Abstract:
A low voltage power-up detection circuit for use includes a programmable resistance biasing network which provides an adjustable voltage to vary a power-up voltage detection point. The programming of the bias network can be set during testing of the device. The low voltage power-up detection circuit may be used with many devices including memory devices such as DRAMs.

Description:
FIELD OF THE INVENTION 
     The present invention relates to power-up detection circuits which have utility in a wide variety of integrated circuit structures, and particularly for detecting a power-up condition in a memory device. 
     DISCUSSION OF THE RELATED ART 
     FIG. 1 depicts a conventional power-up detecting circuit for use in a memory device such as a DRAM. The FIG. 1 circuit indicates a power-up condition when applied Vcc reaches a predetermined value. It includes a differential amplifier  11  which is connected to receive at a negative input a voltage provided at a drain of a transistor  13 , the latter of which is connected to ground through a resistance  15 . The source terminal of the transistor  15  is connected to Vcc. The positive input of differential amplifier  11  is connected to node A which is also connected to the gate of transistor  13 . 
     Node A is also at the junction of a resistor  19  and a transistor  17 , the latter of which is connected as a diode. In operation, when the voltage Vcc is first applied and begins to rise, the voltage at node A between resistor  19  and transistor  17  likewise begins to rise until transistor  13  turns on, clamping the voltage of node A at a Vt. Resistor  15  holds node B at ground until Vcc is a Vt above node A. Once Vcc is greater than a Vt above node A, transistor B turns on charging node B to Vcc. When the voltage at node B crosses the voltage of node A, amplifier  11  switches state and provides a power-up output signal. This is illustrated in FIG. 4 as the crossing point of the node A and node B voltage curves. Note that in FIG. 4, node B charges to Vcc. The instantaneous value of Vcc where switching occurs is typically at Vcc minus the additive threshold voltages Vt of transistors  13  and  17 , e.g., Vcc−2 Vt. 
     Although the FIG. 1 circuit works adequately in detecting a power-up condition for higher values of Vcc, the 2 Vt voltage drop detection point is too high for low voltage Vcc values. Many memory devices, e.g. DRAMS, are now beginning to operate at approximately 1.8 volts. Accordingly, the use of transistors  17  and  13  to set the switching threshold point at 2 Vt does not allow for sufficient variability to accommodate different low levels of operating voltage Vcc. Accordingly, a more versatile power-up circuit is needed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a power-up detection circuit for integrated circuit devices, for example, memory devices such as DRAMS, in which the threshold switching voltage point can be adjusted without regard to a fixed voltage drop set by a transistor or diode, and in which a power-on condition can be detected at a selected one of a plurality of operative voltage levels. A programmable resistor network is used in the detection circuit to establish the switching voltage point. Various resistors of the resistor network can be selectively made operative or inoperative in accordance with a set pattern programmed into the resistance network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will become more apparent from the following detailed description which is provided in connection with the accompany drawings in which: 
     FIG. 1 illustrates a conventional power-up detection circuit; 
     FIG. 2 illustrates a power-up detection circuit in accordance with an exemplary embodiment of the present invention; 
     FIG. 3 illustrates a power-up detection circuit in accordance with an another embodiment of the present invention; 
     FIG. 4 illustrates operation of the FIG. 1 circuit; 
     FIG. 5 illustrates operation of the FIG. 2 circuit; and 
     FIG. 6 illustrates use of the invention in a processor system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 illustrates an exemplary embodiment of the invention. As shown, the invention has particular utility as a power-up detection circuit for a memory device  31  which includes a memory array and access circuitry for reading data from and writing data into said array. A power-up detection signal is supplied to a power-up detection terminal of memory device  31 . Although FIG. 2 shows a memory device  31 , it should be understood that the present invention also has applicability to any other circuit structure, including but not limited to any integrated circuit structure, where it is necessary to determine a power-up condition. 
     The structure of FIG. 2 is similar to that of FIG. 1, except the transistor  17  has been replaced by a resistor network which includes serially connected resistors  21   a ,  21   b ,  21   c  and  23 . These resistors form a resistance divider with resistor  19  to provide a voltage to the gate of transistor  13  at node A. 
     The resistors  21   a ,  21   b  and  21   c  can be selectively shunted by respective transistors  25   a ,  25   b ,  25   c , thereby providing a programming capability for the resistor network. It should be noted that although three resistors and associated transistors are illustrated in FIG. 2, any number of resistors and associated transistors may be used as desired. 
     The transistors  25   a  . . .  25   c  used to selectively shunt a respective resistor  21   a  . . .  21   c  are connected to respective programmable elements  27   a  . . .  27   c . These programmable elements are, in turn, connected to the junction of resistors  33  and  35 , which form a resistance divider connected between Vcc and ground to thereby derive a programming voltage from Vcc. The programming voltage is applied through the respective programmable elements  27   a  . . .  27   c  to the gates of transistors  25   a  . . .  25   c  to turn the transistors  25   a  . . .  25   c  on. The programmable elements  27   a  . . .  27   c  selectively establish a connection to the programming voltage for transistors  25   a  . . .  25   c  and therefore control the selective shunting of resistors  21   a  . . .  21   c.    
