Abstract:
A power-on reset circuit reduces power consumption and layout area by utilizing a time delay to deactivate a reset circuit and clamp a reset signal a period of time after a power supply voltage has reached a predetermined level. The reset circuit includes a reset signal generator which maintains the reset signal in an active state until the power supply voltage has reached the predetermined level. The reset signal generator then deactivates the reset signal which causes a delay circuit to begin a time delay after which a delay signal is asserted. The delay signal deactivates a comparator in the reference voltage generator, a voltage detector, and a reset signal generator within the reset circuit, thereby reducing power consumption. The delay signal also activates a clamp circuit which clamps the reset signal to an inactive state.

Description:
This application corresponds to Korean patent application No. 98-22099 filed Jun. 12, 1998 in the name of Samsung Electronics Co., Ltd., which is herein incorporated by reference for all purposes. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power-on reset circuits for integrated circuits and, more particularly, to a power-on reset circuit suitable for high density integrated circuits. 
     2. Description of the Related Art 
     A power-on reset circuit (sometimes called a power up detection circuit) provides a reset signal for initializing flip-flops, latches, counters, registers and other such internal components of a semiconductor integrated circuit, when power is applied thereto. The reset signal is maintained at a first constant voltage (e.g., a logic low or “0”) for a sufficient time to allow stabilization of the respective components of the circuit. After a predetermined time, the reset signal is switched to a second constant voltage e.g., a logic high or “1”) for as long as the power is applied to the circuit. 
     A large variety of power-on reset circuits have been proposed, such as U.S. Pat. No. 4,885,476, by Mahabadi, issued on Dec. 5, 1989, disclosing a power-on reset circuit including a start-up voltage generator circuit which produces a voltage which is insensitive to changes in field effect transistor threshold voltages, in which a reset signal is fed back to the start-up voltage generator circuit to reduce the steady state current. 
     U.S. Pat. No. 5,386,152, by Naraki, Jan.  31 ,  1995 , discloses a reset signal generating circuit which generates and provides a reset signal for a logic unit during a certain period only when a power component from the output of a differentiator circuit exceeds a threshold voltage of the reset signal generating circuit. 
     U.S. Pat. No. 5,463,335, by Divakaruni et al., issued on Oct. 31, 1995, teaches a power-on reset circuit which includes an output terminal connected through an impedance to a power supply, with the output terminal being further connected through a subthreshold leakage device for a latch to a point of reference potential, the subthreshold leakage device being switched from an initial subthreshold mode to a conduction mode in response to a predetermined level of output voltage developed on the output terminal. 
     U.S. Pat. Nos. 5,612,642 and 5,760,624, by McClintock, issued on Mar. 26, 1997 and Jun. 2, 1998, respectively, disclose power-on reset circuits each of which deasserts a power on reset signal until the power supply voltage level drops to a level low enough to render storage elements in a circuit controlled by the reset signal incapable of holding accurate data. 
     When designing power-on reset circuits, consideration must be given to steady-state power dissipation, chip layout, production costs and the stability of the reset signal. In designing a typical power-on reset circuit, the steady-state power dissipation should be minimized. In addition, to economize the layout area of the chip, the use of passive elements (e.g., capacitors and resistors) which occupy a relatively large area, and depletion mode transistors which add a manufacturing step, should be avoided. 
     With reference to FIG. 1, there is illustrated a conventional monolithic semiconductor integrated circuit chip with a power-on reset circuit  12 , which is disclosed in U.S. Pat. No. 5,376,835, by Van Buskirk et al., issued on Dec. 27, 1994. The power-on reset circuit  12  illustrated in FIG. 1 may be designed so as to be incorporated into a semiconductor chip along with logic and/or memory circuitry  18  and a voltage reference generator  38 . The power-on reset circuit  12  provides a reset signal VCCOK on its first output terminal  14  which is coupled vial line  16  to logic and/or memory circuitry  18  containing a state machine having state registers SR 1 , SR 2 , . . . , SRn. The reset signal VCCOK is a logic signal which resets the state registers SR 1 , SR 2 , . . . , SRn when it is at a low or “0” logic level (active state). 
     The true outputs Q 1  through Qn of the respective registers SR 1  through SRn are connected to corresponding inputs of a NOR gate  20  via lines  22   a - 22   n , respectively. The output of the logic gate  20  on line  24  is fed to an inverter gate  26  on line  28  which provides a state monitoring signal SMON which is connected to a first input terminal  32  of the power-on reset circuit  12 . 
     The power-on reset circuit  12  includes a second output terminal  34  which provides a logic control signal VON which is fed via line  35  to an input terminal  36  of the voltage reference generator  38 . The voltage reference generator  38  generates a stable reference voltage VREFI on its first output terminal  40  which is fed via line  42  to a second input terminal  44  of the power-on reset circuit  12 . The reference generator  38  also provides a start-up voltage VCCDC on its second output terminal  46  which is fed via line  48  to a third input terminal  50  of the power-on reset circuit  12 . 
