Abstract:
Embodiments of the invention provide a power-on reset function that establishes logic circuits in a memory chip at an initial stable state and a power-on read function that triggers a read operation of the memory chip. A first voltage detector output signal transitions when a power supply voltage reaches a first voltage, setting the logic circuits at the initial stable state. A second voltage detector output signal transitions when the power supply voltage reaches a second voltage, placing a latch in a set state that results in activation of a power-on read signal. A power-on read operation is carried out according to the activation of the power-on read signal. If the power supply voltage is not lowered below the first voltage, the second voltage detector output signal does not transition. Accordingly, embodiments are capable of preventing power-on read operations that are unnecessarily performed owing to power noise.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority from Korean Patent Application No. 2002-29493, filed on May 28, 2002, the contents of which are herein incorporated by reference in their entirety for all purposes.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field of the Invention  
           [0003]    This disclosure relates to a power detecting circuit of a semiconductor memory device, and in particular to a power detecting circuit having a power-on reset function and a power-on read function of a semiconductor memory device. Further disclosed is a method for generating a power-on reset signal and a power-on read signal.  
           [0004]    2. Description of the Related Art  
           [0005]    A semiconductor memory device includes an array of memory cells and a number of logic circuits for controlling the array. For example, the logic circuits may be formed of a number of latches and flip-flops. Logic circuits, and in particular, flip-flops and latches, must initially have states that are well-defined. This is accomplished by a power detecting circuit. A conventional power detecting circuit outputs a power-on reset signal that is activated during a predetermined interval of time until a power supply voltage reaches a predetermined voltage at power-on. The power-on reset signal is inactivated when the power supply voltage reaches the predetermined voltage. Logic circuits in a semiconductor memory device are reset to their initial states at activation of the power-on reset signal.  
           [0006]    In the case where non-volatile memory devices are used as a boot-up memory, a read operation is carried out after a power-on reset operation is performed. A voltage detecting circuit is used that detects a power supply voltage and switches the logic state of its output signal when the power supply voltage reaches a predetermined voltage (i.e., a detection voltage). For example, if a power supply voltage reaches a detection voltage, an output signal of a power detecting circuit has a high-to-low transition, and a read operation commences when the signal transition is received by the memory chip. However, noise may be caused by an unstable state of an external power supply voltage or excessive power consumption in the chip. If this occurs, the chip power supply voltage may be suddenly lowered below the detection voltage. If the power supply voltage falls below the detection voltage, it may be raised up to the power supply voltage again. In this case, a voltage detecting circuit detects variation of a power supply voltage, and an output signal of the detecting circuit has a high-to-low transition according to a detection result. This causes the read operation to commence, even though the system was not in a power-up phase. Since low-voltage memory devices are very sensitive to noise, problems such as this abnormal read operation may become more serious.  
           [0007]    Accordingly, a memory device capable of preventing an abnormal read operation due to noise and a power detecting circuit having improved immunity is desirable.  
         SUMMARY OF THE INVENTION  
         [0008]    Embodiments of the invention provide a power detecting circuit of a non-volatile memory device capable of improving noise immunity.  
           [0009]    Embodiments of the invention provide a memory device capable of reading out data stored in a memory cell of a non-volatile memory device without an external command signal (or an external command and an address) at power-on in any system.  
           [0010]    Embodiments of the invention provide a method of generating a power-on reset signal and a power-on read signal.  
           [0011]    Embodiments of the invention also provide a stable power-on reading method for a memory device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components.  
         [0013]    [0013]FIG. 1 is a functional block diagram of a flash memory device according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is a power detecting circuit operable in the flash memory device of FIG. 1.  
         [0015]    [0015]FIGS. 3A, 3B, and  3 C are timing diagrams illustrating example voltages from the power detecting and latch circuits of FIG. 2 as a function of time.  
         [0016]    [0016]FIG. 4 is a circuit diagram illustrating another latch circuit that may be used with the flash memory device of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Embodiments of the invention will be more fully described with reference to the attached drawings of FIGS.  1  to  4 . In the attached drawings, similar constituent elements are marked by the same or similar reference numerals or symbols, respectively.  
