Cross coverage of power states

Power states lie at the core of power aware verification. They capture various operational modes of a design. In order to ensure that a low power design is functionally correct, it becomes essential to ensure that all power states are covered in a comprehensive manner. Hence it becomes important to adopt a coverage-driven methodology for verification. However, success of verification depends on the comprehensiveness of the coverage metrics being used. Traditional code and functional coverage metrics would not suffice here because the power states are defined in the UPF (Unified Power Format) in an abstract manner. Hence, the need arises to bring out new low power specific coverage metrics.

In this paper, we would build on the methodology devised for state and transitions coverage of power states to define a new metric that captures these requirements. We name this metric as cross coverage of power states. In our opinion, this metric is more meaningful in capturing coverage of interdependent systems/subsystems. For the lack of space we would restrict the scope of this paper to power domains only. However, the same approach can be applied for supply sets also.

Power states in UPF

Power states represent supply information along with operational modes of design elements. IEEE Standard 1801-2013 Unified Power Format (UPF) provides a set of commands that can be used to describe power states.

These states can be associated with following UPF objects:

• Supply ports and nets
• Supply sets
• Power domains

A power domain is a collection of HDL instances that are treated as a group for power-management purposes. These instances are powered with the same supplies. Power states of such a domain can be defined in terms of the power states of supply sets associated with the domain. A power domain may sometimes represent an IP. This IP may contain a number of other power domains. Power states of a domain in this case can be defined in terms of power states of lower level power domains.

Similarly, power domains may represent a system sometimes. This system may be composed of a number of subsystems. All these subsystems will have a power domain associated with them. Here, power states of a system can be defined in terms of power states of various subsystems.

To represent power states in such a manner, UPF allows a power state definition of a power domain to reference the power state of any power domain or supply set, or the port state of any supply port or supply net, that is declared in the descendant subtree of the scope of that power domain. UPF defines the add_power_state command to capture information about power states.