Patent Publication Number: US-2002000852-A1

Title: Reset circuit

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a reset circuit incorporated into a electronic equipment and outputting a reset signal to reset a internal circuit in the electronic equipment after power-on.  
       [0003] 2. Description of the Related Art  
       [0004] A semiconductor device is incorporated into electronic equipment, such as desk-top equipments (parsonal computer etc.) or portable equipments (portable telephone etc.). These equipments are act power-on/off. After power on, it is necessary for these equipments to turn back the starting condition to the semiconductor device in each electronic equipment, because of normally operatting of the electronic equipment. Therefore the electronic equipment or the semiconductor device in the electronic equipment have a reset circuit.  
       [0005] The reset circuit wacthes the change of a supply voltage according to power-on. The reset circuit outputs a reset signal temporality when the reset circuit is sensed the change of the supply voltage regard as power-on. The semiconductor device in the electronic equipment receives to the reset signal and turns back the starting condition (hereinafter also called “reset state”) to itself.  
       [0006] For portable equipments, each portable equipments supplyes a supply voltage from battery such as cell. The battery is incorporated into the portable equipment or is able to remove. For the electronic equipment such as portable equipment, The supply voltage unexpectedly reduces due to misdisconnection of a removable battery or the discharge of electrical charges stored in a battery put on charge in operation to the electronic equipment. Such circumstances, the reset circuit is effective to restore the operation of the electronic equipment at high speed.  
       [0007] The semiconductor device is reset for itself by the reset signal from the reset circuit. The reset signal has a ground voltage level or a source voltage level. The voltage level of the reset signal well result the source voltage level at one time after power-on. The semiconductor device is reset correspondance with the source voltage level of the reset signal.  
       [0008] However, the semiconductor device comprises CMOS comprising circuit (hereinafter also called “CMOS circuit”) such as flip-flop circuit and latch circuit. The lowest operating voltage at which the CMOS circuit normally operates is about |VTP|+VTN=VDD. VTP is threshold voltage of P-channel MOS transistor, VTN is threshold voltage of N-channel MOS transistor, and VDD is the source voltage. It is nessecally to assure till the lowest operating voltage at which a CMOS circuit normally operates, a period in which a source voltage level of the reset signal is maintained.  
       [0009] The reset circut only operates immediately after power-on or after reducing the source voltage VDD. Therefore the reset circuit is desirable to comprise a less number of components, to reduce in cost thereof, to reduce in the size of the thereof, or to improve in the degree of freedom of the lyayout of thereof.  
       [0010] An object of the present invention is to provide the reset circuit for reliably resetting a semiconductor device.  
       [0011] A further object of the present invention is to provide the reset circuit for desirabling to comprise a less number of components, to reduce in cost thereof, to reduce in the size of the thereof, or to improve in the degree of freedom of the lyayout of thereof.  
       SUMMARY OF THE INVENTION  
       [0012] A reset circuit of the present invention for sensing to change a level of a supply voltage from a first voltage level to a second voltage level, and for outputting a reset signal from a output node, the supply voltage supplyed a supply voltage source, the reset circuit comprising a first MOS transistor of a first conductive type coupled between said supply voltage source and a control node, the first MOS transistor controlling electlically conductive state between the supply voltage source and the control node corresponding to a voltage level of the control node, a resistive element coupled between said control node and a reference voltage source, a second MOS transisitor of a second conductive type coupled between said output node and said reference voltage source, the second MOS transistor controlling electlically conductive state between the output node and the reference voltage source corresponding to a voltage level of the control node, and a third MOS transistor of said first conductive type coupled between said supply voltage source and said output node, the third MOS transistor controlling electlically conductive state between the supply voltage source and a output node corresponding to a voltage level of the control node.  
       [0013] Typical ones of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:  
     [0015]FIG. 1 is a circuit diagram showing a reset circuit according to a first embodiment of the present invention;  
     [0016]FIG. 2 is a waveform chart for describing the operation of the reset circuit shown in FIG. 1;  
     [0017]FIG. 3 is a circuit diagram illustrating a reset circuit according to a second embodiment of the present invention; and  
     [0018]FIG. 4 is a waveform chart for describing the operation of the reset circuit shown in FIG. 3.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0019] Reset circuits of the present invention will hereinafter be described in detail with reference to the accompanying drawings. FIG. 1 is a circuit diagram of a reset circuit  200  according to a first embodiment of the present invention.  
     [0020] Referring to FIG. 1, the reset circuit  200  comprises three transistors  10 ,  50 ,  240  and a resistive element  30 .  
