Abstract:
A power-on reset circuit and method for generating a reset signal according to the voltage of a power source. The circuit includes an oscillator for generating an oscillation signal. The frequency of the oscillation signal increases with the rising of the voltage of the power source. The circuit further includes a frequency detector for converting the oscillation frequency of the oscillation signal into a first output voltage, and includes a reset signal output circuit for outputting a reset signal according to the first output voltage. Therefore, the power-on reset circuit can be applied in low-voltage chips.

Description:
This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 092106697 filed in Taiwan on Mar. 25, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a power-on reset circuit and a method thereof, and more particularly to a power-on reset circuit for use in a chip with low operation voltage and a method thereof. 
   2. Description of the Related Art 
   In general, there are two methods for generating reset signal at power-on state. One method is to generate the reset signal by a RC delay unit, as shown in the circuit schematic illustration in  FIG. 1  and the associated voltage diagram in  FIG. 2 . The other method is to generate the reset signal by using the threshold voltage of an active device, as shown in the circuit schematic illustration in  FIG. 3  and the associated voltage diagram in  FIG. 4 . 
   Referring to  FIG. 1 , the power-on reset circuit includes an RC (resistor &amp; capacitor) voltage divider  11  and a comparator  12 . The RC voltage divider  11  includes a resistor  111  and a capacitor  112  and generates an output voltage V RC . The RC voltage divider  11  is connected to a voltage source VDD and a ground  13 . The comparator  12  receives an input voltage αVDD proportional to the voltage source VDD and the output voltage V RC  of the RC voltage divider  11  and generates a reset signal Reset by comparing V RC  with αVDD. At the beginning of power on, voltage V RC &lt;αVDD and the comparator enables the reset signal Reset, such as outputting a High State. Then, when voltage V RC &gt;=αVDD, the comparator disables the reset signal Reset, such as outputting a Low state. As shown in the clock diagram in  FIG. 2 , when the power is on, the voltage source VDD outputs a transient voltage having a magnitude increasing from 0 as time elapses. At the beginning of power on, voltage V RC &lt;αVDD and the comparator enables the reset signal Reset. At the time when the condition of (V RC &lt;αVDD) is changed to the condition of (V RC &gt;=αVDD), the reset signal Reset is disabled. 
   Referring to  FIG. 3 , a power-on reset circuit includes a resistor—metal oxide semiconductor voltage divider  21  and a comparator  22 . The resistor—metal oxide semiconductor voltage divider  21  includes a resistor  211  and a metal oxide semiconductor  212  and generates an output voltage V th . The resistor—metal oxide semiconductor voltage divider  21  is connected to a voltage source VDD and a ground  23 . The comparator  22  compares an input voltage αVDD proportional to the voltage source VDD to the threshold voltage V th  and generates a reset signal Reset by comparing V RC  with αVDD. When the threshold voltage V th  is greater than the input voltage αVDD, the comparator  22  enables the reset signal Reset. However, when the threshold voltage V th &lt;=αVDD, the comparator  22  disables reset signal Reset to end the reset state. As shown in  FIG. 4 , at the beginning of power on, the voltage source VDD outputs a transient voltage having a magnitude increasing from 0 as time elapses. At the beginning, the threshold voltage V th &gt;αVDD, the comparator enables the reset signal Reset. At the moment when the condition of (V th &gt;αVDD) is changed to the condition of (V th &lt;=αVDD), the reset signal Reset is disabled. 
   However, the above-mentioned conventional power-on reset circuits have the following drawbacks. Usually, for the RC delay circuit, an external capacitor is needed to have enough delay time. For the circuit with the active device, such as the metal oxide semiconductor, the threshold voltage of the active device tends to be changed with the process variation, environment temperature variation, and other conditions. Thus, the conditions of reset signal being disabled are not consistent and may be changed with the variation of the various environment conditions. Consequently, errors may be caused in which the reset signal cannot be disabled, or is not disabled at the proper time. In addition, as the operation voltage of the IC chip gets lower and lower, the operation voltage VDD gets smaller and smaller. Therefore, when the power is on, the transient voltage variation gets smaller and smaller, and thus the tolerance of the threshold voltage variation gets smaller and smaller. Thus, the conventional power-on reset circuits are not suitable for use in the chip with the low operation voltage. 
   SUMMARY OF THE INVENTION 
   It is therefore one of the many object of the invention to provide a power-on reset circuit adapted to low-voltage chips. The reset circuit may be applied to the low operation voltage without causing errors in the reset operation after power-on owing to the process variation or temperature variation. 
