Patent Publication Number: US-6040722-A

Title: Power-on reset circuit with adjustable interval

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a semiconductor integrated circuit device, and more particularly, to a power-on reset circuit for generating a power-on reset signal to reset sequential logics to desired logic states when a power supply potential is applied. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a circuit diagram showing a conventional semiconductor integrated circuit (IC) device including a power-on reset circuit. In this figure, the conventional IC device includes a power supply potential node 1 supplied with a power supply potential V DD , and a ground potential node 2 connected to the ground potential. The conventional IC device further includes a power-on reset circuit 20 which receives power supply potential V DD , and generates a power-on reset signal. When the power supply potential rises from the ground potential to a prescribed potential V DD  (for example, 5 V), the power-on reset signal is maintained at LO for a prescribed time period, and then is changed to HI. 
     The conventional IC device further includes an internal circuit 10 having, for example, sequential logic circuits or memory devices which are driven by power supply potential V DD . When the power-on reset signal is applied to internal circuit 10, the latter is immediately reset to desired logic states. 
     Referring again to FIG. 1, power-on reset circuit 20 includes a capacitor 3 connected between ground potential node 2 and a connection node 5, a resistor 4 connected between power supply potential node 1 and connection node 5, and a buffer 22 connected between connection node 5 and a reset terminal of internal circuit 10. 
     When power supply potential V DD  is applied, a voltage V --  RC at connection node 5 of time delay circuit 21 rises from zero up to high level V DD , but with a delay in accordance with the time constant RC of resistance R and capacitance C, of the resistor and the capacitor. The delayed voltage V --  RC is provided to buffer 22, which has a threshold voltage V thresh . The buffer outputs a power-on reset signal of low level when V --  RC is smaller than V thresh , and of high level when V --  RC is higher V thresh . 
     Referring to the timing chart of FIG. 2, the conventional IC device operates as follows. At the time of power-up, power supply potential V DD  starts rising from the ground potential at time t0, and reaches the prescribed potential at time t1. At this time, power-on reset signal is LO, and voltage V --  RC starts rising toward V DD  with a time-constant. The time profile of voltage V --  RC can be obtained by the following equation: ##EQU1## 
     Voltage V --  RC will reach the threshold voltage value V thresh  after a delay time t d , which is also known as interval of power-on reset time. Neglecting a delay time of the buffer itself, the value of t d  is found by the following equation: ##EQU2## 
     When the threshold value is reached, power-on reset signal PWR --  ON --  RESET is changed from LO to HI. The power-on reset signal is applied to sequential logics or memory devices of internal circuit 10, to reset them to desired logic states. After this time, the power-on reset signal is maintained at high level and thereby the sequential logics or the memory devices are not influenced by the power-on reset signal. 
     As described immediately above, the power-on reset signal can be obtained using the RC time delay of circuit 21. For a longer delay, a capacitance of capacitor 3 must be increased. Since capacitors having a large capacitance occupy a larger space on a device chip, such capacitors should be formed outside the device chip, as in FIG. 1. This reduces the degree of integration of the device chip, and inevitably requires allocating an input/output pin of the device chip should be for the external capacitor. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems of the prior art. 
     According to an aspect of the present invention, there is provided a power-on reset circuit for generating a power-on reset signal as a power supply potential is applied initially. The signal is applied to sequential logics after an adjustable time interval, so as to allow them to reset to desired logic states immediately thereafter. 
     The power-on reset circuit includes an oscillator for generating a clock signal, and a counter for counting pulses of the clock signal from when the power supply potential is initially applied. The time interval is adjusted by loading a preset data value in the counter. The counter comprises an n-bit counter, and the preset data value is loaded by coding the n bits. The counter outputs a CTR signal when the counted pulses become equal in number to the preset number. 
     The circuit also includes a combinational logic circuit that generates a reset signal for resetting the counter when the power supply potential is initially applied. The combination logic circuit also generates a power-on reset signal when the counter outputs the CTR signal. 
     It is further preferred that the counter is disabled by the power-on reset signal, so as to maintain the CTR signal at the same value. The oscillator can also be disabled by the power-on reset signal. 
     The power-on reset circuit of the invention is incorporated in the same chip as the sequential logics, regardless of how long is the interval of power-on reset time has been adjusted for. Because the power-on reset circuit is embodied in the chip, there is no need for the chip to allocate an input/output pin for the power-on reset circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention may be understood and its objects will become apparent to those skilled in the art by reference to the accompanying drawings as follows: 
