WO2011046981A2

WO2011046981A2 - Architecture for frequency-scaled operation in resonant clock distribution networks

Info

Publication number: WO2011046981A2
Application number: PCT/US2010/052397
Authority: WO
Inventors: Marios C. Papaefthymiou; Alexander Ishii
Original assignee: Cyclos Semiconductor, Inc.
Priority date: 2009-10-12
Filing date: 2010-10-12
Publication date: 2011-04-21
Also published as: WO2011046977A2; US8502569B2; WO2011046977A3; WO2011046974A2; US20140015585A1; US20110090019A1; WO2011046984A3; JP2013507888A; WO2011046984A2; US8659338B2; KR20120095393A; US20130194018A1; WO2011046987A2; WO2011046980A3; US20110084774A1; WO2011046980A2; WO2011046974A3; WO2011046979A2; US20110140753A1; WO2011046987A3

Abstract

An architecture for resonant clock distribution networks is proposed. This architecture allows for the energy-efficient operation of a resonant clock distribution network at multiple clock frequencies through the deployment of flip-flops that can be selectively enabled. The proposed architecture is primarily targeted at the design of resonant clock networks with integrated inductors and exhibits no inductor overheads. Such an architecture is generally applicable to semiconductor devices with multiple clock frequencies, and high-performance and low-power clocking requirements such as microprocessors, ASICs, and SOCs. Moreover, it is applicable to the binning of semiconductor devices according to achievable performance levels.

Description

ARCHITECTURE FOR FREQUENCY-SCALED OPERATION IN RESONANT CLOCK DISTRIBUTION NETWORKS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is a conversion of and claims priority to U.S. Provisional Patent Application No. 61/250,830, entitled SYSTEMS AND METHODS FOR RESONANT CLOCKING INTEGRATED CIRCUITS, filed October 12, 2009, which is incorporated herein in its entirety. This patent application is related to the technologies described in the following patents and applications, all of which are incorporated herein in their entireties:

U.S. Patent Application No. 12/125,009, entitled RESONANT CLOCK AND INTERCONNECT ARCHITECTURE FOR DIGITAL DEVICES WITH MULTIPLE CLOCK NETWORKS, filed October 12, 2009, which claims priority to U.S. Provisional Patent Application No. 60/931,582, entitled Resonant Clock and Interconnect Architecture for Programmable Logic Devices, filed May 23, 2007;

U.S. Patent Application No. , entitled RESONANT CLOCK DISTRIBUTION

NETWORK ARCHITECTURE WITH PROGRAMMABLE DRIVERS, filed concurrently herewith;

U.S. Patent Application No. , entitled Architecture FOR CONTROLLING

CLOCK CHARACTERISTICS, filed concurrently herewith;

U.S. Patent Application No. , entitled METHOD FOR SELECTING NATURAL

FREQUENCY IN RESONANT CLOCK DISTRIBUTION NETWORKS WITH NO INDUCTOR OVERHEAD, filed concurrently herewith;

U.S. Patent Application No. , entitled ARCHITECTURE FOR ADJUSTING

NATURAL FREQUENCY IN RESONANT CLOCK DISTRIBUTION NETWORKS, filed concurrently herewith;

₅6₅67-8000/LEGALl 9380954.1 1 U.S. Patent Application No. , entitled ARCHITECTURE FOR SINGLE- STEPPING IN RESONANT CLOCK DISTRIBUTION NETWORKS, filed concurrently herewith;

U.S. Patent Application No. , entitled ARCHITECTURE FOR OPERATING

RESONANT CLOCK NETWORK IN CONVENTIONAL MODE, filed concurrently herewith; and

U.S. Patent Application No. , entitled Resonant Clock Distribution

Network Architecture for Tracking Parameter Variations in Conventional Clock Distribution Networks filed concurrently herewith.

FIELD OF INVENTION

[0002] This disclosure relates generally to clock distribution network architectures for digital devices with multiple clock networks and various clock frequencies such as microprocessors, application-specific integrated circuits (ASICs), and System-on-a-Chip (SOC) devices.

