Abstract:
A bus has two power consumption modes. A variable bus termination impedance is controlled to provide different bus termination impedances. A controller is coupled to the bus and includes a variable clock having different frequencies that are selectively provided to the controller. The impedance is increased or decreased responsive to the frequency being provided to the controller.

Description:
BACKGROUND  
       [0001]     High speed input/output (I/O) busses cause considerable power to be consumed by I/O buffers in dynamic random access memory (DRAM) chips. Lower power complimentary metal oxide semiconductor (CMOS) busses have not supported the high edge rates used at higher frequencies, such as 400 MHz or 800 MHz data rates currently in use. Such busses have been modified such that they no longer switch rail to rail, causing an increase in current drain, and hence heat generation. Thevenin terminations on the I/O bus have been used to maintain fast edge rates. Such terminations reference the bus output to a low impedance, but cause excess power draw when lower performance is expected from the bus and memory chip.  
         [0002]     Prior methods of power reduction have been suggested for thermal control in memory devices. One simple method involves lowering the speed of the synchronous or clocked bus. This method does relieve thermal stress by lowering the power consumption rate of the system, but it only acts to delay the overall power consumption by delaying fetches from the memory device.  
         [0003]     Another prior method involves momentarily disabling the memory device itself. This is referred to as bandwidth throttling, and acts to periodically disable the memory device in response to either sensed or perceived high temperatures. Bandwidth throttling has also been used in response to an activity detector detecting too much traffic on the bus. Bandwidth throttling reduces potential performance obtained from the memory device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a block diagram of a system including a high speed bus according to an example embodiment of the invention.  
         [0005]      FIG. 2  is a more detailed block diagram of a system including a high speed bus according to an example embodiment of the invention.  
         [0006]      FIG. 3  is a flowchart diagram of an algorithm for placing a bus into a high power mode of operation according to an example embodiment of the invention.  
         [0007]      FIG. 4  is a flowchart diagram of an algorithm for placing a bus into a low power mode of operation according to an example embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0008]     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the described subject matter may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the described subject matter, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the described subject matter. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.  
         [0009]     The functions or algorithms described herein are implemented in software or a combination of software and human implemented procedures in one embodiment. The software comprises computer executable instructions stored on computer readable media such as memory or other type of storage devices. The term “computer readable media” is also used to represent carrier waves on which the software is transmitted. Further, such functions correspond to modules, which are software, hardware, firmware or any combination thereof. Multiple functions are performed in one or more modules as desired, and the embodiments described are merely examples. The software is executed on a digital signal processor, ASIC, microprocessor, or other type of processor operating on a computer system, such as a personal computer, server or other computer system.  
         [0010]     An optimal power configuration is realized by selectively decreasing the power consumed by the I/O of a high speed bus device. The bus may be synchronous or asynchronous. Short transfer times of high speed busses, such as those operating in the gigahertz range use aggressive edge rates. The output drive of a CMOS gate is increased to drive a faster edge rate against a parasitic capacitance (Cp) of the bus. Since the conductor of the bus itself becomes inductive, the high drive strength alone would cause signal quality problems. Thevenin terminations may be used on the bus itself to match the signal impedance and limit ringing of the fast signals.  
         [0011]      FIG. 1  is a block diagram of a system  100  including a synchronous bus  105  according to an example embodiment of the invention. Bus  105 , in one embodiment is a synchronous bus that implements a SSTL (Series Stub Terminated Logic) signaling method. Bus  105  has a variable value termination resistor  110  coupled to it. Bus  105  is used to transmit signals between a controller  115  and a bus device  120 . Bus device  120  comprises a driver  125 , such as a CMOS dual gate FET. Multiple selectable stages of the driver may be used to vary the output power of the driver. The driver  125  is coupled to the bus  105  via a variable drive resistor  130 .  
