Abstract:
Termination impedance of a digital signal bus is adaptively selected as a function of a present or anticipated state of the bus. A variable termination resistor is arranged in series between a termination switch and a common voltage node at the termination end of each bus conductor. Information regarding the current or anticipated bus state is received from an external device such as a bus controller or may be derived by sensing activity on the bus. For example, clock frequency detection logic coupled to clock lines of the bus senses the current operational speed of the bus. A highest-value termination resistance predetermined to be consistent with reliable bus operation under conditions of the current or anticipated bus state is selected for each bus conductor. A bus conductor termination may be taken to a high impedance state by opening the associated termination switch. Decreased average bus power consumption may result.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/910,582 titled “Low Power-Bus Termination Scheme,” filed on Dec. 2, 2013 and incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    Structures and methods described herein relate to digital signal transmission buses such as computer and microprocessor buses, including impedance termination of such buses to reduce interference caused by reflections of bus signals from the end(s) of the bus back toward bus drivers and receivers. 
       BACKGROUND INFORMATION 
       [0003]    In today&#39;s world of portable electronic device computing, digital communication and large server farms, ever-increasing computing speed and decreased power consumption are actively pursued. Complex computing applications, sophisticated communication algorithms, and large server access loading drive the demand for computing speed. User demand for increased time between battery recharge cycles and the large energy requirements of server farms drive the pursuit of decreased power consumption. Advancing the state of the art in computing speed while reducing power consumption is difficult because the two issues are directly related. In general, increasing the clock speed of a given hardware configuration results in higher power consumption. 
         [0004]    A primary data bandwidth bottleneck and power consumption portion of current computing devices are the memory channel and other parallel buses used to access addressable devices. Such channels and buses (e.g., communication channels associated with current-art memory technologies such as double data rate (“DDR”) synchronous dynamic random-access memory (“SDRAM”)) consume a significant amount of power in the termination load associated with each address, control, and clock signal path. The latter termination loads often consist of one or more fixed termination resistors electrically connected at the receiver end of each bus signal path. The termination resistors load the bus at the receiver end to absorb high-speed bus signals that might otherwise be reflected back to receiving devices on the bus. Such reflections can interfere with the coherent reception of the bus signals at the receiving devices. 
         [0005]      FIG. 1  is a prior-art schematic diagram of a computer memory bus  105 . The computer memory bus  105  is an example of a parallel digital signal bus to which embodiments described herein are applicable. In the case of the example memory bus  105 , address and control (“ADD/CTRL”) signals are imposed on bus conductors  110  by a memory controller  115 . The memory controller  115  also transmits one or more clock signals on bus conductors  120 . The clock signals are used to clock address and control words into dynamic random access (“DRAM”) devices  125 . Addressed data words are transmitted to or retrieved from the DRAM devices  125  on a data (“DQ”) bus  130 . 
         [0006]    Address, control and clock bus conductors are typically each individually terminated at the end  135  of the bus  105  that is furthest from the memory controller  115 , at a point past the last bus device (e.g., the DRAM device  140 ). The end  135  of the bus  105  is referred to herein as the “receiver end” of the bus. Each of the bus conductors  110 ,  120  is typically terminated with a fixed-value resistor R_TERM (e.g., the resistor R_TERM  145 ). Each termination resistor R_TERM is connected between the receiver end of the corresponding bus conductor and a common regulated voltage node  150  referred to as the “voltage termination terminal” (“VTT”). The voltage level at the regulated voltage node  150  is maintained by a voltage regulator referred to as the “VTT generator”  160 . 
         [0007]    The described bus termination provides damping for each of the bus conductors  110 ,  120  to reduce bus signal reflections from the receiver ends  135  of the bus conductors as previously described. A lower resistance value of a termination resistor associated with a particular bus conductor provides greater damping as may be required for higher bus clocking frequencies. However, a lower resistance value also increases current flow through that bus conductor and overall power consumption, even when no signals are being transmitted on the bus conductor. Termination resistance values are thus typically chosen as compromises between power consumption and reliable bus operation at highest anticipated bus clocking rates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a prior-art schematic diagram of a computer memory bus. 
           [0009]      FIG. 2  is a schematic diagram of an adaptive digital bus termination apparatus according to various example embodiments of the invention, the bus termination apparatus shown in relation to an example digital memory bus. 
