Abstract:
In a personal computer system utilizing both a main power supply and an auxiliary power supply, the input and output signal lines of auxiliary powered components are selectively isolated from components powered solely by the main power supply. Gating circuitry and buffer circuitry, controlled by various enable signals, are used to isolate the signal lines. External pull-down or pull-up resistors are no longer required, which frees up printed circuit board area for other components and conserves board space in the computer system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to personal computers, and more particularly to an apparatus and method of internally isolating personal computer components powered by an auxiliary power supply from components powered only by a main power supply. 
     2. Description of the Related Art 
     Recently, in certain personal computers an auxiliary power supply has been used to keep part of the system functional when a main computer power supply is down. The components in such a part of the system receiving power from the auxiliary power supply are used to “wake up” the machine at the appropriate time. 
     When the main power turns off, an auxiliary powered component with an output connected to an input of a component powered only by the main power supply should be prevented from driving a signal on that signal line to the “high” state. The auxiliary powered component must be prevented from driving a signal to the high state, because applying a voltage to the input of the component without power may supply enough voltage/current to cause the component to start operating randomly. Random operation is not desirable as randomly active signals can potentially disturb internal circuitry of the auxiliary powered components. 
     Further, inputs to auxiliary powered components connected to a component powered by the main power supply would float in voltage level in certain cases. This would occur, for example when the main power turns off if the inputs to the auxiliary powered components were not properly terminated. Many personal computer components are complementary metal oxide semiconductor (CMOS) integrated circuits. Leaving unused inputs of a CMOS component floating causes problems. A CMOS component with an input left floating may enter the active mode of operation with only minimal leakage current, due to the component&#39;s high input impedance. Normally, leakage currents are such that a CMOS component with an input floating will enter the active mode. Also, the unpowered input signal line can act like an antenna and begin to absorb signal energy present in the machine. After doing so, it can supply a valid logic voltage to the auxiliary powered component. This can disturb circuits not expecting such levels on that signal line. In addition, switching of the internal CMOS circuitry is inefficient as it needlessly consumes power. 
     One solution to this problem has been to couple an external pull-down resistor between ground and the signal line being isolated, or an external pull-up resistor between a positive rail of an active power supply and the signal line being isolated. 
     However, this approach had several undesirable features. The pull-up or pull-down resistors were separate or discrete circuit elements. If a large number of signal lines needed isolation, large amounts of board space could be consumed by the added resistor elements. Further, the additional resistors increased the computer system cost in terms of both the cost of the added resistors and the burden to the system of extra components. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides a technique for selectively isolating a signal line in a computer system using multiple power supplies. When a main power supply is inactive, an auxiliary power supply provides power to a component powered by the auxiliary power supply. Isolation circuitry according to the present invention implements gating circuitry and buffer circuitry, that is controlled by various enable signals, to isolate the signal line. The signal line is isolated by coupling the signal line to an appropriate power rail through a resistance provided by the buffer circuitry. In an embodiment of the present invention, the isolation circuitry is implemented on a monolithic integrated circuit with the component powered by the auxiliary power supply. 
     In one embodiment, the isolation circuitry selectively isolates an input signal line of the component powered by the auxiliary power supply from an output signal line of a component powered by the main power supply. In another embodiment, the isolation circuitry selectively isolates an output signal line of the component powered by the auxiliary power supply from an input signal line of the component powered by the main power supply. In either of the previous embodiments, the isolation circuitry is electrically coupled between the signal line and internal logic of the component powered by the auxiliary power supply. Alternatively, when the isolation circuitry is integrated within the component powered by the auxiliary power supply, the isolation circuitry is electrically coupled between a bonding pad and the internal logic of the component. 
