Abstract:
There is provided a power-on reset circuit and method for a semiconductor integrated circuit device using a plurality of power sources, in which a power-on reset operation is stable and reliable, where the power-on reset circuit includes voltage detection circuits for generating at least two voltage detection signals with respect to the power sources, the power-on reset circuit generates a plurality of power-on reset signals using combination logic circuits for performing logic operations of the voltage detection signals, and internal latches and flip-flops are stably reset in response to the plurality of power-on reset signals.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to power-on reset circuits, and more particularly, to a power-on reset circuit and power-on reset method of a semiconductor integrated circuit using a plurality of power sources.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Semiconductor integrated circuit devices generally include a power-on reset circuit to prevent abnormal operations when a power source is applied thereto. Generally, once the power source is applied to the semiconductor integrated circuit device and reaches a predetermined voltage, the power-on reset circuit provides a reset signal for initializing components of a semiconductor memory device, such as flip-flops, latches, counters, registers, and the like. A typical voltage detection circuit outputs an activated power-on reset signal if the power source voltage reaches a predetermined voltage when the power is on. Internal components of the semiconductor integrated circuit are reset to an initial state in response to the activated power-on reset signal. Then, if the power source voltage reaches a normal operating voltage, the power-on reset signal is deactivated. As demands for high-speed integrated circuits are increasing, integrated circuits utilizing a plurality of power sources are more widely used. For example, separate power sources are used for input/output pins, input/output buffers, memory cells, and the like.  
           [0005]    [0005]FIG. 1 is a schematic diagram showing a problem caused in a conventional power-on reset circuit of a semiconductor memory device  100  using a plurality of power sources. If a voltage level of a power source VDD 1  supplied to a typical memory device reaches a predetermined voltage, a power-on reset circuit  10  detects the voltage level and generates a pulse with a constant width, i.e., a power-on reset signal POR. The power-on reset signal POR sets all the nodes needing to be initialized within the memory device to logic high states. Here, the nodes include a node N 1  connected with a first latch  30  and a node N 2  connected with a second latch  50 .  
           [0006]    Unfortunately, although the power source VDD 1  reaches the predetermined voltage level to generate the power-on reset signal and then reaches a stable voltage level, a power source VDD 2  might not reach a stable voltage level prior to the POR pulse. Therefore, in a PMOS transistor (PM1)  70  having a drain electrode connected to the node N 1 , a source electrode connected to the power source VDD 2  and a gate electrode receiving the power-on reset signal, the source electrode of the PMOS transistor  70  does not reach a stable voltage level and thus the PMOS transistor  70  does not activate. As a result, the initialization of the node N 1  fails.  
           [0007]    In addition, after the power-on reset signal POR generated in response to the power source VDD 1  by the power-on reset circuit  10  is delayed for a predetermined time, a noise may occur in the power source VDD 3  that reaches a stable state when the power-on reset signal POR reaches a gate electrode of a PMOS transistor (PM2)  90  connected between a power source VDD 3  and the node N 2 . In other words, in a case where a noise occurs after the power source VDD 1  reaches a stable voltage level or in a case where a noise occurs at the power source VDD 3  itself, since the power source VDD 3  is unstable at a point when the power-on reset signal POR reaches the gate electrode of the PMOS transistor  90 , the PMOS transistor  90  is not activated. As a result, the initialization of the node N 2  fails.  
           [0008]    Accordingly, it is desirable to provide a stable power-on reset circuit and method for a semiconductor memory device using a plurality of power sources.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is directed towards a power-on reset circuit and poweron reset method that substantially obviate one or more of the problems, limitations and disadvantages of the related art.  
           [0010]    An advantage of the present invention is to provide a power-on reset method of a semiconductor integrated circuit using a plurality of power sources.  
           [0011]    Another advantage of the present invention is to provide a stable power-on reset circuit of a semiconductor integrated circuit using a plurality of power sources.  
           [0012]    These and other features and advantages of the present invention may be understood from the description that follows, and will become apparent to those having ordinary skill in the pertinent art upon examination of the following, or may be learned from practice of the invention. The features and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof, as well as in the appended drawings.  
           [0013]    To achieve these features and other advantages and in accordance with a purpose of the invention, as embodied and broadly described herein, a power-on reset method of an integrated circuit device comprises the steps of: providing a power-on reset pulse whenever a plurality of respective power source voltages supplied from individual power sources reach a plurality of predetermined detection voltages, respectively; and repeatedly initializing internal circuits of the integrated circuit device in response to the power-on reset pulse.  
