Project description: |
Objectives
The objective of this MSc thesis work is to investigate optimal design under
the presence of static gate leakages, and to device how design rules and
trade-offs are altered.Description
A main concern during the design of System-on Chips (SOCs) is the power
budget, especially battery supplied systems are considered. In general,
dynamic and static contributions constitute total power dissipation.
Dynamic power is primarily consumed by the information processing in the
charging and discharging of internal capacitances. As such, dynamic power
consumption is proportional to these capacitances, the switching frequency
and the supply voltage. Static power consumption, on the other hand, is
caused by leakage currents while the circuit is idle, i.e. not performing
computations.
One key attraction of CMOS is negligible static power consumption. However,
with decreasing device sizes this property is no longer satisfied due to
subtreshold conduction. The reason for this is that for smaller devices,
supply voltages are reduced. For speed, this in turn forces a reduction in
threshold voltages. As a consequence, transistors are no longer turned off
satisfactorily, i.e. drain currents contributes significantly to power
losses in the transistor non-conductive state. For a 0.13µm process, the
static losses may constitute almost 50% of the total power consumption.
The issue has been addressed by offering libraries of gates and cells in
both low-VT and high-VT versions. This offers the option of fast, low-VT
cells with high static power losses where timing is critical, and a slower,
high-VT design for other parts. Traditional synthesis tools do not offer the
means to optimize for multiple-VT libraries to reduce static power
consumption. The solution, using such known synthesis tools, consists of
synthesizing a design using a low-VT library, under the constraint that
timing and performance requirements are met. Then, in a post-synthesis phase,
the back-annotated circuit is analyzed with respect to power consumption and
the circuit modified, replacing low-VT by high-VT library cells whereever
possible. The update process does not involve any re-synthesis steps.
This thesis work addresses how to incorporate static power consumption as
part of the synthesis process, utilizing the detailed output of a power
simulation in the synthesis.
A method for multiple-VT synthesis is derived, and the capability
of the method is compared to the capabilities of the Synopsys design tool.
The thesis work will be performed in parallel with two other MSc thesis
works in a collaborative but independent effort. One work focusses on the
design of libraries offering various speed to power alternatives, while the
other work concentrates on the architectural aspects of multiple-VT
libraries. |