BACKGROUND OF THE INVENTION

[0003] Resonant clock distribution networks have been proposed for the energy- efficient distribution of clock signals in synchronous digital systems. In these networks, energy-efficient operation is achieved using one or more inductors to resonate the parasitic capacitance of the clock distribution network. Clock distribution with extremely low jitter is achieved through the reduction in the number of clock buffers. Moreover, extremely low skew is achieved among the distributed clock signals through the design of relatively symmetric all-metal distribution networks. Overall network performance depends on operating speed and total network inductance, resistance, size, and topology, with lower- resistance symmetric networks resulting in lower jitter, skew, and energy consumption when designed with adequate inductance.

₅6₅67-8000/LEGALl 93809₅4.1 2 [0004] In practice, digital devices are often specified and designed to operate at multiple clock frequencies. For example, a high-performance microprocessor may be designed to operate at multiple clock frequencies ranging from 100MHz to 3 GHz. The technique of operating a clock signal at different clock frequencies over time is commonly referred to as frequency scaling and is motivated by the need to reduce power consumption in semiconductor devices. Power consumption in digital semiconductor devices grows in proportion with the rate at which these devices switch between their digital values. When performance requirements decrease, this rate can be reduced by reducing the frequency of the clock signal, thereby reducing power consumption. [0005] The operation of clock signals at more than a single frequency also arises in the context of device binning, that is, the practice of selling at a premium a device that, due to manufacturing variations, is capable of operating at a higher peak clock frequency than another device of identical design and functionality. For example, a batch of microprocessors that was fabricated on a "fast" semiconductor manufacturing corner may contain microprocessors capable of running at clock frequencies of up to 3GHz, while an identical-in-design batch of microprocessors that was fabricated on a "typical" semiconductor manufacturing corner may contain microprocessors that can run at clock frequencies of at most 2GHz. While of identical design, the microprocessors in the first "fast" batch can be sold at significantly higher prices, due to their better achieved performance.

[0006] The challenge with the deployment of resonant clock distribution networks in multi-frequency operation contexts is that these networks typically achieve their highest energy efficiency for a relatively narrow range of clock frequencies centered around the natural frequency of the resonant network. For clock frequencies outside this narrow range, energy efficiency degrades significantly, and to an extent that can outweigh the inherent energy advantages of resonant clocking. For example, consider a microprocessor that has been designed with a target frequency of 3 GHz, but its digital logic can only achieve a peak clock rate of 2GHz after manufacturing. In a non-resonant clock implementation of the microprocessor, the clock network can be operated at 2GHz,

₅6567-8000/LEGAL193809₅4.1 3 consuming power in proportion to its 2GHz operating frequency. In a resonant clock design, however, if the resonant clock network operates at 2GHz, instead of its natural frequency of 3 GHz, its power consumption can significantly exceed the power consumption of the non-resonant design at 2GHz. [0007] In addition to excessive power consumption, when a resonant clock network operates away from its natural frequency, the shape of the clock waveform is increasingly deformed, as the mismatch between the natural and the operating frequency increases. In extreme situations, the peak clock frequency after manufacturing may be so far from the natural frequency of the resonant clock network that the clock waveform at the peak clock frequency becomes deformed to such an extent that incorrect operation of the clocked elements results, and the function of the overall device becomes incorrect.

[0008] It is possible to address the above challenges in a number of straightforward yet impractical ways. One such approach is to enable the adjustment of the natural frequency by providing for the selective introduction of capacitance to the resonant clock network. Since the energy efficiency of the resonant clock network decreases as its capacitance increases, adjusting the natural frequency by introducing capacitance compromises power savings at lower operating frequencies. Moreover, the area overhead of the spare capacitance may be prohibitively high.

[0009] Another approach to the adjustment of the natural frequency is the deployment of series or parallel combinations of multiple distinct inductors that can be selectively engaged. In general, however, the use of multiple distinct inductors results in potentially prohibitive area overheads.

[0010] An alternative approach to addressing the above challenges is to design resonant clock drivers that are capable of also operating in conventional (i.e., non- resonant) mode. In this approach, the resonant clock network can be operated at any clock frequency in conventional mode. However, these delivers typically rely on the introduction of a switch in series to the inductor and the clock load, thus increasing overall

₅6₅67-8000/LEGALl 93809₅4.1 4 resistance of the resonant clock network and degrading its energy efficiency when operating in resonant mode.