           [0014]    The step of providing the power-on reset pulse includes the steps of (a) providing a plurality of voltage detection circuits receiving the power source voltages, the respective voltage detection circuits corresponding to the respective power sources, (b) generating a plurality of voltage detection signals corresponding to a plurality of predetermined detection voltages by the respective voltage detection circuits, (c) providing a plurality of pulse generation circuits corresponding to the plurality of predetermined detection voltages, (d) generating a plurality of power-on reset pulses corresponding to the plurality of predetermined detection voltages by the respective pulse generation circuits, (e) providing a pulse output means receiving the power-on reset pulses generated from the pulse generation circuits, and (f) providing the power-on reset pulses by the pulse output means. Logic states of the respective voltage detection signals change whenever the power source voltages reach corresponding detection voltages. The pulse output means is an OR gate.  
           [0015]    To achieve the above feature, there is provided a power-on reset method for resetting internal circuits of an integrated circuit device using a plurality of power source voltages, which comprises the steps of: generating voltage detection signals in response to the plurality of power source voltages when the power source voltages go beyond a plurality of detection voltages; generating power-on reset pulses using the voltage detection signals; and initializing the internal circuits in response to the power-on reset pulses.  
           [0016]    The internal circuits are initialized whenever the power-on reset pulses are generated, and finally initialized by the last power-on reset pulse.  
           [0017]    In accordance with an embodiment of the present invention, the plurality of detection voltages are a first detection voltage and a second detection voltage, and the voltage detection signals are first voltage detection signals and second voltage detection signals of which logic states changes when the power source voltages go beyond the first detection voltage and the second detection voltage.  
           [0018]    The step of generating the power-on reset pulses includes the steps of: generating first and second detection pulses from the first and second voltage detection signals generated with respect to the respective power source voltages; and outputting the first and second detection pulses via an OR gate.  
           [0019]    To achieve another feature, there is provided a power-on reset circuit which comprises: M voltage detection circuits for generating first detection signals and second detection signals in response to M power source voltages when the power source voltages reach first and second detection voltages, M being an integer equal to or greater than two; first and second pulse generation circuits for receiving the M first detection signals and the M second detection signals from the M voltage detection circuits and generating maximum M first power-on reset pulses and maximum M second power-on reset pulses, respectively; and a pulse output means for receiving the first and second power-on reset pulses and outputting final power-on reset pulses, the maximum number of the final power-on reset pulses being 2×M. Preferably, the pulse output means is an OR gate.  
           [0020]    In an embodiment of the present invention, each of the M voltage detection circuits includes: a first voltage detection circuit for generating the first detection signals of which logic states changes when the corresponding power source voltages go beyond the first detection voltage; and a second voltage detection circuit for generating the second detection signals of which logic states changes when the corresponding power source voltages go beyond the second detection voltage.  
           [0021]    In an embodiment of the present invention, each of the first and second pulse generation circuits includes: a first AND gate receiving the detection signals; an inversion delay unit connected to an output terminal of the first AND gate; and a second AND gate connected to the output terminal of the first AND gate and the an output terminal of the inversion delay unit.  
           [0022]    In an embodiment of the present invention, each of the first and second pulse generation circuits includes: M pulse generators for generating pulses with respect to the M detection signals; and an OR gate connected to output terminals of the M pulse generator, each of the M pulse generator including: an inversion delay unit receiving the corresponding detection signal; and an AND gate receiving an output of the inversion delay unit and the corresponding detection signal.  
           [0023]    In an embodiment of the present invention, the first pulse generation circuit includes: a first AND gate receiving the detection signals; an inversion delay unit connected to an output terminal of the first AND gate; and a second AND gate connected to the output terminal of the first AND gate and an output terminal of the inversion delay unit, wherein the second pulse generation circuit includes: M pulse generation circuits for generating pulses with respect to the M second detection signals; and an OR gate connected to output terminals of the M pulse generation circuits, each of the M pulse generation circuits including: an inversion delay unit receiving the corresponding second detection signal; and an AND gate receiving an output of the inversion delay unit and the corresponding detection signal.  