           [0010]      FIG. 3  is a flow diagram illustrating an example method of adaptive impedance termination of a digital signal bus by an electronic circuit according to various example activities. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0011]    Apparatus and methods disclosed herein adaptively select bus termination impedance as a function of a present or anticipated state of a digital signal bus. As used herein, the term “state of the digital signal bus,” “state of the bus” and “bus state” shall mean a frequency of a clock signal used to clock bus signals into or out of a device connected to the bus and/or bus signal activity on one or more bus conductors (also referred to herein as “bus lines”). A state of bus signal activity may be associated with a bus sleep state or power-down state. Bus signal states may also be associated with one or more portions of the bus or the entire bus being selected by a bus controller (e.g., a memory controller in the example case of a computer memory bus as described herein). The term “bus state information” shall mean information relating to a bus state. Bus state information may be sent across a communication channel from a controller to some embodiments of the apparatus disclosed herein. In some embodiments, bus state information may be derived by one or more components of the disclosed apparatus and sent to other portions of the disclosed apparatus. 
         [0012]    A variable termination resistor is arranged in series between a termination switch and a common voltage node at the termination end of each bus conductor. It is noted that the term “variable termination resistor” as used herein shall mean either a single resistive element whose value may be varied or a group of resistive elements arranged in a series and/or parallel network from which one or more elements may be selected to vary the resistance value of the termination resistor. Embodiments herein select a resistance value and an open or closed state of the termination switch associated with the corresponding bus conductor to control the termination impedance of that bus conductor. A bus conductor termination may be taken to a high impedance state by opening the associated termination switch. With the termination switch in a closed state, termination loading for a particular bus conductor may be increased by selecting a lower-value termination resistor. Doing so may enable the successful transmission and reception of bus signals on that conductor at higher speeds but also increases current flow through the conductor and increases power consumption. Thus, embodiments herein enable tuning of a digital signal bus for lowest power consumption consistent with reliable bus operation. 
         [0013]    Some embodiments process current or anticipated bus state information to make bus conductor termination impedance decisions. The bus state information may be sent to the disclosed apparatus by a device external to the disclosed apparatus such as a bus controller. The bus state information may be sent across one or more lines of the bus to be terminated or across a special-purpose termination bus linking the controller to the disclosed apparatus. Alternatively or additionally, some embodiments may act upon bus state information sensed directly from the digital signal bus by the disclosed apparatus. For example, some embodiments may include clock frequency detection logic coupled to clock lines of the bus. The clock frequency detection logic may sense the current operational speed of the bus and send such bus state information to other logic within the disclosed apparatus to use in making bus termination impedance decisions. Alternatively or additionally, some embodiments may simply act upon commands to set particular termination resistances for one or more bus conductors. Such commands may be received from a device external to the disclosed apparatus, such as a bus controller, or may be internally generated by a bus state sensing device such as the afore-mentioned clock frequency detection logic. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 2  is a schematic diagram of an adaptive digital bus termination apparatus  200  according to various example embodiments of the invention. The bus termination apparatus  200  is shown in relation to an example digital memory (DRAM) bus  105  as previously described. It is noted, however, that structures and methods described herein apply to many types of digital signal buses and that a DRAM bus is merely an example of a digital signal bus. Accordingly, the terms “DRAM bus  105 ,” “digital memory bus  105 ,” and “digital signal bus  105 ” are used interchangeably herein. 
         [0015]    The adaptive bus termination apparatus  200  includes a plurality of termination switches  210  (e.g., the switches  210 A,  210 B,  210 C, and  210 D). Each termination switch  210  is to be singly coupled in series with a bus signal conductor (e.g., with each of the ADD/CTRL signal conductors  110  and with each of the bus clock signal conductors  120 ). Each termination switch  210  is coupled to a corresponding bus conductor at a receiver end  135  furthest from a driver end of the digital signal bus  105 . Each termination switch  210  is configured to leave the corresponding bus signal conductor electrically open at the receiver end  135  of the bus  105  when the termination switch  210  is open. 
         [0016]    The adaptive bus termination apparatus  200  also includes a plurality of variable termination resistors  215  (e.g., the variable termination resistors  215 A,  215 B,  215 C, and  215 D). Each variable termination resistor  215  is coupled in series between a termination terminal  220  (e.g., the termination terminals  220 A,  220 B,  220 C, and  220 D) of a corresponding termination switch  210  and a common voltage node  150 . The termination terminal  220  of the termination switch  210  is electrically connected to the receiver end  135  of the bus  105  only when the termination switch  210  is closed. It is noted that each variable termination resistor  215  may consist of a switched serial and/or parallel combination of a plurality of fixed-value resistors. 
