Dynamic Voltage (IR) Drop Analysis and Design Closure: Issues and
Challenges
Nithin S K, Gowrysankar Shanmugam, Sreeram Chandrasekar
Texas Instruments India
E-mail: {nithin,gowrysankar,sreeram}@ti.com
Abstract—Dynamic voltage (IR) drop, unlike the static
voltage drop depends on the switching activity of the design, and hence it is vector dependent. In this paper we
have highlighted the pitfalls in the common design closure
methodology that addresses static IR drop well, but often fails to bound the impact of dynamic voltage drops
robustly. Factors that can affect the accuracy of dynamic
IR analysis and the related metrics for design closure are
discussed. A structured approach to planning the power
distribution and grid for power managed designs is then
presented, with an emphasis to cover realistic application
scenarios, and how it can be done early in the design cycle. Care-about and solutions to avoid and fix the Dynamic
voltage drop issues are also presented. Results are from industrial designs in 45nm process are presented related to
the said topics.
Keywords—Dynamic voltage Drop, DvD, Dynamic IR,
Peak power, Power switch, VCD, Power gate, SDF.
I. Introduction
Designing an optimal power grid which is robust across
multiple operating scenarios of a chip continues to be a major challenge.[1][2][3] The problem has magnified with technology shrinking allowing more performance to be packed
in a smaller area, from one node to another [4]. The power
distribution on a chip needs to ensure circuit robustness
catering to not only to the average power / current requirements, but also needs to ensure timing or reliability is
not affected due to Dynamic IR drop, caused by localized
power demand and switching patterns. [5]
Further, amongst today’s devices power management
techniques like power gating and switch power supplies are
the norms [6][7][8]. In the case of switched power supplies, typically, power switch cells are uniformly distributed
across the standard cell logic (logic gates) area of the floorplan. There may be further sub-divisions in the switched
power grid in the form of power domains, depending on the
granularity of power gating [10]. These power switches add
an additional dimension to the power distribution problem
as they often limit the response of the power grid to dynamic power or current needs. While the power distribution robustness can be improved easily by increasing the
number of power switches, it has an impact on the offmode leakage (Iddq) and hence battery life in handheld
applications. So clearly, the requirement is also to minimize the number of switches used as well as minimize the
signal routing resources utilized on the power grid.
This paper discusses the issues related to design closure
and signoff (timing, IR Drop, EM, reliability etc.) comprehending Dynamic IR drop effects realistically. On one
hand, the factors that introduce pessimism in Dynamic
voltage drop analysis have to be removed, while on the
other we must ensure the methodology ensures robust coverage of various silicon conditions and design operating
scenarios. We then discuss power distribution and power
grid planning methodology, and highlight the various aspects that need to be taken care of, from the early stages
of design implementation. We also demonstrate some of
the systematic power grid enhancements like robust automated switch placement and switched supply resistance
minimization through DRC-aware power metal fill. All
the discussions and results are based on production implementations of low power application processors for mobile and hand-held devices. The designs include high frequency CPU cores, multimedia subsystems (like imaging
and video). The numbers quoted are from the analysis
and/or simulation.
The structure of the paper is as follows. In section II ,
the commonly followed Dynamic IR methodology and its
pitfalls are highlighted with design results. In section III
the issues related to analysis accuracy and signoff methodology are discussed. Section IV then elaborates how we
went about planning the power distribution and the techniques used to ensure silicon robustness in the tolerant to
Dynamic IR drop.
II. Common Design Closure Methodology and Its
Pitfalls
A. Overview Of Static Vs Dynamic IR Drop
Static IR drop is average voltage drop for the design.[12][13], whereas Dynamic IR drop depends on the
switching activity of the logic[11], hence is vector dependent. Dynamic IR drop depends on the switching time of
the logic, and is less dependent on the a clock period. This
nature is illustrated in Fig 1. The Average current depends
totally on the time period, where as the dynamic IR drop
depends on the instantanious current which is higher while
the cell is switching.
