Circuit that automatically switches between supplying a microprocessor with a first voltage and a second voltage
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The invention relates to controlling a power supply voltage provided to a microprocessor, and more particularly, to a circuit that automatically switches between supplying the microprocessor with a first voltage or a second voltage. 
     2. Description of the Related Art 
     Over the past decade, the performance and speed of microprocessors have improved dramatically. The number of transistors that can be fitted onto a microprocessor chip number is in the millions. Such densities are allowed primarily with the use of complementary metal oxide silicon (CMOS) technology, which allows for significantly lower power consumption over prior bipolar and NMOS technologies. However, even with the use of CMOS technology, power dissipation by today&#39;s high performance microprocessors is a significant problem. To reduce power consumption, microprocessors such as the 486SL and Pentium® CPUs from Intel Corporation include built-in power management functions. The energy efficient CPUs include a stop clock and auto halt mode. In stop clock mode, the clock speed of the microprocessor can be reduced or even stopped altogether, which reduces the power dissipation of the microprocessor when it is idle. The CPU enters into the auto halt mode after execution of a halt instruction. This allows the microprocessor to be placed into low power mode via software. 
     Neither of the above features, however, address how the power consumption of a microprocessor can be reduced while it is active. One popular method of reducing power consumption while the microprocessor is running is to reduce its power supply voltage from the industry standard 5 volts down to 3.3 volts. Lower voltages have also proved to be necessary as die sizes continue to shrink. The small dimensions that exist on a chip require a lower operating voltage to avoid problems associated with high current densities. With the trend towards 3.3 volt microprocessors chip manufacturers have had to provide both microprocessors that operate at 5 volts and 3.3 volts. 
     This presents the computer manufacturers with a problem, however, as the computer system must be capable of providing two different power supply voltages to the microprocessor. One solution is to include a 5-volt power supply voltage on all computer systems, adding a conventional regulator for providing a 3.3 volt power supply voltage in the 3.3-volt computer systems. However, this requires that the manufacturing process be different for 3.3-volt and 5-volt computer systems. Single-part solutions exist where the same part can be used with both 3.3-volt and 5-volt microprocessors. However, these parts include linear regulators, which are relatively expensive components. Thus the use of the separate, conventional linear regulator is undesirable, as it adds cost to the manufacture of the computer systems for several reasons. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a low-cost power supply control circuit that can be used with microprocessors that operate at either a first or second supply voltage. The power supply control circuit according to the present invention includes regulator circuitry that selectively provides a microprocessor supply voltage at a first voltage or a second voltage, those voltages preferably being 3.3 volts and 5 volts, respectively. The computer system preferably includes a socket for receiving a microprocessor, and the socket includes a pin referred to as the voltage detect sense pin. Microprocessors operating at 3.3 volts drive the voltage detect sense pin low, whereas 5-volt microprocessors leave the voltage detect sense pin floating, allowing a pullup resistor to pull the pin high. The voltage detect sense pin from the microprocessor socket is connected to the regulator circuitry in the power supply control circuit. The regulator circuitry includes a power field effect transistor (FET), whose drain is preferably connected to a 5-volt source, and whose source provides the power supply voltage to the microprocessor. The gate of the power FET 200 in the preferred embodiment is connected to a voltage reference source, which drives the gate of the power FET to approximately 12 volts if the voltage detect sense pin is pulled high by the pullup resistor, and which drives the gate to one threshold voltage of the power FET above 3.3 volts if the sense pin is driven low. Thus, if the voltage detect sense pin is pulled low, the power FET drives the microprocessor supply voltage to 3.3 volts. Otherwise, if the voltage detect sense pin is pulled high, the power FET is completely turned on and allows the 5-volt voltage to pass to the microprocessor supply voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description of 5 the preferred embodiment is considered in conjunction with the following drawings, in which: 
     FIG. 1 is an exemplary computer system incorporating the power supply control circuit according to the present invention; and 
     FIG. 2 is a schematic diagram of portions of the power supply control circuit of Figure I that regulates the voltage provided to a microprocessor in the computer system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, an exemplary computer system C incorporating a power supply control circuit according to the present invention is shown. The computer system C includes a socket 100 for receiving a host CPU or processor 101, which is conventionally a microprocessor such as a 486 or Pentium® processor from Intel Corporation. It is contemplated that instead of using the socket 100, the CPU 101 can be directly soldered to the motherboard. The type of microprocessors that can be used with the computer system C include the 486SX, 486DX, 486DX2, 487SX, 486DX4, and Pentium® P24T processors. In addition, microprocessors from other manufacturers compatible with the 486 processor can also be used. The 486SX, 486DX, 486DX2, and P24T processors operate at 5 volts, while the 486DX4 operates at 3.3 volts. Certain of the microprocessors from the other manufacturers also operate at 3.3 volts. The socket 100 receives a pin or plurality of pins PVDD, which is the microprocessor power supply pin or pins driven to 5 volts or 3.3 volts, depending on which microprocessor is used. The socket 100 also includes a voltage detect sense pin VOLDET. When a 3.3-volt microprocessor is inserted into the socket 100, the microprocessor drives the pin VOLDET low. If a 5-volt microprocessor is inserted, the pin VOLDET is left floating. 
