# Patent

## Title
Method and system for functional verification and power analysis of clock-gated integrated circuits

## Abstract

An apparatus for monitoring operation of a design under test (DUT) comprises a plurality of inputs comprising: an incoming clock edge input connected to detect active clock edges provided to a monitored clock gate; an outgoing clock edge input connected to detect active clock edges sent from the monitored clock gate; an enable input connected to detect enable signals provided to the monitored clock gate and any leaf clock gates connected to receive clock edges through the monitored clock gate; and a protocol input connected to receive protocol signals specifying when the monitored clock gate is required to output active clock edges. The apparatus also comprises a memory in communication with the inputs for storing values from the inputs, and a processor in communication with the memory and the inputs, the processor programmed to determine protocol compliance and to calculate energy consequences of dropping of active clock edges.

## Background
FIELD
The present disclosure relates to analysis of integrated circuit designs.
BACKGROUND
The design of an integrated circuit typically includes, among other aspects, functional verification and power analysis. Functional verification refers to a practice of testing the circuit and analyzing the results of the test to determine whether the circuit is performing to specification. For example, given a set of inputs, does the circuit generate the expected output? Functional verification can be executed with a relatively large degree of automation to cover all of the various operation conditions of the circuit. Briefly, functional verification ensures that the logical design of the circuit is correct.
In contrast, power analysis is an aspect of circuit design that is directed to the physical requirements of the design specification. Therefore, power analysis is generally performed separately from functional verification, and the tools for power analysis are different from the tools for functional verification.
Conventional power analysis can report power consumption of each cell and activity in each net of a design, given a design and netlist activity file. However, these power reports do not indicate whether the power consumption of a cell is correlated to the functional workload of the cell. In practice, a cell may be consuming power but not producing useful work. In this case, conventional power analysis would not indicate whether power consumption could be reduced.
It is desirable to obviate or mitigate these shortcomings of conventional power analysis.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.
FIG. 1 is an example of a clock-gated circuit for connecting to a clock gate monitor according to an embodiment of the present disclosure.
FIG. 2 schematically illustrates an example clock gate monitor for performing functional power analysis according to an embodiment of the present disclosure.
FIG. 3 is an example of a clock-gated circuit for connecting to a clock gate monitor according to an embodiment of the present disclosure.
FIG. 4 is an example of a clock-gated circuit for connecting to a clock gate monitor according to an embodiment of the present disclosure.
FIG. 5 schematically illustrates another example clock gate monitor for performing functional power analysis according to an embodiment of the present disclosure.
FIG. 6 is an example of a clock-gated circuit for connecting to a clock gate monitor according to an embodiment of the present disclosure.
FIG. 7 is an example of a clock-gated circuit for connecting to a clock gate monitor according to an embodiment of the present disclosure.
FIG. 8 is a flowchart illustrating an example method of processing inputs received at a clock gate monitor according to an embodiment of the present disclosure.