[0011] Architectures for resonant clock distribution networks have been described and empirically evaluated in several articles, including "A 225MHz Resonant Clocked ASIC Chip," by Ziesler C, et al., International Symposium on Low-Power Electronic Design, August 2003; "Energy Recovery Clocking Scheme and Flip-Flops forJJltra Low-Energy Applications," by Cooke, M., et al., International Symposium on Low-Power Electronic Design, August 2003; and "Resonant Clocking Using Distributed Parasitic Capacitance," by Drake, A., et al., Journal of Solid-State Circuits, Vol. 39, No. 9, September 2004; "900MHz to 1.2GHz two-phase resonant clock network with programmable driver and loading," by Chueh J.-Y., et al., IEEE 2006 Custom Integrated Circuits Conference, September 2006; "A 0.8-1.2GHz frequency tunable single-phase resonant-clocked FIR filter," by Sathe V., et al., IEEE 2007 Custom Integrated Circuits Conference, September 2007; "A Resonant Global Clock Distribution for the Cell Broadband Engine Processor," by Chan S., et al., IEEE Journal of Solid State Circuits, Vol. 44, No. 1, January 2009. In all these articles, the resonant clock distribution networks are restricted to a single natural frequency. No attempt is made and no methods are proposed for operating the resonant clock networks in a way that addresses the aforementioned challenges.

[0012] Overall, the examples herein of some prior or related systems and their associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.

₅6₅67-8000/LEGALl 9380954.1 5 SUMMARY OF THE DESCRIPTION

[0013] An architecture for resonant clock distribution networks is proposed. This architecture allows for the energy-efficient operation of a resonant clock distribution network at multiple clock frequencies through the deployment of flip-flops that can be selectively enabled. The proposed architecture is primarily targeted at the design of resonant clock networks with integrated inductors and exhibits no inductor overheads. Such an architecture is generally applicable to semiconductor devices with multiple clock frequencies, and high-performance and low-power clocking requirements such as microprocessors, ASICs, and SOCs. Moreover, it is applicable to the binning of semiconductor devices according to achievable performance levels.

[0014] Disclosed herein is a clock driver that can selectively be operated in a resonant mode or a non-resonant mode, the clock driver comprising: a resonance element electrically coupled to a clock node of the clock driver, the resonance element configured to enable the clock driver to operate in a resonant mode; a drive element electrically coupled to the clock node, the drive element configured to receive and propagate a reference clock of the clock distribution network based on a logical input signal, wherein the logical input signal is a logical combination of the reference clock and a control signal, wherein the drive element is enabled for operation when the control signal is in an active state; a clocking element electrically coupled to the clock node, the clocking element gated by a gating signal; wherein, the clock driver selectively operates in a resonant mode or in a non-resonant mode based on the values of the control signal and the gating signal, wherein: the clock driver operates in a resonant mode when the control signal is in an active state and the gating signal is an inactive state, wherein the clock driver operates at a frequency relative to a natural resonating frequency of the resonance element; the clock driver operates in a non-resonant mode when the control signal is in an inactive state and the gating signal is in an active state, wherein the clock driver operates at a frequency relative to a gating frequency of the gating signal.

[0015] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not

₅₆₅₆7-8000/LEGAL193809₅4.1 6 intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other advantages and features will become apparent from the following description and claims. It should be understood that the description and specific examples are intended for purposes of illustration only and not intended to limit the scope of the present disclosure.

₅6567-8000/LEGALl 93809₅4.1 7 BRIEF DESCRIPTION OF DRAWINGS

[0016] These and other objects, features and characteristics of the present invention will become more apparent to those skilled in the art from a study of the following detailed description in conjunction with the appended claims and drawings, all of which form a part of this specification. In the drawings:

Fig. 1 shows a typical architecture of a resonant clock distribution network.

Fig. 2 shows a typical driver design for resonant clocking with the clock load modeled as a lumped capacitor.

Fig. 3 shows an approach to adjusting natural frequency by selective introduction of capacitance in parallel to the clock load.

Fig. 4 shows an approach to adjusting natural frequency by selective introduction of inductance in parallel to the original inductor.

Fig. 5 shows an approach for supporting operation at multiple clock frequencies by introducing a switch in series to the inductive element of the resonant clock network.

Fig. 6 shows an embodiment of the proposed resonant clock driver for operating at multiple clock frequencies.

Fig. 7 shows an alternative embodiment of the proposed resonant clock driver for operating at multiple clock frequencies. Fig. 8 shows an embodiment of the proposed clock gater for operating at multiple clock frequencies.

Fig. 9 shows an embodiment of the proposed flip-flop for operating at multiple clock frequencies.