     In this manner, the overall resistance of the resistance network formed by resistors  21   a  . . .  21   c  can be programmably adjusted. Programmable elements  27   a  . . .  27   c  can be formed as programmable fuses or programmable anti-fuses during manufacture of the integrated circuit containing the FIG. 2 power-up circuit. 
     With the circuit of FIG. 2, the voltage which is applied to the gate of transistor  13  can be selected by the selective programming of programmable elements  27   a  . . .  27   c  to select the switching voltage at which amplifier  11  will switch. The applied voltage to gate  13  does not depend on the voltage drop of a transistor or other active device, and therefore can be made variable without being limited by voltage drops associated with an active device, such as the transistor  17  of FIG.  1 . 
     FIG. 2 illustrates negative and positive inputs of differential amplifier  11  respectively connected to the drain and gate of transistor  13 . It is also possible, however, to use the invention with only a single connection of the amplifier  11  to the drain of transistor  13  and using a fixed voltage at the other amplifier  11  input. In either case, the amplifier  11  will change output state when the voltage Vcc applied to the power-up circuit reaches a preselected magnitude at which transistor  13  turns on to indicate a power-up condition. 
     The invention has particular utility in tuning a power-up detecting circuit during a testing phase, at which time the resistor network formed by resistors  21   a  . . .  21   c  can be programmed in accordance with a desired detection characteristic for the power-up detection circuitry. The value of resistors  21   a  . . .  21   c  can be selected to provide coarse and fine adjustments of the Vcc signal level at which amplifier  11  switches to indicate a power-up condition. This can be particularly useful when trimming the operation of the power-up circuit during a circuit testing mode. 
     Further, as shown in FIG. 3, an inverter  41  could be used in place of the amplifier  11  and the programmable elements  27   a  . . .  27   c  could be replaced with switches  47   a  . . .  47   c.    
     The manner of operation of the invention as depicted in the exemplary embodiment of FIG. 2, compared with the conventional circuit illustrated in FIG. 1, is shown with reference to FIGS. 4 and 5. 
     FIG. 4 illustrates the operation of the conventional FIG. 1 circuit. The node A and B voltages are illustrated. As shown, as the voltage in node A begins to increase as the voltage Vcc is applied, there is a lag before transistor  13  turns on and the voltage at node B charges to Vcc. Switching of amplifier  11  occurs when the node B voltage equals the node A voltage, which corresponds to an instantaneous Vcc voltage equal to the final Vcc−2Vt, where Vt is the threshold voltage of each of transistors  17  and  13 . As shown, the circuit of FIG. 1 has a minimum trip point of 2 Vt. 
     By contrast, FIG. 5 illustrates a plurality of node A voltages which are now linear. Each of the linear voltage graphs represents a different resistance value caused by the shunting of various ones of resistors  21   a ,  21   b ,  21   c . As illustrated, by adjusting which of the resistors  21   a  . . .  21   c  is shunted, the turn-on characteristics of transistor  13  are, in effect, adjusted within an adjustment range V 1  to V 3  which allows for power-up detection to occur at a selected instantaneous Vcc voltage value within a range of instantaneous Vcc voltages. Thus, the FIG. 2 circuit provides more flexibility in setting the power-up detecting circuit initially, for example, during a testing phase, and for detection at various levels of Vcc during its power-up phase. In addition, the FIG. 2 circuit permits trimming the voltage switch point to a precise value during the testing of a circuit containing the FIG. 2 power-up detection circuit. 
     FIG. 6 illustrates a processor system which employs the invention. As shown in FIG. 6, a processor system, such as a computer system, for example, generally contains a central processing unit (CPU)  210 , for example, a microprocessor, that communicates with one or more input/output (I/O) devices  240 ,  250  over a bus  270 . The computer system  200  also includes random access memory (RAM)  260 , a read only memory (ROM)  280  and, in the case of a computer system may include peripheral devices such as a floppy disk drive  220  and a compact disk (CD) ROM drive  230  which also communicate with CPU  210  over the bus  270 . At least one of CPU  210 , RAM  260  and ROM  280 , are preferably constructed as integrated circuits which include a power-up detection circuit as previously shown and described with respect to FIGS. 2 and 4. It may also be desirable to integrate the processor  210  and memory  260  on a single IC chip and have one or both of processor  210  and memory  260  employ the power-up detection circuit shown and described with reference to FIGS. 2,  3 , and  5 . 
     Although the invention has been described above in connection with exemplary embodiments, it is apparent that many modifications and substitutions can be made without departing from the spirit or scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description, but is only limited by the scope of the appended claims.