           [0024]    It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    In the drawings:  
         [0026]    [0026]FIG. 1 is a schematic diagram explaining a problem caused in a conventional power-on reset method;  
         [0027]    [0027]FIG. 2 is a block diagram explaining a power-on reset method in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 3 is a circuit diagram of the voltage detection circuit of FIG. 2 in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 4 is a graphical diagram of an output signal waveform of the voltage detection circuit of FIG. 3;  
         [0030]    [0030]FIG. 5 is a block diagram explaining the pulse generation circuit of FIG. 2 and a method of generating the power-on reset pulses using the pulse generation circuit in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 6 is a graphical diagram of the power-on reset pulse waveform according to FIG. 5; and  
         [0032]    [0032]FIG. 7 is a graphical diagram of a power-on reset pulse waveform in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]    Preferred embodiments of the present invention will now be described with respect to the examples illustrated in the accompanying drawings. However, the present invention is not limited to the exemplary embodiments illustrated herein, and such embodiments are introduced to facilitate understanding of the scope and spirit of the present invention.  
         [0034]    As shown in FIG. 2, a block diagram explaining a power-on reset method in accordance with the present invention is indicated generally by the reference numeral  200 . In FIG. 2, power sources VDD 1 , VDD 2  and VDD 3  can be external power sources supplied from an exterior of a semiconductor integrated circuit, for example. In addition, some power sources (e.g., the power source VDD 3 ) can be internal power sources generated from external power sources (e.g., the power source VDD 2 ). To facilitate understanding of the present invention, only three power sources are illustrated in the drawings. However, it shall be understood that two or more power sources can be used within the scope and spirit of the present invention.  
         [0035]    In the diagram  200 , three voltage detection circuits  110 ,  120  and  130  are used corresponding to the three power sources VDD 1 , VDD 2  and VDD 3 , respectively. The voltage detection circuits  110 ,  120  and  130  generate three voltage detection signals POR 1   — 1/POR 2   — 1, POR 1   — 2/POR 2   — 2 and POR 1   — 3/POR 2   — 3, respectively. In other words, the first voltage detection circuit  110  generates a primary voltage detection signal POR 1   — 1 if the power source VDD 1  reaches a primary detection voltage, and generates a secondary voltage detection signal POR 2   — 1 if the power source VDD 1  reaches a secondary detection voltage. In the same manner, the second voltage detection circuit  120  generates a primary voltage detection signal POR 1   — 2 if the power source VDD 2  reaches a primary detection voltage, and generates a secondary voltage detection signal POR 2   — 2 if the power source VDD 2  reaches a secondary detection voltage. Likewise, the third voltage detection circuit  130  generates a primary voltage detection signal POR 1   — 3 if the power source VDD 3  reaches a primary detection voltage, and generates a secondary voltage detection signal POR 2   — 3 if the power source VDD 3  reaches a secondary detection voltage.  
         [0036]    According to another embodiment of the present invention, two or more voltage detection signals can be generated. Further, the primary and secondary voltage detection signals can be various types. For example, the detection signals can be signals whose logic states change when corresponding power source goes beyond the detection voltage (e.g., signals whose logic states change from a logic high state to a logic low state, or from a logic low state to a logic high state) or pulse signals having constant width and height.  
         [0037]    The primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3 respectively generated from the voltage detection circuits  110 ,  120  and  130  are inputted into a first pulse generation circuit  140 . In the same manner, the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 respectively generated from the voltage detection circuits  110 ,  120  and  130  are inputted into a second pulse generation circuit  150 . The first and second pulse generation circuits  140  and  150  respectively receiving the primary and secondary voltage detection signals generate a first power-on reset pulse P_POR 1  and a second power-on reset pulse P_POR 2 , respectively. At this time, the number of the first and second power-on reset pulses P_POR 1  and P_POR 2  is at least one, and equal to the same number as the inputted signals, up to the maximum. The first power-on reset pulse P_POR 1  has a height corresponding to the magnitude of a primary voltage detection signal, and the second power-on reset pulses P_POR 2  has a height corresponding to magnitude of a secondary voltage detection signal. The power-on reset pulses P_POR 1  and P_POR 2  outputted from the first and second pulse generation circuits  140  and  150  are inputted into a pulse output unit  160 , and the pulse output unit  160  generates a plurality of power-on reset pulses POR.  
