Abstract:
The invention relates to a method and a circuit for regulating a pulse synchronization signal (ATD) for the memory cell read phase in semiconductor integrated electronic memory devices. The pulse signal (ATD) is generated upon detection of a change in logic state of at least one of a plurality of address input terminals of the memory cells, so as to also generate an equalization signal (SAEQ) to a sense amplifier. The SAEQ pulse is blocked (STOP) upon the row voltage reaching a predetermined sufficient value to provide reliable reading. Advantageously, the pulse blocking is produced by a logic signal (STOP) activated upon a predetermined voltage value being exceeded during the overboost phase of the addressed memory row.

Description:
TECHNICAL FIELD 
     This invention relates to a method and a circuit for regulating the duration of an ATD pulse synchronization signal for the read phase of non-volatile memory cells in electronic memory devices integrated in a semiconductor. 
     In particular, the invention relates to a circuit for regulating an ATD pulse synchronization signal in order to regulate the read phase of non-volatile memory cells in electronic memory devices integrated in a semiconductor, the circuit being of a type which is controlled by a change of logic state on at least one of a plurality of address input terminals of said memory cells, and comprises a NOR type of structure between said address terminals and an output node whence an equalization signal to a sense amplifier is derived. 
     BACKGROUND OF THE INVENTION 
     As is well known, the read mode, for reading the contents of the cells of a semiconductor integrated electronic memory circuit, is entered by completing a predetermined sequence of operations known as the reading cycle. 
     A reading cycle begins with the memory address of data to be read being presented to the input terminals of a memory circuit. An input stage senses the switching of an address presented to these terminals, thereby to initiate a reading operation. 
     Row and column decoding circuits will select the memory word that has been addressed. 
     The circuit portion arranged to read the contents of the memory cells and convert the analog data read to digital data is referred to as the sense or read amplifier. This amplifier usually is of the differential type and has a pair of inputs which are connected to a cell of the memory matrix and a reference cell, respectively. Reading is enabled by an unbalance in the loads of the matrix leg and the reference leg. 
     The data sensed by the sense amplifier is then output through a buffer output stage. 
     Each of the above phases of the reading cycle must have a preset duration consistent with the memory access times specified by the memory circuit specifications. 
     All of the various phases of the reading cycle are clocked by synchronization pulses derived from a single main or ATD (Address Transition Detection) pulse. The ATD pulse is generated within the memory circuit whenever a change in address is detected on the input terminals. 
     In general, the ATD pulse is generated by a NOR structure whose output is at a normally high logic level. 
     Upon the occurrence of a change in logic level at even one only of the input terminals, the NOR structure switches its output to allow a terminal from which the ATD pulse is delivered to be discharged toward ground. 
     Shown schematically in the accompanying FIG. 1 is circuitry for generating the ATD signal according to the prior art. 
     FIG. 1 shows an ATD cell or circuitry  11  comprising two N-channel MOS input transistors, indicated at M 1  and M 2 , which are highly conductive on account of their high W/L ratio. 
     The cell  1  further comprises a pair of inverters I 1 , I 2 , each including a CMOS complementary pair having a pull-up transistor and a pull-down transistor. The pull-up transistors of the inverters I 1 , I 2  are highly resistive, and therefore little conductive, they having a reduced W/L ratio. 
     The structure resulting from the coupling of the inverters I 1  and I 2  is that of a latch register  3  having outputs Q and {overscore (Q)}, wherein the former, Q, is at a normally high logic level. 
     The latch  3  is input a signal AX and the corresponding negated signal AX_N from one of said input terminals, as smoothed by capacitors C 1  and C 2 . These signals are enabled to pass on to the latch  3  by the respective NMOS transistors M 1  and M 2 . 
     During the wait phase, only one of the input signals will be at a high logic value, e.g., AX_N. The capacitor C 2  will be discharged by the pull-up of the first inverter I 1 . 