         [0018]    [0018]FIG. 1 schematically shows a flash memory device  100  according to an embodiment of the invention. A flash memory device  100  includes a power detecting circuit  135  and a memory chip  140 .  
         [0019]    The flash memory device  100  in FIG. 1 can be used in any system as a boot-up memory for storing boot-up information and as a general memory for storing information except for boot-up information. In a case where the flash memory device  100  is used as the boot-up memory, if logic circuits in the memory are initialized by the power detecting circuit  135  and a power-on read signal D pre  is enabled, an operation of reading out data (e.g., boot-up information) from a memory cell array  161  commences with reference to a previously designed algorithm. Namely, if the power-on read signal D pre  is enabled, commands and addresses are generated in the memory chip  140  according to the previously designed algorithm. Also, if a user gives an external address command to the memory chip  140 , data can be read out from the memory cell array according to the given address. After the power-on read signal D pre  is enabled, a read operation commences only according to an input of an address without a read command. Accordingly, it is possible to perform a read operation according to a user&#39;s choice at power-on.  
         [0020]    The power-on detecting circuit  135  according to the embodiments of the invention generates a power-on reset signal (which will be subsequently referred to as the first buffered detection signal D B1 ) that resets logic circuits in a flash memory device installed on a system to an initial stable state. The power-on detecting circuit  135  further generates a power-on read signal D pre  that triggers a power-on read operation where data is read out from specific cells of a memory cell array. The power-on read operation means that a read operation starts without a read command input when a power supply voltage VDD rises greater than a predetermined voltage after power-on.  
         [0021]    The power detecting circuit  135  includes a voltage detecting circuit  115  and a latch circuit  130 . The voltage detecting circuit  115  includes first voltage detector  110  and second voltage detector  120 . The power detecting circuit  135  resets logic circuits in the memory chip  140  through the first voltage detector  110  (by generating the first buffered detection signal D B1 ) when the power supply voltage VDD increases at power-on and reaches the first voltage. If the power supply voltage VDD reaches the second voltage, the power detecting circuit  135  outputs a power-on read signal D pre  from the latch circuit  130  to trigger a power-on read operation of the memory chip  140 .  
         [0022]    The memory chip  140  includes a memory cell array  161 , control logic  141  for outputting a control signal in response to the power-on read signal D pre , an address buffer  143  for generating row and column addresses, a row decoder  145 , a word line control circuit  149 , a column decoder  147 , a bit line control circuit  151 , a data input/output buffer  153 , a data input/output and address input block  157 , a control signal input block  159 , and a high voltage generator  155 .  
         [0023]    Control logic  141  controls the row decoder  145 , the column decoder  147 , the word line control circuit  149 , the bit line control circuit  151 , and the data input/output buffer  153  during a read operation. The row decoder  145  decodes a row address from the address buffer  143  in response to a control signal from control logic  141 . The word line control circuit  149  receives a decoded row address from the row decoder  145  and a high voltage from the high voltage generator  155 , and selects at least one of a number of rows in the memory cell array  161  in response to a control signal from control logic  141 . In conclusion, the row decoder  145  and the word line control circuit  149  function as a row selecting circuit that selects one or more rows corresponding to a row address.  
         [0024]    The bit line control circuit  151  receives a high voltage from the high voltage generator  155 , and reads and stores data from memory cells in a row, which is selected by the row decoder  145  and the word line control circuit  149 , in response to a control signal from control logic  141 . The column decoder  147  decodes a column address from the address buffer  143  in response to a control signal from the control logic  141 , and provides a decoded column address to the bit line control circuit  151 . Data in the bit line control circuit  151  is transferred to the data input/output buffer  153  according to the decoded column address from the column decoder  147  and a control signal from control logic  141 .  
         [0025]    In the flash memory device  100 , an address from the address buffer  143  can be generated by inputting an external address directly to a data input/output and address input block  157 . Alternatively, an address from the address buffer  143  can be generated internally by control logic  141 . In both cases, as the power-on read signal D pre  is enabled, the power-on read operation of a memory chip  140  commences by an address input (or without an address input) and without a read command.  