     The power-on reset circuit  12  operates in response to the monitoring signal SMON, the start-up voltage VCCDV, and the reference voltage VREFI, and generates and maintains the reset signal VCCOK at an active low state during power-up until the power supply voltage exceeds a predetermined level. More specifically, the power-on reset circuit  12  is active during power-up only if one of the outputs of the state registers SR  1 -SRn is high (i.e., in the non-reset state). Otherwise, if the outputs of the state registers SR  1 -SRn happen to come up in the reset state (all outputs being low), then the power-on reset circuit  12  is never activated at all since the logic control signal VON remains low. When the state registers SR 1 -SRn are powered up in the non-reset state, the reset signal VCCOK continues to be applied to the reset inputs of the state registers SR  1 -SRn until the power supply voltage VCC has reached a predetermined level so as to insure proper operation of the logic and/or memory circuitry  18 . Thereafter, the power-on reset circuit  12  shuts itself off in response to the monitoring signal SMON, thereby reducing power consumption. 
     However, the above-described arrangement, in which the outputs of the state registers SR 1 -SRn are used for shutting off the power-on reset circuit  12  through logic gates  20  and  24 , requires a considerable increase in the layout area for the power-on reset circuit as the number of the state registers increases because the number of logic gates must necessarily increase due to limitations on logic circuit construction such as fan-in. Therefore, the density of an integrated circuit chips having the above-described arrangement is reduced. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, accordingly, is to overcome the drawbacks of prior art power-on reset circuits, and to provide a power-on reset circuit suitable for high integration density integrated circuits. 
     It is another object of the present invention to provide a power-on reset circuit having a small layout area. 
     These and other objects, features and advantages of the present invention are provided by a power-on reset circuit which includes a reset circuit that generates a reset signal and a delay circuit that generates a delay signal which is a delayed version of the reset signal. The reset signal is maintained in a first logic state (e.g., logic 0) until the power supply voltage has reached a predetermined level (e.g., about 2 volts). The reset circuit is deactivated in response to the delay signal so that the reset signal is maintained in a second logic state (e.g., logic 1). The delay circuit preferably provides a delay time ranging from several tens of microseconds to several milliseconds. Specifically, the reset circuit includes a reference voltage generator which generates a reference voltage, a voltage detector which generates a start-up voltage proportional to the power supply voltage, and a reset signal generator which generates the reset signal in response to the reference voltage and the start-up voltage. The reset signal generator includes a differential comparator which generates the reset signal at a first logic state while the start-up voltage is smaller than or equal to the reference voltage, and a clamp circuit which clamps the reset signal to a second logic state in response to the delay signal. 
     The foregoing features and advantages of the invention will be more fully described in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art monolithic semiconductor integrated circuit chip which includes a power-on reset circuit; and 
     FIG. 2 is a circuit diagram of a preferred embodiment of a power-on reset circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an improved power-on reset circuit which can be incorporated into a high density integrated circuit. In the following description, specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these particulars. Accordingly, the specification and a drawing are to be regarded in an illustrative, rather than a restrictive sense. 
     A preferred embodiment of the present invention will now be described with reference to FIG. 2 of the drawings. 
     Referring to FIG. 2, the power-on reset circuit includes a reset circuit  100  and a delay circuit  200 . The reset circuit  100  generates a power-on reset signal VCCOK at a first state (e.g., a logic “low” or “0”) until the power supply voltage VCC has reached a predetermined level (e.g., about 2.26 volts). The delay circuit  200  generates a delay signal VON which is a delayed and inverted version of the reset signal VCCOK. The delay circuit provides a delay time ranging from several tens of microseconds (e.g. 10 microseconds) to several milliseconds (e.g., 1 milliseconds). The reset circuit  100  is deactivated in response to the delay signal VON from the delay circuit  200  so that the reset signal VCCOK is maintained in a second logic state (e.g., a logic “high” or “1” state). 
     The reset circuit  100  includes a reference voltage generator  110  which generates a reference voltage VREF (e.g., 1.2 volts), a power supply voltage detector  120  which generates a start-up voltage VCCDC proportional to the power supply voltage VCC, and a reset signal generator  130  which generates the reset signal VCCOK in response to the reference voltage VREF and the start-up voltage VCCDC. 