     [0021] The transistor  10 , which serves as a first MOS transistor, is a P-channel MOS transistor. A source electrode of the transistor  10  is supplied with a source voltage VDD from a supply voltage source or voltage source. Further, drain and gate electrodes thereof are electrically connected to a node  20  which serves as a control node. The resistive element  30  has one terminal electrically connected to the node  20  and the other terminal electrically grounded to a reference voltage source. Thus, the other terminal of the resistive element  30  is supplied with a ground voltage VSS from the reference voltage source. Incidentally, the threshold voltage of the transistor  10  will be defined as V TP .  
     [0022] The transistor  50 , which serves as a second MOS transistor, is an N-channel MOS transistor. A drain electrode of the transistor  50  is electrically connected to an output node  60 , a gate electrode thereof is electrically connected to the node  20 , and a source electrode thereof is electrically grounded. The threshold voltage of the transistor  50  will be defined as V TN .  
     [0023] The transistor  240 , which serves as a third MOS transistor, is a P-channel MOS transistor. A source electrode of the transistor  240  is supplied with the source voltage VDD. A drain electrode of the transistor  240  is electrically connected to the output node  60 , and a gate electrode thereof is electrically connected to the node  20 . Therefore, the transistors  10  and  240  constitute a current mirror circuit. The threshold voltage of the transistor  240  will be defined as V TP . A change in the value of a voltage developed at the output node  60  is utilized as a reset signal for the reset circuit  200 .  
     [0024] Incidentally, the source voltage VDD employed in the reset circuit  200  includes, as supply voltage sources, for example, one supplied from the outside of electronic equipment, one supplied from a battery such as a cell or the like incorporated into electronic equipment. Various types or forms are known as supply sources for the reset circuit  200 . In particular, the battery may be a chargeable type or may be removable from the electronic equipment.  
     [0025] Even if the resistive element  30  is placed in any configuration, the resistance value of the resistive element  30  is set so as to be sufficiently higher than an on resistance of the transistor  10 .  
     [0026] The operation of the reset circuit  200  constructed in this way will be described below with reference to the drawings. FIG. 2 is a waveform chart for describing the operation of the reset circuit  200 . In FIG. 2, the vertical axis indicates the voltage and the horizontal axis indicates time. A solid line indicates the value of a voltage developed at the output node  60  from which a reset signal is outputted, and a dotted line indicates the value of the source voltage VDD. The relationship between VTP and VTN will be explained as |VTP|&gt;VTN in the following description. Prior to power-on, any of the transistors  10 ,  50  and  240  will be kept in an off state (i.e., there is electrically non-continuity between their sources and drains). Further, any of the voltage values of the source voltage VDD, the node  20  and the output node  60  will be defined as a reference voltage VSS.  
     [0027] When power is turned on at time t 1  in FIG. 2, the value of the source voltage VDD starts to increase based on a time constant held by the power. When the source voltage VDD is given as VDD&lt;|VTP| immediately after power-on, the transistor  10  is in the off state. Therefore, the node  20  is set to the reference voltage VSS through the resistive element  30 . Thus, the transistor  50  is kept in the off state. Since the transistor  240  is placed under the same condition as that for the transistor  10 , it is held in the off state. Accordingly, the value of the voltage developed at the output node  60  is undefined (held in a high-resistance state).  
     [0028] Thereafter, when |VTP| VDD&lt;VTN, the transistor  10  is brought to an on state. Thus, the value of the voltage at the node  20  results in VDD−|VTP|. At this time, the transistor  240  is also turned on. Since the value of the voltage applied to the gate of the transistor  50  is given as VDD−|VTP| (&lt;VTN), the transistor  50  is in the off state. Accordingly, the voltage value of the output node  60  results in VDD (at time t 2 ). Incidentally, if the transistors  10  and  240  constitute the current mirror circuit and are identical to each other in transistor characteristic, the same amount of current as the transistor  10  flows in the transistor  240 .  
     [0029] Thereafter, when VDD |VTP|+VTN, the voltage value of the node  20  results in VDD−|VTP| (VTN) and hence the transistor  50  is turned on. Therefore, the output node  60  is grounded via the transistor  50 . Thus, the voltage value of the output node  60  is brought to the reference voltage VSS. Time t 3  indicates timing provided immediately after VDD |VTP|+VTN.  
     [0030] When the source voltage VDD is lowered, the reset circuit  200  enters into operation opposite to the above operation. Therefore, when VDD&lt;|VTP|+VTN, the output node  60  changes from the reference voltage VSS to the source voltage VDD. In FIG. 2, time t 4  indicates timing provided immediately after VDD&lt;|VTP|+VTN.  