[0017] The headings provided herein are for convenience only and do not necessarily affect the scope or meaning of the claimed invention.

₅6₅67-8000/LEGALl 93809S4.1 8 [0018] In the drawings, the same reference numbers and any acronyms identify elements or acts with the same or similar structure or functionality for ease of understanding and convenience.

56₅67-8000/LEGALl 93809₅4.1 9 DETAILED DESCRIPTION OF THE INVENTION

[0019] Various examples of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. One skilled in the relevant art will understand, however, that the invention may be practiced without many of these details. Likewise, one skilled in the relevant art will also understand that the invention can include many other obvious features not described in detail herein. Additionally, some well-known structures or functions may not be shown or described in detail below, so as to avoid unnecessarily obscuring the relevant description. [0020] The terminology used below is to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the invention. Indeed, certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. [0021] Figure 1 shows a typical resonant clock distribution network architecture for a semiconductor device. In this network, a buffered distribution network is used to distribute a reference clock signal to multiple resonant clock drivers that are in turn used to drive the clock signal across an all-metal clock distribution network. Typically, this all-metal network has an approximately symmetric topology, delivering the clock signal to the clocked elements (for example, flip-flops and clock gaters) of the semiconductor device with very low skew. Each resonant clock driver incorporates an inductor that is used to provide additional drive strength with low energy consumption by resonating the parasitic capacitance of the load seen by the driver.

[0022] Figure 2 shows a typical resonant clock driver design, in which the part of the overall clock distribution network serviced by the resonant clock driver is modeled as a lumped capacitor C in series with a lumped resistance R. This driver comprises a pull-up PMOS and a pull-down NMOS device for driving the clock distribution network. The PMOS device is connected between the clock node and the power supply terminal. The

₅6567-8000/LEGAL193809₅4.1 . 10 NMOS device is connected between the clock node and the ground terminal. Both devices are driven by the reference clock signal. An inductor L is connected between the clock node and a supply node with voltage at approximately the mid-point of the clock signal oscillation. For example, if the clock signal oscillates between 0V and IV, the mid-point supply voltage is approximately 0.5V. In the driver of this figure, the mid-point is implemented using two capacitors Cdd and Css. Capacitor Cdd is connected between the mid-point and the power supply terminal. Capacitor Css is connected between the midpoint and the ground terminal. To maximize energy savings, the value of the inductor is approximately chosen so that the LC tank set-up by the inductor and the parasitic capacitance of the clock distribution network has a natural frequency that is approximately equal to the frequency of the reference clock signal.

[0023] The energy efficiency of the resonant clock driver depends on various design and operating parameters. The quality factor Q of the resonant system is an indicator of its energy efficiency. This factor is proportional to (L / C) / R. In general, energy efficiency decreases as R increases, due to the / R losses associated with the flow of the current / that charges and discharges the parasitic clock load C through the resistance R. Also, for a fixed natural frequency, energy efficiency decreases as capacitance C increases, since the current flowing through resistance R increases.

[0024] The mismatch between the natural frequency of the resonant LC-tank system and the frequency of the reference clock signal is another important factor that affects the energy efficiency of the resonant clock network. As the frequency of the reference clock that drives the resonant clock driver moves further away from the natural frequency of the resonant clock driver's LC-tank, energy efficiency decreases. When the mismatch between the two frequencies becomes too large, the energy consumption of the resonant clock driver becomes excessive and impractically high. Moreover, the shape of the clock waveform can become so distorted that it cannot be reliably used to clock flip-flops or other clocked elements. Consequently, resonant clock drivers tend to have a narrower range of clock frequencies within which they operate efficiently than the range of clock frequencies typically supported by a semiconductor device that uses frequency scaling. In

56567-8000/LEGAL19380954.1 \ \ practice, to support the broad range of operating frequencies used in a frequency-scaled semiconductor device, it is desirable for the resonant clock network to be capable of operating at more than one frequency.