DETAILED DESCRIPTION
Generally, the present disclosure provides methods and systems for verifying a clock-gated integrated circuit using tools that perform both functional verification and power analysis on cells of the clock-gated integrated circuit.
An example tool examines the power consumption of cells under a set of functional workloads. Examining power consumption across a set of workloads enables prediction of power consumption under related, but unmeasured, workloads. Thus, by correlating the functional workload to the power consumption, the power analysis performed by this tool may be considered a functional power analysis. The tool can further determine, from the correlation between the functional workload and the power consumption of a cell, whether the power consumption of the cell, or set of cells, may be reduced.
One aspect of the present disclosure provides an apparatus for monitoring operation of a design under test (DUT) comprising a plurality of combinational logic elements, a plurality of clocked sequential logic elements, and a plurality of clock gate elements connected to selectively provide clock edges to the clocked sequential logic elements. The apparatus comprises a plurality of inputs comprising: an incoming clock edge input connected to detect active clock edges provided to a monitored clock gate; an outgoing clock edge input connected to detect active clock edges sent from the monitored clock gate; an enable input connected to detect enable signals provided to the monitored clock gate and any leaf clock gates connected to receive clock edges through the monitored clock gate; and a protocol input connected to receive protocol signals specifying when the monitored clock gate is required to output active clock edges. The apparatus also comprises a memory in communication with the plurality of inputs for storing values from the plurality of inputs, and a processor in communication with the memory and the plurality of inputs, the processor programmed to determine protocol compliance and to calculate energy consequences of dropping of active clock edges at the monitored clock gate.
One aspect of the present disclosure provides a method for monitoring operation of a DUT. The method comprises detecting active clock edges provided to a monitored clock gate, detecting active clock edges sent from the monitored clock gate, detecting enable signals provided to the monitored clock gate and any leaf clock gates connected to receive clock edges through the monitored clock gate, receiving protocol signals specifying when the monitored clock gate is required to output active clock edges, determining protocol compliance by comparing the active clock edges sent from the monitored clock gate to a set of required edges specified by the protocol signals; and calculating energy consequences of dropping of active clock edges at the monitored clock gate by comparing the active clock edges provided to a monitored clock gate with the active clock edges sent from the monitored clock gate.
Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.
FIG. 1 is an example of a clock-gated circuit which can be used to demonstrate the functional power analysis operations of various embodiments of the present disclosure. The circuit 100 comprises a first clock gate 101 connected to a second clock gate 102 and a third clock gate 103. In the illustrated example, clock signals must pass through the first clock gate 101 before reaching the second or third clock gate 102 or 103, and as such the first clock gate 101 may be referred to as a “root”, and the second and third clock gates 102 and 103 may each be referred to as a “leaf”, of a clock gate “tree”.
The first clock gate 101 is connected to each enable input of a first flip-flop 111 and a second flip-flop 112. The second clock gate 102 is connected to each enable input of a third flip-flop 113 and a fourth flip-flop 114. The data inputs of the first and second flip-flops 111 and 112 are triggered by other upstream combinational logic elements (combinational cloud 121) of which the exact nature is unimportant for the purpose of the present disclosure. The data inputs of the third and fourth flip-flops 113 and 114 are triggered by a second combinational cloud 122, which is connected to the outputs of the first and second flip-flops 111 and 112. Thus, the data inputs of the third and fourth flip-flops 113 and 114 are indirectly connected to the outputs of the first and second flip-flops 111 and 112.
Clock gating is a technique that selectively disables synchronous flip-flops from switching states, which reduces the power consumption of the flip-flops and consequently also power dissipation of combinational cells driven by these flip flops. If the circuit 100 did not have any clock gates, the clock inputs of the flip-flops would be triggered by a common clock, and each flip-flop would switch state on each active clock edge. (As one of skill in the art will appreciate, depending on the design of the circuit in question positive or negative edges may be active clock edges.)
Clock gating selectively passes the clock signal to the clock input of the flip-flop. If a certain flip-flop does not need to change states (to pass the state of data input to the output) then the clock signal can be gated off by the clock gate in order to reduce power consumption in the flip-flop, as well as in the fanout of the flip-flop (e.g., the combinational logic elements receiving data from the flip-flop).
It is difficult to assess the impact of clock gating in the design under representative operation. From a functional verification perspective, the implementation of the clock gating technique in a design should not destroy critical information (as defined by relevant protocols for that design) that would otherwise propagate through the design if clock edges were provided. The destruction of this information would change the required functional behavior of the circuit and would in effect be a violation of relevant protocols (either design specific protocols or industry protocols) applicable to the design.
From a power analysis perspective, clock gating should not provide additional clock edges over what is minimally necessary to move critical information through the design. Otherwise, the dynamic power consumed consequential to delivering these edges is wasted.
Ideally, clock gating should only add minimal complexity to the clock tree—individual clock gates for every flip-flop would not typically save enough power to justify their insertion. It is an optimization problem to find a set of clock gates and enable logic that saves power by reducing clock edges and discarding propagated information at a small incremental cost in added clock gate cells and combinational cells that define the enable logic for each clock gate.
FIG. 2 shows an example clock gate monitor 200 for performing functional power analysis according to an embodiment of the present disclosure. The clock gate monitor 200 can be used to help find the optimal clock gating logic that is protocol compliant yet power efficient.
In an embodiment, the monitor 200 connects to a design under test or device under test (DUT). The DUT comprises a plurality of combinational logic elements (e.g. combinational clouds 121 and 122 of FIG. 1), a plurality of clocked sequential logic elements (e.g. flip-flops 111, 112, 113, and 114 of FIG. 1), and a plurality of clock gate elements (e.g. clock gates 101, 102 and 103 of FIG. 1) connected to selectively provide clock edges to the clocked sequential logic elements.
