Abstract:
A power reset signal generator provides a power reset signal having a minimum predetermined pulse width independent of the ramp time of the applied power.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a power reset signal generator, and more particularly, to a power reset signal generator suitable for power application to an electronic circuit. 
     Electronic systems typically include a power reset circuit that generates a reset signal when the power applied to the system is either initially turned on or cycled from being turned off and then turned on. The electronic system uses the power reset signal to initialize various subsystems at power up. Because the power reset circuit is occasionally used, portable electronic systems typically require a low current draw for the power reset circuit in order to conserve battery power. 
     Manufacturers typically test electronic systems under controlled test conditions, which includes power reset testing. These controlled test conditions typically include a slow ramp time for the applied power. However, in user systems, the user frequently plugs the electronic system into an already powered system, and this provides what is commonly called “hot plug in”. In such hot plug in situations, the power signal has a fast ramp time. Accordingly, devices that the manufacturer has tested under controlled conditions may fail in the field. 
     SUMMARY OF THE INVENTION 
     The present invention provides a power reset signal generator that is independent of the ramp time of the applied power. The present invention also provides a power reset signal generator that draws low current. 
     The present invention provides a power reset signal generator that includes a first voltage divider that provides a first reference signal in response to an applied power signal having a ramp time. The first reference signal is substantially equal to the voltage of the applied power signal for at least a portion of the ramp time in the event that the voltage of the applied signal is less than a threshold voltage and is substantially proportional to the voltage of the applied power signal in the event that the voltage of the applied signal is greater than the threshold voltage. The power signal generator also includes an inverter coupled to the output of the first voltage divider, and includes a second voltage divider having a first input coupled to an output of the first voltage divider, having a second input coupled to the output of the inverter, having an output for providing a reference signal. An NAND gate has a first input coupled to the output of the first voltage divider, a second input coupled to the output of the second voltage divider and an output for providing a power reset signal in response to the first and second reference signals. 
     The power reset generator may include a capacitor coupled to the second input of the second voltage divider for maintaining the voltage on the reference signal at a predetermined voltage level for a predetermined time. The pulse width of the power reset signal may be the greater of the predetermined time and a time of the voltage level of applied power signal becoming greater than the threshold voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a power reset signal generator according to an embodiment of the present invention. 
     FIGS. 2 and 3 are timing diagrams of the power reset signal generator shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram of a power reset signal generator  100  according to an embodiment of the present invention. The power reset signal generator  100  comprises a control signal generator  102 , a pulse generator  104 , and a delay element  106 . The power reset signal generator  100  comprises n-channel metal oxide semiconductor field effect transistors (NMOS transistors) N 1 , N 2 , N 3 , N 4 , N 5 , and N 6 , p-channel metal oxide semiconductor field effect transistors (PMOS transistors) P 1 , P 2 , P 3 , P 4 , P 5  and P 6  and a capacitor C. 
     The control signal generator  102  comprises the PMOS transistors P 1 , P 2 , and P 3  and the NMOS transistors N 1 , N 2 , N 3 , and N 4 . The drain-source terminals of the PMOS transistor P 1  and the NMOS transistors N 1  and N 2  are coupled in series between a power supply line and a ground line. A node  108  is formed of the common node of the source of the PMOS transistor P 1  and the drain of the NMOS transistor N 1 . The PMOS transistor P 1  and the NMOS transistors N 1  and N 2  form a voltage divider at the node  108 . In one embodiment of the present invention, one of the NMOS transistors N 1  and N 2  may be omitted from the power reset signal generator  100 . In another embodiment of the present invention, a resistor may be used instead of the NMOS transistors N 1  and N 2 . In this embodiment, the resistance of the resistor preferably is large for a low current draw. 
     The drain-source terminals of the PMOS transistor P 3  and the NMOS transistor N 3  are coupled in series between the power supply line and the ground line to form a node  110  at the common node formed by the source terminal of the PMOS transistor P 3  and the drain terminal of the NMOS transistor N 3 . The gates of the series coupled PMOS transistor P 3  and the NMOS transistor N 3  are coupled together and to the node  108 . The series coupled PMOS transistor P 3  and NMOS transistor N 3  are arranged as an inverter. The signal from the node  108  is applied to the gates of the PMOS transistor P 3  and the NMOS transistor N 3  to provide an inverted signal at the node  110 . In one embodiment of the present invention, the PMOS transistor P 3  is weak relative to the NMOS transistor N 3  to allow the NMOS transistor N 3  to quickly ground the node  110 . As used in the art, a first transistor is “weak” relative to a second transistor if the absolute value of the magnitude of the current provided by the first transistor is less than the absolute value of the magnitude of the current provided by the second transistor for a given absolute value of the applied voltage (for FETS, the voltage is VGS). 