[0025] Figure 3 shows a possible approach for widening the range of operating frequencies of a resonant clock network by supporting a second natural frequency through selective introduction of capacitance in parallel to the clock load. A switch P is used to selectively connect capacitor Cp in parallel to the parasitic capacitance C of the clock network. When switch P is turned off by control signal ENP, the total capacitance seen at the clock node is C, and the natural frequency fl of the resonant clock network is proportional to 1 / (L C)^l/2. When switch P is turned on, the total capacitance seen at the clock node increases to C + Cp, resulting in a lower natural frequency f2, which is proportional to 1 / (L (C + Cp) )^1/2. The main drawback of this approach is that due to the additional capacitance Cp and the resistance introduced by switch P, operation at f2 has a lower Q factor than at fl, thus resulting in decreased relative energy savings. Another drawback of this approach is that the implementation of capacitance Cp using an integrated capacitor results in significant area overheads. For example, to obtain β = fl¹¹², capacitance Cp must be approximately equal to the capacitance C of the clock distribution network.

[0026] Figure 4 shows another possible approach for widening the range of operating frequencies of a resonant clock network by supporting a second natural frequency in a resonant clock driver. In this approach, an inductor Lp is selectively introduced in parallel to the original L of the resonant clock driver using a pair of switches PI and P2. When the two switches are turned off, the total inductance in the resonant clock network is L, and the natural frequency fl of the resonant clock network is proportional to 1 / (L C)¹'². When the two switches are turned on, the total inductance decreases to L Lp / (L + Lp), resulting in a higher natural frequency f2, which is proportional to 1 / ((L+Lp) C) . The main drawback of this approach is that due to the decrease in total inductance, and the additional resistance introduced by switches PI and P2, operation at f2 has a lower Q factor than at fl, thus resulting in decreased relative energy savings. For clock networks operating at GHz

56₅67-8000/LEGAL193809₅4.1 12 frequencies, this decrease in energy savings is exacerbated by the fact that total resistance at the higher operating frequency fi will be higher than at fl, due to skin effect. Another drawback of this approach is that inductance Lp must be implemented using an inductor in parallel to L, generally resulting in significant area overheads. For example, to obtain f2 = fl^1/2, inductance Lp must be approximately equal to the original inductance L in the resonant clock driver.

[0027] Figure 5 shows another possible approach for supporting operation at multiple clock frequencies. In this case, a switch T is introduced in series to the inductive element of the resonant clock network. Switch T can be used to selectively decouple the inductor from the mid-point supply, thus providing the option of driving the clock network in conventional (i.e., non-resonant) mode. When the control signal EN turns the switch on, the driver operates in resonant mode. When the switch is turned off, the driver operates in conventional mode at the frequency of the reference clock. In this mode, the capacitance of the clock network is not resonated and, therefore, the clock network can operate at the frequency of the reference clock without any concern about the mismatch of this frequency from the natural frequency of the LC tank. In this figure, the switch is shown conceptually as a single NMOS transistor. In general, this switch will be typically implemented as a transmission gate that incorporates both NMOS and PMOS transistors.

[0028] The main advantage of the approach shown in Figure 5 is that it allows for the operation of the clock network at any arbitrary frequency, as dictated by the frequency of the reference clock signal. However, the drawback of this approach is that switch T is in series to the inductive element of the resonant clock network, adding to its total resistance and, thus, significantly degrading the energy efficiency of the clock network when operating in resonant mode. [0029] Figure 6 shows an embodiment of the proposed approach for enabling the operation of the resonant clock network at multiple clock frequencies. In this embodiment, a control signal S is introduced in the basic resonant clock driver design of Figure 2 to selectively pull the clock node high, independently of the reference clock signal. In this 6₅67-8000/LEGAL193809₅4.1 13 figure, this function is accomplished by combining the control signal S and the reference clock signal through an AND gate whose output drives the NMOS and PMOS devices of the resonant clock driver. Alternative embodiments that provide the same functionality are possible. [0030] In addition to signal S, the proposed embodiment includes a flip-flop design that can be selectively disabled using a gating signal g. Figure 6 shows a representative such flip-flop that is gated by a signal gj. While the gating signal is inactive, the proposed flip-flop is enabled and its state is updated on every rising edge of its clock input. While the gating signal is active, the state of the proposed flip-flop remains unaltered. The state of the proposed flip-flop also remains unaffected when the clock is low. When the clock is high, however, the state of the proposed flip-flop is updated whenever the gating signal transitions from active to inactive. This property is critical for the operation of the proposed flip-flop in the context of the proposed approach for operating the resonant clock network at multiple clock frequencies. [0031] The embodiment of Figure 6 enables the operation of the resonant clock network at its natural frequency in resonant mode. It also enables its operation at any other frequency in conventional mode. In this specific embodiment, when signal S set high and the gating signals are inactive, the resonant clock network can be operated in resonant mode. In this case, to ensure high energy efficiency and a clock waveform that conforms to its specification, the frequency of the reference clock should be relatively close to the natural frequency of the LC tank system.