In a typical implementation, a DUT would be provided with a plurality of clock gate monitors 200, with one clock gate monitor 200 connected to each clock gate of the DUT. In some implementations, additional clock gate monitors 200 may be connected to the clock gate input of each un-clock-gated flip-flop in the DUT (i.e., at locations where additional clock gates could be added to the DUT), for example in order to assist in evaluation of whether or not to add additional clock gates.
The monitor 200 comprises a plurality of inputs, comprising: an incoming clock edge input 201, an outgoing clock edge input 202, an enable input 203, and a protocol input 204. The monitor 200 also comprises a memory 205; and a processor 206.
The incoming clock edge input 201 is connected to the DUT to detect active clock edges provided to a monitored clock gate.
The outgoing clock edge input 202 is connected to the DUT to detect active clock edges sent from the monitored clock gate.
The enable input 203 is connected to the DUT to detect enable signals provided to the monitored clock gate and any leaf clock gate connected to receive clock edges through the monitored clock gate.
The protocol input 204 is connected to the DUT to receive protocol signals specifying when the monitored clock gate is required to output active clock edges. A protocol signal active edge is preferably provided prior to the delivery time of each required output active clock edge.
In some embodiments, the protocol input 204 comprises two bits and when either bit is high a required output active clock edge is indicated. With such a configuration, a continuous series of required output active clock edges can be represented in the protocol input as {[0,1], [1,0], [0,1], [1,0], . . . }.
The memory 205 is in communication with the plurality of inputs 201-204 and stores values from the plurality of inputs.
The processor 206 is in communication with the memory 205 and the plurality of inputs 201-204. The processor 206 is programmed to determine protocol compliance and to calculate energy consequences of dropping of active clock edges at a monitored clock gate.
The monitor 200 provides dynamic analysis of a clock gate to allow confirmation that, cycle to cycle, the clock gate is well coordinated with other clock gates and conforms to relevant protocols. Monitor 200 provides advantages over conventional static analysis techniques, which are only based on toggle counts per net over a time interval, not cycle to cycle behavior of the design.
FIG. 3 shows an example of using a monitor (not shown), such as the monitor 200, to determine a protocol violation. For example, the flip-flop 111 may be part of the write address slave AXI interface of a design. This flip-flop 111 must follow protocol defined for a slave on the AXI write address channel, governed by AWVALID and AWREADY. The behavior of AWVALID and AWREADY define when this sequential cell must receive a clock edge to capture the attributes of the AXI write address. The clock gate 102 for the flip-flop 111 inherits the requirements for the specific flip-flop.
The monitor connected to the clock gate 102 will determine whether the clock gate 102 violates the AXI protocol. In the example shown in FIG. 3, an incoming clock edge input (not shown), similar to the incoming clock edge input 201 of the clock gate monitor 200, receives the input of the clock gate 102 (which is the output of clock gate 101), an outgoing clock edge input (not shown), similar to the outgoing clock edge input 202 of the clock gate monitor 200, receives the output of clock gate 102, an enable input (not shown), similar to the enable input 203 of the clock gate monitor 200, receives the enable single input to clock gate 102, and a protocol input (not shown), similar to the protocol input 204 of the clock gate monitor 200, receives the protocol signal 301.
A processor (not shown), similar to the processor 206 of the clock gate monitor 200, determines, from the information provided by the outgoing clock edge input and the protocol input, that a protocol violation occurred. In particular, in the example of FIG. 3, the protocol signal 301 indicates that two edges should be output from the clock gate 102, but only one edge is actually output as indicated by the signal above protocol signal 301 in FIG. 3.
FIG. 4 shows an example of using a monitor (not shown), such as the clock gate monitor 200, to determine a protocol spurious clock edge. For example, the AXI protocol requires that flip-flops 111 and 112 receive two clock edges, as indicated by protocol signal 401. The AXI protocol requires that flip-flops 113 and 114 receive one clock edge, as indicated by protocol signal 402. Consequently, the monitor can determine that clock gate 103 is outputting an additional unrequired or “spurious” clock signal that is not correlated to the AXI protocol. Consequently, the flip-flops 113 and 114 are consuming more power than necessary given protocol.
A processor (not shown), similar to the processor 206 of the clock gate monitor 200, can also calculate how much energy is saved by dropping a clock edge at the monitored clock gate. The processor calculates the savings by comparing and incoming clock edge input (not shown), similar to the incoming clock edge input 201 of the clock gate monitor 200, to an outgoing clock edge input (not shown), similar to the outgoing clock edge input 202 of the clock gate monitor 200. For example, when the incoming clock edge input has two edges and the outgoing clock edge input has one edge the difference between the incoming clock edge input and the outgoing clock edge input shows a single clock edge energy savings credited to the monitored clock gate. In the case where the monitored clock gate has one or more leaf clock gates further downstream in its clock gate tree, the processor can also attribute additional energy savings to the monitored clock gate for dropping an edge when an enable input (not shown), similar to the enable input 203 of the clock gate monitor 200, indicates that such leaf clock gates are enabled, such that but for dropping of the edge at the monitored clock gate that edge would have also propagated to the leaf clock gates, and the monitored clock gate is credited with saving the energy that would have been consumed by the leaf clock gates and their fanouts.
FIG. 5 shows a clock gate monitor 500 according to a further embodiment of the present disclosure. The clock gate monitor 500 can be used to help find the optimal clock gating logic that is protocol compliant yet power efficient.
The monitor 500 connects to a design under test or device under test (DUT). The DUT comprises a plurality of combinational logic elements (e.g. combinational clouds 121 and 122), a plurality of clocked sequential logic elements (e.g. flip-flops 111, 112, 113, and 114), and a plurality of clock gate elements (clock gates 101 and 102) connected to selectively provide clock edges to the clocked sequential logic elements.
In a typical implementation, a DUT would be provided with a plurality of clock gate monitors 500, with one clock gate monitor 500 connected to each clock gate of the DUT. In some implementations, additional clock gate monitors 500 may be connected to the clock gate input of sets of un-clock-gated flip-flop in the DUT (i.e., at locations where additional clock gates could be added to the DUT), for example in order to assist in evaluation of whether or not to add additional clock gates.