Static IR drop was good for signoff analysis in older
technology nodes where sufficient natural decoupling capacitance from the power network and non-switching logic
were available. Where as Dynamic IR drop Evaluates the
IR drop caused when large amounts of circuitry switch simultaneously, causing peak current demand[1][14]. This
current demand could be highly localized and could be
brief within a single clock cycle (a few hundred ps), and
Fig. 1. Average Current Over A Window
Fig. 2. Effect Of Low Switch Density In Notch
could result in an IR drop that causes additional setup or
hold-time violations. Typically, high IR drop impact on
clock networks causes hold-time violations, while IR drop
on data path signal nets causes setup-time violations.
B. Deficiencies Found By Dynamic Analysis On A
“Good” Power Grid
A typical power grid and power switches (count and distribution) are designed for average power or in other words
they are designed to meet static IR drop targets and not
for Dynamic IR drop. In the initial stage of the design,
the grid robustness is checked only with the Static IR drop
result. This is because of late availability of use case scenarios (Voltage change dump (VCD) files). For the example,
the switch and metal grid densities in the notches region
can satisfy the static IR drop criteria, because the average
power density in this region is not significant.
But when a particular application is run, notch area
could have higher power density because of localized
switching in that area and the switches combined with
metal grid (Switched supply is distributed to cells by lower
layers like MET2 and MET3) may not be enough to support the current density in the notch area. Because of
which there can be very high dynamic IR drop. Refer to
Notch area as shown in Fig. 2, Here due to less number of
switch cells combined with not so robust power grid is the
main cause of high dynamic IR drop. As described by the
figure, Switch Voltage drop and MET3 voltage drop are
the dominant factors in the overall voltage drop. A similar
analogy on the power density can be extended to larger
region.
Refer to Fig. 3, With the original MET3 grid, static IR
drops was within the budget. However, to meet the dynamic IR drop goals, an increase of the MET3 (MET3 Grid
is Vertical) grid density by 3 times, was needed. The drop
across the MET3 and related vias reduced by 50%, after
the improvement. This is another example of a robustness
issue which was missed in static analysis.
As discussed earlier, the number of power switches is cal-
Fig. 3. Effect Of MET3 Grid On Dyanmic IR drop
Fig. 4. Closer View Of Dynamic IR Drop
culated based on the static IR drop requirement. For our
design, with the switch density that is calculated as per
average power, and with “calculated” optimal cell density
and optimal decap density, our expectation is to have a
good dynamic voltage drop. Static IR drop and vectorless
dynamic results runs were within the budgets. Vectorless
dynamic IR drop was 70mV, but vector based dynamic IR
drop was 153 mV, which is beyond the budget. The main
cause for such high voltage drop was localized switching.
The high dynamic IR drop region has very high power density and hence this region has high current requirement,
which is not fulfilled by the existing power switch density
in that region, and as a result there is high Dynamic voltage
drop. The High IR drop region has reasonably good decap
density and has low utilization as shown in the Fig. 4. This
indicates that the affected region is not really a case of a
poorly designed power grid, but more of an exceptionally
high power density, due to the design architecture combined with the placement of cells. In any case, the power
grid has to eventually be able to support the design’s power
demands in that region, which requires a different approach
as will be discussed later.
III. Accuracy Of Analysis
A. Comprehending Delays In Gate Simulation
Fig. 5. Dynamic IR Drop: SDF Annotated VCD
There are several factors that affect the accuracy of the
dynamic IR analysis, and how closely it represents the nature of actual Silicon behavior. One of the key requisites is
to generate a realistic VCD (a file format that captures the
switching information) which accounts for the real cell and
interconnect delays (typically done by annotating an SDF
in the gate simulation). Such a simulation captures the realistic spread of switching activity in the design. The other
common approach is to use a VCD from a zero-delay simulation, along with the timing windows from STA analysis,
which often results in non-realistic Dynamic IR drop that
can be pessimistic or optimistic. Refer to Fig. 5 (SDF Annotated VCD) and Fig. 6 (Without SDF annotated VCD).
It shows a drop close to 175 mV with a VCD generated
with SDF, versus Vs 141 mV from analysis using a VCD
without SDF annotation. In this case, 175mV is the more
realistic result for the given application. Also, the analysis
needs to be done for more than 1 cycle because this would
expose more weak spots and allow sufficient pre-simulation
time for the decap effects to be comprehended more accurately.