         [0026]    [0026]FIG. 2 shows an example of the power detecting circuit  135  that can be used in the memory device illustrated in FIG. 1. Referring to FIG. 2, the power detecting circuit  135  includes a first voltage detector  110  formed by a first voltage detecting circuit  111  and a first buffer circuit  113 , a second voltage detector  120  formed by a second voltage detecting circuit  121 , a synchronization component  123 , and a second buffer circuit  125 . The power detecting circuit  135  also includes a latch  130 .  
         [0027]    The first voltage detecting circuit  111  senses variation of a power supply voltage VDD, and outputs the first detection signal D 1  at a logic low state when the power supply voltage VDD reaches the first voltage. The first buffer circuit  113  includes two CMOS inverters  115  and  117  that are connected in series to each other. The first buffer circuit  113  is connected to an output terminal of the first voltage detecting circuit  111  and outputs the first buffered detection signal D B1 . The first buffered detection signal D B1  from the first voltage detector  110  triggers the power-on reset function for resetting latches and flip-flops (that is, logic circuits) in a memory chip  140 , for example, control logic, a row decoder, a column decoder, and so on. The control logic  141  of the memory chip  140  operates responsive to a power-on read signal D pre  from the latch  130 , so that a power-on read operation starts.  
         [0028]    The first voltage detecting circuit  111  includes a number of enhancement-type PMOS transistors P 1 -Pn connected in series between the power supply voltage VDD and the first node N 1 , a number of depletion-type NMOS transistors DN 1 -DNn connected in series between the first node N 1  and a ground voltage VSS, a capacitor C 1  connected between the first node N 1  and the ground voltage VSS, and a CMOS inverter  112  connected between the first node N 1  and the ground voltage VSS. The gates of the transistors P 1 -Pn and DN 1 -DNn are grounded. A logic threshold voltage of the inverter  112  is equal to the first voltage. The CMOS inverter  112  includes a PMOS transistor I_P 1  and an NMOS transistor I_N 1  connected in series between the power supply voltage VDD and the ground voltage VSS. Gates of the transistors I_P 1  and I_N 1  are commonly connected to the first node N 1 , and drains of the transistors I_P 1  and I_N 1  are interconnected to form an output terminal for outputting the first detection signal D 1 .  
         [0029]    The first buffer circuit  113  includes two CMOS inverters  115  and  117  that are connected serially each other. The CMOS inverter  115  includes PMOS transistor I_P 2  and NMOS transistor I_N 2  that are connected in series between the power supply voltage VDD and the ground voltage VSS, and the CMOS inverter  117  includes PMOS transistor I_P 3  and NMOS transistor I_N 3  that are connected in series between the power supply voltage VDD and the ground voltage VSS. The gates of the transistors I_P 2  and I_N 2  are interconnected to form an input terminal that is connected with an output terminal of the first voltage detecting circuit  111 , that is, an output terminal of the inverter  112 . The drains of the transistors I_P 2  and I_N 2  are interconnected to form an output terminal of the inverter  115 . Likewise, gates of the transistors I_P 3  and I_N 3  are interconnected to form an input terminal that is connected with an output terminal of the inverter  115 . The drains of the transistors I_P 2  and I_N 2  are interconnected to form an output terminal for outputting the first buffered detection signal D B1 .  
         [0030]    The second voltage detecting circuit  121  senses variation of the power supply voltage VDD, and outputs the second detection signal D 2  at a logic low state when the power supply voltage VDD reaches the second voltage. The synchronization component  123  includes a NOR gate  123 ′, which receives as inputs the first buffered detection signal D B1  and the second detection signal D 2 . The second buffer circuit  125  is connected to an output of the synchronization component  123 , and outputs the second buffered detection signal D B2  that is provided to the latch  130 . The second buffer circuit  125  includes one CMOS inverter.  
