Circuit device and corresponding method for resetting non-volatile and electrically programmable memory devices

Abstract: A method for generating a reset signal in an electrically programmable non-volatile storage device (1) of a type which comprises a matrix (2) of memory cells and a control logic portion (3) being supplied a supply voltage (Vcc) and a programming voltage (Vpp), and a threshold detection circuit (5) adapted to detect a decrease in the supply voltage (Vcc), provides for the signal applied to the control logic (3) to be obtained as a change-over function between the output signal from the threshold detector (5) and a reset signal (POR) generated during the power-on transient of the device.

Description: Field of the Invention This invention relates to a circuit device and associated method for generating a reset signal in storage devices. The invention is particularly, but not exclusively, concerned with a non-volatile storage device, integrated to a semiconductor and electrically programmable, and the ensuing description will make reference to that application for convenience of illustration. Prior Art As is known, many of the storage devices currently available commercially provide for the memory internal logic control circuitry to be reset. The reset signal is normally generated within the integrated storage circuit, and the activation of this signal is responsive to a decrease in the circuit supply voltage Vcc. This manner of operation is of special importance to devices being supplied a programming voltage Vpp from outside which usually has a relatively high value, in excess of 10 volts. The most dangerous situation likely to be encountered by such devices is, in fact, a decrease in the supply voltage Vcc while the value of the programming voltage Vpp is held at a high. Under this condition, lacking an ability to control the logic circuitry operation, erasure or incorrect programming of the storage elements may occur. To avoid such problems, currently available devices on the market are arranged to include threshold detection devices which are supplied the programming voltage Vpp and effective to detect a drop in the supply voltage Vcc below a predetermined minimum value. Such circuits activate the device resetting by disabling all the circuits connected to the relatively high programming voltage Vpp. The operation of the detection circuits is only ensured, however, in the presence of a programming voltage Vpp which has a positive value above a predetermined minimum value. An example of a detection circuit based on the above-outlined operation principles is to be found in US Patent No. 4,975,883 to INTEL. The circuit described therein has an output which would signal a drop in the supply voltage Vcc below a threshold value of 3.7 volts, but only for higher values of the programming voltage than 4.5 volts. However, the reset mode provided by the prior art circuits affords no control at the device power-on stage, i.e. as the supply voltage Vcc is brought from 0 up to a steady-state value. This deficiency is quite serious because on turning on, and in the absence of the programming voltage Vpp, no reset signal would be generated. In this way, the initial state of the internal control logic remains indetermined. To obviate this drawback, some devices provide for the storage reset signal to be driven from outside. For instance, the integrated storage circuit currently available commercially has a PWD (Deep Power Down) pin intended to minimize the circuit consumption. However, that PWD pin may also receive a reset signal activated by the user. The inadequacy of such a solution, depending on the device user's proper recollection, is self-evident. The underlying technical problem of this invention is to provide a uniquely simple circuit solution which allows the storage device to be reset automatically even during the power-on transient, thereby overcoming all the drawbacks with which the prior art approaches are beset. Summary of the Invention The solutive idea on which the invention stands is one of obtaining a logic sum between a logic signal responsive to a decrease in the supply voltage Vcc, being only effective in the presence of a programming voltage Vpp above a predetermined value, and a signal generated by a purposely provided circuit which follows the power-on transient. Based on this solutive idea, the technical problem is solved by a circuit as defined in Claim 1. The technical problem is also solved by a method for resetting a storage device, as defined in Claim 4. The features and advantages of a circuit and a method according to the invention will be apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 diagramatically shows a non-volatile storage device incorporating a reset signal generating circuit according to the invention; and Figures 2 and 3 diagramatically illustrate the pattern vs. time of a set of electric signals present in the device shown in Figure 1. Detailed Description With reference to the drawing Figures, generally and schematically shown at 1 is a non-volatile storage circuit which is integrated to a semiconductor and electrically programmable. The circuit 1 is in particular, but not limited thereto, a storage device of the "flash" type. This device comprises a memory matrix 2 and a logic control circuit portion 3, as well as electric connections 4 of the bus type, between the matrix 2 and the control logic 3. The circuit 1 is supplied by a supply voltage Vcc of about 5 V and by a programming voltage Vpp of about 12 V. These voltages are supplied to the storage circuit through respective input terminals or pins. The device 1 further comprises a threshold detection circuit 5 connected between the supply voltage Vcc, the programming voltage Vpp, and a signal ground GND. The circuit 5 has an output 6 at which a logic signal LowVcc having a "high" logic value will appear upon detection of a decrease in the supply voltage Vcc below a predetermined minimum value Vccmin. The circuit 5 would only be operative in the presence of a programming voltage Vpp higher than a predetermined minimum value Vppmin. Under this condition (Vpp > Vppmin), the signal LowVcc would be brought to a "high" logic value upon the supply voltage Vcc dropping below the minimum threshold Vccmin. The signal LowVcc pattern in relation to the threshold value Vccmin is illustrated by the graph in Figure 2. Also provided is a circuit 7 generating a reset logic signal POR (Power On Reset). The circuit 7 is supplied the supply voltage Vcc, and is also operative in the absence of the programming voltage Vpp. It plays its most important role as the device 1 is turned on. The signal POR is passed to an output 8 of the circuit 7, and during the power-on or power-off transient it essentially tracks the value of the supply voltage Vcc until the voltage Vcc occurs between two predetermined values Va and Vb, the latter limit lying below the minimum threshold Vccmin. The pattern of the signal POR is illustrated by the graph in Figure 3. Advantageously according to the invention, the circuit 1 comprises a logic gate 10 having two inputs 9, 11 and an output 12. The logic gate 10 is of the OR type. The inputs 9 and 11 of said logic gate 10 are respectively connected to the output 6 of the circuit 5 and the output 8 of the circuit 7. As a result, the first input 9 of the gate 10 will be applied the logic LowVcc, whilst the second input 11 will be applied the logic signal POR. The output 12 of the logic gate 10 supplies a reset signal which is determined by the logic sum of the input signals and applied to the control logic 3. The method of this invention for generating the reset signal will now be described. As already brought out by the foregoing description, the method of this invention provides for application of the reset signal to the control logic 3 as a function of the change-over between the output signal from the threshold detector 5 and the signal POR generated by the circuit 7 during the power-on transient of the device 1. In particular, the change-over function is a logic sum (OR). The reset signal at the output of the logic gate 10 behaves like the signal LowVcc in the presence of programming voltage Vpp (of course, only if Vccmin is greater than Vb), and duplicates the pattern of the signal POR in the absence of programming voltage Vpp. Thus, on turning on, and absent the programming voltage Vpp, the whole logic circuitry 3 of the device 1 is held to a reset state for as long as the value of the supply voltage Vcc is held between Va and Vb. Thus, the whole circuitry will exhibit a logic state as defined before the operation is started, thereby overcoming the prior art problems previously pointed out. The values of the threshold voltages Va and Vb used in the circuit are selected to meet the requirements for proper operation of the storage circuit. For instance, the threshold value Vb is to be lower than Vccmin but in all cases equal to a voltage whereby the entire logic of the device is expected to perform satisfactorily. Furthermore, in a storage circuit provided with a PWD (Deep Power Down) terminal, this terminal would just serve the single function of lowering the device consumption, and no external resetting would become necessary. Thus, using the solution provided by the invention, a user in no need for low consumption may connect the PWD terminal directly to ground. This makes for more convenient management of the device.

