Abstract:
The protection circuit of the present invention addresses the problem of indeterminate logic levels caused by loss of one of the power supplies in a two-power-supply CMOS integrated circuit. The circuit of the present invention replaces the typical scheme of power supply sequencing to fix the problem. The circuit disclosed herein detects the state of the core voltage and disables the output drivers when the core voltage is detected as being off. The disabled drivers are put into a high impedance state, thereby eliminating the potential for damage and eliminating the need for power supply sequencing. The invention also protects against the sudden loss of the integrated circuit core voltage, VDD, power supply during normal operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to the field of integrated circuits. In particular, this invention relates to power supplies for integrated circuits and to an improved and simplified method for providing a stable source of power for a complementary metal oxide semiconductor (CMOS) integrated circuit utilizing a split-rail or dual power supply. 
     2. Related Art 
     The field of integrated circuitry is a rapidly developing field of technology. Integrated circuits are continually being made smaller with the attendant requirements of increasing both device speed and circuit density. The miniaturized devices built within and upon a semiconductor substrate are spaced very closely together and the integrated circuit density, that is, the number of integrated circuits per unit of surface area, continues to increase significantly. The highest integrated circuit density is achieved using Field Effect Transistors (FETs). A FET is a device having a source, gate, and drain arranged such that when a high logic signal voltage is applied to the gate, current may pass from the source to the drain. Similarly, the FET does not allow current to pass between the source and the drain when a low logic signal is applied to the gate. 
     As the integrated circuit density increases, the amount of power dissipated by the integrated circuits on the substrate increases proportionally. The amount of power dissipation is a concern because complicated heat sinks and circuit packaging may be required to prevent the chip temperature from rising above its rated operational temperature limit. Further, many devices containing integrated circuits typically operate on stored power. One example is a portable computer operating on battery power. As power dissipation increases, battery life decreases, and the shorter the operational life of the electronic device. Therefore, reducing the power consumption for a given integrated circuit density is important to the design of integrated circuits. 
     One way to decrease this power consumption is to reduce the voltage at which the circuits operate. However, decreasing the operational voltage level creates a compatibility problem because some integrated circuits are designed to operate at predetermined, specific voltage levels. For example, some circuits may interface with low voltage circuits, and these same circuits may need to operate at higher voltage levels to operate electromechanical devices. Also, there are many existing integrated circuits that cannot have their operating voltage altered, yet, new, lower voltage circuits must interact with them. Therefore, to lower the voltage of integrated circuits to dissipate less power, and still permit interaction with different existing hardware, some form of interface circuit is required. 
     In general, the related art has provided a variety of interface circuits for translating lower voltage levels into higher voltage logic levels and vice versa. This is because the logic voltage levels implemented in integrated circuits have been generally decreasing. 
     Many complementary metal oxide semiconductor (CMOS) integrated circuits require more than one power supply per chip. Such designs are known in the art as “split rail designs.” For instance, a split rail design is utilized when the internal or core chip voltage, VDD, operates at a different voltage level than the input/output (I/O) interface voltage or output driver voltage, OVDD. The integrated circuit core voltage, VDD, is limited by the integrated circuit technology or power dissipation requirements of the chip and the driver output voltage, OVDD. 
     Split rail designs create many challenges that must be addressed by both integrated circuit designers and system designers. For a typical split rail integrated circuit to operate properly, both of the power supplies must be in the powered up state. Numerous problems can occur when one supply is off while the other is on. Problems can also occur when the sequence in which the two supplies are powered up or powered down becomes critical. 
     One example of such a problem occurs when the integrated circuit core voltage, VDD, is in an off state and the output drivers are powered up via the output driver voltage, OVDD. In this situation, the output drivers will have lost all the control signals from the integrated circuit core which are derived from the integrated circuit core voltage, VDD. With no control signals to the drivers, the driver&#39; output stages may try to pull the output pad both up and down at the same time. This scenario is characterized by a high crossover current effect from the output driver voltage, OVDD, to ground, which can be multiplied by hundreds of drivers throughout the chip thereby causing permanent equipment damage. 