         [0031]    As illustrated in FIG. 2, the second voltage detecting circuit  121  is configured the same as the first voltage detecting circuit  111 . Namely, the second voltage detecting circuit  121  includes a number of PMOS transistors P 1 -Pn connected in series between the power supply voltage VDD and the second node N 2 , a number of depletion-type NMOS transistors DN 1 -DNn connected in series between the second node N 2  and the ground voltage VSS, a capacitor C 2  connected between the second node N 2  and the ground voltage VSS, and a CMOS inverter  122  connected between the second node N 2  and the ground voltage VSS. The gates of the transistors P 1 -Pn and DN 1 -DNn are grounded. An output of the CMOS inverter  122  is the second detection signal D 2 . A logic threshold voltage of the inverter  122  is equal to the second voltage. The CMOS inverter  122  includes a PMOS transistor I_P 4  and an NMOS transistor I_N 4 . The gates of the transistors I_P 4  and I_N 4  are commonly connected to the second node N 2 , and the drains of those transistors are interconnected to form an output terminal for outputting the second detection signal D 2 .  
         [0032]    In a case where the memory device operates at a low power supply voltage (e.g., 1.8V), the first voltage is 1.3V and the second voltage is 1.6V.  
         [0033]    The depletion-type NMOS transistors DN 1 -DNn in the respective circuits  111  and  121  form a current-controlled resistor that allows a constant amount of current to flow into the ground voltage VSS. Accordingly, the first or second voltage has a voltage obtained by dividing the power supply voltage VDD. If the PMOS transistors P 1 -Pn in the respective circuits  111  and  121  are turned on according to the increase of the power supply voltage VDD, the capacitors C 1  and C 2  are charged.  
         [0034]    The latch  130  outputs a power-on read signal D pre  in response to the first buffered detection signal D B1  and the second buffered detection signal D B2 . According to this embodiment, the latch  130  includes an inverter  41  and first and second NOR gates  43  and  45 . The inverter  41  has its input terminal connected to receive the second buffered detection signal D B2 . The first NOR gate  43  has a first input terminal connected to an output terminal of the inverter  41 . The second NOR gate  45  has a first input terminal connected to receive the first buffered detection signal D B1 , a second input terminal connected to an output terminal of the first NOR gate  43 , and an output terminal connected to the second input terminal of the first NOR gate  43 .  
         [0035]    The operation of the power detecting circuit  135  according to the embodiment of the invention will be more fully described below.  
         [0036]    At power-off where a power supply voltage VDD is not supplied, that is, when the power supply voltage VDD is 0V, since voltages of the first node N 1  and an output terminal of the inverter  112  have a ground voltage VSS, a voltage of the first detection signal D 1  is 0V. Namely, the first buffered detection signal D B1  from the first buffer means  113  is at 0V, the ground voltage VSS. As the power supply voltage VDD is gradually increased, the PMOS transistors P 1 -Pn of the first voltage detecting circuit  111  are turned on. As charges are transferred to a capacitor Cl through the turned-on transistors P 1 -Pn, a voltage V 1  of the first node N 1  follows the power supply voltage VDD. At this time, a PMOS transistor I_P 3  of the inverter  117  in the first buffer circuit  113  is turned on, so that the first buffered detection signal D B1  follows the power supply voltage VDD. When the power supply voltage VDD reaches the first voltage, a voltage of the first node N 1  becomes a logic threshold voltage of the inverter  112 . This causes an NMOS transistor I_N 1  to be turned on. Thus, the first detection signal D 1  has a logic low state and the buffer circuit  113  outputs the first buffered detection signal D B1  at a logic low state.  
         [0037]    Meanwhile, an output signal D B2  of the second voltage detector  120  has the same logic state (i.e., logic ‘1’) as an output signal D B1  of the first voltage detector  110  in a range where the power supply voltage VDD is lower than the first voltage. The reason is because the first buffered detection signal D B1 , the output signal from the first voltage detector  110 , is supplied to the NOR gate  123 ′ in the synchronization means  123  of the second voltage detector  120 .  