   Another object of the invention is to provide a power-on reset circuit adapted to low-voltage chips. The circuit utilizes a ring oscillator, which provides an oscillation frequency that rises as the transient voltage rises, to control the ON/OFF of the switch, to charge/discharge capacitors, and to generate a first voltage by conversion. The first voltage is compared to a second voltage, which is generated after the transient voltage is processed by the voltage divider. Then, it is determined whether or not the circuit has to be reset. 
   According to one aspect of the invention, a power-on reset circuit adapted to a low operation voltage chip includes an oscillator, a frequency detector, and a reset signal output circuit. A power source provides a transient voltage when it is on, and the transient voltage has the magnitude that rises as time elapses. The oscillator is coupled to the power source. The oscillator generates an oscillation signal having an oscillation frequency that increases as the transient voltage increases. The frequency detector is coupled to the power source and the oscillator. The frequency detector outputs a corresponding first output voltage according to the oscillation frequency of the oscillation signal. The reset signal output circuit outputs a reset signal according to the first output voltage. The magnitude of the reset signal is one of a first level and a second level. 
   According to another aspect of the invention, a power-on reset method applied to a power-on reset circuit is provided. The power-on reset circuit includes an oscillator, a frequency detector, and a comparator. The method includes the steps of: receiving a transient voltage when a power source is on, wherein the magnitude of the transient voltage increases as time elapses; providing a corresponding oscillation signal according to the transient voltage, wherein the oscillation signal has an oscillation frequency that increases as the transient voltage increases; outputting a corresponding first output voltage according to the oscillation signal; comparing the first output voltage to a second output voltage; enabling a reset signal when the first output voltage is greater than the second output voltage; and disabling the reset signal when the first output voltage is equal to or smaller than the second output voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic illustration of a conventional power-on reset circuit using a RC delay unit. 
       FIG. 2  shows an associated voltage clock diagram of the conventional power-on reset circuit in  FIG. 1 . 
       FIG. 3  shows a schematic illustration of a conventional power-on reset circuit using a threshold voltage of a metal oxide semiconductor. 
       FIG. 4  shows an associated voltage clock diagram of the conventional power-on reset circuit in  FIG. 3 . 
       FIG. 5  shows a schematic illustration of a power-on reset circuit according to an embodiment of the invention. 
       FIG. 6  shows an associated voltage and frequency clock diagram of the power-on reset circuit according to the embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 5 , the circuit of the embodiment of the present invention includes an oscillator  31 , a frequency detector  32  and a comparator circuit  33 . The oscillator  31  can be a ring-oscillator and includes a set of at least three odd-numbered and serially connected inverters  311 , and the output terminal of the last inverter is connected to the input terminal of the first inverter. The oscillator  31  may also be a voltage-controlled oscillator (VCO). Each inverter is powered by a voltage source VDD. The oscillator  31  generates an oscillation signal ck. The frequency detector  32  is coupled to the voltage source VDD and a ground  34  and includes a current source  321 , a first capacitor  322 , a second capacitor  323 , a first switch  324  and a second switch  325 . The frequency detector  32  outputs a first output voltage V FD  according to the oscillation signal ck generated by the oscillator  31 . The current source  321  is coupled to the voltage source VDD. The first capacitor  322  has a first terminal coupled to the output terminal of the current source  321 , and a second terminal coupled to the ground  34 . The second capacitor  323  and the first capacitor  322  are commonly connected to the ground  34 . The first switch  324  is coupled to the first terminal of the first capacitor  322  and another terminal of the second capacitor  323 . The second switch  325  and the second capacitor  323  are connected in parallel. The comparator circuit  33  includes a resistor—voltage divider  331  and a comparator  332  and generates a reset signal Reset. The resistor—voltage divider  331  is coupled to the voltage source VDD and generates a second output voltage αVDD proportional to the voltage source VDD with a first resistor  3311  and a second resistor  3312 . The comparator  332  compares the first output voltage V FD  to the second output voltage αVDD. When the first output voltage V FD  is greater than the second output voltage αVDD, the comparator  332  enables the reset signal Reset, such as outputting a High state; and when the first output voltage V FD  is smaller than or equal to the second output voltage αVDD, the comparator  332  disables the reset signal Reset, such as outputting a Low state. 