Claims: An electrically programmable non-volatile memory device (1) having a matrix (2) of memory cells, a control logic portion (3), supplied by a supply voltage (Vcc) and a programming voltage (Vpp), a reset signal generator (7) and a threshold detection circuit (5) adapted to detect a decrease in the supply voltage (Vcc) below a first predetermined minimum value (Vccmin), characterized in that: the memory device (1) further comprises an OR logic gate (10) having a first input (9) connected to an output (6) of the threshold detection circuit (5), a second input (11) and an output (12) connected to said control logic portion (3); and that the reset signal generator (7) has an output (8) connected to the second input (11) of said or logic gate (10) and provides a reset signal (POR) when the supply voltage (Vcc) is in the range between a minimum threshold (Va) and a maximum threshold value (Vb), said minimum and maximum threshold values (Va, Vb) being smaller than the first predetermined minimum value (Vccmin), and the threshold detection circuit (5) being only operative when the programming voltage (Vpp) is higher than a second predetermined minimum value (Vppmin) and the reset signal generator (7) being operative and the reset signal (POR) being generated also in the absence of the programming voltage (Vpp). A device according to Claim 1, characterized in that said reset signal generator (7) is only supplied the supply voltage (Vcc). A device according to Claim 1, characterized in that said reset signal (POR) has the same value as the supply voltage (Vcc). A method for generating a reset signal in an electrically programmable non-volatile memory device (1) having a matrix (2) of memory cells, a control logic portion (3), supplied by a supply voltage (Vcc) and a programming voltage (Vpp), a reset signal generator (7) and a threshold detection circuit (5) adapted to detect a decrease in the supply voltage (Vcc) below a first predetermined minimum value (Vccmin), characterized in that: said reset signal is applied to the control logic portion (3) and is generated by means of a logic sum (OR) between the output signal from the threshold detection circuit (5) and a power-on-reset signal (POR) generated during the power-on transient of the memory device (1), said threshold detection circuit (5) being only operative when the programming voltage (Vpp) is higher than a second predetermined minimum value (Vppmin), and said power-on-reset signal (POR) being generated when the supply voltage (Vcc) is in the range between a minimum threshold (Va) and a maximum threshold value (Vb), said minimum and maximum threshold values (Va, Vb) being smaller than the first predetermined minimum value (Vccmin), also in the absence of the programming voltage (Vpp).

Inventors: OLIVO MARCO, PADOAN SILVIA, OLIVO, MARCO, PADOAN, SILVIA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G11C16
IPC codes: G11C17, G06F1, G11C16