         [0012]     A variable clock  135  is coupled to the controller  115 . Clock  135  in one embodiment has two different frequencies, which may be referred to as a high setting and a low setting. The high setting provides a higher frequency, such as 533 MHz, and the low setting provides a lower frequency, such as 400 MHz. The controller also controls the values of the variable termination resistor  110  and the variable drive resistor  130  depending on whether the frequency is high or low.  
         [0013]     In the high setting, the signaling on the bus utilizes high static power. Current is continuously sourced to maintain a logic high or “1”, and constantly sunk to maintain a logic low or “0”. The current passed from a termination voltage  140 , referred to as VTT may be significant. Since some busses are now 64 to 128 bits wide, the amount of power used in output termination can create several watts of power consumption.  
         [0014]     To reduce the power consumption when lower performance may be appropriate, the clock is set to the low frequency, resulting in a decrease in the number of switching times on the bus. This may be done by controller  115  in one embodiment in response to user commands, or sensed or expected utilization of the bus. The output drive of the bus device  120  may also be decreased to a lower level. This decreases both the AC and DC drive current on the bus. Optionally, the bus terminations themselves are changed or even removed so as to remove the DC load component.  
         [0015]     When the bus device is a DRAM, during normal data reads, the DRAM device outputs data to the memory controller in high power mode. The clock is fast. In low power mode, the memory controller is slowed down by the clock, easing the timing expectations on the bus. The DRAM output drive can be decreased, and the termination loading reduced or eliminated.  
         [0016]     In one embodiment, the bus is bidirectional. During writes to DRAM, data flows from the controller to the DRAM. The memory controller drive current may also be reduced as long as the clocking speed is slow.  
         [0017]     The lower power mode may be dynamically and reversibly applied. Typical stimulus for changing the power consumption of the bus include a manual power conservation being asserted. For example, in a notebook computer, the bus may be set up for high power, high performance while coupled to AC power. Lower performance may be utilized when the notebook computer is operating off of battery power.  
         [0018]     Power conservation may be applied in the event that a computer system incorporating the bus and multiple devices is detected as overheating. For example, DRAM may be added to the bus in a fairly dense manner, with little provision made for proper conduction of heat. This may put an entire computer system at risk. Temperature sensors may be used to identify such risk, and apply power conservation by implementing the low power mode. Power conservation may also be applied when little bus traffic is observed.  
         [0019]      FIG. 2  is a block diagram of an example computer system  200  incorporating an embodiment of the invention. System  200  has a central processing unit CPU  205  coupled by a front side bus  210  to a bridge device  215 . In one embodiment, bridge device  215  is a North bridge, and interfaces a controller  220 , such as a memory controller to CPU  205 . In some embodiments, the memory controller  220  is integrated into the CPU itself.  
         [0020]     The controller  220  is coupled via an interface driver  225  and bus  230  to a bus device, such as dynamic random access memory (DRAM)  235 . In one example embodiment, the DRAM  235  is a Micron Inc. DDR-2 DRAM. The DDR-2 DRAM has “on die termination,” with 2 possible termination resistor networks indicated at  240  and  245 . Each resistor is approximately 150 ohms in one embodiment. A pair of switches indicated at  250  and  255 , when closed, provides a bus termination that is effectively 75 ohms terminated to Vss (ground) and to VDDQ (the operational voltage of the DRAM). When switches  250  and  255  are closed, the resistor network  240  is coupled to the bus, effectively doubling the termination resistance in one embodiment. In further embodiments, the resistor values are other than equal.  
         [0021]     The operating speed of the memory controller  220  is set by a clock  260  and third switch  265  to either approximately 400 mhz or 533 mhz. These frequencies may be significantly varied for different types of devices and busses. Future busses are likely to have even higher operating frequencies available. The operating frequency is related to the speed of switching utilized for high and low states on the bus. Higher frequencies use faster switching.  
