Abstract:
A method of implementing bi-directional level translation in an information handling system includes coupling a first device port via a first resistor to a first voltage. A second device port is coupled via a second resistor to a second voltage different from the first voltage. Lastly, a single bipolar junction transistor (BJT) couples the first device port to the second device port, the single bipolar junction transistor including an emitter coupled to the first device port, a base coupled via a third resistor to the first voltage, and a collector coupled to the second device port, wherein the bipolar junction transistor operates as a bi-directional level translator between the first and second device ports.

Description:
BACKGROUND  
         [0001]    The present disclosure relates generally to information handling systems, and more particularly to an information handling system featuring a BJT-based bi-directional level translator.  
           [0002]    As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.  
           [0003]    In conjunction with information handling systems, there are multiple chipsets and subsystems powered off multiple power rails with different voltage levels. Often a design engineer is faced with a task of interfacing subsystems signaling at different voltage levels (such as System Messaging Bus (SMB) level conversion). Typically to accomplish this task, voltage translator circuits based on Metal Oxide Field Effect Transistors (MOSFETs) are used. However, at lower signaling voltages, the problem becomes more difficult to resolve due to the lack of inexpensive MOSFETs with low gate threshold voltages (Vgs).  
           [0004]    MOSFETs have traditionally been used to translate voltages from 2.5V to 5V. They easily lend themselves to these applications due to the indifference of MOSFETs to the direction of current in its conducting channel. In other words, a signal can propagate in either direction, forward or reverse. FIG. 1 illustrates a schematic diagram view for a typical MOSFET-based voltage translator  10 . However, as signaling voltages drop lower than 2.0V, the typical MOSFET-based voltage translator circuit can no longer work reliably due to a minimum Vgs voltage required to maintain the conducting channel in the MOSFET. Unfortunately, due to the manufacturing process limitations, discrete MOSFETs with lower than 1.5V Vgs are difficult to produce at reasonable costs.  
           [0005]    Accordingly, it would be desirable to provide method and apparatus providing level translation in an information handling system absent the disadvantages found in the prior methods discussed above.  
         SUMMARY  
         [0006]    According to one embodiment, a method of implementing bi-directional level translation in an information handling system includes coupling a first device port via a first resistor to a first voltage. A second device port is coupled via a second resistor to a second voltage different from the first voltage. Lastly, a single bipolar junction transistor (BJT) couples the first device port to the second device port, the single bipolar junction transistor including an emitter coupled to the first device port, a base coupled via a third resistor to the first voltage, and a collector coupled to the second device port, wherein the bipolar junction transistor operates as a bi-directional level translator between the first and second device ports. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 illustrates an electrical schematic diagram view of a field effect transistor based bidirectional level translator known in the art; and  
         [0008]    [0008]FIG. 2 illustrates a block diagram view of an information handling system featuring a BJT-based bi-directional level translator according to an embodiment of the present disclosure; and  
         [0009]    [0009]FIG. 3 illustrates an electrical schematic diagram view of a BJT-based bi-directional level translator according to an embodiment of the present disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0010]    According to one embodiment of the present disclosure, a method and system apparatus are disclosed for implementing a BJT-based bi-directional level translator in an information handling system. The method and system can be better understood by reference to the flow charts, drawing figures, and additional discussion included herein.  
         [0011]    [0011]FIG. 2 depicts a high level block diagram of an information handling system  100  in which the disclosed technology is practiced. For purposes of this disclosure, an information handling system may include any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, an information handling system may be a personal computer, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The information handling system may include random access memory (RAM), one or more processing resources such as a central processing unit (CPU) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the information handling system may include one or more disk drives, one or more network ports for communicating with external devices as well as various input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The information handling system may also include one or more buses operable to transmit communications between the various hardware components.  
         [0012]    The particular information handling system  100  depicted in FIG. 2 is a portable computer which includes a processor  105 . An Intel Hub Architecture (IHA) chip  110  provides system  100  with memory and I/O functions. More particularly, IHA chip  110  includes a Graphics and AGP Memory Controller Hub (GMCH)  115 . GMCH  115  acts as a host controller that communicates with processor  105  and further acts as a controller for main memory  120 . GMCH  115  also provides an interface to Advanced Graphics Port (AGP) controller  125  which is coupled thereto. A display  130  is coupled to AGP controller  125 . IHA chip  110  further includes an I/O Controller Hub (ICH)  135  which performs numerous I/O functions. ICH  135  is coupled to a System Management Bus (SM Bus)  140  which is coupled to one or more SM Bus devices  145 .  