         [0038]    Accordingly, in a power-on reset method of the present invention, a plurality of power-on reset pulses are generated such that the nodes that need to be initialized can be initialized whenever the power-on reset pulses are generated. The nodes can be initialized by a generated POR pulse. Thus, if a voltage level of a power source rises to reach a secondary detection voltage, a power-on reset pulse is generated and the initialization is completed.  
         [0039]    Turning to FIG. 3, a voltage detection circuit in accordance with the present invention is indicated generally by the reference numeral  300 . Here, only the first voltage detection circuit  110  corresponding to the power source VDD 1  is shown for ease of explanation. The remaining voltage detection circuits corresponding to the remaining power sources have the same configuration as the first voltage detection circuit  110 , but are not shown in order to avoid repetition of explanation.  
         [0040]    Referring also to FIG. 4, a graphical diagram is indicated generally by the reference numeral  400 , showing signal waveforms of the voltage detection signals outputted from the voltage detection circuit  300  of FIG. 3. The voltage detection circuit  110  of FIG. 3 is divided into a first detection circuit  301  for generating a primary voltage detection signal POR 1   — 1 and a second detection circuit  401  for generating a secondary voltage detection signal POR 2   — 1. When the power source voltage supplied form the power source VDD 1  goes beyond the predetermined primary detection voltage, the first detection circuit  301  detects the power source voltage and generates the primary voltage detection signal POR 1   — 1 having an updated logic state (e.g., from a logic low state to a logic high state). In the same manner, when the power source voltage supplied form the power source VDD 1  goes beyond the predetermined secondary detection voltage, the second detection circuit  401  detects the power source voltage and generates the secondary voltage detection signal POR 2   — 1 having an updated logic state (e.g., from a logic low state to a logic high state). The first detection circuit  301  and the second detection circuit  401  have configurations similar to each other. A difference is that the MOS transistors constituting the respective circuits have different sizes and resistances. In other words, the first and second detection voltages may be determined by adjusting the sizes and resistances of the MOS transistors constituting the respective circuits.  
         [0041]    After the power is on and a voltage of the power source VDD 1  rises, if the voltage of the power source VDD 1  goes beyond a primary detection voltage, the first detection circuit  301  detects the corresponding voltage and generates the primary voltage detection signal POR 1   — 1. Then, if the voltage of the power source VDD 1  continues to increase and goes beyond the secondary detection voltage, the second detection circuit  401  detects the corresponding voltage and generates the secondary voltage detection signal POR 2   — 1.  
         [0042]    The voltage detection circuit  110  of FIG. 3 includes voltage division units  300  and  400  for dividing the voltage supplied from the power source VDD 1  when current paths are formed, and first and second detection circuits  301  and  401  for receiving two voltages divided by the voltage division units  300  and  400 . The voltage division unit  300  includes two resistors, R 1  and R 2 , connected in series between the power source VDD 1  and a second node N 2 . If a transistor MN 2  of the first detection circuit  301  is turned on, the voltage of the power source VDD 1  is dropped by the two resistors R 1  and R 2  so that a first voltage V1 and a second voltage V2 are applied at the first node N 1  disposed between the resistors R 1  and R 2 , and the second node N 2  disposed between the resistor R 2  and the NMOS transistor MN 1 , respectively. The first detection circuit  301  includes one PMOS transistor MP 1 , two NMOS transistors MN 1  and MN 2 , and three CMOS inverters INV 1 , INV 2  and INV 3  connected in series. The two NMOS transistors MN 1  and MN 2  are connected in series between the second node N 2  and a ground terminal VSS. Here, a gate of the NMOS transistor MN 1  is connected to the first node N 1  and receives the first voltage V 1 , and a gate of the NMOS transistor MN 2  is connected to the power source VDD 1 . A source of the NMOS transistor MN 1  is connected to the second node N 2  and receives the second voltage V2. A source of the NMOS transistor MN 2  is connected to the ground terminal. Drains of the two NMOS transistors MN 1  and MN 2  are connected to each other to form a third node N 3 . The PMOS transistor MP 1  has a source connected to the node N 1  for receiving the first voltage V1, a gate connected to the second node N 2  for receiving the second voltage V2, and a drain connected to the third node N 3 . Three inverters INV 1 , INV 2  and INV 3  are sequentially connected in series to a fourth node N 4  that is connected to the gate of the PMOS transistor MP 1 . The primary voltage detection signal POR 1   — 1 is outputted via a fifth node N 5  connected to an output of the last inverter INV 3  among the serially connected inverters INV 1 , INV 2  and INV 3 .  