     [0031] Thus, when the node  20  is used as a control node for controlling the operation of the transistor  50  and a semiconductor device built in electronic equipment is reset according to the state of the source voltage VDD of the output node  60 , which is outputted as a reset signal, a period in which a source potential level of the reset signal is maintained, is assured till the lowest operating voltage at which a CMOS circuit normally operates. Thus, even if the CMOS circuit exists in the semiconductor device supplied with the reset signal, the semiconductor device can be reliably reset.  
     [0032] Further, since the reset circuit  200  comprises a less number of components, a reduction in cost thereof, a reduction in the size of the semiconductor device itself and an improvement in the degree of freedom of the layout of the reset circuit can be expected.  
     [0033] Although the relationship between VTP and VTN has been explained as |VTP| VTN as mentioned above, the operation of the reset circuit slightly differs from the above-described operation in the following points when |VTP|&lt;VTN.  
     [0034] Namely, the operation thereof at VDD&lt;VTP is similar to the above description. Thereafter, when |VTP| VDD&lt;VTN and VTN VDD |VTP|+VTN, the transistor  10  is turned on so that the voltage value of the node  20  is raised. Since, however, the voltage value of the node  20  is represented as VDD−|VTP| (&lt;VTN), the transistor  50  is kept in an off state. Thus, the voltage value of the output node  60  will result in the source voltage VDD. Thereafter, the operation thereof from VDD&gt;|VTP|+VTN is similar to the aforementioned description.  
     [0035] If the reset circuit  200  of the present invention is applied to, for example, cellular or portable electronic equipment after power-on, then a semiconductor device incorporated into the electronic equipment can be reliably reset and the operation of the electronic equipment can be restored at high speed where the source voltage VDD is unexpectedly reduced due to misdisconnection of a removable battery or the discharge of electrical charges stored in a battery put on charge.  
     [0036] Since the transistor  10  and the transistor  240  constitute the current mirror circuit in the reset circuit  200 , the mutual conductance gm between the transistor  10  and the transistor  240  can be adjusted so as to reduce currents which constantly flow in the transistors  10  and  50 , whereby current consumption can be reduced. Since the transistor  240  is used as the resistive element, the area of the reset circuit can be reduced.  
     [0037] In the reset circuit  200 , the reset signal outputted from the output node  60  may be supplied to respective circuits to be reset through two inverters. Theses inverters comprise a P-channel MOS transistor and an N-channel MOS transistor respectively.  
     [0038] More specifically, a reset signal outputted from the output node  60  is supplied to gate electrodes of two transistors which constitute a first inverter. One electrode of a P-channel MOS transistor, which constitutes the first inverter, is supplied with the source voltage VDD. One electrode of an N-channel MOS transistor, which constitutes the first inverter, is supplied with the reference voltage VSS. The other electrodes of the two transistors, which constitute the first inverter, are electrically connected to one another. A signal outputted from the first inverter becomes a signal transmitted to the other electrodes of these transistors. The signal outputted from the first inverter is supplied to gate electrodes of two transistors, which constitute a second inverter. One electrode of a P-channel MOS transistor, which constitutes the second inverter, is supplied with the source voltage VDD, and one electrode of an N-channel MOS transistor, which constitutes the second inverter, is supplied with the reference voltage VSS. The other electrodes of the two transistors, which constitute the second inverter, are electrically connected to one another. A signal outputted from the second inverter becomes a signal transmitted to the other electrodes of these transistors. The output signal of the second inverter will result in a reset signal used for each circuit.  
     [0039] Owing to such a construction, the level of the voltage at the output node  60  can be further amplified when the voltage level thereof increases. In a state in which the source voltage VDD has risen sufficiently, the voltage level of the reset signal supplied to each circuit can be set to an L level more reliably. Thus, the effects of the present invention can be obtained more reliably.  
     [0040] A reset circuit according to a second embodiment of the present invention will next be described below with reference to the drawings. FIG. 3 is a circuit diagram of a reset circuit  300  according to the second embodiment. In FIG. 3, the same elements of structure as those illustrated in the reset circuit  200  according to the first embodiment shown in FIG. 1 are identified by the same reference numerals.  
     [0041] Referring to FIG. 3, transistors  10 ,  50  and  240  and a resistive element  30  are similar to those employed in the reset circuit  200  shown in FIG. 1. Namely, the reset circuit  300  shown in FIG. 3 is constructed in such a manner that a P-channel MOS transistor  350  used as a fourth MOS transistor is further added to the reset circuit  200  shown in FIG. 1.  
