XPLANATION:FPGA101
with 90-nm technology. (Note: While
dynamic power varies with the square of
V CCINT, it is largely temperature and process
invariant for the core of the FPGA.)
FPGA Power Analysis Tools
Xilinx has two types of power analysis tools.
We designed the first, the XPower
Estimator (XPE) spreadsheet tool, for use
before a designer employs implementation
tools. Use the second tool, XPower
Analyzer, after you have implemented your
design to check how the changes you’ve
made affect power consumption.
The XPower Estimator gives you a
quick power estimation based on user
descriptions of resource utilization in the
FPGA, toggle rates, loading, etc. in a
spreadsheet environment. This is the tool
to use for your initial power evaluation,
selection of power supplies and regulator,
as well as any cooling solutions for the system
(heat sinks, fans and the like).
With this Microsoft Excel-based tool,
system architects can make device-, designand
system-oriented power decisions. You
simply enter the estimated design parameters,
such as resource utilization, operating
environment, and clock and toggle rates.
XPE then calculates estimated power for a
given design and reports total power and
maximum junction temperature as well as
rail-based and block-based power.
In setting up the estimation run, the
tool’s Process function is an important feature.
It allows you to see typical or worstcase
power consumption by various blocks.
Primarily, the static power from leakage on
the V CCINT supply is very process dependent.
Further, the Voltage Source Summary
lets you quickly see the effect on power
consumption when voltages are varied.
That is especially important to understand
relative to V CCINT, which is one of the
power supplies representing all the core
logic. Both the process variation and voltage
variation selections in the XPE tool
ensure that you can determine proper
worst-case power supply sizing.
One other valuable feature of XPE is the
Thermal Information/Summary, which
allows you to specify heat sink, PCB properties
and temperature information. This
ensures that the design will also meet the
thermal specifications for commercialgrade
or industrial-grade devices. The
Block Summary, meanwhile, shows the
power from each block and the Power
Summary displays the sum of the quiescent
and dynamic power.
Each of the tabs in the XPE tool allows
you to enter utilization and toggle rates for
a given type of resource, such as clocks,
logic, I/O, block RAM (BRAM), PLLs,
DSP and so on.
Finally, XPE’s Graphs tab/sheet gives
you a graphical look at power by function,
process, voltage and temperature varia-
tion. The Power by Function graphic, in
particular, lists each feature and shows its
power consumption, allowing you to
identify features that could best benefit
from optimization.
The second Xilinx power analysis tool,
XPower Analyzer, provides an even more
accurate view of the power breakdown
based on exact resource information it
extracts during the implementation. You
can supply the tool with test and simulation
vectors, or perform vectorless power
estimation. This tool uses characterized
capacitance data for physical resources in
the FPGA design.
XPower Analyzer is tied into the Xilinx
Integrated Software Environment (ISE ®),
and it accepts post-place-and-route information
from several internal Xilinx file formats.
It also accepts industry-standard Value
38 Xcell Journal First Quarter 2009
Figure 3 – Xilinx XPower Analyzer summary page
Change Dump (VCD) and Switching
Activity Interchange Format (SAIF) files.
If you are using either the VCD or SAIF
formats, you need to create representative
simulation vectors so the tool can record the
toggle rates of nodes in the system, which in
turn allows you to access the data later. In
the absence of these simulation files, the
user can have the XPower Analyzer tool perform
a vectorless simulation. This type of
simulation uses mathematical and statistical
modeling to propagate starting toggle rates
through the actual design logic. It then generates
a result containing toggle rates of each
node in the design.
With both the vector-based (from
VCD and SAIF) files and the vectorless
variety, XPower considers physical connectivity
of the placed-and-routed design and
exact resource usage. The tool cross-references
the activity or toggle rate at each
node with characterized capacitance data
for physical resources and individual
dynamic power consumption of each
block at given toggle rates. The result,
shown in Figure 3, represents total power
and maximum junction temperature, and
contains rail-based, block-based and hierarchically
based power reporting.
XPower gives you a detailed look at
where your design is consuming power and
lets you do “what if” analysis to make
more-informed choices for which blocks
could most benefit from optimizations,
ranging from simple ones through rearchi-
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