     A technique of isolation that does not require external resistors is highly desirable, since added external resistors result in additional cost and can potentially deplete considerable amounts of printed circuit board space that could be used for other circuitry. When there are multiple components that require isolation, the reduction in the number of external isolation resistors and the availability of the printed circuit board area previously used by the external isolation resistors can result in significant benefits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which: 
     FIG. 1 is a block diagram of the computer system S implemented according to the present invention; 
     FIG. 2 is a schematic electrical circuit diagram illustrating a prior art technique of signal line isolation; 
     FIG. 3 is a schematic diagram illustrating the circuitry added to an output according to an embodiment of the present invention; 
     FIG. 4 is a schematic diagram illustrating the circuitry added to an input according to an embodiment of the present invention; 
     FIG. 5 is a simplified schematic electrical circuit diagram of a new technique of isolation according to the present invention; and 
     FIG. 6 is a timing diagram of an embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Computer System Overview 
     Turning to FIG. 1, illustrated is a typical personal computer system S implemented according to the invention. In the embodiment of FIG. 1, the computer system S is a microprocessor-based system. While this system is illustrative of one embodiment, the present invention can be implemented in a wide variety of systems. The computer system S in the illustrated embodiment is a PCI bus/ISA bus based machine, having a peripheral component interconnect (PCI) bus  10  and an industry standard architecture (ISA) bus  12 . The PCI bus  10  is controlled by PCI controller circuitry located within a memory/accelerated graphics port (AGP)/PCI controller  14 . This controller  14  (the “host bridge”) couples the PCI bus  10  to a processor socket  16  via a host bus, an AGP connector  18 , and a memory subsystem  20 . A second bridge circuit, a PCI/ISA bridge  24  (the “ISA bridge”) bridges between the PCI bus  10  and the ISA bus  12 . 
     The host bridge  14  in the disclosed embodiment is a 440LX Integrated Circuit by Intel Corporation, also known as the PCI AGP Controller (PAC). The ISA bridge  24  is a PIIX4, also by Intel Corporation. The host bridge  14  and ISA bridge  24  provide capabilities other than bridging between the processor socket  16  and the PCI bus  10 , and the PCI bus  10  and the ISA bus  12 . Specifically, the disclosed host bridge  14  includes interface circuitry for the AGP connector  18 , the memory subsystem  20 , and the AGP  22 . The ISA bridge  24  further includes an internal enhanced IDE controller for controlling up to four enhanced IDE drives  26 , and a universal serial bus (USB) controller for controlling USB ports  28 . 
     The host bridge  14  is preferably coupled to the processor socket  16 , which is preferably designed to receive a Pentium II processor module  30 , which in turn includes a microprocessor core  32  and a level two (L 2 ) cache  34 . The processor socket  16  could be replaced with different processors other than the Pentium II without detracting from the spirit of the invention. 
     The host bridge  14 , when the Intel 440LX North Bridge is employed, supports extended data out (EDO) dynamic random access memory (DRAM) and synchronous DRAM (SDRAM), a 64/72-bit data path memory, a maximum memory capacity of one gigabyte, dual inline memory module (DIMM) presence detect, eight row address strobe (RAS) lines, error correcting code (ECC) with single and multiple bit error detection, read-around-write with host for PCI reads, and 3.3 volt DRAMs. The host bridge  14  support up to 66 megahertz DRAMs, whereas the processor socket  16  can support various integral and nonintegral multiples of that speed. 
     The ISA bridge  24  also includes enhanced power management. It supports a PCI bus at 30 or 33 megahertz and an ISA bus  12  at ¼ of the PCI bus frequency. PCI revision 2.1 is supported with both positive and subtractive decoding. The standard personal computer input/output (I/O) functions are supported, including a direct memory access (DMA) controller, two 82C59 interrupt controllers, an 8254 timer, a real time clock (RTC) with a 256 byte complementary metal oxide semiconductor (CMOS) static RAM (SRAM), and chip selects for system read only memory (ROM), RTC, keyboard controller, an external microcontroller, and two general purpose devices. The enhanced power management within the ISA bridge  24  includes full clock control, device management, suspend and resume logic, advanced configuration and power interface (ACPI), and system management bus (SMBus) control, which implement the inter-integrated circuit (I 2 C) protocol. 
     The PCI bus  10  couples a variety of devices that generally take advantage of a high speed data path. This includes a small computer system interface (SCSI) controller  36 , with both an internal port  38  and an external port  40 . In the disclosed embodiment, the SCSI controller  36  is a AIC-7860 SCSI controller. Also coupled to the PCI bus  10  is a network interface controller (NIC)  42 , which preferably supports the ThunderLan™ power management specification by Texas Instruments. The NIC  42  is coupled through a physical layer  44  and a filter  46  to an RJ- 45  jack  48 , and through a filter  50  to a AUI jack  52 . 
