Abstract:
An add-on card is provided for use within a computer system that has an expansion slot connected to a bus. The bus has a first supply line for supplying a first predetermined voltage and a second supply line for supplying a second predetermined voltage which is higher than the first predetermined voltage. The add-on card is adapted to operate properly regardless of whether the respective predetermined voltages are supplied on (1) the first supply line only, (2) the second supply line only, or (3) both supply lines. In a PCI bus implementation, where 3.3V and 5V are the predetermined voltage levels, the add-on card operates properly regardless of whether: only a 5V level is provided, only a 3.3V level is provided, or both 3.3V and 5V levels are provided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to computer systems. More particularly, the present invention relates to voltage supply circuits for add-on cards in PCs and other computer systems. 
     2. Brief Description of the Related Art 
     Personal Computers (PCs) and other types of computer systems commonly include expansion slots for receiving add-on cards. The expansion slots are typically provided on the host computer&#39;s motherboard, and provide access to signal lines of the computer&#39;s main bus. Any of a variety of different types of cards can be added to the computer system using the expansion slots, including host adapter boards, network cards, and hardware accelerators. 
     The add-on cards receive power from the host computer&#39;s power supply using power supply lines of the main bus. In computers that use a Peripheral Component Interconnect (PCI) type bus, these power supply lines have traditionally provided a voltage level of 5V. Recently, however, there has been a trend in the industry towards using 3.3V components that consume less power. As a result, manufacturers of PC systems sometimes provide a PCI bus that includes an additional, 3.3V power supply line. When this extra voltage supply level is provided, both 3.3V and 5V type cards can be used within the system. 
     Unfortunately, because not all PCs are provided with a 3.3V power level, add-on cards are commonly designed to use only the 5V supply, and to step down to 3.3V internally. This step-down method causes the cards to consume more system power than is necessary. In addition, the provision of such cards impedes the ability of PC manufacturers to eventually eliminate the 5V supply, as is desirable for reducing manufacturing costs. The elimination of the 5V supply also has the effect of reducing the cost of power supplies since only a single voltage level is provided. 
     SUMMARY OF THE INVENTION 
     An add-on card for a computer bus that automatically selects the low voltage line available on a bus is provided. The add-on card includes a voltage selection circuit that detects the presence of a low voltage and uses that voltage to supply power to the add-on card. The voltage selection circuit includes a detector for every voltage level that may be supplied on a bus. The voltage selection circuit is constructed such that a voltage regulator for a given voltage level is automatically disabled by a detection of any voltage lower than that voltage but higher than a minimum voltage required for proper operation of the add-on card. 
     The add-on card is preferably used within a computer system having an expansion slot connected to a bus. The bus has a first supply line for supplying a first predetermined voltage and a second supply line for supplying a second predetermined voltage which is higher than the first predetermined voltage. The add-on card is adapted to operate properly regardless of whether the respective predetermined voltages are supplied on (1) the first supply line only, (2) the second supply line only, or (3) both supply lines. The add-on card includes a selection circuit which connects to the first and second supply lines. The selection circuit detects the presence of the first predetermined voltage on the first supply line and selects between the first and second supply lines as a power source for at least one integrated circuit of the card. The selection circuit selects the first supply line as the power source when the detection circuit detects the first predetermined voltage on the first power supply line. A step-down circuit steps down a voltage provided on the second supply line when the second power supply line is selected as the power source. In a 5V/3.3V PCI embodiment, the card operates in the following supply voltage configurations: 5V only, 3.3V only, and 5V/3.3V. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features and other features of the present invention will now be described with reference to the drawings of a preferred embodiment of an add-on card. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate, but not to limit the invention. The drawings include the following figures: 
     FIG. 1 illustrates one arrangement of a computer system; 
     FIG. 2 illustrates a computer bus and an add-on card constructed in accordance with the present invention; 
     FIG. 3 is a circuit diagram of an embodiment of the power source selection circuit of the present invention; 
     FIG. 4 is a logic diagram illustrating an embodiment of the power source selection circuit of the present invention when more than two supply voltage levels may be provided by a bus; and 
     FIG. 5 is a circuit diagram of a higher output current embodiment of the power source selection circuit of the present invention; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To facilitate an understanding of the preferred embodiment, the general architecture and operation of a computer system that includes add-on cards will initially be described. The specific architecture and operation of the preferred embodiment will then be described with reference to the general architecture and operation of an add-on card power source selection circuit. FIG. 1 illustrates a typical computer system  100  such as a PC. In FIG. 1, the computer system  100  comprises a central processing unit (CPU)  102 , Random Access Memory (RAM)  104 , a local bus  106 , an add-on video card  108 , an add-on sound card  110 , and an I/O device controller card  112 . The computer system of FIG. 1 is an example of a configuration using a single bus. A network card and any of a variety of other types of cards may additionally be provided. The system of FIG. 1 may also include an additional expansion bus to communicate with additional I/O devices. 