Fig. 6. Dynamic IR Drop: Without SDF Annotated VCD
B. Comprehending Realistic Glitch Propagation
Glitches arising out of combinational logic switching can
cause a large amount of instantaneous switching. It is important to factor the effect of such switching, with consideration to which of these glitches would die down or
propagate, considering cell and interconnect delays under
realistic conditions. If the glitches are very narrow, the
chances of them getting filtered out by the inertial delay
of the path stages (cell + interconnect) is very high. We
filtered out glitches much smaller than the stage delay, and
let those comparable to (or larger than) the stage delay
propagate. The glitches in between were kept as ’x’. We
found that the pessimism in the dynamic voltage drop reduced by 20% by using this approach.
C. Choice Of Technology Specs For Signoff
Often, worst case conditions are chosen for timing, electrical and reliability checks to ensure robust silicon operation. However, it is also critical to strike a balance between
picking bounding conditions and being overly pessimistic.
In an effort to get results closer to realistic silicon conditions, and to detect potentially silicon fails, we selectively
evaluated designs under both worst case and non-worstcase conditions. For example if we compare the effect of
the worst via resistance spec against the nominal via specs,
the drop across vias alone reduce by 50%, as show in Fig. 7.
With Via resistance and Metal resistances typically being
Fig. 7. Via Drop: Worst corner Vs Nom Corner
uncorrelated, it is a pessimistic assumption to consider that
all vias and metal layers would be in the worst case corner.
With sufficient characterization data, we can apply a less
pessimistic analysis condition for dynamic IR analysis.
D. Voltage Annotated Timing Closure
Timing impact has been analyzed with dynamic voltage
annotation in the STA tool. The voltage annotated timing
violations on one particular design before any fixes can be
seen in Table I. It was ensured that the frequency goals
were met by fixing these violations, either addressing the
voltage drop itself, or at least by improving timing slack
on those paths.
Design
IP1
IP2
IP3
IP4
Worst Slack (ps)
-251
-347
-30
-37
Failing End Points
370
95
8
2
TABLE I
Dynamic IR Drop Annotated Timing
IV. Methodology For power Grid Design For Robust Dynamic IR
In this section, the care-about in planning the power distribution (grid, switches) for power managed designs are
discussed. Knowledge of the design operating scenarios
and architecture play a key role in ensuring the robustness across scenarios. Some techniques to improve power
grid robustness through simple physical implementation
schemes such as power metal fill and decap planning are
also touched upon.
A. Choosing The Right Average Power
The choice of the average power value for which the
power distribution is designed for is critical. It is common
practice to design for the average power seen in the use case
that consumes the highest power. However, there can be a
sub-window within the application window, for which the
average power is much higher than that of the entire use
case time. It is obvious that the grid has to support this
Fig. 8. Average Power Vs Peak Average Power
higher average power during the high-power sub-window,
else the device would not function as per design. An example of this is shown in Fig. 8, where the application average
power is about 214 mW where as the average power over a
sub-window is 367mW. This sub window extends over a few
hundred clock cycles. In this case, the grid has to support
367 mW of average power and not 214mW. Hence, choosing the right average power for designing the grid would
help the design scale up to not just dynamic voltage drop
issues, but even to sustain the average cases more robustly.
B. Early Dynamic IR Analysis
One of the difficulties in evaluating the dynamic IR impact on SOCs or complex designs (IPs) is to get vectors
for sufficient scenarios, and to get them in time to detect
issues before the design tapes out. Our Early Analysis flow
addresses this issue. In this flow, the switching activity of a
sub IP is integrated at the top level, and switching activity
at the top level is created, for use in dynamic IR analysis.
Using this flow, we were able to identify certain architectural hot-spots for dynamic IR drop, like cases of crossbar
interconnects interacting with shared memories having very
high power density. The results obtained from this flow
Fig. 11. Region Based Switch Density
Fig. 9. Dynamic IR Drop Profile Using Early Analysis Flow
list of IPs/Modules which consume more power than the
rest of the design. This means that these IPs/Modules
need higher current. Which implies that there is a need for
more switches in these modules. Typically standard cells of
sub IPs/Modules are placed within close proximity. Hence
planning a higher switch density in this area will make the
area better in terms of dynamic IR drop. Covering more
scenarios (More VCD) will excite different parts of design
and hence will show any weakness in the power network.