         [0043]    Since the second detection circuit  401  has the same configuration as the first detection circuit  301 , a detailed description for the second detection circuit  401  will be omitted. Hereinafter, an operation of the first detection circuit  301  configured as above will be described in detail. Since the power source VDD 1  is not high in an earlier state after the power is on, the NMOS transistor MN 2  is not turned on so that a current path is not formed via the resistors R 1  and R 2 . Accordingly, the voltage of the power source VDD 1  is applied on the two nodes N 1  and N 2  so that the fourth node N 4  is set to a logic high state. As a result, the primary voltage detection signal POR 1   — 1 of the ground voltage is generated at the output node N 5  of the first detection circuit  301  via the three inverters INV 1 , INV 2  and INV 3  connected in series to the fourth node N 4 , and then inputted into the first pulse generation circuit  140 . In the same manner, since the NMOS transistor MN 2 ′ is also not turned on, the second detection circuit  401  generates the secondary voltage detection signal POR 2   — 1 of the ground voltage at an output node N 5 ′ and then the secondary voltage detection signal POR 2   — 1 is inputted into the second pulse generation circuit  150 .  
         [0044]    Referring again to the graphical diagram  400  of FIG. 4, if the voltage of the power source VDD 1  continuously rises to turn on the NMOS transistor MN 2  of the first detection circuit  301 , the current path is formed via the resistors R 1  and R 2 . Therefore, at a time t1 when the power source VDD 1  reaches the first detection voltage (i.e., a time when the power source VDD 1  becomes a normal state), the voltage of the fourth node N 4  becomes near the ground voltage and a pull-up PMOS transistor (not shown) of the inverter INV 3  is turned on. As a result, the primary voltage detection signal POR 1   — 1 following the power source VDD 1  is generated. In other words, at a time when the power source VDD 1  reaches the primary detection voltage, a signal changing from a logic low state to a logic high state is generated. At this time, in the second detection circuit  401 , since resistances and sizes of the transistors are set so that the time when the above procedures are performed can be t3 later than t1, the secondary voltage detection signal POR 2   — 1 is still the ground voltage. At the time t3 when the voltage of the power source VDD 1  continuously rises and reaches the secondary detection voltage, the voltage of a fourth node N 4 ′ of the second detection circuit  401  becomes near the ground voltage so that a signal following the power source VDD 1  is generated at an output node N 5 ′ of the inverter INV 3 ′.  
         [0045]    In the same manner, the voltage detection circuit  120  connected to the power source VDD 2  also generates the primary and secondary detection signals POR 1   — 2 and POR 2   — 2, each having logic states change in response to the voltage of the power source VDD 2 . In addition, the voltage detection circuit  130  connected to the power source VDD 3  generates the primary and secondary detection signals POR 1   — 3 and POR 2   — 3, each having logic states change in response to the voltage of the power source VDD 3 .  
         [0046]    The present voltage detection circuits are illustrated and described for exemplary purposes. Other voltage detection circuits can be used to generate detection signals (e.g., a signal whose logic state changes from one state to another state) when a power source voltage reaches a specific voltage. Accordingly, those skilled in the art can use various voltage detection circuits without departing from the scope of the present disclosure.  
         [0047]    A method for generating a plurality of power-on reset pulses using a pulse generation circuit and a pulse generation method will be described. Outputs of the pulse generation circuits are pulses that are input into pulse generation units so that a plurality of power-on reset pulses are generated.  
         [0048]    As shown in FIG. 5, a block diagram circuit is indicated generally by the reference numeral  500 . The circuit  500  includes the pulse generation circuits  140  and  150  of FIGS. 2 and 3, illustrated in further detail, and an OR gate corresponding to the pulse output unit  160 . The pulse generation circuits  140  and  150  receive the detection signals from the detection circuits of FIG. 3 and output the pulses.  
         [0049]    Turning to FIG. 6, a graphical waveform diagram of the power-on reset pulses outputted from the OR gate  160  of FIG. 5 is indicated generally by the reference numeral  600 . The primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3 generated from the voltage detection circuits  110 , 120  and  130  are inputted into the first pulse generation circuit  140 , and the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 are inputted into the second pulse generation circuit  150 .  