     Upon the occurrence of an input transition, the capacitor C 1  of the transistor M 1  is quickly discharged, while the capacitor C 2  begins to be charged by the pull-up of the second inverter I 2 . In consequence of this, the first output Q of the latch  3  is at once brought to a low logic level. The other output Q# will instead take a little longer to change its state because the pull-up transistors of the inverters I 1 , I 2  are highly resistive. Thus, there will be a time period when both said outputs are at a low logic level. 
     With the outputs Q and {overscore (Q)} connected directly to the respective inputs of a logic gate I 3  of the NOR type, the output of the gate I 3  will be driven to a high logic level, thereby allowing an NMOS transistor M 3  connected to the output node  4  of the circuit  11  to be turned on. 
     Associated with each address input terminal of the memory circuit is a cell  11 , and a plurality of cells have their outputs tied to a common line as shown schematically in FIG.  2 . 
     In this approach, referred to in the art as the distributed NOR, the output  18 , for each cell, isconnected to a single ATD-LINE line  7  which is usually in the form of a metallization line taken to the supply Vdd through a PMOS transistor M 4  having its control terminal connected to a ground GND. 
     An ATD pulse is delivered from this line  7  through an inverter  5 . 
     Each ATD cell  11  can bias the line  7  to ground on the occurrence of an input transition. This line  7  being relatively long, it exhibits resistance and intrinsic capacitance of relatively high values, and if the switching involves all the addresses in parallel, the line  7  will be discharged at a very fast rate; otherwise, when the switching only affects the farthest terminal from the output node, the line  7  would be discharged at a slower rate. 
     The ATD signal performs two basic functions: a first is that of initiating the supply voltage boost operations, and the second is that of initiating the equalization of the sense amplifier nodes. 
     Shown schematically in the accompanying FIG. 3 are the main features of a conventional sense amplifier. 
     A circuit block A represents that portion of the circuit which is intended for the current/voltage conversion, and a circuit block B represents the circuit portion which drives the output stages. The block A is to convert the value of the current, taken up by the non-volatile memory cells being written or erased, to a voltage. 
     The sense amplifier can discriminate between the logic contents, a “0” or a “1”, of a selected memory cell by verifying the unbalance of the matrix and reference nodes Vref and Vmat, respectively. 
     The conventional technique is to equalizing the nodes Vref and Vmat before effecting a reading. 
     The ATD signal is to generate an equalization signal, designated SAEQ hereinafter, to prepare the sense amplifier for a reading operation to be completed within the shortest possible time. 
     FIG. 4 is a plot vs. time of a plurality of voltage signals present in the memory circuit immediately after a transition on an address input. 
     A rise in the signal SAEQ reflects in a similar rise of the boost signal for the memory row or word line, and in a short phase of equalization of the nodes Vref and Vmat. The read phase proper takes place on the falling edge of the signal SAEQ, when the control of the nodes Vref and Vmat is shifted from the equalization network to the reference and the matrix cell. 
     From all of the above considerations, it can be appreciated that the duration of the pulse SAEQ is vital to the memory access time. 
     To avoid harmful oscillations of the nodes Vref and Vmat, the equalization network should only be released once the selected cell is biased at the highest gate voltage and, accordingly, able to take up maximum current. Otherwise, an erased cell could be mistaken for a written one. 
     In this respect, it should be considered that memories operated at low supply voltages require that the cell be biased with a boosted voltage. 
     Furthermore, in the prior art solutions, the ATD pulse is extended through a chain of inverters to provide the signal SAEQ; the consequent timing is highly responsive to temperature and supply voltage variations, and for modern memory devices, variations of as much a 150° C. and a few volts are to be anticipated. 
     More complicated solutions provide a pulse of preset duration to a fixed time for conditions of minimum voltage and maximum temperature, so as to have a sufficient margin in every condition of operation. Thus, in the opposite operating conditions, i.e., minimum temperature and maximum supply voltage, the pulse duration will be much longer, resulting in a waste of memory access time. 