Refer to Fig. 11 for region based switch density. Covering
more scenario will also show the area where the voltage
drop is low (cool area), the regions which do not have high
IR drop region. In the cool area, switch density can be reduced by removing some of the switches. This will help in
reducing the leakage power of the design in standby mode.
D. Switch Placement In Floorplan Channels /
Boundaries
Fig. 10. Dynamic IR Drop Profile From Full Subsystem Simulation
were found to correlate well with the analysis done with
the complete simulation done at the top level of the subsystem itself. Both cases are shown in Fig. 9 and Fig. 10,
where we can notice both the magnitude and the profile
of the dynamic IR results match closely (The first map is
based on the sub-design switching ported to the top level
while the second map is with switching information from
full design simulation ). This technique can be extended
to SOC’s, to do vector based dynamic IR drop analysis
accurately.
C. Power Switch Density And Placement
For designs with power switches, in most cases, high
voltage drop is because of lesser number of switches than
needed for localized power density in certain regions. From
common power analysis methods, it is possible to get a
Channels (between macro cells) and floorplan edges or
boundaries are often weak spots in a design’s power distribution scheme. It was highlighted earlier how a channel
with power switches placed a bit far from the high switching activity logic gave rise to a dynamic IR hot spot (Refer
to Fig. 2). To address such issues, we have implemented
an automated bounding scheme where all the standard cell
logic area in the floorplan is surrounded by power switches
at the boundaries. Refer to Fig. 12. The switch cell bounding is done over the corners of channel, making it more
robust to voltage drop variations.
E. Using Design Knowledge To Reduce Dynamic
IR
In one of our design, the architecture of the design was
such that, a group of registers banks switching simultaneously, and these banks would switch in every cycle. Also
these groups of register banks and associated cells are physically placed close to each another. The Clock to some of
the flops were skewed so as to stagger the switching which
will reduce the switching activity (These timing paths had
high positive slacks). This will reduce the peak current
requirement and hence reduce the peak drop. Refer to
Fig. 12. Switch Density In Notches
Fig. 14. Metal density without Vs with Metal fill
Fig. 13. Staggering Switching Activity To Reduce Dynamic IR Drop
the metal fill is done on 2 layers and 2% improvement when
metal fill was done on all layers.
Fig. 13, the switching activity last for around 200ps, where
as the clock period is higher, Thus we have used the design
knowledge to reduce switching activity.
F. Power/Ground Metal Fill
Experiment
Without Metal Fill
Metal Fill on 2 Layers
Metal Fill on All layers
%Drop
9.3
8.4
7.3
% Improvement
0.9
2
TABLE II
Using Power/Ground Metal Fill to improve power grid
robustness
Another technique we followed was Power/Ground Metal
fill. After the design is frozen, final step is to add metal
fill in the areas where the free metal tracks are available.
These inserted metal straps are connected power or ground.
Refer to Fig. 14. By doing so, the power and ground grid
becomes stronger and hence would help in reducing voltage
drop. Refer to Table.1. We have seen that as much as 0.9%
(0.9% of supply voltage) improvement in voltage drop when
G. Other Methods To Reduce Dynamic IR Drop
Load and Slew violation will not only cause crosstalk
but also cause high power. This is because, there will be
high current requirement for higher loads/slews. Hence
fixing load/slew violation will help in reducing dynamic
voltage drop. Another method to reduce Dynamic IR drop
is haloing of Clock tree cells, and adding decaps near these
cells. This will help in reducing the voltage drop in clock
tree cells due to switching.
V. Conclusion
We have highlighted the common issues faced in the design closure of power managed designs . Key accuracy
and signoff methodology issues were addressed and improvements made in replicating actual device operating
conditions in analysis. A comprehensive set of techniques
adopted in our designs to create a robust power grid, and to
ensure device timing robustness considering dynamic voltage drop, was presented. This covered the choice of the
correct power values, power switch planning, using design
knowledge and power routing techniques.
A. Future Work
The main area of our ongoing work is with respect to
comprehending the impact of dynamic IR on timing behavior of the device - path level, and timing yield. Another
area of study is on the coverage of multiple scenarios without having to simulate each of them (which is impossible,
and hence vector based analysis is not complete today).
Further, dynamic IR impact on test modes are presently
being studied. Efforts are on to correlate analysis and silicon measurements to establish a close link between analysis
and real device operation.
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