         [0050]    Referring again to both FIGS. 5 and 6, if the respective power sources VDD 1 , VDD 2  and VDD 3  go beyond the primary detection voltage at the respective times t1, t3 and t5, the respective first detection circuits of the voltage detection circuits generate the primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3, which follow the corresponding power sources, respectively, and then the primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3 are inputted into the first pulse generation circuit  140 . In addition, if the respective power sources VDD 1 , VDD 2  and VDD 3  continuously rise and go beyond the secondary detection voltage at the respective times t2, t4 and t6, then the second detection circuits of the voltage detection circuits generate the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3, which follow the corresponding power sources, respectively, and then the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 are inputted into the second pulse generation circuit  150 .  
         [0051]    The first pulse generation circuit  140  includes a first AND gate  141  receiving the primary detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3 and a pulse generation unit  142  for generating pulses using the output of the first AND gate  141 . The pulse generation unit  142  includes an inversion delay unit  143  for inverting an output of the first AND gate  141 , and a second AND gate  144  for receiving the output of the inversion delay unit  143  and the output of the first AND gate  141 . Accordingly, the last voltage detection pulse P_POR 1  is generated at a time t5′ of FIG. 6 by the detection signal POR 1   — 3, which, in turn, was generated in response to the power source VDD 3  reaching its primary detection voltage. In other words, the primary voltage detection signal POR 1   — 3 is generated at the latest time (t5) via the output terminal of the first AND gate  141 , and this signal (e.g., a signal changing from a logic low state and to a logic high state) is inputted into the second AND gate  144  so that the second AND gate  144  outputs the first pulse P_POR 1  at the time t5′.  
         [0052]    The second pulse generation circuit  150  generates the second pulses P_POR 2   — 1, P_POR 2   — 2 and P_POR 2   — 3 corresponding to the secondary detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3. For this, only the pulse generation unit  142  is used instead of the first AND gate  141  of the first pulse generation circuit  140 . In other words, the second pulse generation circuit  150  includes a first pulse generation unit  152 , a second pulse generation unit  152 ′, a third pulse generation unit  152 ″, which correspond to the respective secondary detection signals, and an OR gate  151  connected to output terminals of the pulse generation units. The second pulse generation units  152 ,  152 ′ and  152 ″ of the second pulse generation circuit  150  have the same configuration as the pulse generation unit  142  of the first pulse generation circuit  140 . Therefore, detailed description for those operations previously described for the pulse generation unit  142  will be omitted. As shown in FIG. 6, if the power source VDD 1  reaches the secondary detection voltage at the time t2, the secondary detection signal POR 2   — 1 is generated and inputted into the first pulse generation unit  152  of the second pulse generation circuit  150 . Therefore, the second pulse P_POR 2   — 2 is generated at the time t2′. In the same manner, the secondary voltage detection signal POR 2   — 2 generated at the time t4 is inputted into the second pulse generation unit  152 ′ of the second pulse generation circuit  150  and the second pulse P_POR 2   — 2 is generated at the time t4′. The secondary voltage detection signal POR 2   — 3 generated at a time t6 is inputted into the third pulse generation unit  152 ″ of the second pulse generation circuit  150  and the second pulse P_POR 2   — 3 is generated at the time t6′. The generated second detection pulses P_POR 2   — 1, P_POR 2   — 2 and P_POR 2   — 3 are inputted into the OR gate  151 .  
         [0053]    As a result, the pulses outputted from the pulse generation circuits  140  and  150  are inputted into the OR gate  160  acting as the pulse output unit, in which the pulses are the first voltage detection pulse P_POR 1  having the height corresponding to the magnitude of the primary detection voltage and the second voltage detection pulses P_POR 2   — 1, P_POR 2   — 2 and P_POR 2   — 3 having heights corresponding to the magnitude of the secondary detection voltage. Consequently, four power-on reset pulses POR are generated at the times t2′, t4′, t5′ and t6′, respectively. Accordingly, when a given power source is unstable and does not generate a corresponding power-on reset pulse, the power-on reset pulse is generated by other power sources so that a stable initialization is achieved.  
         [0054]    The OR gate  151  of the second pulse generation circuit  150  need not be used. In other words, the pulses outputted from the pulse generation units  152 ,  152 ′ and  152 ″ of the second pulse generation circuit  150  can be directly inputted into an OR gate  160 .  