     A host bus 102 is connected to the CPU 101 to act as a first bus in the computer system C. A main memory system 104 is also connected to the host bus 102 to act as the main memory of the computer system C. A video system 106 is further connected to the host bus 102 to allow for a high performance video system. 
     A controller 110 is connected between the host bus 102 and an ISA or Industry Standard Architecture bus 112. The controller 110 includes a bus controller portion, a memory controller portion and a system peripheral control portion. The system peripheral control portion includes certain common peripheral devices used in the computer system C such as timers, an interrupt controller, and a DMA controller. The bus controller portion of the controller 110 controls bus cycles on the host bus 102 and the ISA bus 112. The memory controller portion provides appropriate signals to the memory 104, which preferably is made up of dynamic random access memories (DRAMs). A number of ISA slots 116 for receiving interchangeable circuit cards are present on the ISA bus 112. 
     Appropriate buffer and transceiver logic 120 is connected between the ISA bus 112 and an X bus 122, which forms an additional input/output (I/O) bus in the computer system C. A read-only memory or ROM 124, which contains the instructions forming the BIOS and other fundamental operations, is connected to the X bus 122. A combo peripheral chip 126 is also connected to the X bus 122. The combo peripheral chip 126 includes a keyboard controller to receive keyboard and pointing device inputs from a user, an interface for connection to parallel and serial ports 130 to provide for certain I/O capabilities, and a hard disk controller for interfacing to a hard disk drive 128. The combo peripheral chip 136 also includes a real-time clock (RTC) and basic and extended CMOS memory. The RTC provides various real-time clock functions, and the CMOS memory provides storage for various system configuration information. Additionally, a floppy disk controller 132 is connected to the X bus 122. The floppy disk controller 132 acts as an interface between the X bus 122 and a floppy disk drive 134. 
     A power supply circuit 136 is also located in the computer system C. The outputs provided by the power supply circuit 136 include a 12-volt supply signal +12 V, a 5-volt supply signal +5 V, and the supply signal PVDD, which varies between 3.3 volts and 5 volts depending on the type of the CPU 101 used. The voltage detect sense pin VOLDET is provided to the power supply circuit 136. The power supply signal PVDD is provided to the CPU 101, and in accordance with the present invention, is automatically set at 3.3 volts or 5 volts depending on the state of the pin VOLDET. 
     Referring now to FIG. 2, regulator circuitry in the power supply circuit 136 for generating the supply signal PVDD is shown. The regulator circuitry includes a power n-channel enhancement field effect transistor (FET) 200, whose drain is connected to the supply signal +5 V and whose source is connected to the supply signal PVDD. A decoupling capacitor 204 is connected between the drain of the power FET 200 and ground to remove high frequency noise. Decoupling and bulk capacitors 224 are connected to the supply signal PVDD to maintain the voltage level at PVDD stable. The gate of the power FET 200 is connected to the output pin of a voltage reference source 202. Any one of the standard voltage reference source chips generally available can be used, provided that the reference voltage is as described below, though the TL431 is preferred. The gate of the power FET 200 is also coupled to the supply signal +12 V through a resistor 203. The reference input of the voltage reference source 202 is connected to the collector of a bipolar junction transistor (BJT) 210, and the ground pin of the voltage reference source 202 is connected to ground. The emitter of the BJT 210 is connected to ground and its base is coupled to the voltage detect sense pin VOLDET through a resistor 212. A pullup resistor 214 is connected between the voltage detect sense pin VOLDET and the supply signal +5 V. When a 5-volt CPU 101 is placed into the socket 100, the sense pin VOLDET is left floating. As a result, the pullup resistor 214 is able to pull the sense pin VOLDET to 5 volts. If a 3.3-volt CPU 101 is inserted into the socket 100, then the sense pin VOLDET is driven low by the CPU 101. 
