FIELD OF THE INVENTION  
         [0001]    The invention is generally related to very large scale integrated circuits. More particularly, the invention is related to compensating for voltage transients in very large scale integrated circuits.  
         BACKGROUND OF THE INVENTION  
         [0002]    As silicon technology is scaled down in integrated circuit (“IC”) design, the voltage at which the integrated circuit operates is also reduced. However, power consumption tends to increase for the scaled down ICs, increasing the current going through the power supply and the power delivery network.  
           [0003]    Because of this large amount of current in integrated circuits, such as, for example, very large scale integrated (“VLSI”) circuits used for microprocessor design, a large transient may occur in the power supply network due to switching events and instantaneous changes in the current function. This change in the current may cause the voltage to vary by a large percentage of the supply. A reduction in the operating voltage due to the change in current is known as a “voltage droop”. Voltage droops may cause delays in circuit operation.  
           [0004]    Traditionally, processors and most VLSI circuits operate at a fixed frequency, such as, for example, 1 GHz. Because of the frequency is fixed, the VLSI circuits should maintain the frequency of operation for the lowest voltage point that may be seen in the circuit. Thus, a voltage droop may require a VLSI circuit to operate at lower frequency than it could support if the frequency were based on the average voltage of operation.  
         SUMMARY OF THE INVENTION  
         [0005]    A method for compensating for voltage droop in an integrated circuit is described. The method may include detecting a voltage droop in an integrated circuit driven by a clock signal and determining an optimum frequency change to compensate for the voltage droop. The method may further include adapting cycle time of the clock signal in an incremental manner to achieve the optimum frequency change.  
           [0006]    An integrated circuit having voltage droop compensation capability is also described. The integrated circuit may include a plurality of chip circuits, a clock control system, a clock distribution network including at least one delay element and a voltage droop detector. In one embodiment, the clock control system may adapt cycle time in the clock distribution network through use of the at least one delay element when a voltage droop is detected. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention is illustrated by way of example and not limitation in the accompanying figures in which like numeral references refer to like elements, and wherein:  
         [0008]    [0008]FIG. 1 is a schematic diagram illustrating an exemplary embodiment of a clock distribution system;  
         [0009]    [0009]FIG. 2 is a circuit diagram illustrating an exemplary embodiment of a clock distribution circuit employing principles of an embodiment of the invention;  
         [0010]    [0010]FIG. 3 is a circuit diagram illustrating one embodiment of a droop indicator for use with the clock distribution circuit of FIG. 2;  
         [0011]    [0011]FIG. 4 is a block diagram illustrating one embodiment of a system for compensating for voltage droop in a VLSI circuit;  
         [0012]    [0012]FIG. 5 is a flow diagram of one embodiment of a process for adapting a VLSI clocking circuit to a voltage droop; and  
         [0013]    [0013]FIG. 6 is a graphic representation of the operation of an exemplary embodiment of the clock distribution circuit adapting in response to a voltage droop.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that these specific details need not be used to practice the invention. In other instances, well known structures, interfaces, and processes have not been shown in detail in order not to obscure unnecessarily the invention.  
         [0015]    [0015]FIG. 1 is a schematic diagram illustrating an exemplary embodiment of a clock distribution system. The clock distribution system  100  is a typical clock distribution system for a VLSI circuit. The system  100  may include a central primary buffer  101 , at least one second level control buffer (“SLCB”) 102 , and quadrant repeaters  103 .  
         [0016]    This figure indicates the complexity and span of an advanced microprocessor clock distribution. The buffer  101  initiates the clock distribution which proceeds along matched delay paths to the four quadrant repeaters  103 . Each of these quadrant repeaters  103  includes a matched route out to a number of SLCBs  102  which provide the final level of buffering before contact with the local clock gating circuits. The span across this distribution is approximately 14 mm×18 mm. This is enough distance to accumulate large voltage differentials across a VLSI chip with transfer times of at least a nanosecond, which is larger than the clock cycle for high speed microprocessors.  
         [0017]    [0017]FIG. 2 is a circuit diagram illustrating an exemplary embodiment of a clock distribution circuit  200  employing principles of an embodiment of the invention. The clock distribution circuit  200  may receive a clock signal  220  as an input. The clock signal  220  is received by core primary driver  221  which transmits the clock signal to repeaters  203 . The repeaters  203  transmit the clock signal to SLCBs  202 . Although three SLCBs  202  are shown, the clock distribution circuit may include any number of SLCBs.  
         [0018]    Each SLCB  202  may include at least one controllable delay element (not shown). In one embodiment, the controllable delay element(s) of the SLCB  202  may provide delays in the range of 0 to 1 cycle. Each SLCB  202  is coupled to a circuitry using the clock signal  220 . The circuitry using the clock signal may include gates  225  and latches  227 . The clock signal  220  is also forwarded to a phase locked loop  229 , where the signal is taken as the clock feedback, enabling the elimination of the distribution delay from the clock phase.  
         [0019]    [0019]FIG. 3 is a circuit diagram illustrating one embodiment of a droop indicator  300  for use with the clock distribution circuit of FIG. 2. In one embodiment, the droop indicator  300  may detect voltage droop by monitoring the time relative changes of a voltage of operation of the integrated circuit  100 . The droop indicator  300  may include an operational amplifier  335  to determine when a voltage droop occurs.  