The monitor 500 comprises a plurality of inputs, comprising: an incoming clock edge input 501, an outgoing clock edge input 502, an enable input 503, a protocol input 504, a data-in input 507, a data-out input 508, an upstream clocking input 509, a downstream clocking input 510, and a time window input 511. The monitor 500 also comprises a memory 505 and a processor 506.
The incoming clock edge input 501 is connected to the DUT to detect active clock edges provided to a monitored clock gate.
The outgoing clock edge input 502 is connected to the DUT to detect active clock edges sent from the monitored clock gate.
The enable input 503 is connected to the DUT to detect enable signals provided to the monitored clock gate and any leaf clock gate connected to receive clock edges through the monitored clock gate.
The protocol input 504 is connected to the DUT to receive protocol signals specifying when the monitored clock gate is required to output active clock edges. A protocol signal is preferably provided just prior to the time each required for outputting each active clock edge. In some embodiments, the protocol input 504 comprises two bits as described above with reference to FIG. 2.
The memory 505 is in communication with the plurality of inputs and stores values from the plurality of inputs.
The processor 506 is in communication with the memory and the plurality of inputs. The processor is programmed to determine protocol compliance and to calculate power consequences of clock gating.
The data-in input 507 is connected to detect signals on data input pins (D-pins) of sequential logic elements within a fanout of the monitored clock gate. The fanout of the monitored clock gate comprises all of the clocked sequential elements connected to receive clock signals through the monitored clock gate.
The data-out input 508 is connected to detect signals on data output pins (Q-pins) of sequential logic elements within the fanout of the monitored clock gate.
The upstream clocking input 509 is connected to detect active clock edges output from the clock gates controlling the sequential logic elements upstream from the sequential logic elements controlled by the monitored clock gate.
The downstream clocking input 510 is connected to detect active clock edges output from the clock gates controlling the sequential logic elements downstream from the sequential logic elements controlled by the monitored clock gate.
The time window input 511 receives a time window range instructing the processor 506 to perform certain operations for that time window range. The time window range may, for example, be a fixed or adjustable number of clock cycles. In an embodiment, the processor 506 determines power saving based on the energy saved due to dropping of active clock edges at the monitored clock gate for the time window. In another embodiment, the processor 506 determines power saving based on energy saved due to dropping of active clock edges at the monitored clock gate, and energy saved in the fanout of the monitored clock gate, for the time window. In yet another embodiment, the processor 506 determines power savings based on energy saved due to dropping of active clock edges at the monitored clock gate, energy saved in the fanout of the monitored clock gate, and also determines potential additional power savings realizable through elimination of the unnecessary active clock edges for the time window. In yet another embodiment, the processor 506 determines power savings based on energy saved due to dropping of active clock edges at the monitored clock gate, energy saved in the fanout of the monitored clock gate, potential additional power savings realizable through elimination of the unnecessary active clock edges for the time window. In yet another embodiment, the processor 506 determines power savings based on energy saved due to dropping of active clock edges at the monitored clock gate, energy saved in the fanout of the monitored clock gate, potential additional power savings realizable through elimination of the unnecessary active clock edges for the time window, and also determines additional power savings realizable through elimination of unnecessary combinational activity.
The monitor 500 provides the ability to determine if clock gates for flip-flops upstream and downstream of each other are well coordinated so that required information propagates with a minimum number of clock edges. Protocol violations and compliance, spurious clock edges, actual energy/power savings due to clock gating and potential additional energy/power savings may be determined by the monitor 500 in substantially the same manner as described above with respect to the monitor 200 of FIG. 2. Energy consumption in the fanout of the monitored clock gate may be determined, for example, based on the data-in and data-out inputs 507 and 508 which indicate the set of sequential elements in the fanout that change output values in response to a clock edge.
FIG. 6 shows an example of using the monitor 500 to determine coordination of upstream and downstream clock edges. For example, the clock gate 101 propagates an edge, while the clock gate 102 drops the edge. The clock gate 103, however, propagates the same edge that was dropped by clock gate 103, but clock gate 103 has a fanout that is located downstream of the fanout of clock gate 102. Downstream means that the fanout of the clock gate 103 (the clocked sequential logic elements connected to receive clock edges through clock gate 103) receives data from the fanout of the clock gate 102.
In this case, the monitor 500 is connected to the clock gate 103 and the processor 506 compares the upstream clock input 509 and the outgoing clock edge input 502 and determines that the upstream clock gate 102 dropped an edge and no new information will be propagated to the fanout of the clock gate 103 for that edge. Therefore, the processor will determine that the clock gate 103 could have dropped the edge that was dropped by clock gate 102, such that additional energy/power savings could be realized.
FIG. 7 shows another example of using the monitor 500 to determine coordination of upstream and downstream clock edges. For example, the clock gate 101 propagates an edge, while the clock gate 103 drops the edge. The clock gate 102, however, propagates the same edge but has a fanout that is located upstream of the fanout of clock gate 103. Upstream means that the fanout of the clock gate 102 (the clocked sequential logic elements connected to clock gate 103) are providing data to the fanout of the clock gate 103.
In this case, the monitor 500 is connected to the clock gate 102 and the processor 506 will compare the downstream clock input 510 and the outgoing clock edge input 502 and determine that the downstream clock gate 103 dropped an edge and the information propagated by the fanout of clock gate 102 to the fanout of the clock gate 103 for that edge will simply be discarded. Therefore, the processor will determine that the clock gate 102 could have dropped the edge that was dropped by clock gate 103, such that additional energy/power savings could be realized.
In some embodiments, one or more clock gate monitors (such as monitor 200 or 500 described above) are implemented on a chip, with the inputs implemented on pins of the chip. The following table describes the pins of an example clock gate monitor, with reference to corresponding inputs of monitors 200/500 described above where applicable:









TABLE 1





Pin
Width
Purpose/Notes







ECK
1
Provides an ability to attribute all past activity to a net power




savings or power loss, and check compliance to protocol.




(Corresponds to outgoing clock edge input 202/502.)


D_PINS
R bits
Together with CK, provides an ability to sample values at the



determined by
inputs of sequential fanout that may not propagate to the



total sequential
outputs of the sequential fanout because of clock gating (CK



cell input pins.
moved but not ECK), thereby attributing the reduction in




power to the clock gate behavior.




(Corresponds to data-in input 507.)


Q_PINS
Q bits
Provides an ability to observe activity at the outputs of



determined by
sequential fanout that did stimulate the combinational fanout,



total sequential
which in turn provides the ability to attribute all dynamic



cell output
power in the design to the behavior of individual clock gates.



pins.
Without Q PINS it is not possible to accurately model




combinational power consumption nor power dissipated




internal to sequential cells.




(Corresponds to data-out input 508.)


U_ECK
N bits, one bit
Provides an ability to observe whether or not previous to this



per upstream
ECK active edge at least 1 upstream clock gate provided a



clock gate that
clock edge to its own fanout, thereby propagating information



propagates
that could be captured by this clock gate. Lacking at least 1



information to
upstream clock gate ECK active edge, there is no new



the sequential
information for this clock gate ECK to capture.



fanout of the
(Corresponds to upstream clocking input 509.)



monitored




clock gate.



D_ECK
P bits, one bit
Provides an ability to observe whether or not after this ECK



per
active edge at least 1 downstream clock gate provided a



downstream
clock edge to its own fanout, thereby capturing information



clock gate that
propagated by this clock gate. Lacking at least 1



propagates
downstream clock gate ECK active edge, the information



information
propagated by this clock gate has been discarded and hence



from the
the ECK edge produced wasted power.



monitored
(Corresponds to downstream clocking input 510.)



clock gate.



MUST_ECK
2
Provides an ability to observe whether or not this ECK edge




is required by protocol. The count of past MUST_ECK active




edges is either equal to 0 (protocol spurious) 1 (protocol




required) >1 (some protocol required edges were not




delivered).




One of a pair of bits transitions low-high ahead of the CK




active edge to indicate that protocol requires an ECK active




edge before the end of the current clock cycle. Two bits are




used so that an OR indicates times when clocks should be




provided by the clock gate. Two bits are used so that back




to back clock edges can produce positive edges on




WINDOW near inactive edges of CK, and ahead of the




active edge of CK.




(Corresponds to protocol input 204/504.)


CK
1
Provides an ability to count the number of input clock active




edges dropped before ECK has an active edge. For leaf




clock gates in particular this ensures they are not credited




with dropping edges that were dropped by root clock gates




because ECK didn't toggle because CK input didn't toggle either.




(Corresponds to incoming clock edge input 201/501.)


E
S bits, one bit
Provides an ability to credit a root clock gate with power



per fanout leaf
savings in leaf clock gates when CK does not propagate to



clock gate in
ECK because specific leaf clock gates have their enable pin



the fanout of
active.



the monitored
(Corresponds to enable input 203/503.)



clock gate.



WINDOW
1, both edges
Typical practice is to define static time windows in which



active,
power is computed for the activity in the design. In contrast,




WINDOW allows the clock gate to track the power




consequence of each pin toggle as they occur.




WINDOW can be arbitrarily set to the clock period itself or to




the duration of a packet passing through the design and




similar. The WINDOW pin exposes the time-accuracy of the




monitor so that a user can see power dissipated in each




packet etc. This flexibility means that unlike current practice




it is not necessary for a user to, for example, work




backwards from reported power consumed between 100 and




120 nanoseconds to specific clock cycles nor specific design




specific events.




Ultimately the clock gate monitor is still a digital, event




driven, apparatus. It cannot look within a single clock cycle




and determine that there is high power dissipation because




the clock wave form is very crisp, with a lot of high frequency




components. But within limitations of digital event-based




modelling the clock gate monitor can be made arbitrarily time




accurate.




(Corresponds to time window input 511.)