         [0055]    In addition, in the pulse generation circuit of FIG. 5, the respective pulse generation circuits can have the same configuration as other pulse generation circuits. For example, if the second pulse generation circuit  150  of FIG. 5 has the same configuration as the first pulse generation circuit  140 , two power-on reset pulses P_POR 1  and P_POR 2  are generated from the OR gate acting as the pulse output unit. In other words, the respective pulse generation circuits generate one pulse. Meanwhile, if the first pulse generation circuit  140  of FIG. 4 has the same configuration as the second pulse generation circuit  150  of FIG. 5, six power-on reset pulses are generated from the OR gate acting as the pulse output unit. In other words, the respective pulse generation circuits generate three pulses.  
         [0056]    Turning now to FIG. 7, a graphical waveform diagram of power-on reset pulses outputted from an alternate embodiment power-on reset circuit and method in accordance with the present invention is indicated generally by the reference numeral  700 . In this embodiment, the first pulse generation circuit is the same as that shown as  140  in FIG. 5, but the second pulse generation circuit is also like that shown as  140  in FIG. 5, rather than that shown as  150  in FIG. 5. For this embodiment, the second pulse generation circuit is indicated generally by the reference numeral  140 ′, and has a structure comparable to that defined by the circuit  140  of FIG. 5.  
         [0057]    Since the second pulse generation circuit has the same configuration as the first pulse generation circuit in this alternate embodiment, two power-on reset pulses P_POR 1  and P_POR 2  are supplied to the OR gate acting as the pulse output unit. In other words, the respective pulse generation circuits each generate one pulse. Accordingly, the signal descriptions for the first pulse generation circuit  140  are as described with respect to FIG. 6, but the signal descriptions for the second pulse generation circuit are different from those described for FIG. 6, as described below.  
         [0058]    Here, The secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 generated from voltage detection circuits  110 ,  120  and  130  are inputted into the first pulse generation circuit  140 . If the respective power sources VDD 1 , VDD 2  and VDD 3  go beyond the primary detection voltage at the respective times t1, t3 and t5, the respective first detection circuits of the voltage detection circuits generate the primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3, which follow the corresponding power sources, respectively, and then the primary voltage detection signals POR 1   — 1, POR 1   — 2 and POR 1   — 3 are inputted into the first pulse generation circuit  140 . In addition, if the respective power sources VDD 1 , VDD 2  and VDD 3  further rise and go beyond the secondary detection voltage at the respective times t2, t4 and t6, then the second detection circuits of the voltage detection circuits generate the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3, which follow the corresponding power sources, respectively, and then the secondary voltage detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 are inputted into the second pulse generation circuit  140 ′.  
         [0059]    The second pulse generation circuit  140 ′ includes a first AND gate  141 ′ receiving the primary detection signals POR 2   — 1, POR 2   — 2 and POR 2   — 3 and a pulse generation unit  142 ′ for generating pulses using the output of the first AND gate  141 ′. The pulse generation unit  142 ′ includes an inversion delay unit  143 ′ for inverting an output POR 2   — 3 of the first AND gate  141 ′, and a second AND gate  144 ′ for receiving the output of the inversion delay unit  143 ′ and the output of the first AND gate  141 ′. Accordingly, the last voltage detection pulse P_POR 2  is generated at a time t6′ of FIG. 7 by the detection signal POR 2   — 3, which, in turn, was generated in response to the power source VDD 3  reaching its secondary detection voltage. In other words, the secondary voltage detection signal POR 2   — 3 is generated at the latest time (t6) via the output terminal of the first AND gate  141 ′, and this signal is inputted into the second AND gate  144 ′ so that the second AND gate  144 ′ outputs second pulse P_POR 2  at the time t6′.  
         [0060]    Accordingly, in a power-on reset method for a semiconductor integrated circuit device using a plurality of power sources in accordance with the present invention, a stable and noise-resistant power-on reset can be provided using a plurality of power-on reset pulses.  
         [0061]    It will be apparent to those of ordinary skill in the pertinent art that various modifications and variations may be made to embodiments of the present invention without departing from the scope or spirit thereof. Thus, it is intended that the present invention cover such modifications and variations defined within the scope of the appended claims and their equivalents.