     The CPU  102  is coupled to the RAM  104  as to store data and instructions executed by the CPU. The CPU  102  is coupled to the bus  106  to communicate with add-on cards coupled to the bus. The CPU  102  may alternatively be coupled to the bus  106  by a bridge controller that serves as an interface between a processor bus (not shown) and the bus. The video add-on card  108  is coupled to a display device  116  by a communication line  109 . The video card may include memory that is used to provide screen refresh information to the display  116 . The sound card  110  is coupled, for example, to a pair of speakers  118 ,  120 . The sound card may include a controller and memory used to process digital sound information and convert it to analog sound that is delivered to the speakers  118 ,  120 . The I/O device controller  112  may be an IDE controller, a SCSI controller, or a Fibre Channel controller. The I/O controller card  112 , also commonly referred to as a host adapter board, may be coupled to one or more I/O devices, such as an array of disk drives  122 ,  124 . The I/O controller may be used, for example, to receive requests from the CPU and process those requests by accessing the I/O devices  122 ,  124 . The bus  106  is preferably a Peripheral Component Interconnect (PCI) bus. The bus  106  may also be a Video Electronics Standards Association (VL) local bus or an EISA/ISA type bus. The PCI bus specification includes a dual voltage line requirement such that PCI bus manufacturers are required to at least supply a 5V supply line. The manufacturer can optionally provide a 3.3V supply line on a designated pin on the bus. PCI peripherals can exist as chips embedded on a motherboard, as add-on cards, or as external devices with a card interface. Future implementations of the PCI specification may eliminate the 5V supply to enable systems to use lower voltage power supplies. In addition, it is contemplated that supply levels lower than 3.3V (e.g. 2.5V) will be introduced to the PCI and other bus specifications in the future, and that the present invention will facilitate transitions to such new supply levels. 
     FIG. 2 illustrates an add-on card that includes a power selection circuit according to the present invention. A power supply  202  of the computer is used to provide power to a pair of lines  206 ,  208  on the bus  106 . Each line may be composed of several physical power traces on the bus that supply the same voltage level. Providing multiple traces permits a higher level of bus current, since each physical trace has a limited current capacity. A first line  206  carries a first potential, and a second line  208  carries a second potential. The line  204  is a ground potential line. The add-on card interfaces with the bus  106  by an edge connector (not shown) commonly used in the industry. The edge connector includes pins (not shown) that interface with data lines and includes pins  212 ,  214 , and  216  that interface with power supply bus lines  208 ,  206 , and  204 , respectively. The line from pin  212  is coupled to a “low voltage potential” input of the power source selection circuit  210 . The line from pin  214  is coupled to a “high voltage potential” input of the power source selection circuit  210 . The line from pin  216  is coupled to the power source selection circuit  210  ground level input. In the preferred PCI implementation, the power supply lines  212 ,  214  can have one of three possible voltage configurations, depending upon system design: 5V only, 5V and 3.3V, or 3.3V only. In accordance with the invention, the card supports all three of these voltage configurations without the need to set switches on the card or other manual configuration steps. Data, address, and control pins  220  from the bus connector are associated with data lines  218  which are coupled to an integrated circuit  201 . 