[0032] Operation at other clock frequencies is performed in conventional mode. Specifically, the gating signals are first activated to ensure the integrity of the data stored in the flip-flops while the clock is transitioning from resonant mode to conventional mode. Subsequently, signal S transitions from high to low, pulling the clock node to the supply voltage Vdd. With the clock node at Vdd, the gating signals are periodically operated from active to inactive and back to active at the target operating frequency. For each active to inactive transition, the flip-flops update their state. The transitions of the gating signals

56₅67-8000/LEGALl 93809₅4.1 J 4 should be performed with relatively low relative skew, since the network of gating signals is essentially operated as a clock network. In general, the skew of the gating signals should be sufficiently low to ensure correct operation at the target clock frequency. In general, the skew requirements on the gating signals will not be as stringent as the ones on the resonant clock network, since the gating signals will typically operate at a lower clock frequency than the natural frequency of the resonant clock network.

[0033] Figure 7 shows an alternative embodiment of the proposed approach. In this embodiment, the NMOS devices can be driven by a separate reference clock from the PMOS devices. Moreover, a subset of the NMOS and PMOS devices can be selectively activated using the control signal ΕΝΙ,. , .,ΕΝη. All NMOS and PMOS devices are controlled by signal S, that can be used to pull the clock node to the supply voltage Vdd, independently of the level of the reference clock signal.

[0034] In alternative embodiments of the resonant clock drivers shown in Figures 6 and 7, the control signal S can be combined with the reference clock using an OR gate. In this case, when signal S is set high, the clock node is driven low. Such an embodiment needs to be combined with a flip-flop embodiment whose state remains unaffected when the clock is high, whereas when the clock is low, the state of the proposed flip-flop is updated whenever the gating signal transitions from active to inactive.

[0035] Figure 8 shows an embodiment of a clock gater device to be used with the proposed approach. In this embodiment, if the gating signal gj is active (i.e., at a low voltage level), then the resonant clock signal is prevented from propagating to the output of the clock gater device, in which case the output port clock also presents a low voltage, regardless of the level of the resonant clock signal. If the gating signal gi is inactive (i.e., at a high voltage level), then the resonant clock propagates to the output of the clock gater, driving the port clock low / high whenever it makes a falling / rising transition. While the resonant clock is at a low voltage level, the output of the clock gater remains low, regardless of the level of the clock gater signal gj. While the resonant clock is at a high voltage level, however, then the output port clock is driven low / high whenever the gating

₅6₅67-8000/LEGAL193809₅4.1 \ 5 signal gj makes a falling / rising transition. Therefore, when the resonant clock signal is high, the clock gating signal gj can be used as an alternative clock signal.

[0036] Figure 9 shows an embodiment of a flip-flop device to be used with the proposed approach. This flip-flop is an adaptation of a set-reset topology that has been extended to include a gating signal gj. When this flip-flop is not gated (i.e., signal gj is low, and signal EN is high), it updates its state on the rising edge of the clock signal RC. The state of the flip-flop is stored in the cross-coupled pair of NAND gates. Specifically, while the clock signal RC is low, the internal nodes XT and XF are charged to the level of the supply voltage Vdd through the PMOS devices P3 and P4. When both XT and XF are at a high voltage level, the value stored by the pair of cross-coupled NAND gates is not affected, since for NAND gates, high input values are non-controlling. On the rising edge of the clock signal RC, one of the internal nodes XT and XF is driven to a low voltage, depending on the data input value DT. (When DT is high/low, then XT is set low/high and XF high/low.) This pair of opposite values on nodes XT and XF results in a pair of opposite values at the outputs Q T and QF. (When DT is high/low, then QT is set high/low and XF low/high.) This pair of output values is stored in a stable manned by the pair of NAND gates, due to their cross-coupled topology. When this flip-flop is gated (i.e., signal gj is high, and signal EN is low), PMOS devices PI and P2 keep the internal nodes XT and XF at the voltage level of the supply Vdd, regardless of the level of the clock signal RC. [0037] In general, the flip-flop in Figure 9 is enabled (i.e., gating signal gj is low), when the resonant clock network is operated in resonant mode, and the clock signal RC swings from high to low level every clock cycle. However, this flip-flop can also be operated in an alternative manner that enables operation at frequencies other than the natural frequency of the resonant clock network. In this alternative approach, the clock signal RC remains stable, while the gating signal gj is used to update the state of the flip- flop. Therefore, the signal gj is essentially used as a clock signal that can be operated at any target clock frequency. Specifically, by keeping the clock signal RC at a high voltage level, the internal nodes XT and XF can be charged by driving the gating signal gj high. The clock signal RC can be driven high by driving the control signal S low in the resonant