     Between the PCI Bus  10  and the ISA Bus  12 , an ISA/PCI backplane  54  is provided which include a number of PCI and ISA slots. This allows ISA cards or PCI cards to be installed into the system for added functionality. 
     Further coupled to the ISA Bus  12  is an enhanced sound system chip (ESS)  56 , which provides sound management through an audio in port  58  and an audio out port  60 . The ISA bus  12  also couples the ISA bridge  24  to a Super I/O chip  62 , which in the disclosed embodiment is a National Semiconductor Corporation PC87307VUL device. This Super I/O chip  62  provides a variety of input/output functionality, including a parallel port  64 , an infrared port  66 , a keyboard controller for a keyboard  68 , a mouse port for a mouse port  70 , additional series ports  72 , and a floppy disk drive controller for a floppy disk drive  74 . These devices are coupled through connectors to the Super I/O  62 . 
     The ISA bus  12  is also coupled through bus transceivers  76  to a flash ROM  78 , which can include both basic input/output system (BIOS) code for execution by the processor  32 , as well as an additional code for execution by microcontrollers in a ROM-sharing arrangement. 
     The ISA bus  12  further couples the ISA bridge  24  to a security, power, ACPI, and miscellaneous application specific integrated circuit (ASIC)  80 , which provides a variety of miscellaneous functions for the system. The ASIC  80  includes security features, system power control, light emitting diode (LED) control, a PCI arbiter, remote wake up logic, system fan control, hood lock control, ACPI registers and support, system temperature control, and various glue logic. 
     Finally, a video display  82  can be coupled to the AGP connector  18  for display of data by the computer system S. Again, a wide variety of systems could be used instead of the disclosed system S without detracting from the spirit of the invention. 
     Prior Technique Of Signal Line Isolation 
     FIG. 2 illustrates a prior art technique for isolating a signal line between components within the computer system S. In FIG. 2, a component  219  powered by a main power supply  220  is indicated. Signal lines from component  219  are isolated from signal lines of component  221  using external isolation resistors  224  and  226 . Component  221  is powered by an auxiliary power supply  222 . The isolation resistors  224  and  226  are shown coupled between the signal lines and a voltage reference level, either an electrical ground or a positive power supply rail. If pull-down resistors are needed the resistors are coupled between the signal lines and ground. If pull-up resistors are desired, the resistors would be coupled between the signal lines and the positive power supply rail. 
     As stated previously, this prior art approach had several undesirable features. The pull-up or pull-down resistors were separate or discrete circuit elements. If a large number of signal lines needed isolation, large amounts of board space could be consumed by the added resistor elements. Further, the additional resistors increased the computer system cost in terms of both the cost of the added resistors and the burden to the system of extra components. 
     Signal Line Isolation According To The Present Invention 
     Moving to FIGS. 3 and 4 shown are embodiments according to the present invention for isolating a signal line. FIG. 3 illustrates circuitry added to an output of an auxiliary powered component according to one embodiment of the present invention. FIG. 4 shows circuitry added to an input of the auxiliary powered component according to another embodiment of the present invention. Both FIGS. 3 and 4 detail how an external resistor is replaced with isolation circuitry which is preferably internal to the component powered by an auxiliary power supply. If a pull-down resistor is needed, a first input of multiplexer  206  is coupled to ground. If a pull-up resistor is needed, the first input of multiplexer  206  is coupled to a positive rail of the auxiliary power supply. 
     In FIG. 4, if multiplexer  206  is not utilized then buffer  212 &#39;s input is directly coupled to ground or the positive power supply rail (as appropriate). If the additional circuitry is implemented on a monolithic integrated circuit with the auxiliary powered component, the additional circuitry is electrically coupled between a bonding pad (not shown) and internal logic of the component. 