     SUMMARY OF THE INVENTION 
     The present invention solves these problems in the related art by detecting the state of the core voltage and disabling the output drivers when the core voltage is detected to be off. The disabled drivers are put into a high impedance state, thereby eliminating the potential for damage and eliminating the need for power supply sequencing requirements. The disclosed invention also protects against the sudden loss of the integrated circuit core voltage power supply, VDD, during normal operation. 
     It is therefore an advantage of the present invention to provide a semiconductor chip comprising: a first plurality of circuits connected to a first voltage contact and a ground contact; a second plurality of circuits connected to a second voltage contact and said ground contact; a disabling circuit connected to said first voltage contact and said second voltage contact, and having an output node, said disabling circuit adapted to operate by pulling said output node to said ground contact only when a second voltage source is connected to said second voltage contact and no voltage source is connected to said first voltage contact; and wherein at least one of said second plurality of circuits is connected to said output node of said disabling circuit and wherein said at least one of said second plurality of circuits is adapted to enter a high impedance state when said output node is pulled to said ground contact. 
     Another aspect of the invention is to provide a method of protecting circuitry in a semiconductor chip, comprising: providing a first plurality of circuits connected to a first voltage contact and a ground contact; providing a second plurality of circuits connected to a second voltage contact and said ground contact; providing a disabling circuit connected to said first voltage contact and said second voltage contact, and having an output node, said disabling circuit operating by forcing said output node to said ground contact only when a second voltage source is connected to said second voltage contact and no voltage source is connected to said first voltage contact; connecting at least one of said second plurality of circuits to said output node of said disabling circuit. 
     The foregoing and other objects, features and advantages of the invention will be apparent in the following and more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 depicts a schematic diagram of a typical level-shifting CMOS output driver circuit of the related art; and 
     FIG. 2 depicts a schematic diagram of an input/output (I/O) protection circuit for detecting the loss of the VDD voltage supply in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, FIG. 1 illustrates an example of the related art problem discussed supra. The driver circuit  100  shown in FIG. 1 illustrates a typical two-power-supply level-shifting CMOS output driver circuit. The driver circuit  100  includes an input stage  110 , a pre-drive stage  120 , and an output stage  130 . The internal integrated circuit core voltage, VDD, is nominally about 2.5 volts, and the output driver voltage, OVDD, is nominally about 3.3 volts. A first input, labeled DATA, is the driver input and a second input, labeled ENABLE, is used to switch the output stage  130  into a high impedance state. Under normal operation, 2.5 volt logic data from the integrated circuit core is present at the DATA and ENABLE inputs to the driver circuit  100 . Buffers B 1  and B 2  pass the data to buffers B 3  and B 4 , respectively, which convert, or level shift, the 2.5 volt logic to 3.3 volt logic. From this point, the remaining circuitry is a typical 3.3 volt driver consisting of the pre-drive stage  120  and the output stage  130 . The pre-drive stage  120  is a NAND/NOR pre-drive used to control the rate of change of the driver output current (di/dt). 
     In operation, when the 3.3 volt supply, OVDD, powers up before the 2.5 volt supply, VDD, or when there is a sudden loss of the 2.5 volt supply, VDD, during otherwise normal operation, the logic levels at the inputs, DATA, ENABLE, to the driver circuit  100  become indeterminate. Because the driver circuit  100  output devices, transistor T 9  and transistor T 10 , are powered by the 3.3 volt supply, the driver circuit  100  is capable of supplying current from 3.3 volts to ground through transistor T 9  and transistor T 10 . 
     According to the present invention, a novel method of preventing the inputs (DATA, ENABLE) to the driver circuit  100  from becoming indeterminate involves detecting the loss of the 2.5 volt supply and forcing a logic zero at nodes N 1  and N 5 . A logic zero at nodes N 1  and N 5  is a valid input to buffers B 3  and B 4 , respectively, which are powered by the 3.3 volt supply while it is still active. The outputs of buffers B 3  and B 4  are also at a logic zero, or ground potential, which is passed to the NAND/NOR pre-drive stage  120 . Transistors T 1 , T 2 , T 3 , and T 4  form the NAND gate  140  of the pre-drive stage  120  that controls the output P-channel field effect transistor (PFET) T 9 . A zero logic level at the gates of transistor T 1  and transistor T 4  forces the output of the NAND gate  140  to 3.3 volts at the gate of transistor T 9 , independent of the indeterminate voltage at the gates of transistor T 2  and transistor T 3 . The 3.3 volt level at the gate of transistor T 9  shuts off the output PFET transistor T 9  thus preventing any current through this device. 
