Pest This program produces a quick, rough estimate of the
energy consumed by a given circuit for a given simulation, as
represented by the .out trace file produced by a TimeMill® simulation, together with
the corresponding capacitance information. A list is produced of the
energy consumption at each node in the design, together with the total
energy consumed by the circuit. The power may then be obtained by
dividing these energy results by the simulated duration.
The results produced are only an estimate and even though the results
a presented in terms of Joules, the actual numbers should be treated
with a pinch of salt since there are a number of inaccuracies in the
calculations used. Nevertheless, the results do make it clear which
are the most energy consuming sections of a circuit and can be used to
give a rough feel for the total energy consumption. If you require
more accurate power analyses and you have the money, the producers of TimeMill® have a companion tool called
pest tracefile [capfile]
Output is written to stdout. Errors are logged on stderr.
- The name of the design unit you wish to process. The name may
include the .out extension for the trace file, but the .out
extensions is assumed if it is not given. This filename may be used
to derive the name of the capacitance file.
- The name of the file that contains the capacitance at each node
(as produced by TimeMill®). The name may
include the .cap extension for this file, but that will be assumed
if it is not explicitly included. If this optional argument is not
given on the command line the capacitance information is searched
for in tracefile.cap. The capacitances are assumed to be
measured in femtofarads.
The output produced by the tool should be self explanatory, but can be
very long winded since it lists the energy consumption for every
node. To avoid having to read the whole file, the most important
information is placed at the start of the file, starting with the
total energy consumption, followed by the list of the energy
consumed at each node, sorted in descending order of consumption.
estimation is based on the calculation that the energy consumed by a
single signal transition is: 1/2 * C * V^2. This represents the energy
required to change the voltage at this particular node in the face of
the capacitance attached to it. By summing this result for all the
transitions, for all the nodes, a total energy consumption may be
This calculation is based on the dynamic power consumption which is
the largest factor in the total power consumption for CMOS
circuits. It ignores the static leakage current and the crowbar
current that flows through the transistor stack during the short time
that both the P and N transistors are active. It has been suggested
that these sources of power consumption account for 10-20% of the
total power, so the tool may be expected to produce an under estimate
of the power consumption.
Other inaccuracies also creep into the estimate:
- The first transition on every node is ignored, since this is
assumed to be a statement of the initialisation state of that node,
and this power model is intended for use in measuring a running
circuit, not the start-up costs.
- The calculation is dependent on the accuracy of the capacitance
figures passed to the tool.
- Precise information about the actual voltage swing for a
transition is not available, so the tool assumes that all
transitions cover the full 0V to VDD range. The value of VDD is
automatically extracted from the .out file. This full swing
assumption is not unreasonable for CMOS circuits, where most signals
will involve full range transitions. In practice, however, some signals
will not show a full range transition, so the tool will produce an
overestimate of the dynamic power consumption.
The information about signals that is actually available to the tool
indicates when the voltage crosses the low and high logic thresholds
in place for the given simulation. Therefore the
alternative to assuming VDD swings would be to base the calculation
on the difference between the two logic thresholds, but that would
be likely to produce a considerable underestimate.
- The tool only considers full transitions between the two logic
thresholds. A signal might, for example, start at logic 0 move up to
an undefined state, then be driven back to 0. Those voltage changes
would result in power consumption, but they will be ignored by the
Perl release 5.001d or greater.
The copyright and any intellectual property rights associated with this
work are retained by the author and the University of Manchester, but
permission is granted to freely copy, distribute, modify and use this
software under certain conditions, as described in the Artistic Licence (with the deletion of
clause 8). This licence is obtained from the Perl distribution.
Subject to the licensing arrangements described above, the source code may
be obtained from http://www.cs.man.ac.uk/amulet/projects/horn/pest
TimeMill is a registered trademark of EPIC Design Technology
Inc, 2901 Tasman drive, Suite 212, Sata Clara, CA 95054.
This software was developed as part of the OMI HORN
project at the University
of Manchester Department of Computer Science, funded as part of
the European Union ESPRIT initiative (project number 7249).
Rhod Davies (email@example.com)
This page last modified
Tue Apr 9 1996 10:45:22