         [0038]    In particular, if the power supply voltage VDD is not supplied, that is, when the power supply voltage VDD is 0V, the second detection signal D 2  is 0V, because the voltages of the second node N 2  and an output terminal of the inverter  122  are both at the ground voltage. As previously described, when the power supply voltage VDD is 0V, the first buffered detection signal D B1  is also at the ground voltage VSS. For this reason, the output of the NOR gate  123 ′ has a logic high state. Accordingly, the second buffered detection signal D B2  from the second buffer circuit  125  is at the ground voltage VSS. As the power supply voltage VDD gradually increases, the PMOS transistors P 1 -Pn of the second voltage detecting circuit  121  are turned on and charge is supplied to the capacitor C 2  connected to the second node N 2 . This allows a voltage V 2  of the second node N 2  to follow the power supply voltage VDD. Since the first buffered detection signal D B1  following the power supply voltage VDD indicates a logic high level, an output signal of the NOR gate  123 ′ becomes low (i.e., 0V). As the PMOS transistor I_P 5  of the inverter in the second buffer circuit  125  is turned on, the second buffered detection signal D B2  follows the power supply voltage VDD and has the same logic state as the first buffered detection signal D B1 . In other words, when the power supply voltage VDD is lower than the first voltage, the first and second buffered detection signals D B1  and D B2  have the same logic state.  
         [0039]    As the power supply voltage VDD continues to increase, a voltage V 2  of the second node N 2  increases by accumulation of charges in the capacitor C 2 . This means that the PMOS transistor I_P 5  of the inverter  125  is turned on so that the second buffered detection signal D B2  follows the power supply voltage VDD. If the power supply voltage VDD reaches the second voltage, an NMOS transistor I_N 4  of an inverter  122  is turned on, and the second detection signal D 2  is at a logic low state. The second detection signal D 2  is supplied to one input terminal of the NOR gate  123 ′. At this time, the first voltage detector  110  outputs the first buffered detection signal D B1  at a logic low state, which is supplied to the other input terminal of the NOR gate  123 ′. The NOR gate  123 ′ outputs a signal at a logic high state in response to its input signals. That is, the first buffered detection signal D B1  has a logic low state and the second detection signal D 2  has a logic low state. Accordingly, the second buffer means  125  outputs the second buffered detection signal D B2  at a logic low state.  
         [0040]    An output signal of the latch means  130 , that is, the power-on read signal D pre , has its logic state determined by output signals of the first and second voltage detectors  110  and  120 , that is, the first and second buffered detection signals D B1  and D B2 .  
         [0041]    [0041]FIGS. 3A, 3B, and  3 C are timing diagrams illustrating the variation of the power supply voltage VDD, the first and second buffered detection signals D B1  and D B2 , and the power-on read signal D pre , respectively, of the power detecting and latch circuits of FIG. 2.  
         [0042]    As explained above and with reference to FIGS. 3A and 3B, before the power supply voltage VDD reaches the first voltage, that is, before a time t 1 , the first and second buffered detection signals D B1  and D B2  have the same logic state, a logic high state (refer to FIG. 3B). Since the second buffered detection signal D B2  at a logic high state is provided to the first NOR gate  43  via the inverter  41  of the latch  130  and the first buffered detection signal D B1  at a logic high state is provided directly to the second NOR gate  45 , the latch  130  is reset so that a power-on read signal D pre  at a logic low state is output.  
         [0043]    As illustrated in FIGS. 3A and 3B, at a time t 1  when the power supply voltage VDD reaches the first voltage, the first buffered detection signal D B1  transitions from a logic high state to a logic low state. As the latch  130  maintains its initial state, that is, a reset state, the power-on read signal D pre  is maintained at a logic low state (FIG. 3C). At this time, the latches and flip-flops in the memory chip  140  are maintained at an initial stable state by the first buffered detection signal D B1  being at a logic low state.  
         [0044]    At a time t 2  when the power supply voltage VDD reaches the second voltage, the second buffered detection signal D B2  transitions from a logic high state to a logic low state (FIG. 3B). The second buffered detection signal D B2  at a logic low state is provided to the first NOR gate  43  via inverter  41  of latch  130 , and the first buffered detection signal D B1  at a logic low state is provided to the second NOR gate  45 . The state of the latch  130  transitions to a set state from the reset state, causing the power-on read signal D pre  to transition from a logic low state to a logic high state (FIG. 3C). Accordingly, the power-on read operation of the memory chip commences.  