In some embodiments, the clock gate monitor assesses all dropped and propagated clock edges for power impact. The power impact of a dropped or propagated clock edge is defined as the dynamic power consumed (or saved) by the clock gate, the sequential fanout of the clock gate, and the combinational fanout of the sequential fanout of the clock gate consequential to the propagated (or dropped) clock edge.
In some embodiments, the clock gate monitor also assesses all enable toggles for power impact. The power impact of an enable toggle may be defined as the dynamic power dissipated in the clock gate and combinational cloud that solely provides the clock gate enable signal consequential to the enable toggle. The word solely here allows a clock gate monitor to, in extreme cases, attribute very small power consequence to enable toggles when that enable is generated by a combinational cloud that drives other design cells than the clock gate alone.
By assessing the output clock edges and enable toggles for total power impact, a set of clock gate monitors, one per clock gate in the design, is able to assess dynamic power consumption in the entire design on the basis firstly of enable toggles and output clock edges. In such a full design assessment, ungated sequential cells may be provided with a set of virtual clock gates that pass all clock edges.
To increase the accuracy of the power consequences attributed to the clock gate, in some embodiments the D (input) and Q (output) pins of the sequential fanout of each clock gate are also monitored. These additional inputs allow the specific power consequence within each sequential cell of propagation of D (input) edges to Q (output) edges to be assessed, as well as the specific power consequence of Q (output) edges on the combinational fanout.
The total dynamic power impact of dropped or propagated clock edges, and in aggregate the entire design, may be defined as either required or wasted on the basis of: protocol requirements; the previous active clock edges of upstream clock gates; the subsequent active clock edges of downstream clock gates; the current enable state of fanout clock gates; and, activity at D (input) and Q (output) pins of fanout sequential cells.
The power consequence of the clock gate may be assessed as follows:
1. For all clock gates:
a) A missing protocol-required output active clock edge is defined as missing required power. This indicates that the design does not meet requirements. A dropped output active clock edge is defined as saving power otherwise.
b) An additional output active clock edge that is not required by protocol is defined as producing wasted power.
c) All other output active clock edges are defined as producing required power unless the previous behavior of upstream clock gates dictates otherwise.
2. For a clock gate with upstream clock gates:
a) An output active clock edge without at least one previous upstream active clock edge is defined as wasted power as it is not possible for this output active clock edge to capture any new information propagated by an upstream active clock edge.
b) All other output active clock edges are defined as producing required power unless the subsequent behavior of downstream clock gates dictates otherwise.
3. For a clock gate with downstream clock gates:
a) An output active clock edge without at least one subsequent downstream active clock edge is defined as wasted power as it is not possible that any information propagated by this clock gate was captured by downstream sequential elements.
b) All other output active clock edges are defined as producing required power.
4. For a root clock gate with leaf clock gates that drops an output clock edge:
a) The power consequence of this dropped output active edge is increased for each leaf clock gate with an asserted enable signal. The root clock gate is credited with larger power impact for each leaf clock gate that would otherwise have propagated the clock edge.
The power consequence of the clock gate behavior is increased as follows:
1. For all dropped active clock edges, the power consequence of the dropped active edge is increased by: the set of D input toggles that are not propagated to Q toggles (power which would have been dissipated by sequential fanout); and, the set of Q toggles that do not excite the combinational cloud (power which would have been dissipated by combinational fanout).
2. For all propagated active clock edges, the power consequence of the propagated active edge is increased by: the set of D input toggles that are propagated to Q toggles (internal power dissipated by sequential fanout); and, the set of Q toggle that do excite the combinational cloud (power dissipated by combinational fanout).
In some embodiments, the activity of the monitored clock gate is viewed by the clock gate monitor as a sequence of transactions. Each transaction is defined by a single output active edge of the clock gate. All activity of the clock gate and cells grouped with the clock gate is associated with specific output active edges of the clock gate in a manner that indicates the net positive (negative) impact of the clock gate on device power. A set of transactions may be grouped together within a window defined by a time window input as discussed above.
In some embodiments, a clock gate monitor maintains statistics for each of a total window, a previous transaction, and a current transaction. Such statistics may, for example, be determined by incrementing various counters for the window, previous transaction and current transaction, as discussed further below. The total window statistics indicate the energy consumed and saved by the clock gate is accumulated in a period that spans all ECK transactions completed within a power window defined by WINDOW pin edges. The previous transaction statistics are maintained because some pin activity is only possible due to an ECK active edge and occurs after the ECK edge. Consequently, this activity can only be recorded for the previous ECK transaction when the current ECK active edge is observed. The current transaction statistics may be used to track pin activity that is not due to an ECK active edge, and as such may be recorded for the current ECK transaction.
As an extension to basic functionality, if a clock gate monitor is observing netlist activity with functional timing and many non-zero transition delays, then ECK edges may be used to define delays at which point pins are examined for activity.
In some embodiments, a clock gate monitor retains a set of 3 statistics only (i.e., window, previous transaction and current transaction), replacing content as events occur. In some embodiments, statistics could be forwarded or copied to an external agent or memory prior to replacing content.
The following table illustrates examples of how pin activity is mapped to transactions in some embodiments:









TABLE 2





Pin
Transaction
Notes







ECK
Definition of
Each active edge of ECK defines a new transaction,



Boundaries
completes the previous transaction.


D_PINS
Current
Toggles on D pins are sampled on ECK active edges and




recorded for the current transaction. Some or none of these




toggles will later propagate to Q pin toggles.


Q_PINS
Previous
Toggles on Q pins occur after ECK active edges and are




recorded for the previous transaction.


U_ECK
Current
Output active edges from upstream clock gates must have




occurred prior to the output active edge of this clock gate




otherwise the current output active edge is spurious - not




possibly capturing new information.


D_ECK
Previous
Active edges of downstream clock gates must occur after the




previous output active edge of this clock gate and before the




current output active edge of this clock gate otherwise the




previous output active edge is spurious - information was




discarded by downstream clock gates.


MUST_ECK
Current
Prior to the current output active edge, protocol monitors




have either provided 1, none, or more than 1 MUST_ECK




positive edge indicating that the current output active edge is




either required, spurious, or insufficient with regards to protocol.


CK
Current
Additional CK active edges prior to the output active edge of




the clock gate are claimed as saved energy for the current




transaction.


E
Current
Any enable edges prior to the output active edge are claimed




for the current transaction to provide indication of when the




enable pin is toggling too frequently to allow the clock gate to




save any net energy.


WINDOW
Total Window
Edges of the window pin define the end of the total window,




triggering computation of energy to power followed by




clearing all statistics for the next window.









FIG. 8 is a flowchart showing an example method 800 carried out by a clock gate monitor according to one embodiment. Throughout method 800, the clock gate monitor monitors the outgoing clock edge input for an active clock edge at 802. When an outgoing active clock edge is detected (block 802 YES output), the previous transaction statistics are replaced with the current transaction statistics at 804, and the consequences of the previous transaction are resolved at 806. After the previous transaction has been resolved, counters for the total window are incremented based on the previous transaction statistics at 808.
Incrementing counters for the total window may, for example, involve any or all of the following counters:

    
    
        Protocol_spurious_fJ: incremented if the last ECK edge is spurious with respect to protocol.
        Protocol_ck_edges_lost_fJ: incremented if missing protocol required clock cycles were detected.
        No_receiver_fJ: incremented if no downstream clock gate accepted data from last ECK edge.
        No_transmitter_fJ: incremented if no upstream clock gate sent data for capture by last ECK edge.
        Q_edges_saved_fJ: incremented by difference between observed D edges and Q edges across fanout for last ECK edge using fJ_PER_Q_EDGE[k] for Q bit k, all k.
        Combo_edges_saved_fJ: incremented by difference between observed D edges and Q edges across fanout for last ECK edge.
        ECK_edges_saved_fJ: incremented across fanout clock gates for all edges that otherwise would have been delivered to their fanout.
        E_fJ: incremented for each E (enable) edge between the last active ECK edge and the previous active ECK edge.
        Total_fJ, power consumed by the cluster of cells grouped with the clock gate: incremented by:
        