         [0020]    In one embodiment, the operational amplifier  335  may be a subtracting op amp having a positive input and a negative input. In the example shown, the operational amplifier  335  receives a voltage signal from a local supply as the first (positive) input  331  and a voltage signal from a reference voltage source as the second (negative) input  333 . The output  337  of the operational amplifier  335  may indicate the polarity of a comparison of the first input  331  and the second input  333  (i.e. a high output may indicate input  331  is at a greater voltage potential than input  333 , and a low output may indicate the reverse).  
         [0021]    In one embodiment, these voltage droop indicator(s)  300  may be placed around the chip in sufficient quantity to quickly and accurately detect voltage droops that may originate from any circuits on the chip. Since SLCB 102 placements have already been arranged on the chip for the clock distribution and there are typically a large number (32 in FIG. 1), a voltage droop indicator may be included along with each SLCB in one embodiment.  
         [0022]    [0022]FIG. 4 is a block diagram illustrating one embodiment of a VLSI clocking adapting system  400 . Adapting system  400  may include a droop indicator  402 , a control system  404  and at least one delay element  406 . The droop indicator  402  may be similar to the droop indicator  300  described above. The control system  404  may be similar to or include the control system for clock distribution circuit  200 . Delay element(s)  406  may be or include a switched capacitor element, a current starved inverter, a switchable delay element or any other appropriate delay element that may be used with VLSI clock distribution circuit  200 .  
         [0023]    The droop indicator  402  may detect a droop in the operating voltage of clock distribution circuit  200 . The control system  404  may receive the indication of a voltage droop from the droop indicator  402 . The control system may determine the optimum frequency to compensate for the voltage droop, and then activate one or more delay elements  406  as described below with respect to processing block  530  of FIG. 5.  
         [0024]    [0024]FIG. 5 is a flow diagram of one embodiment of a process for adapting a VLSI clocking circuit to a voltage droop. At processing block  510 , the droop indicator  300 ,  402  detects a voltage droop as described above with reference to FIG. 3. Since each droop indicator  300 ,  402  is associated with a SLCB  202 , the following process will be performed with reference to the SLCB  202  with which the droop indicator  300 , 402  indicating the droop is associated.  
         [0025]    At processing block  520 , the control system  404  determines the optimum frequency for the SLCB  202  to operate as a result of the voltage droop. In one embodiment, the control system  404  may determine the optimum frequency change for the clock signal to compensate for the voltage droop.  
         [0026]    At processing block  530 , the control system  404  adapts the cycle time of the clock signal through the SLCB  202  associated with the droop indicator  300 ,  402  indicating the voltage droop. In one embodiment, the control system  404  uses delay elements  406  to implement changes in cycle time. For example, the control system  404  may add delays to the cycle time using delay elements  406  so that the cycle time of the clock signal through the clock distribution network is increased, thereby producing a temporary frequency reduction.  
         [0027]    In one embodiment, the control system  404  may implement the change in frequency incrementally. For example, the control system  404  may increase the cycle time for all chip circuits in a progressive manner using a large range delay line including delay elements  406 . Thus, if the frequency is to be decreased by 1%, the cycle times may be increased by 1%, then 2%, then 3%, etc. to effect the desired decrease in frequency. In one embodiment, the control system  404  may determine the number of cycles the delay lasts based on at least one of the amount of time the voltage droop lasts and the amount of time needed by the phase locked loop (“PLL”), or the source of the clock frequency, to respond to a request to reduce frequency. Thus, in this embodiment, either the voltage transient or droop goes away before the delay line range is consumed, or the clock system adjusts the actual clock frequency. The temporary decrease in frequency allows the PLL to have enough time to adjust the actual clock frequency if the voltage droop does not go away.  
         [0028]    [0028]FIG. 6 is a graphic representation of the operation of an exemplary embodiment of the clock distribution circuit adapting in response to a voltage droop. At time  642 , a voltage droop is detected by droop indicator  300 ,  402  associated with a SLCB  202 .  
         [0029]    As shown by signal  644 , the normal clock signal at this time has a period of T. When the control system  404  receives indication of the voltage droop at time  642 , the control system  404  determines an optimum frequency change. The control system  404  implements the optimum frequency change by progressively increasing the clock cycle time distributed by the SLCB  202  with which the voltage droop indicator  300 ,  402  indicating the voltage droop  642  is associated, as shown by signal  646 . For example, the clock system adds a delay of ΔT to the first cycle after the droop is detected. The clock system then continues to add a delay of 2ΔT to the second cycle, and a delay of 3ΔT to the third cycle.  
         [0030]    While this invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. These changes may be made without departing from the spirit and scope of the invention.
A system and method of compensating for voltage droop in an integrated circuit. The integrated circuit may include a plurality of chip circuits, a clock control system, a clock distribution network including at least one delay element and a voltage droop detector. The clock control system adapts cycle time in the clock distribution network through use of the at least one delay element when a voltage droop is detected. The method may include detecting a voltage droop in an integrated circuit where the integrated circuit is driven by a clock signal, determining an optimum frequency change to compensate for the voltage droop, and adapting cycle time of the clock signal in an incremental manner to achieve the optimum frequency change.