   In this embodiment, when the power is on, the voltage source VDD is at a transient voltage having the magnitude that increases from 0 as time elapses. The oscillation frequency of the oscillation signal ck of the oscillator  31  increases with the rising of the transient voltage. The oscillation frequency thereof also decreases with the increasing of the number of the cascaded inverters in the oscillator  31 . Thus, the magnitude of the transient voltage input to the inverters and the number of the cascaded inverters will determine the oscillation frequency of the oscillation signal ck. In the frequency detector  32 , the switching operations of the first switch  324  and the second switch  325  are controlled by the oscillation signal ck. The state of the first switch  324  is opposite to that of the second switch  325 . That is, when the first switch  324  is ON, the second switch  325  is OFF and vice versa. Therefore, the ON/OFF states of the first switch  324  and the second switch  325  alternate with the oscillation frequency of the oscillation signal ck and the switching states of the first switch  324  and the second switch  325  are different. 
   In the circuit implementation of this embodiment, the first switch  324  substantially switches according to the oscillation signal ck while the second switch  325  substantially switches according to an inverse signal of the oscillation signal ck, as shown in  FIG. 5 . The frequency detector  32  has a current source  321  and two capacitors  322 ,  323  coupled in parallel via the first switch  324 . When the states of the first switch  324  and the second switch  325  alternate with the oscillation signal ck, the current source  321  charges/discharges the first/second capacitor  322 / 323  according to the states of the first switch  324  and the second switch  325 , respectively. When the oscillation frequency of the oscillation signal ck is lower, the charge/discharge time of the first/second capacitor  322 / 323  is longer. In this case, the magnitude of the first output voltage V FD  approximates the transient voltage of the voltage source VDD. When the oscillation frequency of the oscillation signal ck is higher, the charge/discharge time of the first/second capacitor  322 / 323  is shorter. In this case, the first output voltage V FD  is smaller than the transient voltage of the voltage source VDD, and decreases with the increasing of the oscillation frequency of the oscillation signal ck. If the current of the current source  321  in the frequency detector  32  is I, and the first capacitor  322  has a capacitance C 1 , the second capacitor  323  has a capacitance C 2 , the oscillation signal ck has an oscillation frequency f ck , then the magnitude of the first output voltage V FD  generated by the frequency detector  32  is:
 
 V   FD =( I/f   ck )*((2 *C   1   +C   2 )/(C 1   *C   2 )).
 
   According to the above-mentioned equation, the first output voltage V FD  of the frequency detector  32  is inversely proportional to the oscillation frequency f ck  of the oscillation frequency ck. So, when the value f ck  of the oscillation frequency ck is higher, the first output voltage V FD  is lower. 
   Since the operation principle and manner of the comparator circuit  33  are similar to those of the conventional power-on reset circuit, detailed description thereof may be found in the above-mentioned description and will be omitted. 
   It is to be noted that in this invention, the comparator circuit  33  also may be implemented by an inverter. The inverter receives the first output voltage V FD  and determines the level of the output reset signal according to the magnitude of the first output voltage V FD . When the power source is just started, the value of the first output voltage V FD  is smaller than a default threshold value of the inverter, At this time, the inverter regards the first output voltage V FD  as a low-level signal and inversely outputs a high-level signal to enable the reset signal Reset. Because the value of the first output voltage V FD  increases as time elapses, when the value of the first output voltage V FD  is greater than the default threshold value of the inverter, the inverter regards the first output voltage V FD  as a high level signal and inversely outputs a low-level signal to disable the reset signal Reset. In this embodiment, the working principle of the power-on reset circuit in  FIG. 5  is illustrated in  FIG. 6 . 
   The oscillation frequency of the oscillation signal ck output from the oscillator  31  relates to the magnitude of the voltage source VDD. When the voltage source VDD is lower, the oscillation frequency f ck  of the oscillation signal ck output from the oscillator  31  is also lower. According to the above-mentioned equation for the first output voltage V FD , the first output voltage V FD  approximates to the voltage source VDD, so the first output voltage V FD  is greater than αVDD. When the first output voltage V FD  is greater than αVDD, the comparator  332  enables the reset signal Reset. At this time, the digital circuit that needs to be reset on the chip is in a reset state. With the rising of the voltage source VDD, the oscillation frequency f ck  of the oscillation signal ck increases. In this case, the first output voltage V FD  gradually decreases. When the first output voltage V FD  decreases to be smaller than or equal to αVdd, the comparator  332  disables the reset signal Reset. That is, the reset is disabled while the digital circuit may start to work. In practice, in the above-mentioned circuit, the current source  321  may be replaced by a resistor, the comparator  332  also may be replaced by an inverter to make the circuit operable under a lower operation voltage. 
   While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.