     SUMMARY OF THE INVENTION 
     An embodiment of this invention provides a method and a circuit for regulating the duration of the ATD signal pulse, which method and circuit have such respective functional and structural features as to overcome the drawbacks which are besetting the solutions according to the prior art. 
     The embodiment stops or blocks the ATD, or SAEQ, pulse upon the row voltage reaching a sufficient value or height to provide reliable reading. 
     In this way, the ATD signal is substantially adaptive and unaffected by parameters outside the actual execution of the read phase. 
    
    
     The features and advantages of the method and the circuit of this invention will be apparent from the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a circuit for generating a pulse ATD signal. according to the prior art. 
     FIG. 2 shows schematically a set of circuits as in FIG. 1, connected into a distributed NOR structure for generating the ATD signal. 
     FIG. 3 shows schematically a read amplifier incorporated to an electronic memory device and utilizing the ATD signal to initiate the read phase. 
     FIG. 4 shows a set of curves of voltage signals plotted against time, as present in the amplifier of FIG.  3 . 
     FIG. 5 shows schematically a row decoding circuit portion incorporated to an electronic memory device. 
     FIG. 6 shows schematically a circuit embodying this invention. 
     FIG. 7 shows voltage (V) vs. time (ns) plots representing signals present in the regulator circuit of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawing views, in particular to the example of FIGS. 5 and 6, generally and schematically shown at  1  is a circuit of this invention for regulating the duration of an ATD (Address Transition Detection) signal to be used during a memory cell reading cycle. 
     Memory cells are here a plurality of memory elements incorporated to a conventional semiconductor integrated electronic device which is constructed as a matrix of cells organized into rows and columns. Associated with the matrix are corresponding row and column decoding circuit portions and sense amplifiers. 
     These circuit components are powered between a first supply voltage reference Vcc and a second voltage reference GND. 
     The memory cells may be of a type whichever, those of the non-volatile type being preferred. 
     Shown in FIG. 5 is a final portion  10  of the row decoder, together with a portion of the voltage booster circuitry for use with the present invention. 
     A first circuit node A is shown in the portion  10  as having one end of a capacitor Cboost 1  connected thereto which is adapted to be charged and discharged during the row voltage boost phase. 
     The other end of the capacitor Cboost 1  is connected to a second circuit node B corresponding to the supply reference of the row drivers of the memory matrix. 
     The second node B is connected toward a voltage reference Vpcx through a PMOS transistor M 10  having its body terminal connected to the node B and having the control terminal arranged to receive a signal CONTROL. 
     A parasitic capacitor Cp is present between the node B and the second ground reference. 
     A complementary pair of transistors, PMOS transistor MBP and NMOS transistor MBN, further connect the node B to the addressed memory row. The source terminal of the NMOS transistor MBN in this complementary pair is connected to a potential Vgc representing a signal ground. 
     The capacitor Cboost 1  should be precharged to the supply voltage Vcc prior to receiving the boosted voltage pulse. For this to occur, the node A should be at ground potential and the node B biased at Vcc. 
     The control signal CONTROL turns off the PMOS transistor M 10  such that the node B can be voltage boosted and the boosted voltage transmitted to the addressed row through the transistor MBP, the other transistor MBN being turned off. 
     Depicted in FIG. 6 is the basic construction of the circuit  1  of this invention, which is connected to the input of the node B of FIG. 5 to pick up a boosted voltage signal, indicated at BULK in FIG.  6 . 
     The circuit  1  has an input terminal IN and an output terminal OUT. The input IN is, as previously stated, connected to the node B and has a PMOS pass transistor M 1  cascade connected thereto which has its control terminal connected to the first supply reference Vcc. 
     A capacitor C 1  is connected in series with this pass transistor M 1  and is to charge an internal node A 1  at a predetermined time constant. 
     Between the node A 1  and the second supply reference GND, a pair of NMOS transistors M 2 , M 3  are connected in series with each other and have their gate terminals connected to the supply Vcc. 