     The power source selection circuit  210  includes a voltage level output line  222  providing a predetermined output voltage to an integrated circuit  201 , and possibly multiple integrated circuits of the add-on card. The power source selection circuit also provides a ground potential line  224  to the integrated circuit  201 . In the preferred PCI implementation, this output voltage is 3.3V. The integrated circuit  201  may be a controller ASIC that is used to process I/O commands received by the host adapter add-on board. 
     FIG. 3 is a circuit diagram of the power source selection circuit  210  of FIG.  2 . The power source selection circuit  210  includes a low voltage input line  212 ; a high voltage input line  214 , a ground input  216 , a ground output  224 , and a voltage output line  222 . The voltage output line provides the predetermined voltage level (e.g. 3.3V) regardless of which of the three voltage configurations is provided. The circuit  210  includes a driver  302  (such as part number LTC1157) that is used as a sensor to detect the presence of the low voltage on the line  212 . The driver  302  has an input connected to one end of a pull-up resistor  304  and to the inflow end of an inverter  306  that has an open collector output. The driver  302  has a voltage line connected to the low voltage input  212 . The driver  302  has an output voltage line coupled to the gates of a pair of switches  310 ,  312  which are MOSFET switches in the preferred embodiment. The switches  310 ,  312  are coupled to the low voltage line  212  and to the voltage output  222  as to connect the low voltage input line  212  to the voltage output when the switches are on. The driver  302 , in combination with the switches  310 ,  312  and the associated components, serves as a selection circuit to decide whether the low voltage input line  212  or the high voltage input line  214  is used to provide the output voltage. A voltage regulator  308  (e.g. part number LT1129-3.3) has a voltage input coupled to the high voltage input line  214  and to a positive end of a ground capacitor  316 . The shutdown input of the voltage regulator  308  is coupled to the outflow end of the inverter  306  and one end of a pull-up resistor  314 . A sense input and a voltage output of the voltage regulator  308  are both coupled to the voltage output  222 . A positive end of a ground capacitor  318  is coupled to the voltage output  222 . The voltage regulator  308  along with the associated components such as the pull-up resistor  314  and the ground capacitor  318  serve as a step down circuit which steps down the voltage provided on the high voltage supply line  214 . 
     In operation, the circuit of FIG. 3 selects between the power supply lines  212 ,  214  such that the lowest available supply voltage is used to provide the predetermined output voltage. For example, in the above mentioned PCI embodiment, a 5V/3.3V or 3.3V only configuration causes the 3.3V line  212  to be used, while a 5V only configuration causes the 5V line  214  to be used. If a supply voltage is available on line  212 , the driver  302  is activated such that a gate drive voltage is generated at the switches  310 ,  312 . This causes the switches  310 ,  312  to turn ON so that the low voltage from line  212  is gated to the voltage output  222 . Additionally, the voltage at the outflow end of the inverter  306  causes the voltage regulator  308  to shut off Therefore, only the low voltage is used to provide the output voltage. When the low voltage is not sensed by the driver  302 , the voltage at the outflow end of the inverter  306  does not shut down the voltage regulator  308 . The voltage regulator  308  then steps down the high voltage level from line  214  to that of the low voltage and provides the stepped down voltage to the output line  222 . The low voltage line  212  is isolated from the output  222  in this event because the switches  310 ,  312  are OFF. In this manner, the low voltage level is provided to the voltage output  222  regardless of the availability of the low voltage on line  212 . 
