29 is such as not to alter the value of the discharge time constant RC, this value of resistance being in fact considerably lower than the value of the resistance of the discharge element 6.

[0069] Basically, the solution described enables increase in the performance and reliability of the reading operations of the LTC stage 1.

[0070] The aforesaid characteristics thus render use of the LTC stage 1 and of the associated reading circuit 19 in an electronic device 40 advantageous, for example, for secure applications, as illustrated schematically in FIG. 5.

[0071] The electronic device 40 comprises a control unit 41, for example of the microprocessor or microcontroller type, which supervises its general operation, and a memory 42, of a non-volatile type, operatively coupled to the control unit 41.

[0072] The control unit 41 further comprises the LTC stage 1 and an associated electronic interface circuit 44, including the reading circuit 19 and further biasing circuits (not illustrated herein).

[0073] In particular, the control unit 41, following upon detection of an attempt at attack by an external electronic device 46 (for example, following upon detection of an unauthorised access to the information stored in the memory 42), may determine a blocking state of the electronic device 40. The control unit 41 may further read, for example at regular intervals, the residual charge stored in the storage capacitor 2 of the LTC stage 1, in order to determine exit from the blocking state, at the end of a wait interval of a preset duration (which, as described previously, may be appropriately regulated via the value of the comparison reference voltage V.sub.x).

[0074] The electronic device 40 may advantageously be integrated in a portable mobile-communication apparatus (not illustrated), such as a cell phone, a smartphone, a personal digital assistant (PDA), a digital audio player with voice-recording capacity, a photographic camera or video camera, a controller for videogames, etc., or a wearable apparatus, such as a smartwatch or an electronic bracelet.

[0075] Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

[0076] For example, it is evident that the numeric values indicated for the voltages acting in the reading circuit 19 are to be understood as provided purely by way of example, since in an equivalent way different values may be present, according to the particular operating requirements.

[0077] Furthermore, the LTC stage 1 and the corresponding reading circuit 19 may be used in different electronic devices, in general for secure applications. Other uses may in any case be envisaged, for example in the field of management of the timing for access rights to multimedia contents.