     The combined series resistances of the channels of M 2  and M 3 , in cooperation with the capacitor C 1 , set the time constant RC for charging and discharging the node A 1 . 
     Advantageously, the transistor pair M 2  and M 3  provide a cascodc effect for the connection to the ground GND. 
     An additional NMOS transistor M 7  is connected between the node A 1  and ground, and receives on its gate terminal a signal DIS enabling a fast discharge of the node A 1 , so as to restore the circuit  1  for operation at the next input transition. 
     A short-circuit transistor M 6  is connected in parallel across the capacitor C 1  and receives a signal DIS on its gate terminal for re-distributing the charge onto the capacitor C 1 , causing the discharge phase to occur at a near-zero voltage difference, and accordingly, producing a smaller width pulse toward ground. 
     If C 1  stays charged, the node A 1  may acquire a negative potential, but the transistors M 6  and M 7  provided are effective to avoid this possible problem. 
     The potential at the node A 1  is applied directly to the gate terminal of an NMOS pull-down transistor M 4  included in a buffer stage  9  which also comprises a PMOS transistor M 5 . 
     The stage  9  is connected between the supply Vcc and the ground GND, with the pull-up transistor M 5  having its gate terminal connected to ground. 
     The interconnection node between the transistors M 4  and M 5  of the stage  9  is connected to the output OUT of the circuit  1  through an inverter  8  allowing a signal STOP to be output which is active to turn off the signal SAEQ. 
     As shown in FIG. 6A, the signals STOP and ATD are coupled to first and second inputs of a logic stage  15  which produces the signal SAEQ at its output. The logic stage  15  can be using various logical elements such as aDT flip-flop gate  15 A having a reset input that is coupled to the STOP signal via an inverter  16 A, a timing input coupled to the ATD signal, and an output that provides the signal SAEQ. Such a configuration of the flip-flop  15 A generally implements a logical AND. Alternatively, the logic stage  15  can be implemented using a NOR gate  15 B having a first input coupled to the STOP signal and a second input coupled to the ATD signal via an inverter  16 B. In whichever configuration, the logic stage  15  inactivates the SAEQ signal (goes low) in response to the STOP signal being activated (goes high). 
     It will be appreciated from the foregoing that the inventive circuit acts as a shunter capable of detecting the overboost occurring to the addressed memory row and of producing a stop or interrupt signal which blocks the signal SAEQ and restores it to ground. In this way, the signal SAEQ only remains active for the time required to complete the reading cycle. 
     In other words, it is as if the signal SAEQ were provided with a self-turn-off function that cancels it once the effective memory access sought is obtained. 
     The signal BULK picked up from the node B is normally at the potential Vcc, so that the transistor M 1  is off and the node A 1  held to ground by the transistors M 2  and M 3 . In these conditions, the signal STOP will be at a logic low as clearly shown in FIG.  7 . 
     Upon the signal BULK exceeding by a threshold the supply Vcc, the transistor M 1  begins to conduct and, in view of a suitable ratio having been selected between M 1  and the series of the transistors M 2 , M 3 , the transistor M 4  can be turned on by the potential at the node A 1 , thereby bringing the signal STOP to a logic high. 
     In essence, the signal STOP is activated upon the signal BULK exceeding by a PMOS threshold the value of the supply Vcc. 
     Once the input step is depleted, the node A 1  is discharged at a preset time constant to restore the initial conditions. 
     The waveform of the node A 1  is shown in FIG.  7 . It should be noted that on the falling edge of the signal BULK there may occur a pulse toward ground which is undesired and likely to trigger latchup phenomena. This possible problem has already been corrected by providing the pair of transistors M 2 , M 3  in cascode configuration. 
     The method and circuit of this invention do solve the technical problem and afford a number of advantages, outstanding among which is the adjustability of the AID pulse duration to meet actual memory access requirements. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.