         [0022]     In further embodiments, an algorithm  270  provides outputs that are used to control the frequency selection switch  265 , and resistor network switches  250  and  255 . The algorithm may be implemented on many different logic platforms, including CPU  205  or controller  220 . The algorithm may also be used to control a further switch  273  that is used to include a second stage in interface driver  225 .  
         [0023]     A counter  275  may be used to monitor bus traffic, and provide indications of the amount of bus traffic to the algorithm for use in determining an appropriate frequency of operation. Further, a thermistor  280  is optionally placed proximate the bus device  235  to monitor operating temperatures and to potentially slow down the operating frequency to reduce power consumption and correspondingly heat generation. The thermistor  280  provides information about the operating temperature to the algorithm for use in determining the frequency of operation if desired. A further input may be provided externally to the algorithm for directly selecting the frequency of operation, such as by direct user input or other power management algorithms.  
         [0024]      FIG. 3  is a flow chart example  310  of algorithm  270  configuring the system for a high performance mode. At  320 , the operating speed of the memory controller  220  is at its fast setting. Switch  265  is set to the fast clock speed of approximately 533 MHz. At  330 , the memory controller interface driver  225  is enabled for the highest possible output drive by closing switch  273  to engage parallel transistors. At  340 , the Thevenin termination inside the DRAM  235  is set to the highest current drive by closing switches  250  and  255 . This may be done in one embodiment by setting appropriate bits in an extended mode register of the DRAM to obtain a termination resistance of 75 ohms, or other value depending on the values of resistors used. At  350 , the output drive of the DRAM may also be set to maximum by setting a corresponding bit in the extended mode register of the DRAM.  
         [0025]      FIG. 4  is a flow chart  400  showing operation of the algorithm  270  in configuring the system  200  for low power mode. At  410 , switch  265  is set to the lower frequency, 400 MHz. At  420 , switch  273  is opened, decreasing the output drive of the interface driver  225  by removing one of the parallel transistor stages. At  430 , switches  250  and  255  are opened, increasing the termination source to a 150 ohm load in one embodiment. At  440 , the DRAM output drive is optionally set to low by removing one or more of the drive stages.  
         [0026]     The low power mode may be entered as a function of many different factors. The device  235  temperatures may be too high. Thermister  280  provides an indication, such as a signal representative of temperature which may be used to determine the device is at an unsafe temperature. The algorithm  270  receives information from the thermister  280  and configures the system  200  for low power mode in accordance with the flow chart  400 .  
         [0027]     The amount of activity on the bus from the device, such as DRAM  235  may be either very high or very low. Counter  275  measures the amount of DRAM activity over a duration. If the amount of activity is very high, then DRAM overheating is suggested, and the algorithm may determine to configure the system for low power mode. If the amount of activity is very low, the algorithm may determine that the system may operate at lower than high performance. If so, the algorithm may place the system in the low power mode to lower performance. The system then becomes more efficient in power consumption without sacrificing overall system performance. This aspect of the algorithm may be useful in large systems with multiple bus devices that are all generating heat. If all such devices were operating in high performance mode, more heat may be generated than can be effectively handled by the system.  
         [0028]     Various manual settings may indicate a desire for a power conservation mode as indicated at  285 . Reasons may include detection of battery versus AC line power, lower CPU speed settings, acoustic requirements where less cooling fan noise is desired, or detection of other manual power conservation settings.  
         [0029]     In some instances, the algorithm  270  may decide that the system temperature is very low and can “margin” the timing. When silicon is cool, it will inherently operate faster. The algorithm receives information, such as by reading the thermistor  280  and determines that the termination strength can be reduced—without reducing the speed—of the memory controller. In other words, the Algorithm may decide that the system has adequate margin and may use something other than the high output drive to work reliably. The algorithm will then open switches  273  to decrease the drive, and open switches  250  and  255  to decrease the termination strength. The drive strength of the device may also be decreased by the algorithm via the extended mode register. The decrease may be accomplished in the same manner as the decrease in drive strength of the controller interface driver  225 .