            Q edges*(fJ_PER_D_TO_Q_EDGE+fJ_PER_Q_EDGE)
            +fJ_PER_ECK_EDGE[0]
            +total_leakage_fJ, (leakage is calculated based on time as known in the art)
        
        
        Net_fJ, negative representing net energy savings: leakage_fj+E_fj−q_edges_saved_fj eck_edgs_saved_fj−fanout_saved_fJ−combo_saved_fJ.
    
    


At 810, the current transaction counters are cleared and the current transaction statistics are reset based on the most recent outgoing active clock edge detected. Resetting the current transaction statistics at 810 may, for example, involve: recording the total CK active edges dropped prior to the current ECK active edge (this contributes to power savings by this clock gate); recording total ECK active edges not transmitted to leaf clock gates when they were otherwise enabled (this contributes to power savings by this clock gate); recording total E edges prior to the new ECK edge (this contributes to power cost of this clock gate); and/or, if protocol applies, recording the number of protocol required ECK active edges prior to the current ECK active edge (this determines if the current edge is required by protocol and whether previous required edges were not delivered).
At 812, counters for the current transaction are incremented based on events at the inputs of the clock gate monitor. Incrementing the current transaction counters at 812 continues as long as no new outgoing active clock edge is detected (block 802 NO output). When a new outgoing active clock edge is detected (block 802 YES output), the method 800 returns to 804 and continues as discussed above.
The examples above include descriptions of ideal clock gating behavior. This is defined by the functional specification of the circuit design. Conventionally, comparing ideal design behavior to actual design behavior is a functional verification exercise. The clock gate monitor embodiments of the present disclosure provide functional power analysis by allowing for comparison of the ideal clock gate behavior with actual clock gate behavior through dynamic, cycle-by-cycle, clock gate power monitoring and processing.
In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the understanding. For example, specific details are not provided as to whether the embodiments described herein are implemented as a software routine, hardware circuit, firmware, or a combination thereof.
Embodiments of the disclosure can be represented as a computer program product stored in a machine-readable medium (also referred to as a computer-readable medium, a processor-readable medium, or a computer usable medium having a computer-readable program code embodied therein). The machine-readable medium can be any suitable tangible, non-transitory medium, including magnetic, optical, or electrical storage medium including a diskette, compact disk read only memory (CD-ROM), memory device (volatile or non-volatile), or similar storage mechanism. The machine-readable medium can contain various sets of instructions, code sequences, configuration information, or other data, which, when executed, cause a processor to perform steps in a method according to an embodiment of the disclosure. Those of ordinary skill in the art will appreciate that other instructions and operations necessary to implement the described implementations can also be stored on the machine-readable medium. The instructions stored on the machine-readable medium can be executed by a processor or other suitable processing device, and can interface with circuitry to perform the described tasks.
The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein, but should be construed in a manner consistent with the specification as a whole.

## Claims

1. An apparatus for monitoring operation of a design under test (DUT), the DUT comprising a plurality of combinational logic elements, a plurality of clocked sequential logic elements, and a plurality of clock gate elements connected to selectively provide clock edges to the clocked sequential logic elements, the apparatus comprising:
a plurality of inputs comprising:
an incoming clock edge input connected to detect active clock edges provided to a monitored clock gate of the plurality of clock gate elements of the DUT;
an outgoing clock edge input connected to detect active clock edges sent from the monitored clock gate;
an enable input connected to detect enable signals provided to the monitored clock gate and any leaf clock gates of the plurality of clock gate elements of the DUT connected to receive clock edges through the monitored clock gate; and
a protocol input connected to receive protocol signals specifying when the monitored clock gate is required to output active clock edges;

a memory in communication with the plurality of inputs for storing values from the plurality of inputs; and
a processor in communication with the memory and the plurality of inputs, the processor programmed to determine protocol compliance and to calculate energy consequences of dropping of active clock edges at the monitored clock gate.

2. The apparatus of claim 1 wherein the plurality of inputs comprises:
a data-in input connected to detect signals on D-pins of sequential logic elements within a fanout of the monitored clock gate, the fanout of the monitored clock gate comprising all of the clocked sequential logic elements connected to receive clock signals through the monitored clock gate, and the combinational logic elements that receive data from the clocked sequential logic elements connected to receive clock signals through the monitored clock gate; and
a data-out input connected to detect signals on Q-pins of sequential logic elements within the fanout of the monitored clock gate,
and wherein the processor is programmed to calculate energy consumed in the fanout of the monitored clock gate.

3. The apparatus of claim 2 wherein the plurality of inputs comprises:
an upstream clocking input connected to detect active clock edges output from clock gates controlling sequential logic elements upstream from the sequential logic elements controlled by the monitored clock gate; and,
a downstream clocking input connected to detect active clock edges output to clock gates controlling sequential logic elements downstream from the sequential logic elements controlled by the monitored clock gate,
wherein the processor is programmed to determine unnecessary active clock edges sent from the monitored clock gate and calculate potential energy savings realizable through elimination of the unnecessary active clock edges.