         [0017]    The adaptive bus termination apparatus  200  further includes a termination logic module  225  coupled to the plurality of termination switches  210  and to the plurality of variable termination resistors  215 . The termination logic module  225  receives bus state information and/or one or more bus termination commands as described below. The termination logic module  225  uses the bus state information and/or the bus termination commands to select a termination resistance for each bus signal conductor  110 ,  120 . The module  225  selects the highest termination resistance for each bus signal conductor  110 ,  120  predetermined to be consistent with reliable operation of the digital signal bus  105  for a bus state indicated by the bus state information or the bus termination commands. A bus conductor termination resistance is selected by setting a state of each termination switch  210  and a value of each variable termination resistor  215 . 
         [0018]    In some embodiments, the adaptive bus termination apparatus  200  also includes a VTT generator  160  coupled to the common voltage node  150 . The VTT generator  150  is a voltage regulator and maintains a constant voltage level at the common voltage node  150 . 
         [0019]    The adaptive digital signal bus termination apparatus  200  also includes clock frequency detection logic  235  coupled to the termination logic module  225 . The clock frequency detection logic  235  is to be coupled to one or more of the bus clock signal conductors  120 . The clock frequency detection logic  235  receives one or more bus clock signals, senses a frequency of the bus clock signal(s), and outputs an indication of the present frequency of operation of the digital signal bus  105  or a portion thereof to the termination logic module as the bus state information. 
         [0020]    The adaptive digital signal bus termination apparatus  200  further includes one or more termination control bus input terminal(s)  240  associated with the termination logic module  225 . The termination control bus input terminals  240  are to couple to a termination control bus  243 . The input terminals  240  receive the bus state information or the bus termination command from a controller  115  at the driver end  135  of the digital signal bus  105 . The bus state information and/or termination impedance commands are transferred across the termination control bus  243  and are used by the termination logic module  225  to set a state of each termination switch  210  and a resistance value of each variable termination resistor  215 . 
         [0021]    The adaptive digital signal bus termination apparatus  200  also includes a bus command decoder  250  coupled to the termination logic module  225 . The bus command decoder  250  is also to be coupled to a portion of the digital signal bus  105  or to the entirety thereof. The bus command decoder  250  receives bus state information, one or more bus termination commands, and/or one or more termination resistor calibration commands. The bus state information and/or commands are transmitted across the digital signal bus  105  from a controller  115  configured at the driver end of the digital signal bus  105 . The bus command decoder  250  passes the bus state information and/or the bus termination commands to the termination logic module  225 . 
         [0022]    Some embodiments of the adaptive digital signal bus termination apparatus  200  also include a calibration logic module  260  coupled to the bus command decoder  250  and to each of the variable termination resistors  215 . The calibration logic module  260  receives termination resistor calibration commands from the controller  115 . The calibration logic module  260  compares resistances values of the variable termination resistors  215  to a resistance value of a precision calibration resistor  265  configured externally to the adaptive digital signal bus termination apparatus  200 . The calibration logic module  260  calibrates the resistance values of the variable termination resistors  215  to adjust for temperature, voltage, and/or process induced variations of the resistance values of the variable termination resistors  215  from design values. 
         [0023]    It is noted that some embodiments of the adaptive digital signal bus termination apparatus  200  may include the elements applicable to a particular embodiment such as the termination switches  210 , the variable termination resistors  215 , the termination logic module  225 , the clock frequency detection logic  235 , the termination control bus input terminal  240 , the bus command decoder  250 , the calibration logic  260 , and the VTT generator  160  in a single integrated circuit package. 
         [0024]      FIG. 3  is a flow diagram illustrating an example method  300  of adaptive impedance termination of a digital signal bus by an electronic circuit according to various example activities. The method  300  includes receiving bus state information or a bus termination command, determining the state of the digital signal bus, and selecting an appropriate bus termination resistance for each bus conductor as further described in detail below. The “state of the digital signal bus” means a frequency of clock signals used to clock address and control signals into at least one receiving device coupled to the digital signal bus or bus signal activity on one or more of the bus signal conductors, as previously defined. Examples of bus states include an active bus state associated with data being transmitted across the digital signal bus, a deselected bus state during which no data is transmitted across the digital signal bus while address and control bus signal conductors are actively driven, and a power-down bus state. 
