Power aware verification of ARM-based designs

Power dissipation has become a key constraint for the design of today’s complex chips. Minimizing power dissipation is essential for battery-powered portable devices, as well as for reducing cooling requirements for non-portable systems. Such minimization requires active power management built into a device.
This article is from class ATC-155 at ARM Technology Conference. Click here for more information about the conference.

In a System-on-Chip (SoC) design with active power management, various subsystems can be independently powered up or down, and/or powered at different voltage levels. It is important to verify that the SoC works correctly under active power management.

When a given subsystem is turned off, its state will be lost, unless some or all of the state is explicitly retained during power down. When that subsystem is powered up again, it must either be reset, or it must restore its previous state from the retained state, or some combination thereof. When a subsystem is powered down, it must not interfere with the normal operation of the rest of the SoC.

Power aware verification is essential to verify the operation of a design under active power management, including the power management architecture, state retention and restoration of subsystems when powered down, and the interaction of subsystems in various power states. In this presentation, we summarize the challenges of power aware verification and describe the use of IEEE 1801-2009 Unified Power Format (UPF) to define power management architecture. We outline the requirements and essential coverage goals for verifying a power-managed ARM-based SoC design.

Critical design constraints

The continual scaling of transistors and the end of voltage scaling has made power one of the critical design constraints in the design flow. Trying to maintain performance levels and achieve faster speeds by scaling supply and threshold voltages increases the subthreshold leakage current due to exponential relationships with threshold voltage .

Leakage currents lead to power dissipation even when the circuit is not doing any useful work, which limits operation time between charges for battery-operated devices, and creates a heat dissipation problem for all devices.

Minimizing power dissipation starts with minimizing the dynamic power dissipation associated with the clock tree, by turning off the clock for subsystems that are not in use. This technique has been in use for many years. But at 90nm and below, static leakage becomes the dominant form of power dissipation. Active power management minimizes static leakage through various techniques, such as shutting off the power to unused subsystems or varying the supply voltage or threshhold voltage for a given component to achieve the functionality and performance required with minimum power.

Active power management
Active power management can be thought of as having three major aspects:

-the power management architecture, which involves the partitioning of the system into separately controlled power domains, and the logic required to power those domains; mediate their interactions, and control their behavior;


-the power managed behavior of the design, which involves the dynamic operation of power domains as they are powered up and down under active power management, as well as the dynamic interactions of those power domains to achieve system functionality;

-the power control logic that ultimately drives the control inputs to the power management architecture, which may be implemented in hardware or software or a combination thereof.

All three of these aspects need to be verified to ensure that the design will work properly under active power management. Ideally such verification should be done at the RTL stage. This enables verification of the active power management capability much more efficiently than would be possible at the gate level, which in turn allows more time for consideration of alternative power management architectures and simplifies debugging.

Power management techniques

Several power management techniques are used to minimize power dissipation: clock gating, power gating, voltage scaling, and body biasing are four of them. Clock gating disables the clock of an unused device, to eliminate dynamic power consumption by the clock tree. Power gating uses a current switch to cut off a circuit from its power supply rails during standby mode, to eliminate static leakage when the circuit is not in use.

Voltage scaling changes the voltage and clock frequency to match the performance required for a given operation so as to minimize leakage. Body biasing changes the threshhold voltage to reduce leakage current at the expense of slower switching times.

Power gating is one of the most common active power management techniques. Switching off the power to a subsystem when it is not in use eliminates the leakage current in that subsystem when it is powered down, and hence the overall leakage power dissipation through that subsystem is reduced. However, this technique also results in loss of state in the subsystem when it is switched off. Also, the outputs of a power domain can float to unpredictable values when they are powered down.

Another common technique is the use of different supply voltage levels for different subsystems. A subsystem that has a higher voltage supply can change state more quickly and therefore operate with higher performance, at the expense of higher static leakage and dynamic power.

A subsystem with a lower voltage supply cannot change state as quickly, and consequently operates with lower performance, but also with less static leakage and dynamic power. This technique allows designers to minimize static leakage in areas where higher performance is not required.

Multiple voltage supplies can also be used for a single subsystem, for example, by enabling it to dynamically switch between a higher voltage supply and a lower voltage supply.

This allows the system to select higher performance for that subsystem when necessary, but minimize static leakage when high performance is not required. Multi-voltage and power gating techniques can be combined to give a range of power/performance options.

All of these power management techniques must be implemented in a manner that preserves the intended functionality of the design. This requires creation of power management logic to ensure that the design operates correctly as the power supplies to its various components are switched on and off or switched between voltage levels. Since this power management logic could potentially affect the functionality of the design, it is important to verify the power management logic early in the design cycle, to avoid costly respins.

Power management specification

The power management architecture for a given design could be defined as part of the design, and ultimately it will be a part of the design’s implementation. A better approach, however, is to specify the power management architecture separate from the design. This simplifies exploration of alternative power management architectures, reduces the likelihood of unintended changes to the golden design functionality, and maintains the reusability of the design data.

This is the approach supported by IEEE 1801-2009, "Standard for Design and Verification of Low Power Integrated Circuits." This standard is also known as the Unified Power Format (UPF) version 2.0. Initially developed by Accellera, UPF is currently supported by multiple vendors and is in use worldwide.

UPF provides the concepts and notation required to define the power management architecture for a design. A UPF specification can be used to drive the implementation of power management for a given design, during synthesis or subsequent implementation steps.

A UPF specification can also be used to drive verification of power management, during RTL simulation, gate-level simulation, or even via static verification methods. The ability to use UPF in conjunction with RTL simulation enables early verification of the power management architecture. The ability to use UPF across all of these applications eases implementation and validation by enabling reuse of power management specifications throughout the flow.

UPF syntax is defined as an extension of Tcl, which enables UPF descriptions to leverage all of the control features of Tcl. UPF captures the power management architecture in a portable form for use in simulation, synthesis, and routing, reducing potential omissions during translation of that intent from tool to tool. Because it is separate from the HDL description and can be read by all of the tools in the flow, the UPF side file is as portable and interoperable as the logic design’s HDL code.

Information is shared by www.irvs.info