     FIG. 1 is a circuit diagram showing a conventional semiconductor integrated circuit including a power-on reset circuit; 
     FIG. 2 is a timing chart showing the operation of the power-on reset circuit of FIG. 1; 
     FIG. 3 is a block diagram showing a power-on reset circuit according to a preferred embodiment of the present invention; 
     FIG. 4 is a detailed circuit diagram for the combinational logic of the power-on reset circuit shown in FIG. 3; and 
     FIG. 5 is a timing chart for the circuit of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A novel power-on reset circuit 200 made in accordance with the preferred embodiment of the present invention is now described with reference to FIG. 3. Circuit 200 includes an oscillator 202, an n-bit counter 204 and a combinational logic 206. Power-on reset circuit 200 is integrated in a device chip, together with an internal circuit 10 having sequential logics or memory devices. Power-on reset circuit 200 generates a power-on reset signal when a power supply potential is applied, and enables internal circuit 10 to reset to desired logic states immediately thereafter. 
     When power supply potential VDD is applied to the device chip, oscillator 202 generates a clock signal CLK having a constant frequency. At the same time, n-bit counter 204 is reset, and then starts a counting operation in synchronization with clock signal CLK. The counter counts up, and generates a pulse signal when a counted value becomes equal to a preset loaded data value. 
     Combinational logic 206 resets the counter by providing a RESET signal when power supply potential VDD is initially applied. The combinational logic also outputs a hold signal HOLD that disables further counting up, when the counted value has become equal to the preset loaded data value. Thus output signal CTR of the counter is held at its output level. 
     Combinational logic 206 also outputs a power-on reset signal PWR --  ON --  RESET, which is provided to a reset terminal of internal circuit 10. In the preferred embodiment, the power-on reset signal of high level is advantageously used also as the hold signal for counter 204, and also for oscillator 202. As a result, the oscillator is inactivated by power-down control, and at the same time the output signal CTR of the counter is held at the high level. 
     FIG. 4 is a detailed circuit diagram for combinational logic 206. In this figure, combinational logic 206 includes an AND gate circuit 210 having two input terminals, one of which receives power supply potential VDD, and the other of which receives output signal CTR of counter 204. The power-on reset signal PWR --  ON --  RESET is generated at the output of AND gate circuit 210. 
     The combinational logic further includes a plurality of inverters 211˜213 which are connected in series. First stage inverter 211 receives power supply potential V DD , and third stage inverter 213 outputs counter reset signal RESET. 
     The operation of power-on reset circuit 200 is now described with reference to FIGS. 3 through 5. When external power supply potential V DD  rises from a ground potential toward the prescribed potential, oscillator 202 starts to generate a clock signal CLK having a constant period T clk . In addition, combinational logic 206 produces a counter reset pulse RESET, which resets counter 204. The counter then starts to count up an initial data in synchronization with the clock signal CLK which is generated from time t1. 
     At time t2, a counted data of the counter reaches up to the preset data value. At that time, signal CTR is changed from low level (&#34;LO&#34;) to high level (&#34;HI&#34;), and thus the power-on reset signal PWR --  ON --  RESET is also changed from LO to HI. Sometimes this change is also referred to as providing the power-on reset signal. As a result, sequential logics or memory devices of internal circuit 10 are reset. 
     As described above, the power-on reset signal is advantageously applied to counter 204 and to oscillator 202 as a hold signal. Accordingly, signal CTR is maintained HI, and the oscillator is inactivated. 
     Table 1 shows operational states of oscillator 202 and counter 204 in response to the hold signal HOLD and counter reset signal RESET. 
     
                       TABLE 1                                                     
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Output of Combinational                                                   
                 Operational State                                        
Logic            N-bit Counter                                            
                            Oscillator                                    
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HOLD signal on   hold state off state                                     
RESET Signal     reset state                                              
                            --                                            
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     With power-on reset circuit 200, the interval of power-on reset time td can be represented by the following equation: 
     
         t.sub.d =N·T.sub.clk 
    
     wherein T clk  is the constant period of the clock signal which is generated from oscillator 202. 
     The value of delay time td can be controlled according to the above equation to be a predetermined time interval. Once T clk  is known, controlling is accomplished by proper choice of the number N in the above equation, which is the number of clock pulses between times t1 and t2. The number N is loaded in the counter as the preset data value. For a counter that is an n-bit counter, n being an integer, N must be between 1≦N≦2 n-1 . N is loaded by properly setting the n bits.