     The following discussion uses positive logic in the description of the various enable signals. In this discussion, a low state is in effect a false logic state and a high state is in effect a true logic state. It should be apparent that the circuitry could be implemented using negative logic. If negative logic is used, the high state serves as the false logic state and the low state serves as the true logic state. The enable signals may be supplied by ASIC  80  or by another source. The isolation circuitry may be implemented within ASIC  80  or any other auxiliary powered component. Auxiliary powered components include, for example, the ISA bridge  24 , the NIC  42 , the physical layer  44 , and the Super I/O chip  62 . 
     Turning back to FIG. 3, shown is an embodiment of the isolation circuitry for an output signal line of the auxiliary powered component within the computer system S. The isolation circuitry includes a tri-state output buffer  204 , a two-input OR gate  202 , and a two-input multiplexer  206 . A main power off MPO signal is coupled to a control line of the two-input multiplexer  206  and a first input of the two-input OR gate  202 . Gating circuitry is comprised of the two-input multiplexer  206  and the two-input OR gate  202 . Buffer circuitry includes the output buffer  204 . The main power off MPO signal provides a first enable signal that senses the state of the main power supply. The main power off MPO signal of the disclosed embodiment is high when the main power is off. 
     An output of the output buffer  204  is coupled to the output signal line. An input of the output buffer  204  is coupled to an output of the two-input multiplexer  206 . A first input of the two-input multiplexer  206  is coupled to an auxiliary power supply rail. If a pull-down resistor is required the input of multiplexer  206  is coupled to ground. If a pull-up resistor is required the input of multiplexer is coupled to the positive power supply rail. 
     A second input of multiplexer  206  is coupled to an output OS signal which originates from the internal logic of the auxiliary powered component. A control line of the output buffer  204  is coupled to an output of the two-input OR gate  202 . The output of the two-input OR gate  202  provides a third enable signal which acts as a control signal. Coupled to a first input of the OR gate  202  is the main power off MPO signal. Coupled to a second input of the OR gate  202  is an output enable OE signal. The output enable OE signal provides a second enable signal which acts as a buffer enable signal. 
     Again, when main power is off, the main power off MPO signal is high. When the main power off MPO signal is high the grounded input (or the input connected to the positive auxiliary power supply rail) of the two-input multiplexer  206  is selected to drive the output buffer  204 . Thus, the output buffer  204  is enabled whether the output enable OE signal is high or low when the main power off MPO signal is high. If both the output enable OE signal and the main power off MPO signal are low, then the output buffer  204  is in the tri-state condition (high impedance state). 
     When the main power off MPO signal is high (main power is off) the output buffer  204  couples the output signal line to ground (or to the positive power supply rail). When the main power off MPO signal is low (main power is on) an output OS signal from the internal logic of the component is selected and is passed to the input of the device that is coupled to the auxiliary powered component, if the output enable OE signal is high. 
     Moving to FIG. 4, an embodiment of the isolation circuitry associated with one of the input signal lines within the computer system S is shown. The isolation circuitry includes an output buffer  212  and a two-input OR gate  202 . As in the case of an output signal, the main power off MPO signal is coupled to the first input of the two-input OR gate  202 . Again, the main power off MPO signal senses the state of the main power supply and is high when the main power is off. An output of the output buffer  212  is coupled to the input signal line. 
     In one embodiment, an input of the output buffer  212  is coupled to the output of the multiplexer  206 . One input of the multiplexer  206  is coupled to one of the auxiliary power supply rails. If a pull-down resistor is required, the input of multiplexer  206  is coupled to ground. If a pull-up resistor is required, the input of multiplexer  206  is coupled to the positive auxiliary power supply rail. In another embodiment, the multiplexer  206  is not utilized and the input of the output buffer  212  is directly coupled to the ground rail or the positive power supply rail. A control line of the output buffer  212  is coupled to the output of the two-input OR gate  202 . As above, the output of the two-input OR gate  202  provides the control signal. 
     As previously related, coupled to the second input of the OR gate  202  is the output enable OE signal which acts as the buffer enable signal. When main power is off, the main power off MPO signal is high. When the main power off MPO signal is high, the output buffer  212  is enabled whether the output enable OE signal is high or low. If both the output enable OE signal and the main power off MPO signal are low, then the output buffer  212  is disabled. When the main power off MPO signal is high (main power is off) the output buffer  212  is enabled and the input signal line is coupled to the ground rail or the positive power supply rail. 