         [0025]    The method  300  commences at block  305 A with receiving bus state information, one or more bus termination commands, or both, from a controller configured at the driver end of the digital signal bus. The bus state information and/or bus termination commands may be sent by the controller across the digital signal bus itself or across a specialized termination control bus. Alternatively or additionally, the method  300  may include receiving bus clock signals, at block  305 B. The method  300  may also include determining bus state information as the frequency of operation of one or more portions of the digital signal bus from transitions of the bus clock signals, at block  305 C. 
         [0026]    The method  300  continues at block  310  with determining a state of the digital signal bus from the bus state information and/or from the bus termination commands. For each of a plurality of bus signal conductors associated with the digital signal bus, the method  300  includes choosing a highest termination resistance predetermined to supply sufficient termination load to the bus signal conductor for reliable operation of the digital signal bus consistent with the state of the digital signal bus, at block  315 A. Some variations of the method  300  may include indexing a look-up table using the state of the digital signal bus as an index value, at block  315 B. The method  300  may also include choosing the highest termination resistance based upon a value returned from the look-up table in response to the indexing operation, at block  315 C. 
         [0027]    The method  300  continues at block  320  with generating a regulated VTT voltage at a common voltage node. The method  300  includes selecting a state of a termination switch singly coupled in series with the bus signal conductor at a receiver end of the digital signal bus, selecting a value of a variable termination resistor coupled in series between a termination terminal of the termination switch and a common voltage node, or both to select the highest termination resistance, at block  325 A. The method  300  also includes opening a switch coupled in series with the termination end of the bus signal conductor to select an open circuit as the highest termination resistance, at block  325 B. 
         [0028]    Apparatus and methods described herein may be useful in applications other than adaptive digital signal bus impedance termination. The apparatus  200  and the method  300  are intended to provide a general understanding of the sequences of various methods and the structures of various embodiments. They are not intended to serve as complete descriptions of all elements and features of methods, apparatus and systems that might make use of these example sequences and structures. 
         [0029]    The various embodiments may be incorporated into semiconductor analog and digital circuits for use in receptacle power converters, electronic circuitry used in computers, communication and signal processing circuitry, single-processor or multi-processor modules, single or multiple embedded processors, multi-core processors, data switches, and application-specific modules including multi-layer, multi-chip modules, among others. Such apparatus and systems may further be included as sub-components within a variety of electronic systems such as robotics, medical devices (e.g., heart monitor, blood pressure monitor, etc.), motor vehicles, televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., MP3 (Motion Picture Experts Group, Audio Layer 3) players), set top boxes, household appliances and others. 
         [0030]    Methods and structures disclosed herein act upon derived or externally received digital signal bus state information by adaptively selecting, for each bus conductor, a highest termination resistance predetermined to be consistent with reliable bus operation of a digital signal bus in a particular state as determined from the received bus state information. Power is conserved by increasing bus conductor termination resistance and thus decreasing bus conductor current at times when bus operation frequencies are decreased, one or more portions of the bus are quiesced, etc. Portable electronic device users may experience increased time between battery charges as a result. Operators of large server farms may incur lower energy costs. 
         [0031]    By way of illustration and not of limitation, the accompanying figures show specific aspects in which the subject matter may be practiced. It is noted that arrows at one or both ends of connecting lines are intended to show the general direction of electrical current flow, data flow, logic flow, etc. Connector line arrows are not intended to limit such flows to a particular direction such as to preclude any flow in an opposite direction. The aspects illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other aspects may be used and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense. The breadth of various aspects is defined by the appended claims and the full range of equivalents to which such claims are entitled. 
         [0032]    Such aspects of the inventive subject matter may be referred to herein individually or collectively by the term “invention” merely for convenience and without intending to voluntarily limit this application to any single invention or inventive concept, if more than one is in fact disclosed. Thus, although specific aspects have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific aspects shown. This disclosure is intended to cover any and all adaptations or variations of various aspects. 
         [0033]    The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b) requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In the preceding Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted to require more features than are expressly recited in each claim. Rather, inventive subject matter may be found in less than all features of a single disclosed embodiment. The following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.