     The drain-source terminals of the PMOS transistor P 2  and the NMOS transistor N 4  are coupled in series between the power supply line and the ground line to form a node  112  at the common node formed by the source of the PMOS transistors P 2  and the drain of the NMOS transistors N 4 . The node  110  is coupled to the gate of the NMOS transistor N 4  to selectively couple the node  112  to ground. The node  108  is coupled to the gate of the PMOS transistor P 2  to selectively couple the node  112  to the power supply line. 
     The delay element  106  comprises the capacitor C, which couples the node  112  to ground. 
     In one embodiment of the present invention, the PMOS transistor P 2  is weak relative to the PMOS transistor P 1 . In one example of such embodiment, the PMOS transistor P 1  has dimensions W/L and provides a current I 1  and the PMOS transistor P 2  has dimensions W/(L*N) and provides a current I 1 /N. 
     The pulse generator  104  comprises the PMOS transistors P 4  and P 5  and the NMOS transistors N 5  and N 6 . These transistors are arranged in a NAND gate configuration. Specifically, the drain-source terminals of the PMOS transistor P 4  and the NMOS transistors N 5  and N 6  are coupled in series between the power supply line and the ground line. The source of the PMOS transistor P 4  is coupled to the drain of the NMOS transistor N 5  to form an output node  114 . The gates of the PMOS transistor P 4  and the NMOS transistor N 6  are coupled together to form a common node as a first input of the NAND gate  104 , which is coupled to the node  108 . In one embodiment of the present invention, the trip point voltage of the first input of the NAND gate  104  has substantially the same trip point voltage as the inverter formed of the PMOS transistor P 3  and the NMOS transistor N 3 . The drain-source terminals of the PMOS transistor P 5  are coupled between the power supply line and the output node  114 . The gate of the PMOS transistor P 5  is coupled to the gate of the NMOS transistor N 5  to form a common node as a second input of the NAND gate  104 , which is coupled to the node  112 . The node  108  and the node  112  provide the pair of inputs for the pulse generator  104 . In one embodiment of the present invention, the PMOS transistor P 4  is weak relative to the series connected NMOS transistors N 5  and N 6 . 
     The output node  114  of the power reset signal generator  100  provides a reset signal independent of the rise time of an operational voltage Vcc applied to the power reset signal generator  100 . 
     FIGS. 2 and 3 are timing diagrams of the power reset signal generator  100 . The timing diagrams of FIGS. 2 and 3 are Simulation Program with Integrated Circuit Emphasis (SPICE) simulations of the power reset signal generator  100  for a respective slow and fast ramp time of the operational voltage. Referring now to FIG. 2, a line  200  shows the time relationship of the operational voltage applied to the power supply line for a slow ramp time. For illustration purposes, the timing diagram for a ramp time of 100 microseconds is shown. Lines  202 ,  204 ,  206 , and  208  show the time relationship of the voltage of the nodes  108 ,  110 ,  112 , and  114 , respectively. Line  210  shows the time relationship of the voltage on the common node formed of the source of the NMOS transistor N 1  and the drain of the NMOS transistor N 2 . Line  212  shows the time relationship of the voltage on the common node formed of the source of the NMOS transistor N 5  and the drain of the NMOS transistor N 6 . 
     Initially (t=0) no operational voltage Vcc (Vcc=0) (line  200 ) is applied to the power reset signal generator  100 , and all nodes  108 ,  110 ,  112 , and  114  are at zero voltage. As the operational voltage Vcc is applied to the power reset signal generator  100  (inclined portion of line  200 ), the operational voltage Vcc rises turning on the NMOS transistors N 1  and N 2  thereby grounding the node  108  (line  202 ). The grounding of the node  108  also turns on the PMOS transistor P 1  to pull the node  108  up to the operational voltage Vcc. As noted above, the circuit formed by the PMOS transistor P 1  and the NMOS transistors N 1  and N 2  functions as a voltage divider. As shown in line  202 , the voltage on the node  108  rises with the operational voltage Vcc and then experiences a drop for a period of time before rising linearly with the operational voltage Vcc. 
     During the initial application of the operational voltage Vcc, the grounding of the node  108  turns on the PMOS transistor P 3  (and correspondingly the NMOS transistor N 3  is kept turned off) thereby pulling the node  110  up to the operational voltage Vcc (line  204 ), and also turning on the NMOS transistor N 4 . The turning on of the NMOS transistor N 4  pulls the node  112  to ground (line  206 ), which keeps the NMOS transistor N 5  turned off and starts turning on the PMOS transistor P 5 . The output node  114  is pulled up to the operational voltage as the PMOS transistor P 5  turns on (line  208 ). As the operational voltage rises, the voltage of the output node  114  rises. 