     In the case of the input signal line, the output buffer in FIG. 4 is a bi-directional buffer. The input circuitry consists additionally of an input buffer  214 , an inverter  218 , and a two-input AND gate  216 . Buffer circuitry for the input signal line includes the input buffer  214  and the output buffer  212 . Gating circuitry for the input signal line comprises the inverter  218 , the OR gate  202 , the AND gate  216 , and the multiplexer  206  (if present). 
     When the isolation circuitry is integrated with the component powered by the auxiliary power supply, an input of the input buffer  214  is coupled to an input bonding pad (not shown) of the auxiliary powered component. An output of the input buffer  214  is also coupled to a first input of the two-input AND gate  216 . The main power off MPO signal is coupled to an input of the inverter  218 . An output of the inverter  218  is coupled to a second input of the two-input AND gate  216 . The output of the inverter  218  provides a fourth enable signal which is used to block a signal on the input signal line. The output stage  212  of the bi-directional output buffer is turned on when the main power off MPO signal is high (main power is inactive or off) which results in the input signal line being coupled to the ground rail (or to the positive power supply rail). 
     Also, when the main power off MPO signal is high (main power is off) the output of the inverter  218  is low. A low to one input of the two-input AND gate  216  causes an output of the gate to go low; thereby blocking a signal from the output of an external device powered by the main power supply from toggling the internal logic of the auxiliary powered component. When the main power off MPO signal is low (main power is on) one input to the two-input AND gate  216  is high and the input signal is passed to the internal logic of the auxiliary powered component, if the output buffer  212  is disabled. 
     Moving to FIG. 5, a simplified embodiment for isolating a signal line within the computer system S according to the present invention is illustrated. The drawing depicts external resistors  224  or  226  being replaced with a FET  228 . When the FET  228  is turned on the channel resistance of the device serves as the resistor  224  or  226  between the signal line and the ground rail or the positive auxiliary power supply rail. As discussed previously, if a pull-up resistor is required one end of the FET  224  or  226  is connected to the positive auxiliary power supply rail. In the disclosed embodiment according to the present invention an internal channel resistance of buffer  204  (FIG. 3) or buffer  212  (FIG. 4) provides an isolation resistance between the signal line and the ground rail or the positive power supply rail. 
     Turning to FIG. 6, a timing diagram of one aspect of the present invention is illustrated. The following discussion assumes that the inputs of buffers  204  (see FIG. 3) and  212  (see FIG. 4) are coupled to ground. FIG. 6 is provided to aid one skilled in the art to more readily visualize when power related activity occurs. PSON is a control signal supplied by the ASIC  80 . The ASIC  80  turns the main power supply on or off. When the PSON signal is high, the main power supply for the computer system S turns on. When the PSON signal is low, the main power supply for the computer system S turns off. POWERGOOD is a signal supplied from the main power supply or supplied by components on the system board indicating that a valid voltage level exists on the main power supply rail. The main power off MPO signal is high when both PSON and POWERGOOD are low. The POWERGOOD signal is used by the computer system S to determine when the reset process can begin. The ASIC  80  uses this signal in determining when to activate or deactivate the isolation buffers  204  and  212 . 
     When the main power is off, with both PSON low and POWERGOOD low, the output buffer  212  on the input signal line turns on and drives the signal trace to ground (when the input of the buffer  212  is coupled to the ground rail). When PSON is driven high, the output buffer  212  turns off. The buffer  212  turns off at this point because while the POWERGOOD signal may not be active, indicating a valid voltage level, there can be voltage applied to a component within the computer system S. Therefore, it is possible that some components might begin to drive an input signal line of the auxiliary powered component to the high state. If this happens, there would be a contention with the output buffer  212  which is driving a low. Turning off the output buffer  212  for the input signal line when PSON is driven high prevents this from occurring. The output buffer  212 , for the input signal line, turns on after both the PSON signal and the POWERGOOD signal have returned to a low state. 
     The output signals of the auxiliary powered components are held in the high impedance state unless both the PSON signal and the POWERGOOD signal are high. The reason is that the main system operations is only valid at that time. If the output signals were to drive an input of a part powered by the main power supply to the high state while main power is off they might damage that part. 
     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.