     [0042] The transistor  350  has a source electrode supplied with a source voltage VDD, a drain electrode electrically connected to a node  20  and a gate electrode electrically connected to an output node  60 .  
     [0043] The operation of the reset circuit  300  constructed in this way will be described below with reference to the drawings. FIG. 4 is a waveform chart for describing the operation of the reset circuit  300 . In FIG. 4, the vertical axis indicates the voltage and the horizontal axis indicates time. A solid line indicates the value of a voltage developed at the output node  60  and a dotted line indicates the value of the source voltage VDD. Incidentally, the relationship between VTP and VTN is set as |VTP|&lt;VTN in the reset circuit according to the second embodiment.  
     [0044] Since the transistors  10 ,  240  and  350  are placed under the same condition till VDD&lt;|VTP|+VTN (till time t 2 ) after power-on, the present embodiment is similar to the first embodiment.  
     [0045] When VDD |VTP|+VTN, the transistor  50  is brought to an on state according to the value of a voltage at the node  20 . Thus, the value of the voltage at the output node  60  will result in a reference voltage VSS. Time t 3  shown in FIG. 4 indicates timing provided when VDD |VTP|+VTN.  
     [0046] As the voltage value of the output node  60  becomes the reference voltage VSS, the transistor  350  is completely turned on. When the resistance value of the resistive element  30  is set so as to become sufficiently higher than an on resistance of the transistor  350  at this time, the voltage value of the node  20  is set to the source voltage VDD. Thus, the transistors  10  and  240  both change from an on state to an off state.  
     [0047] A description will be made of the case in which the value of the source voltage VDD is reduced for some reason subsequent to the above state.  
     [0048] Since the gate electrode of the transistor  50  is supplied with the source voltage VDD and the gate electrode of the transistor  350  is supplied with the reference voltage VSS, no changes occur in the on/off states of the respective transistors in the relationship of VDD (I VTP or a higher value of VTN) (immediately before time t 3  in FIG. 4). Therefore, the voltage value of the output node remains at the reference voltage VSS.  
     [0049] When VDD&lt;|VTP| in the case of |VTP| VTN, the transistor  350  changes from the on state to the off state. Therefore, the voltage value of the node  20  reaches the reference voltage VSS and correspondingly the transistor  50  is also turned off. Thus, the output node  60  is undefined (held in a high-resistance state).  
     [0050] On the other hand, when |VTP| VDD&lt;VTN in the case of |VTP|&lt;VTN, the transistor  50  changes from the on state to the off state. Thus, the transistors  10  and  240  are turned on. Therefore, the voltage value of the output node  60  changes from the reference voltage VSS to the source voltage VDD.  
     [0051] As described above, the reset circuit  300  according to the second embodiment can bring about an effect similar to that obtained by the reset circuit  200  according to the first embodiment referred to above.  
     [0052] Further, the reset circuit  300  according to the second embodiment can turn off the transistors  10  and  240  after the voltage value of the output node  60  has been brought to the reference voltage VSS. Thus, when the reset circuit is held in a steady state (corresponding to a state of being stably supplied with the source voltage VDD to activate the semiconductor device stably), a current, which flows in each of the transistors  10  and  240  of the reset circuit  300 , can be interrupted. As a result, the current to be used up or consumed by the reset circuit can be reduced.  
     [0053] In the second embodiment, the voltage value of the output node  60  can follow VDD to reach VDD |VTP|+VTN when the source voltage VDD rises. On the other hand, when the source voltage VDD drops, the voltage value of the output node  60  can be maintained at the reference voltage VSS till VDD&lt;|VTP|+VTN. Therefore, even when a voltage variation occurs in the source voltage VDD due to noise or the like when the semiconductor device being placed in the steady state is in operation, it is possible to exert control on a change of the voltage value of the output node into the source voltage VDD. It is thus possible to restrain the semiconductor device from being erroneously reset.  
     [0054] The signal outputted from the reset circuit according to the second embodiment may be used as a reset signal for each circuit to be reset through two inverters in a manner similar to the first embodiment.  
     [0055] Although the reset circuit of the present invention has been described above in detail, the reset circuit of the present invention is not necessarily limited to the aforementioned configuration. Various modifications can be made thereto.  
     [0056] The reset circuit may be utilized so that, for example, the N-channel MOS transistor and the P-channel MOS transistor are set in reverse and the portions for supplying the reference voltage VSS and the source voltage VDD act in reverse.  
     [0057] The threshold voltages of the respective transistors may be controlled for the lowest operating voltage of a circuit to be reset by the reset signal outputted from the output node so as to obtain the effects of the present invention more reliably.  
     [0058] While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.