4. The apparatus of claim 3 wherein the plurality of inputs comprises a timing input connected to receive a time window, and wherein the processor is programmed to determine power savings due to dropping of active clock edges at the monitored clock gate, power consumed in the fanout of the monitored clock gate, and potential power savings realizable through elimination of the unnecessary active clock edges for the time window.

5. The apparatus of claim 3 wherein the processor is programmed to determine that an unnecessary active clock edge is sent from the monitored clock gate when the upstream clocking input indicates that no active clock edge is sent to sequential logic elements upstream from the sequential logic elements controlled by the monitored clock gate.

6. The apparatus of claim 3 wherein the processor is programmed to determine that an unnecessary active clock edge is sent from the monitored clock gate when the downstream clocking input indicates that no active clock edge is sent to sequential logic elements downstream from the sequential logic elements controlled by the monitored clock gate.

7. The apparatus of claim 2 wherein the plurality of inputs comprises a timing input connected to receive a time window, and wherein the processor is programmed to determine power savings due to dropping of active clock edges at the monitored clock gate, and power consumed in the fanout of the monitored clock gate, for the time window.

8. The apparatus of claim 2 wherein the processor is programmed to calculate energy consumed in the fanout of the monitored clock gate based on a number of sequential logic elements that change signal levels on their Q-pins.

9. The apparatus of claim 1 wherein the plurality of inputs comprises a timing input connected to receive a time window, and wherein the processor is programmed to determine power savings due to dropping of active clock edges at the monitored clock gate for the time window.

10. The apparatus of claim 1 wherein the processor is programmed to attribute energy which would have been consumed by any leaf clock gates connected to receive clock edges through the monitored clock gate as energy saved due to dropping of an active clock edge at the monitored clock gate when the enable input indicates that the leaf clock gates were enabled when the active edge was dropped.

11. A method for monitoring operation of a design under test (DUT), the DUT comprising a plurality of combinational logic elements, a plurality of clocked sequential logic elements, and a plurality of clock gate elements connected to selectively provide clock edges to the clocked sequential logic elements, the method comprising:
detecting active clock edges provided to a monitored clock gate of the plurality of clock gate elements of the DUT;
detecting active clock edges sent from the monitored clock gate;
detecting enable signals provided to the monitored clock gate and any leaf clock gates of the plurality of clock gate elements of the DUT connected to receive clock edges through the monitored clock gate;
receiving protocol signals specifying when the monitored clock gate is required to output active clock edges;
determining protocol compliance by comparing the active clock edges sent from the monitored clock gate to a set of required edges specified by the protocol signals; and
calculating, using a processor, energy consequences of dropping of active clock edges at the monitored clock gate by comparing the active clock edges provided to a monitored clock gate with the active clock edges sent from the monitored clock gate.

12. The method of claim 11 comprising attributing energy which would have been consumed by any leaf clock gates connected to receive clock edges through the monitored clock gate as energy saved due to dropping of an active clock edge at the monitored clock gate when the enable signals indicate that the leaf clock gates were enabled when the active edge was dropped.

13. The method of claim 12 comprising:
detecting signals on D-pins of sequential logic elements within a fanout of the monitored clock gate, the fanout of the monitored clock gate comprising all of the clocked sequential logic elements connected to receive clock signals through the monitored clock gate, and the combinational logic elements that receive data from the clocked sequential logic elements connected to receive clock signals through the monitored clock gate;
detecting signals on Q-pins of sequential logic elements within the fanout of the monitored clock gate; and
calculating energy consumed in the fanout of the monitored clock gate based on the detected signals on the D-pins and Q-pins.

14. The method of claim 13 comprising calculating energy consumed in the fanout of the monitored clock gate based on a number of sequential logic elements that change signal levels on their Q-pins.

15. The method of claim 13 comprising:
detecting active clock edges output from clock gates controlling sequential logic elements upstream from the sequential logic elements controlled by the monitored clock gate;
detecting active clock edges output to clock gates controlling sequential logic elements downstream from the sequential logic elements controlled by the monitored clock gate; and
determining unnecessary active clock edges sent from the monitored clock gate and calculating potential energy savings realizable through elimination of the unnecessary active clock edges.

16. The method of claim 15 comprising determining that an unnecessary active clock edge is sent from the monitored clock gate when the upstream clocking input indicates that no active clock edge is sent to sequential logic elements upstream from the sequential logic elements controlled by the monitored clock gate.

17. The method of claim 15 comprising: determine that an unnecessary active clock edge is sent from the monitored clock gate when the downstream clocking input indicates that no active clock edge is sent to sequential logic elements downstream from the sequential logic elements controlled by the monitored clock gate.

18. The method of claim 15 comprising:
receiving a time window; and
determining power savings due to dropping of active clock edges at the monitored clock gate, power consumed in the fanout of the monitored clock gate, and potential power savings realizable through elimination of the unnecessary active clock edges for the time window.

19. The method of claim 15 comprising:
defining a new transaction as commencing at each detected active clock edge sent from the monitored clock gate; and,
maintaining statistics for at least a current transaction and a previous transaction.

20. The method of claim 19 comprising, upon detecting each active clock edge sent from the monitored clock gate:
replacing a set of previous transaction statistics with a set of current transaction statistics;
resolving energy consequences of the previous transaction based on the set of previous transaction statistics; and,
resetting the set of current transaction statistics based on data corresponding to a newest detected active clock edge sent from the monitored clock gate.