5  EP 1 499 941 B1 6  will be V1 nor even necessarily one of the lower voltages. The decisions about when and how much to alter the frequency and/or the voltage may be made in response to a wide variety of application demands, design con- straints, and so forth. [0017] FIG. 6 illustrates one exemplary embodiment of a system 60 according to this invention. A power supply 12 provides electrical power to a voltage regulator 14, which provides a voltage supply Vcc to a load circuit 16. The load circuit can be, for example, a microprocessor, or any other device in which it is appropriate for the in- vention to be practiced. The voltage regulator may typi- cally be adapted to provide different voltages to different components in the system, but, for ease of illustration, only the Vcc voltage to the load circuit is illustrated here. A clock generator 22 provides a clock signal CLK to the load circuit. [0018] A temperature sensor 62 measures the operat- ing temperature of the load circuit and provides a signal T indicating the present temperature. A voltage sensor 64 is coupled to measure the Vcc voltage provided to the load circuit, and to provide a present voltage signal Vnow indicating the instantaneous present voltage. In some embodiments, the temperature sensor and the voltage sensor may be constructed as one unified module per- forming both functions. [0019] A frequency responder 66 is coupled to receive the outputs Vnow and T of the voltage sensor and the temperature sensor, respectively, and, in response to them, provides a frequency control signal F to the clock generator to control the frequency of the clock signal CLK. As long as the load circuit is cool enough (e.g. below Ttest), the clock frequency can be raised above Flimit. The cooler the circuit is, the faster it can be clocked. At some point, the increased frequency will raise the tem- perature enough that the frequency must be lowered. [0020] The system also includes a voltage responder 68 which provides a voltage identification signal VID to tell the voltage regulator what Vcc voltage it should pro- vide to the load circuit. The voltage responder does this as a function of the temperature signal T from the tem- perature sensor, and as a function of the instantaneous voltage Vnow. In an explanatoryexample, it may be found desirable to operate the voltage responder according to a voltage identification Vtime which has been smoothed over time, rather than according to the instantaneous voltage Vnow itself; in such cases, a voltage integrator 70 may be included to provide this smoothing function. The smoothing allows the voltage responder to make VID changes that make better sense in the long term, rather than simply responding to a possibly wildly swinging Vnow value. GLYPH<3> [0021] An optional mode switch 72 provides a mode signal M to control the voltage responder, such that the system operates in either a high- performance mode or a low- power mode. If the mode switch has selected high- performance mode, the voltage responder will cause the operating voltage Vcc to be raised as high as reliability [0028] The invention may prove useful in operating a wide variety of synchronous load circuits, that is, those which operate according to a clock frequency input. Some such clocked devices operate synchronously with [0027] As future load circuits trend toward larger cur- rent and lower voltage, this invention becomes even more desirable because the droop will become larger as a percentage of the total supply voltage. [0026] Using this invention can, similarly, allow the us- age of lower cost power supplies and voltage regulators, as those will no longer necessarily have to provide the same degree of droop prevention or transient perform- ance that would be required without the invention. [0025] Using this invention can, in many instances, en- able the load circuit and other components to be specified for use with a power supply or voltage regulator which is assumed to be somewhat better than the worst case pow- er supply or voltage regulator; the invention will allow for the lower performance of the power supply or voltage regulator in those few cases where they are sub- par, while enabling the majority of the systems, in which the power supply or voltage regulator are performing well, to operate at a higher performance level. [0024] One scenario in which the invention may be ad- vantageous is in applications in which the load circuit has large, sudden swings in the current it draws (di/dt), which cause voltage droop at the power supply. When the load circuit suddenly increases its current draw, the supply voltage Vcc may sag below the value indicated by VID. and the frequency responder lowers the frequency to keep the load circuit within reliable operating parameters. When the current draw lessens, or when the power sup- ply catches up and provides the requested Vcc, the fre- quency responder reacts and increases the frequency to improve performance. [0023] In one explanatory example, the voltage sensor and the temperature sensor can include analog- to- digital (A/D) converters, which output multi- bit binary signals Vnow and T,  respectively. In one explanatory example, the voltage integrator and the voltage responder output multi- bit binary signals Vtime and VID, respectively. In one embodiment, the voltage responder and the frequen- cy responder can be implemented as lookup tables stored in read- only memory, for example. In one explan- atory example, the clock generator can be a digital fre- quency divider. In one explanatory example, the frequen- cy output can be produced by an analog delay element which responds to voltage and temperature in the same way the load circuit does. [0022] In the low- power mode, the voltage responder will cause the operating voltage Vcc to be lowered as low as possible while still maintaining adequate perform- ance, which will in turn force the frequency responder to lower the frequency, both of which will lower the temper- ature. limits will allow, which will in turn enable the frequency responder to cause the clock signal CLK frequency to be raised. 4