         [0045]    In short, the first voltage detector  110  outputs a power-on reset signal, that is, the first buffered detection signal D B1 , at a logically activated state before the power supply voltage reaches the first voltage. When the power supply voltage VDD reaches the first voltage, the power-on reset signal (first buffered detection signal D B1 ) is logically inactivated. Herein, a logically activated state is a logic high state (logic ‘1’), and a logically inactivated state is a logic low state (logic ‘0’). Likewise, the second voltage detector  120  outputs the second buffered detection signal D B2  at a logically activated state before the power supply voltage VDD reaches the second voltage. When the power supply voltage VDD reaches the second voltage, the second buffered detection signal D B2  is logically inactivated. Also, in a case where the power supply voltage VDD is lower than the first voltage, output signals of the first and second voltage detectors  110  and  120  have the same logic state.  
         [0046]    If the power supply voltage VDD reaches the first voltage, the latch  130  is reset, placing the power-on read signal D pre  in an inactive state. If the power supply voltage VDD reaches the second voltage, the latch  130  is set, placing the power-on read signal D pre  in an active state. When the signal D pre  is activated, the power-on read operation of the memory chip commences.  
         [0047]    Again referring to FIGS.  3 A- 3 C, it is assumed that noise arises at a time t n  after the power supply voltage VDD reaches a stable voltage level VDD_sat. The power supply voltage VDD is lowered below the second voltage owing to the noise, but recovers at the stable voltage level VDD_sat at a time t n+d . The second buffered detection signal D B2  transitions from a logic low state to a logic high state at the time t n , and again transitions from a logic high state to a logic low state at the time t n+d . However, if the logic state of the first buffered detection signal D B1  is not changed, the state variation of the second buffered detection signal D B2  is unable to change an output state of the latch means  130 .  
         [0048]    In the above embodiment, a NOR gate  123 ′ used as the synchronization component  123  in the second voltage detector  120  may alternatively be replaced with a CMOS inverter. In that case, the first buffered detection signal D B1  from the first voltage detector  110  is not supplied to the second voltage detector  120 .  
         [0049]    Alternatively, a latch  130 ′ may also be configured as illustrated in FIG. 4. The latch  130 ′ includes an inverter  41 ′ having its input terminal connected to receive the first buffered detection signal D B1 ; a first NAND gate  43 ′ having a first input terminal connected to an output of the inverter  41 ′; a second NAND gate  45 ′ having a first input terminal connected to receive the second buffered detection signal D B2 , a second input terminal connected to an output terminal of the first NAND gate  43 ′, and an output terminal connected to a second input terminal of the first NAND gate  43 ′. The latch  130 ′ in FIG. 4 operates the same as the latch  130  in FIG. 2, so a detailed description is omitted.  
         [0050]    According to embodiments of the invention, the power detecting circuit simultaneously generates a power-on reset signal for resetting logic circuits in a memory chip and a power-on read signal for triggering a read operation of the memory chip. When a power supply voltage is lower than the first voltage after power-on, the power detecting circuit generates a power-on reset signal that follows the power supply voltage. When the power supply voltage is lower than the first voltage, the power detecting circuit generates a detection signal at the same logic state as the power-on reset signal. When the power supply voltage is lower than the second voltage but is higher than the first voltage, the power detecting circuit generates a detection signal that follows the power supply voltage. The power-detecting circuit generates a power-on read signal in response to the power-on reset signal and the detection signal. At this time, the power-on read signal has a logically inactivated state when the power supply voltage is lower than the first voltage, and has a logically activated state when the power supply voltage is higher than the second voltage.  
         [0051]    Specific embodiments of tine invention will now be discussed.  