     A resistor 206 is connected between the source of the power FET 200 and the collector of the BJT 210. Another resistor 208 is connected between the collector of the BJT 210 and ground. The resistors 206 and 208 form a voltage divider circuit to provide a voltage at their common node based on the voltage of the supply signal PVDD. In addition, a capacitor 216 is connected between the output pin and the reference pin of the reference voltage source 202. One side of a resistor 220 is also connected to the output pin of the voltage reference source 202, and the other side of the resistor 220 is connected to one side of a capacitor 218. The other side of the capacitor 218 is connected to the reference pin of the voltage reference source 202. The capacitors 216 and 218 and the resistor 220 function to stabilize the voltage generated on the output pin of the voltage reference source 202. Any noise glitch occurring on the reference pin of the voltage reference source 202 is filtered by the combination of the resistor 220 and the capacitors 216 and 218. 
     If the sense pin VOLDET is pulled high by the resistor 214, the BJT 210 is turned on, thereby pulling the reference pin of the voltage reference source 202 to approximately the collector-to-emitter saturation voltage of the BJT 210, which is approximately 0.2 to 0.3 volts. This turns the voltage reference source 202 off, which allows the supply signal +12 V to drive the gate of the power FET 200 to approximately 12 volts. As a result, the voltage of the supply signal +5 V completely passes through the power FET 200 to drive the supply signal PVDD to 5 volts. 
     If a 3.3-volt CPU 101 is installed and the sense pin VOLDET is driven low, the BJT 210 is turned off. As a result, the voltage provided to the reference pin of the voltage reference source 202 is developed by the voltage divider formed from the resistors 206 and 208. The voltage at the reference pin of the voltage reference source 202 is thus dependent upon the power supply voltage PVDD. At steady state, the voltage at the reference pin of the voltage reference source 202 is driven to about 2.5 volts. In response to this voltage, the voltage reference source 202 drives the gate of the power FET 200 to approximately one threshold voltage (V T ) above 3.3 volts. The threshold voltage V T  of the power FET 200 ranges from 3 to 10 volts. Consequently, due to the gate-to-source threshold voltage V.sub. of the power FET 200, the voltage at the source of the power FET 200 is limited to approximately 3.3 volts. A feedback path exists from the regulated supply signal PVDD through the resistor 206 to the reference pin of the voltage reference source 202, which allows the supply signal PVDD to be maintained at a relatively stable 3.3 volts. 
     Optional zero ohm resistors 222 are inserted between the supply signal +5 V and the supply signal PVDD when the power FET 200 and all its associated circuitry are not installed. When optional resistors 222 are used instead of the power FET 200, the computer system C can be used only with 5-volt microprocessors. 
     Thus, a circuit has been described that automatically switches the power supply voltage provided to a CPU, referred to as supply signal PVDD, between 3.3 volts and 5 volts. The circuit detects whether the CPU installed in a socket is a 3.3-volt part or a 5-volt part by determining the state of a voltage detect sense pin provided by the socket. If the voltage detect sense pin is driven low, that indicates a 3.3-volt CPU is being used. If a 5-volt CPU is installed, the voltage detect sense pin is left floating by the CPU, which allows a pullup resistor to pull the voltage detect sense pin high. The power supply voltage provided to the CPU is regulated through a power field effect transistor (FET). The gate of the power FET is connected to the output of a voltage reference source and is coupled to a 12-volt supply signal. If the voltage detect sense pin is pulled high, the voltage reference source is turned off, allowing the 12-volt supply signal to drive the gate of the power FET. This in turn allows the power FET to pass a 5-volt supply signal to the CPU supply signal PVDD. If the voltage detect sense pin is pulled low, the voltage reference source is turned on to drive the gate of the power FET to approximately one threshold voltage above 3.3 volts. In response, the power FET passes only 3.3 volts to the CPU supply signal PVDD. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, materials, components, circuit elements, wiring connections and contacts, as well as in the details of the illustrated circuitry and construction and method of operation may be made without departing from the spirit of the invention.