     The forced-zero logic level at the outputs of buffers B 3  and B 4  is also fed to the NOR gate  150  of the pre-drive  120 , comprising transistors T 5 , T 6 , T 7 , and T 8 , through inverter I 1 . Since inverter I 1  is powered from the 3.3 volt supply, this forces a 3.3 volt (high) logic level to the gates of transistors T 5  and T 8 . This in turn forces the output of the NOR gate  150  to ground, independent of the indeterminate voltage at the gates of transistor T 6  and transistor T 7 . The gate of transistor T 10  is at ground potential, which turns off transistor T 10 , preventing any current flow through this device. Because transistors T 9  and T 10  are both turned off, the driver circuit  100  is in a true high impedance state, preventing any crossover current from the 3.3 volt supply, OVDD, to ground, or from the signal connection pad (PAD)  160  to ground, or from the 3.3 volt supply, OVDD, to PAD  160 . 
     FIG. 2 shows the I/O protection circuit  200 , for loss of VDD, of the present invention that detects the loss of the 2.5 volt supply, VDD, and forces the nodes N 1  and N 5  to a logic zero or ground potential. The protection circuit  200  of FIG. 2 operates as follows. Transistors TN 1  and TN 2  are two diode-connected N-channel FETs (NFETs). The function of transistors TN 1  and TN 2  is to lower the maximum voltage at node N 3  from OVDD to a voltage level less than or equal to VDD. This will ensure that transistor TP 1  is off when the gate of TP 1  is held at VDD. The N-well of transistor TP 1  is also tied to node N 3 . 
     In the normal functioning mode, VDD is powered up. In this case, transistors TP 1  and TN 3  form an inverter with its input or gates tied to VDD. When VDD is powered up, the input to the inverter is high, which forces transistor TP 1  off and transistor TN 3  on. Transistor TN 3  pulls down the gates of transistors TN 4  and TN 5  thereby shutting off TN 4  and TN 5 . With TN 4  and TN 5  off, nodes N 1  and N 5  float and have no effect on the driver circuit  100  shown in FIG.  1 . 
     In the failure mode, wherein the VDD supply either drops to ground or fails to turn on, the input to the inverter stage  220  formed by TP 1  and TN 3  is at ground. This turns off transistor TN 3 , and turns on transistor TP 1 , pulling up the gates of transistors TN 4  and TN 5  to the voltage level at node N 3 . This turns on transistors TN 4  and TN 5 , pulling down nodes N 1  and N 5 , and forcing the driver circuit  100  to a high impedance state as described in the description of the driver circuit  100  of FIG.  1 . 
     Note that the effect of transistor TN 4  pulling down node N 1  is sufficient, by itself and without the application of TN 5 , to force the driver circuit  100  to a high impedance state. Also, the effect of transistor TN 5  pulling down node N 5  is to force the input of buffer B 4  to a stable state at logic zero when the loss of the 2.5 volt supply is detected. This condition (i.e., wherein buffer B 4  is in a stable state at logic zero) will prevent buffer B 4  from floating, which would cause unnecessary power dissipation in buffer B 4 . 
     This protection circuit  200  (FIG. 2) dissipates zero DC power because node N 3  is kept below the minimum VDD voltage level. More or fewer diode connected NFETs may be used depending on the range of voltage for VDD and OVDD. The protection circuit  200  illustrated in FIG. 2 describes the preferred embodiment of the invention for a nominal VDD value of about 2.5 volts and a nominal OVDD value of about 3.3 volts. Slight modifications can be made to this protection circuit  200  to accommodate higher or lower values of VDD and OVDD by adding or subtracting the number of diode-connected NFETs, shown here as TN 1  and TN 2  in FIG.  2 . 
     While preferred and particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.