         [0052]    In accordance with an embodiment of the invention, there is provided a power detecting circuit which initializes logic circuits in a flash memory chip and starts a stable read operation of the memory device at power-on. The power detecting circuit includes a first voltage detector, a second voltage detector, and a latch. The first voltage detector includes a first voltage detecting circuit for outputting the first detection signal in response to the variation of a power supply voltage, and a first buffering circuit connected to the first voltage detecting circuit to output the first buffered detection signal. The second voltage detector includes a second voltage detecting circuit for outputting the second detection signal in response to variation of the power supply voltage, and a second buffering circuit connected to the second voltage detecting circuit to output the second buffered detection signal. The latch is set or reset by the first and second buffered detection signals. The first buffered detection signal that is output from the first voltage detector is used as a power-on reset signal for resetting the logic circuits.  
         [0053]    When the power supply voltage reaches a first voltage, the first buffered detection signal has a state transition, so that the logic circuits are reset. When the power supply voltage reaches a second voltage, the second buffered detection signal has a state transition. The latch is set to trigger a power-on read operation after the state transition of the second buffered detection signal.  
         [0054]    In particular, the first buffered detection signal follows the power supply voltage and has a logically activated state (logic high or logic ‘1’) before the power supply voltage reaches the first voltage. The first buffered detection signal transitions to a logically inactivated state (logic low or logic ‘0’) after the power supply voltage reaches the first voltage. In other words, the first detection signal from the first voltage detecting circuit becomes low when the power supply voltage reaches the first voltage, and the first buffer buffers the first detection signal to output the first buffered detection signal at a logic low state. Thus the logic circuits are reset to an initial stable state.  
         [0055]    The second buffered detection signal follows the power supply voltage and has a logically activated state (logic high or logic ‘1’) before the power supply voltage reaches the second voltage. The second buffered detection signal becomes inactive (logic low or logic ‘0’) when the power supply voltage reaches the second voltage. At this time, the latch is set to output the power-on read signal at an active state, which is provided to a memory chip.  
         [0056]    In this embodiment, each of the first and second voltage detecting circuits includes a number of PMOS transistors which are connected in series between the power supply voltage and a first node, the gates of the PMOS transistors grounded; a number of depletion-type NMOS transistors which are connected in series between the first node and the ground voltage, the gates of the depletion-type NMOS transistors grounded; a capacitor that is connected between the first node and the ground voltage; and an inverter that is connected to the first node. A logic threshold voltage of the inverter of the first voltage detecting circuit is equal to the first voltage, and a logic threshold voltage of the inverter of the second voltage detecting circuit is equal to the second voltage. The first buffer and the second buffer include two serially-connected CMOS inverters.  
         [0057]    Preferably, before the power supply voltage reaches the first voltage level, the first and the second buffered detection signals are at the same logic state.  
         [0058]    The first voltage detector includes a first voltage detecting circuit that detects the power supply voltage and produces a first detection signal; and a first buffer circuit that has as its input the first detection signal and outputs a first buffered detection signal that is supplied to a first input terminal of the latch means. The first buffered detection signal is supplied to a number of logic circuits.  
         [0059]    The second voltage detector includes a second voltage detecting circuit which detects the power supply voltage and produces a second detection signal; a synchronization circuit that has as inputs the second detection signal and the first buffered detection signal; and a second buffer circuit that has as its input an output of the synchronization circuit and outputs a second buffered detection signal that is supplied to a second input terminal of the latch.  
         [0060]    Because of the synchronization means, if the power supply voltage is lower than the first voltage, the first and second buffered detection signals have the same logic state, that is, they follow the power supply voltage. During this time, the logic circuits are reset. If the power supply voltage is lower than the second voltage, the second buffered detection signal follows the power supply voltage.  
         [0061]    In this embodiment, the latch includes a first NOR gate that has a first input terminal connected to receive the second buffered detection signal, a second input terminal, and an output terminal; an inverter that has an input terminal connected to receive the first buffered detection signal and an output terminal; and a second NOR gate that has a first input terminal connected to the output terminal of the inverter, a second input terminal connected to the output terminal of the first NOR gate, and an output terminal connected to the second input terminal of the first NOR gate.  
