Regulating power consumption of digital circuitry using a multi-layer ring oscillator

[0001] Regulating power consumption of digital circuitry has become an important consideration in designing computing and consumer devices such as desktop and laptop computers, televisions, audio equipment, smartphones, cameras, etc. In portable devices, regulating power consumption helps increase the life of battery power, and in general, regulating power consumption helps reduce operating costs, particularly for large enterprise companies that may continuously operate hundreds or even thousands of computing and storage devices.

[0002] FIG. 1A illustrates a prior art technique for regulating power consumption of digital circuitry including a critical path circuit 2. A propagation delay circuit 4 generates a propagation delay frequency 6 representing a propagation delay of the critical path circuit 2. A frequency generator 8 generates a reference frequency 10 and a frequency comparator 12 for generating a frequency error 14 representing a difference between the reference frequency 10 and the propagation delay frequency 6. An adjustable circuit 16, responsive to the frequency error 14, adjusts at least one of a supply voltage and a clocking frequency 18 applied to the critical path circuit 2 in order to maintain a target performance and/or a target power consumption.

[0003] For example, if the frequency error 14 indicates the propagation delay frequency 6 has fallen bellow a threshold, the adjustable circuit 16 may increase the supply voltage to the digital circuitry to maintain error free operation and performance by maintaining a target propagation delay. Alternatively, the adjustable circuit 16 may decrease the clocking frequency applied to the digital circuitry to maintain error free operation at a lower performance while decreasing the power consumption.

[0004] The propagation delay frequency 6 of FIG. 1A may be generated by a ring oscillator shown in FIG. 1B which comprises a plurality of inverters 20₀-20_N connected in a ring configuration and an NAND gate 22 for enabling the ring oscillator in response to an enable signal 24. The output frequency of the ring oscillator depends on the number of inverters 20₀-20_N, the amplitude of the supply voltage 18, and other factors, such as variations in fabrication process, ambient temperature, etc.

[0015] FIG. 2 shows a computing device according to an embodiment of the present invention comprising digital circuitry 26 fabricated on a multi-layer integrated circuit including a first layer and a second layer, and a multi-layer ring oscillator 28 operable to generate a propagation delay frequency 30 representing a propagation delay of the integrated circuit, wherein the multi-layer ring oscillator 28 comprises a first interconnect fabricated on the first layer and a second interconnect fabricated on the second layer. The propagation delay frequency 30 is compared 36 to a reference frequency 38 to generate a frequency error 40, and at least one of a supply voltage and a clocking frequency 42 applied to the digital circuitry 26 is adjusted 44 in response to the frequency error 40.

[0016] Each layer of the multi-layer integrated circuit may comprise any suitable material, such as metal, metal silicide, or polysilicon. In one embodiment, the multi-layer ring oscillator 28 of FIG. 2 comprising a plurality of inverting elements that are fabricated on a first layer, wherein at least one interconnect for connecting the inverting elements is fabricated on a second layer. In other embodiments, the inverting elements may be fabricated on more than one layer which may be the same or different from the interconnect layers.

[0017] Using a multi-layer ring oscillator to adjust at least one of the supply voltage and the clocking frequency applied to the digital circuitry helps compensate for anomalies in the fabrication process that can increase the propagation delay of the layers. For example, uneven etching or other physical anomalies can increase the capacitance between adjacent layers, thereby slowing the interconnect signal paths. The adjustable circuit 44 of FIG. 2 compensates for these anomalies by adjusting the supply voltage to maintain a target propagation delay across the layers, or by adjusting the clocking frequency of the digital circuitry to account for the longest propagation delay across the layers.

[0018] FIG. 3 shows a multi-layer ring oscillator 28 according to an embodiment of the present invention comprising a number of inverting elements 46₀-46_N, and a plurality of interconnects in the form of feedback paths 48₀-48_N fabricated on multiple layers, including a first feedback path 48₀ fabricated on a first layer, and a second feedback path 48₁ fabricated on a second layer. The first inverting element 46₀ comprises a number of logic gates that are configured to wait until the slowest feedback path changes state before the output of inverting element 46₀ changes state so that the longest propagation delay through the multiple layers governs the output frequency (the delay frequency 30). In the embodiment of FIG. 3, the multi-layer ring oscillator comprises a short path 50 that sets up the receiving gates of the first inverting element 46₀ to select the proper polarity of the propagated oscillation signal.

[0019] FIG. 4 shows a multi-layer ring oscillator 28 according to an embodiment of the present invention comprising a configurable number of inverting elements 52₀-52_N. A multiplexer 54 selects the output of one of the inverting elements as the signal applied to the feedback paths 48₀-48_N. Accordingly, the length of the feedback paths for each inverting element 52₀-52_N is configurable to substantially match any length of interconnect path through the multiple layers. For example, in one embodiment the length of the feedback paths 48₀-48_N may correspond to the length of a critical path circuit implemented within the digital circuitry. Accordingly, the multiplexer 54 can be configured so that the length of the feedback paths 48₀-48_N substantially match the length of the critical path circuit. In another embodiment, the length of the feedback paths 48₀-48_N may be selected (scaled) to match a multiple of a target interconnect path.