56₅67-8000/LEGALl 9380954.1 16 clock drivers whose exemplary embodiments are shown in Figures 6 and 7. When signal gj is driven low, one of the two nodes XT and XF is driven low, depending on the value of the data input DT. (When DT is high/low, then XT is set low/high and XF high/low.) This pair of opposite values on nodes XT and XF results in a pair of opposite values at the outputs QT and QF. (When DT is high/low, then QT is set high/low and XF low/high.) This pair of output values is stored in a stable manned by the pair of NAND gates, due to their cross-coupled topology.

[0038] Alternative embodiments of the flip-flop in Figure 9 are possible. For example, this flip-flop can be extended in a straightforward manner to include scan and set/reset functionality.

[0039] In its most general alternative embodiment, the flip-flop in Figure 9 will include a gating signal gj and a clock input RC. When the clock input RC remains at some predetermined fixed value (high or low), it is possible to update the state of the flip-flop by performing a pre-determined transition (high to low, or low to high) of the gating signal gj. [0040] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense (i.e., to say, in the sense of "including, but not limited to"), as opposed to an exclusive or exhaustive sense. As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements. Such a coupling or connection between the elements can be physical, logical, or a combination thereof. Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

₅6₅67-8000/LEGAL193809₅4.1 17 [0041] The above Detailed Description of examples of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific examples for the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. While processes or blocks are presented in a given order in this application, alternative implementations may perform routines having steps performed in a different order, or employ systems having blocks in a different order. Some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or sub-combinations. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed or implemented in parallel, or may be performed at different times. Further any specific numbers noted herein are only examples. It is understood that alternative implementations may employ differing values or ranges.

[0042] The various illustrations and teachings provided herein can also be applied to systems other than the system described above. The elements and acts of the various examples described above can be combined to provide further implementations of the invention.

[0043] Any patents and applications and other references noted above, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the invention can be modified, if necessary, to employ the systems, functions, and concepts included in such references to provide further implementations of the invention.

[0044] These and other changes can be made to the invention in light of the above Detailed Description. While the above description describes certain examples of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing

₅6567-8000/LEGAL] 9380954.1 18 certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention under the claims.

[0045] While certain aspects of the invention are presented below in certain claim forms, the applicant contemplates the various aspects of the invention in any number of claim forms. For example, while only one aspect of the invention is recited as a means- plus-function claim under 35 U.S.C. § 1 12, sixth paragraph, other aspects may likewise be embodied as a means-plus-function claim, or in other forms, such as being embodied in a computer-readable medium. (Any claims intended to be treated under 35 U.S.C. § 112, 6 will begin with the words "means for.") Accordingly, the applicant reserves the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the invention.

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Claims

CLAIMS We claim:

1. A clock driver that can selectively be operated in a resonant mode or a non- resonant mode, the clock driver comprising: a resonance element electrically coupled to a clock node of the clock driver, the resonance element configured to enable the clock driver to operate in a resonant mode; a drive element electrically coupled to the clock node, the drive element configured to receive and propagate a reference clock of the clock distribution network based on a logical input signal, wherein the logical input signal is a logical combination of the reference clock and a control signal, wherein the drive element is enabled for operation when the control signal is in an active state; a clocking element electrically coupled to the clock node, the clocking element gated by a gating signal; wherein, the clock driver selectively operates in a resonant mode or in a non- resonant mode based on the values of the control signal and the gating signal, wherein: the clock driver operates in a resonant mode when the control signal is in an active state and the gating signal is an inactive state, wherein the clock driver operates at a frequency relative to a natural resonating frequency of the resonance element; the clock driver operates in a non-resonant mode when the control signal is in an inactive state and the gating signal is in an active state, wherein the clock driver operates at a frequency relative to a gating frequency of the gating signal.

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