         [0062]    In this embodiment, before the power supply voltage reaches the first voltage, the first and second buffered detection signals are synchronized with each other to have the same logic state. During this period, the latch is stably reset. That is, before the power supply voltage reaches the first voltage, the first and second buffered detection signals have a logic high state and follow the power supply voltage. At this time, the second buffered detection signal is inverted by the inverter included in the latch, and the latch is reset.  
         [0063]    In an alternative embodiment, the latch includes a first NAND gate which has a first input terminal connected to receive the second buffered detection signal, a second input terminal, and an output terminal; an inverter which has an input terminal connected to receive the first buffered detection signal and an output terminal; and a second NAND gate which has a first input terminal connected to the output terminal of the inverter, a second input terminal connected to the output terminal of the first NAND gate, and an output terminal connected to the second input terminal of the first NAND gate.  
         [0064]    According to embodiments of the invention, if a power-on read operation commences followed by power noise, the latch is not set again as long as the power supply voltage remains above the first voltage. The power detecting circuit stably triggers a power-on read operation and provides a power-on reset function. Since the first and second buffered detection signals provided to the latch have the same logic state when the power supply voltage is lower than the first voltage, the latch is stably reset when the power supply voltage is lower than the first voltage related to a power-on reset operation.  
         [0065]    According to other embodiments of the invention, a flash memory device is provided that includes a first voltage detector that outputs a first signal of a disable state when a power supply voltage reaches a first voltage; a second voltage detector that outputs a second signal of a disable state when the power supply voltage reaches a second voltage, the second voltage being higher than the first voltage; a latch which generates a power-on read signal for triggering a power-on read operation in response to the first and second signals; a memory cell array that has a number of memory cells arranged in a matrix of rows and columns; an address generator that generates row and column addresses; control logic that generates a control signal in response to the power-on read signal; and a read circuit that reads out from the memory cell array in response to the addresses from the address generating means and the control signal from the control logic. The latch produces a reset signal in response to the first signal and a set signal in response to the second signal, so that the power-on read operation commences.  
         [0066]    In the flash memory device according to embodiments of the invention, addresses from the address generating can be generated by an external address command. Accordingly, in a case where the flash memory device is used in any system, it is capable of reading out data at power-on. Addresses from the address generator can also be generated internally by the control logic. In that case, the flash memory device may be used as a boot-up memory for any system.  
         [0067]    In accordance with still other embodiments of the invention, a method for generating a power-on reset signal for resetting logic circuits in a flash memory device and a power-on read signal for triggering a read operation of the memory device is, provided. The method includes generating the power-on reset signal that follows a power supply voltage when the power supply voltage is lower than a first voltage at power-on; generating a detection signal; and generating the power-on read signal in response to the power-on reset signal and the detection signal. The detection signal is at the same logic state as the power-on reset signal when the power supply voltage is lower than the first voltage, and the detection signal follows the power supply voltage when the power supply voltage is lower than a second voltage, where the second voltage is greater than the first voltage. The power-on read signal has a logically inactivated state when the power supply voltage is less than the first voltage and a logically activated state when the power supply voltage is greater than the second voltage.  
         [0068]    In accordance with yet other embodiments of the invention, a method for reading a memory device at power-on that operates stably in the presence of noise is provided. The method includes generating a power-on reset signal that follows a power supply voltage when the power supply voltage is lower than a first voltage after power-on; generating a detection signal; generating a power-on read signal in response to the power-on reset signal and the detection signal; and reading out data from a memory cell array in response to an address generated by an external address command. The power-on read signal has a logically activated state when the power supply voltage is higher than a second voltage, the second voltage is greater than the first voltage. The detection signal indicates the same logic state as the power-on reset signal when the power supply voltage is less than the first voltage, and the detection signal follows the power supply voltage when the power supply voltage is less than the second voltage. The power-on read signal is logically re-activated only when the power supply voltage falls below the first voltage due to power noise and then returns to the level of the second voltage.  
         [0069]    In accordance with embodiments of the invention, unnecessary power-on read operations of a memory device caused by power noise are prevented. Thus, power consumption of a system may be reduced.  
         [0070]    The invention has been described using exemplary embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.