         [0013]    ICH  135  is coupled to a Peripheral Component Interconnect (PCI) bus  155  which is coupled to mini PCI connector slots  160  which provide expansion capability to portable computer  100 . A super I/O controller  170  is coupled to ICH  135  to provide connectivity to input devices such as a keyboard and mouse  175  as shown in FIG. 2. A firmware hub (FWH)  180  is coupled to ICH  135  to provide an interface to system BIOS  185  which is coupled to FWH  180 . A General Purpose I/O (GPIO) bus  195  is coupled to ICH  135 . USB ports  200  are coupled to ICH  135  as shown. USB devices such as printers, scanners, joysticks, etc. can be added to the system configuration on this bus. An integrated drive electronics (IDE) bus  205  is coupled to ICH  135  to connect IDE drives  210  to the computer system. System  100  may further include a network interface card (NIC)  215 .  
         [0014]    Referring now to FIG. 3, the BJT-based bi-directional level translator  300  according to an embodiment of the present disclosure will be discussed in further detail. Bipolar Junction Transistors (BJTs) are current controlled devices as opposed to voltage-controlled devices, that is, MOSFETs. As long as the appropriate base current is maintained, a BJT will be able to conduct collector current at very low controlling base-emitter voltage. Typically, the base emitter voltage is on the order of 0.6V. However, a drawback of BJTs preventing them from being used in level translators in the past is the sensitivity of BJTs to the direction of current between emitter and collector terminals. Typical applications of BJTs always assume that the BJTs will conduct collector current in one direction only.  
         [0015]    In practice, however, for certain types of BJTs, it is possible to build a bi-directional level translating circuit  300  using unconventional reverse conduction characteristics of BJT  302 . The BJT translator circuit as shown in FIG. 3 is an example of such circuit. When a logical zero (0) is asserted on the PORT 1  side  304 , the BJT  302  operates as expected with base-emitter current controlling collector current. A low emitter potential causes the base-emitter junction to be forward biased, allowing conduction of collector current determined by resistor R3 and a current gain coefficient of the transistor  302 . The current gain coefficient of the transistor is referred to as the Beta (β) of the transistor. In turn, collector current causes a voltage drop across R3, resulting in a voltage on PORT 2   306  to be recognized as a logical zero (0).  
         [0016]    However, when a logical zero (0) is asserted on the PORT 2  side  306 , the BJT  302  operates in a reverse conduction mode. In the reverse conduction mode, the base-collector junction performs functions of a base-emitter junction controlling the emitter current, instead of the collector current as in the conventional case. The magnitude of the emitter current is set by pull-up resistor R1 and a reverse current gain coefficient of the transistor (i.e., reverse Beta).  
         [0017]    According to one embodiment, the types of BJTs suitable for use in the circuit  300  of FIG. 3 are include BJTs having relatively high reverse Beta, on the order of 10 and above. This can be achieved by selecting basic low-cost BJTs without additional silicone doping used to enhance switching characteristics, such as gold doping. Examples of such transistors include Fairchild part MMBT100.  
         [0018]    Resistor values R1 and R3 are selected based on the load impedances and load capacitances, as well as, the rise times required on PORT 1   304  and PORT 2   306 . Once selected, R1 and R3 will set emitter and collector currents. Resistor R2 value is selected based on the emitter current, VCC1, Vbc, Vil_max [[ What does Vil_max stand for?—Voltage at maximum current input?]] on PORT 2 , Vol_max [[What does Vol_max stand for?—Voltage at maximum current output?]] on PORT 1  and reverse Beta at the lowest operational temperature. Since the base-collector junction is more susceptible to avalanche breakdown, it is preferable to connect PORT 2  to the part of the circuit where an electrostatic discharge (ESD) event is more likely to occur (e.g., a VGA connector).  
         [0019]    Although only a few exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the embodiments of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the embodiments of the present disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.