     More specifically, the reference voltage generator  110  includes three NMOS transistors NM 1 , NM 2  and NM 3 , each of which has a controlling electrode (i.e., gate electrode) and a pair of controlled electrodes (i.e., source/drain electrodes). The reference voltage generator  110  is also provided with three resistors R 1 , R 2  and R 3 . The resistors R 1  and R 2 , the drain and source electrodes of the NMOS transistor NM 1 , the resistor R 3 , and the drain and source electrodes of the NMOS transistor NM 2  are serially connected between the power supply terminal VCC (first power supply voltage) and the ground terminal VSS (second power supply voltage) in the order recited so as to define a first node N 1  between the resistors R 1  and R 2 , a second node N 2  (i.e., reference voltage output node VREF) between the resistor R 2  and the transistor NM 1 , a third node N 3  between the transistor NM 1  and the resistor R 3 , and a fourth node N 4  between the resistor R 3  and the transistor NM 2 . Specifically, the gate electrodes of the NMOS transistors NM 1  and MN 3  are coupled to the first and third nodes N 1  and N 3 , respectively. Further, the source and drain electrodes of the transistor NM 3  are connected to the first and fourth nodes N 1  and N 4 , respectively. 
     Resistor R 3  sets the gate-to-source voltage of NMOS transistor NM 3  so that the NMOS transistor NM 3  conducts in its subthreshold region. Thus, NMOS transistor NM 3  has a negative temperature coefficient. In contrast, NMOS transistor NM 1  has a positive temperature coefficient in its conduction region. 
     The operation of the reference voltage generator  110  will now be described with reference to FIG.  2 . As the power supply voltage VCC increases, the voltage at node N 1  (the gate voltage of NMOS transistor NM 1 ) increases, and the amount of current flowing through resistor R 2  increases. The increase in gate voltage of NMOS transistor NM 3  due to the higher voltage at node N 3  causes an increase in the current flowing through NMOS transistor NM 3 . Thus, the voltage at node N 1  decreases, and current flowing through the resistor R 2  is reduced, which causes the drain-to-source current of NMOS transistor NM 1  to decrease. As a result, reference voltage VREF remains relatively constant despite an increase in the power supply voltage VCC. 
     Conversely, when the first power supply voltage VCC decreases, the reduced voltage level at node N 1  decreases the current that flows through the resistor R 2 . The voltage at node N 3  and the reference voltage VREF also decrease. However, as the voltage of node N 3 , corresponding to the gate voltage of NMOS transistor NM 3 , is reduced, the voltage at node N 1  increases, and the current flowing through the NMOS transistor NM 1  increases. 
     Thus, the NMOS transistors NM 1  and NM 3  adjust to changes in the power supply voltage VCC in a complementary manner, so that the reference voltage VREF is relatively insensitive to power supply voltage variations. Stated differently, NMOS transistor NM 1  controls the voltage level at node N 2  and NMOS transistor NM 3  controls the voltage level at node N 1 , so that the reference voltage at node N 2  is relatively stable notwithstanding changes in the power supply voltage VCC. 
     As described above, the reference voltage generator  110  provides a stable reference voltage VREF (e.g., about 1.2 volts for a VCC of about 3 volts) that is relatively insensitive to variations in the threshold voltages of the transistors therein. The reference voltage generator  110  is also relatively insensitive to variations in power supply voltage VCC and temperature. 
     The power supply voltage generator  120  includes two serially connected resistors R 4  and R 5  acting as a voltage divider, and an NMOS transistor NM 4  having a controlling electrode (i.e., gate electrode) and a pair of controlled electrodes (i.e., source and drain electrodes). The resistors R 4  and R 5  and the drain and source electrodes of the transistor NM 4  are connected in series between the first and second power supply voltages VCC and VSS in the order recited so as to define a node N 5  (i.e., start-up voltage output node VCCDC) between the resistors R 4  and R 5 . When transistor NM 4  is turned on, the power supply voltage VCC is divided by resistors R 4  and R 5  so as to obtain the start-up voltage VCCDC at node N 5 . 
     The reset signal generator  130  includes a differential amplifier  132  serving as a comparator. The differential amplifier  132  includes two PMOS transistors PM 1  and PM 2  and three NMOS transistors NM 5 , NM 6  and NM 7 , each of which has a controlling electrode (i.e., gate electrode) and a pair of controlled electrodes (i.e., source and drain electrodes). The source electrodes of the PMOS transistors PM 1  and PM 2  are commonly connected to the first power supply terminal VCC and the gate electrodes thereof are connected to each other. Also, the gate electrodes of the PMOS transistors PM 1  and PM 2  are commonly connected to the drain electrodes of the PMOS transistor PM 1  and NMOS transistor NM 5 . The drain electrodes of the PMOS transistor PM 2  and NMOS transistor NM 6  are connected to each other. The source electrodes of the NMOS transistors NM 5  and NM 6  are connected to the drain of the NMOS transistor NM 7 . The source electrode of the NMOS transistor NM 7  is connected to the second power supply terminal VSS (i.e., the ground voltage). The differential amplifier  132  generates the reset signal VCCOK at a low state while the start-up voltage VCCDC is lower than or equal to the reference voltage VREF. 
     The trip voltage of the differential amplifier  132  can be obtained by following equation: 
     