     The method illustrated by the circuit of FIG. 3 can be used to select between a greater number of supply voltages, such as between 5V, 3.3V and 2.5V supply voltages that may be provided in future PCI systems. By way of example, FIG. 4 is a logic diagram of an automatic detection circuit that is able to sense four voltage levels and automatically select the lowest level available for use in providing a predetermined output voltage. The circuit includes four voltage level inputs  402 - 408  from a low level input  402  to a high level input  408  respectively. The low level input  402  is associated with a sensor  410  such that the sensor provides a signal in response to a detection of the low voltage on line  402 . A switch  412  is coupled to the sensor  410  such that the switch provides a connection between the low level input  402  and the voltage output  222  in response to a detection signal from the sensor  410 . A second voltage level input  404  is associated with a voltage regulator  414 . The voltage regulator  414  has an enable input coupled to the low voltage line  402 , a voltage input coupled to the second voltage level input  404 , and a voltage output coupled to the output line  222 . A third voltage level input  406  is associated with a voltage regulator  418 . The voltage regulator  418  has an enable input coupled to the output of an OR gate  416 , a voltage input coupled to the third voltage level input  406 , and a voltage output coupled to the output line  222 . The OR gate  416  has one input coupled to the line from the enable input of the lower voltage regulator  414  and an input coupled to the line from the voltage input of the lower voltage regulator  414 . A fourth voltage level input  408  is associated with a voltage regulator  422 . The voltage regulator  422  has an enable input coupled to the output of an OR gate  420 , a voltage input coupled to the third voltage level input  408 , and a voltage output coupled to the output line  222 . The OR gate  420  has one input coupled to the line from the enable input of the lower voltage regulator  418  and an input coupled to the line from the voltage input of the lower voltage regulator  418 . 
     In operation, detection of the low level voltage by the sensor  410  causes the switch  412  to conduct the low voltage to the output  222 . The presence of the low level voltage on line  402  also disables the voltage regulator  414  by de-asserting the signal on the enable input of the regulator  414 . The low level voltage also disables voltage regulators  418  and  422  as the outputs of OR gates  416 , and  420  are asserted by the propagation of the low level voltage. In this manner, it can be appreciated that the presence of any voltage level below that of the input to a voltage regulator disables the same voltage regulator since the enable input is connected to the logical OR of all lower voltage lines. Although, the illustration of FIG. 4 employs logic gates, actual implementation of the circuit varies in operation since analog, as opposed to digital signals, are employed. Nevertheless, the overall logical configuration of the circuit remains, in the analog implementation, as can be appreciated from the circuit of FIG. 3 which is the analog implementation of the first two stages of the power selection circuit of FIG.  4 . In can be further appreciated that the circuit can be extended to detect any number of voltage levels by incorporating additional modules of two input OR gates and voltage regulators. 
     FIG. 5 is an alternate embodiment of the circuit of the present invention which provides a higher level of output current by using different switching transistors and a different voltage regulator. Additionally, when a 12V supply line  515  is available on the bus an inexpensive transistor can replace the driver  302  of FIG.  3 . The voltage selection circuit of FIG. 5 includes an inverter  502 , which has an open collector output, coupled to a pull-up resistor connected to the 3.3V supply line  510  and the base of a transistor  504  (e.g., part number 2N2222). The collector of the transistor  504  is connected to a pull-up resistor connected to a 12V supply line  515 , and to the gates of a pair of transistors  506 ,  508 . The emitter of the transistor  504  is connected to the ground line. The transistors  506 ,  508  are coupled to the 3.3V supply line  510  and to the voltage output  512  as to connect the low voltage supply line to the voltage output when the switches are on. A voltage regulator  514  (e.g., part number LT1528) has a voltage input coupled to the 5V supply line  516  and to a positive end of a ground capacitor  518 . The shutdown input of the voltage regulator  514  is coupled to the outflow end of an inverter  522  that has an open collector output, and one end of a pull-up resistor  524  that is connected to the 5V supply line  516 . A sense input and a voltage output of the voltage regulator  514  are both coupled to the voltage output  512 . A positive end of a ground capacitor  526  is coupled to the voltage output  512 . The voltage regulator  514  along with the associated components serve as a step down circuit which steps down the voltage provided on the 5V supply line  214 . The use of a different type of voltage regulator  514  provides a higher level of output current. 
     Although the invention has been described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined by the claims that follow.