[0020] FIG. 5 shows a multi-layer ring oscillator 28 according to an embodiment of the present invention similar to the embodiment of FIG. 4 except that the feedback paths 48₀-48_N comprise a serpentine pattern which increases the length of the paths along each layer. Only the first layer and corresponding feedback path is shown in FIG. 5, whereas the other feedback paths (not shown) are fabricated in separate layers underneath the first layer.

[0021] In one embodiment, the digital circuitry 26 of FIG. 2 is fabricated in the integrated circuit such that on even numbered layers the interconnects are routed in a first direction (e.g., vertically), and on odd numbered layers the interconnects are routed in a second direction (e.g., horizontally). Accordingly, an embodiment of the present invention may comprise a first multi-layer ring oscillator having interconnects (e.g., feedback paths) fabricated on the even numbered layers, and a second multi-layer ring oscillator having interconnects (e.g., feedback paths) fabricated on the odd numbered layers (i.e., the layers of the two ring oscillators are interleaved). For example, a first multi-layer ring oscillator (FIG. 5) may comprise feedback paths fabricated vertically on the even numbered layers, and a second multi-layer ring oscillator (FIG. 6) may comprise feedback paths fabricated horizontally on the odd numbered layers.

[0022] FIG. 7A shows an alternative embodiment of a multi-layer ring oscillator wherein the interconnects between the inverting elements are fabricated on different layers. For example, a first interconnect 56₁ fabricated on a first layer couples an output of a first inverting element 46_o to an input of a second inverting element 46₁, and a second interconnect 56_N fabricated on a second layer couples the output of the second inverting element 46₁ to an input of a third inverting element 46_N. This embodiment adjusts the supply voltage to maintain a target average propagation delay across the layers (or adjusts the clocking frequency to compensate for changes in the average propagation delay).

[0023] FIG. 7B shows an embodiment wherein the multi-layer ring oscillator of FIG. 7A is configurable using multiplexers 58₀-58_N to selectively omit one or more layers. That is, the multiplexers are configured to bypass the interconnect 56_i and the inverting element 52_i in order to remove it from the multi-layer ring oscillator. In one embodiment, the bypass path within each multiplexer 58_i may include an inverted path and a non-inverted path so that the number of inverting elements in the ring oscillator remains an odd number.

[0024] FIG. 8 shows yet another embodiment of a multi-layer ring oscillator comprising a first set of inverting elements 46A₀-46A_N connected in a ring using interconnects fabricated on a first layer, and a second set of inverting elements 46B₀-46B_N connected in a ring using interconnects fabricated on a second layer. A logic circuit 60 selects the slower frequency between the outputs 30A and 30B as the propagation delay frequency 30 compared 36 to the reference frequency 38 in FIG. 2. The first and second set of inverting elements may be fabricated together on a single layer, or they may be fabricated on different layers (e.g., on the same as or different from the interconnect layers). Also in this embodiment (as well as the above embodiments), the interconnects fabricated on the different layers may be aligned vertically over on another, or they may be fabricated at different locations on their respective layers. The embodiment of FIG. 8 may also be modified to included more than two sets of inverting elements having interconnects fabricated on different layers. In addition, the logic circuit 60 may comprise configuration circuitry for selectively omitting one or more of the layers as desired.

[0025] Similar configuration circuitry may also be added to the above embodiments in order to selectively omit one or more layers as desired (e.g., omit one or more feedback paths in the embodiment of FIG. 3). For example, in one embodiment only a subset of the layers may be active during a particular operating mode of the digital circuitry 26, and therefore the non-active layers may be omitted from the multi-layer ring oscillator 28 while the digital circuitry 26 is operating in a respective mode.

[0026] An embodiment of the present invention may employ different types of multi-layer ring oscillators (e.g., FIG. 4 and FIG. 7A) together with a logic circuit that selects between the frequency errors 40 that is used to adjust 44 at least one of the supply voltage and the clocking frequency 42. In other embodiments, different types of ring oscillators may be employed, such as the multi-layer ring oscillator described above together with a single layer ring oscillator matched to a critical path circuit fabricated on a particular layer, as well as a logic circuit for selecting between the resulting frequency errors. In yet another embodiment, the multi-layer ring oscillator described above may also be matched to a critical path circuit in the digital circuitry (or matched to a multiple of the critical path circuit) with respect to the interconnect delays and/or the gate delays of the critical path circuit.