       
         VCCDC=VREF  (1) 
       
     
     where VCCDC=VCC×(R 5 /(R 4 +R 5 ). Assuming the ratio R 5 /(R 4 +R 5 )=0.53 and VREF=1.2 volts, equation (1) can be rewritten as follows: 
     
       
         VCC×0.53=1.2 volts  (2) 
       
     
     Solving equation (2) for VCC, we have: 
     
       
         VCC=1.2 volts/0.53=2.26 volts  (3) 
       
     
     Accordingly, when the power supply voltage VCC is less than about 2.26 volts, the reset signal VCCOK remains low. 
     The reset signal generator  130  further includes a clamp circuit  134 . This circuit  134  includes a PMOS transistor PM 3  whose source electrode is connected to the first power supply terminal VCC and whose drain is connected to both drain electrodes of the transistors PM 2  and NM 2  (i.e., node N 6 ), respectively. The clamp circuit  134  clamps the reset signal VCCOK to a voltage of the logic high state (i.e., VCC) in response to the delay signal VON. 
     The delay circuit  200  includes a capacitor C and an odd number of serially connected inverters, for example, three inverters I 1 , I 2  and I 3 . The first inverter I 1  has its input connected to the node N 6  (i.e., reset signal output node) of the reset signal generator  110 . The last inverter  13  has its output commonly connected to the gate electrodes of the transistors NM 4 , NM 7 , NM 2  and PM 3 . 
     The operation of the power-on reset circuit of the invention will now be described in more detail. The differential amplifier  132  initially maintains the reset signal VCCOK at the low state. This forces the delay signal VON from the delay circuit  200  remain high. The high state of the delay signal VON renders the transistors NM 2 , NM 4  and NM 7  conductive. Also, the high state of the VON signal makes the transistor PM 3  nonconductive. As the power supply voltage VCC ramps up towards the steady-state level of approximately 3 volts after power-on, the reference voltage VREF and the start-up voltage VCCDC follow the power supply voltage VCC. When the power supply voltage VCC reaches about 2.26 volts, the reference voltage VREF and the start-up voltage VCCDC will be about 1.2 volts. At this time, the reset signal VCCOK is still maintained in the low state. Thereafter, when the startup voltage VCCDC goes beyond 1.2 volts with the continuous ramp up of the power supply voltage VCC, the reset signal VCCOK goes high since the reference voltage VREF is maintained at a constant level of about 1.2 volts as mentioned earlier. After a given delay time (e.g., several tens of microseconds through several milliseconds) from the low-to-high transition of the reset signal VCCOK, the delay signal VON from the delay circuit  200  goes low. This makes the NMOS transistors NM 2 , NM 4  and NM 7  nonconductive, resulting in the automatic deactivation of the reference voltage generator  110 , the power supply voltage detector  120  and the differential amplifier  132 , thereby reducing power consumption. Also, the low state of the signal VON makes the PMOS clamp transistor PM 3  conductive so that the VCCOK signal is clamped to the power supply voltage VCC, i.e., logic high state. 
     Based on the above, it can be appreciated that a power-on reset circuit according to the present invention occupies a small area compared to the prior art circuit shown in FIG. 1, since a circuit according to the invention needs no logic gates for checking the outputs of the status registers. Accordingly, the power-on reset circuit of the invention is suitable for high density integrated circuits. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein. We claim all modifications and variations coming within the spirit and scope of the following claims.