     Grounding the node  108  also turns on the PMOS transistors P 1 , P 2  and P 4  and turns off NMOS transistor N 6 . Specifically, when the operational voltage applied to the PMOS transistor P 3  rises, the applied voltage to the PMOS transistor P 3  allows the PMOS transistor to function as a transistor and consequently turn on because the voltage applied to the gate thereof from the node  108  is sufficiently low. The voltage on the node  110  is correspondingly Vcc. As the voltage on the node  108  rises, the PMOS transistor P 3  turns off and the node  110  is grounded. In one embodiment of the present invention, the PMOS transistor P 3  is much weaker than the NMOS transistor N 3 . As the voltage of the node  108  rises, even though the PMOS transistor P 3  does not completely turn off, the NMOS transistor turns on sufficiently to cause the voltage of the node  110  to be sufficiently grounded to turn off the NMOS transistor N 4 . As the operational voltage Vcc reaches a trip point voltage of the circuit (Vtrip), the voltage on the node  108  becomes Vtrip (line  202 ) and the PMOS transistors P 2  and P 4  turn off and the NMOS transistor N 6  turns on to thereby couple the output node  114  to ground so that the output node  114  provides a zero voltage signal (line  208 ). Thus the output node  114  provides a power reset signal during the ramp up time of the operational voltage Vcc. 
     Referring now to FIG. 3, the operation of the power reset signal generator  100  for a fast ramp time of the power signal is now described. For illustrative purposes, the timing diagram for a ramp time of  10  nanoseconds is shown. A line  300  shows the time relationship of the operational voltage applied to the power supply line for a fast ramp time. Lines  302 ,  304 ,  306  and  308  show the voltage on the nodes  108 ,  110 ,  112 , and  114 , respectively. Line  310  shows the time relationship of the voltage on the common node formed of the source of the NMOS transistor N 1  and the drain of the NMOS transistor N 2 . Line  312  shows the time relationship of the voltage on the common node formed of the source of the NMOS transistor N 5  and the drain of the NMOS transistor N 6 . 
     If the ramp time of the operational voltage is sufficiently fast, the voltage level on the node  108  is high for a very short time (line  302 ) and the power reset signal generator  100  does not reset without the capacitor C. Specifically, the voltage level (line  302 ) on the node  108  is high or overshoots for a short time until the NMOS transistors N 1  and N 2  are sufficiently turned on and the PMOS transistor P 1  is sufficiently turned off so that these transistors can operate as a voltage divider. The voltage level on the node  108  reaches a steady state divided voltage level (line  302 ). Likewise, the voltage level (line  304 ) on the node  110  is high for a short time before rapidly falling to zero, and the NMOS transistor N 3  is turned on and the PMOS transistor P 3  is substantially turned off. Because the ramp time of the operational voltage (line  300 ) is fast, the PMOS transistor P 1  and the NMOS transistors N 1  and N 2  are turned on quickly which quickly pulls the voltage on the node  108  (line  302 ) to the operational voltage Vcc (line  300 ) before rapidly becoming the divided voltage when the NMOS transistors N 1  and N 2  are turned on. Likewise, the voltage on the node  110  (line  304 ) quickly becomes 0 when the operational voltage reaches Vcc. 
     The capacitor C retains the voltage of the node  112  at a sufficiently high voltage level to reset the power reset signal generator  100 . The PMOS transistor P 2  provides a current to the node  112  to thereby charge the capacitor C from a zero voltage to a trip voltage Vtrip in a pre-selected time. This charging keeps the output signal on the output node  114  at Vcc for a time T. In one embodiment of the present invention, the PMOS transistor P 2  provides a current I 1 /N and the PMOS transistor P 1  provides a current I. The current I 1 /N is selected to be small, and may be, for example, approximately about 10 nanoamps. In this embodiment, the capacitor C holds the output signal on the output node  114  high for a time T=(C*N*Vtrip)/I 1 . Thus, by coupling the capacitor C between the node  112  and ground, the voltage on the node  112  rises much less quickly to the operational voltage Vcc (line  306 ). Accordingly, the output signal on the node  114  remains high until the voltage on the node  112  is sufficiently high to turn off the PMOS transistor P 5  and turn on the NMOS transistor N 5  (line  308 ). The NMOS transistor N 5  is turned on after the NMOS transistor N 6  to thereby couple the output node  114  to ground (line  308 ). 
     The power reset signal generator of the present invention provides a power reset signal that is substantially independent of the ramp time of the application of power to the power reset signal generator  100 . This allows the circuit to be used in applications that have rapid power on such as hot plug and/or have slow turn on in normal power up conditions. The current draw of the power reset signal generator  100  is small so that the generator when used in portable electronic systems to thereby conserve battery power.