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5. Power states - Eda-Stds.org

By Don Martinez,2014-11-08 06:58
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5. Power states - Eda-Stds.org

5. Power states

    This clause details how power states are used in UPF.

    Note required: Just because a state is named ―on‖ or ―off‖ does not give it semantic meaning.

Note required: Negating a state in a Boolean expression evaluates to True whenever the

    conditions of the state do not exist. Negating a state does not imply any other particular state is True. For example, if two states are defined on a supply set and they are named ―on‖ and ―off‖, ―! on‖ does not imply ―off‖ because ―! on‖ includes conditions that are

    not included in the definitions of ―on‖ or ―off.‖

    5.1 Overview

    Supply nets, supply ports, supply sets and power domains have power states. The definition of the power state of each object is different.

    5.1.1 Power states of supply nets and ports

    Supply nets and ports form the foundation of the power specification. Each supply port and net maintains two pieces of information: a supply state and a voltage value, which together constitute the power state of the supply net or port. The power state information for supply nets and ports is defined by the supply_net_type; the state of being

    FULL_ON, OFF, PARTIAL_ON, or UNDETERMINED and the voltage level (which

    is relevant only for the FULL_ON and PARTIAL_ON states). A supply net may be

    included in a supply set. Each supply set has a reference ground. The default reference ground for a supply set is an implicit common ground of 0 volts. The default reference ground can be explicitly overridden by specifying a supply net that is used as the reference ground for every supply net in the set. The voltage value of the supply net is in reference to this ground within the context of a supply set.

    5.1.2 Power states of supply sets

    Continuing up from the foundation of supply nets, the next step in defining the supply network is the supply set a collection of supply nets that together define a complete

    power supply. The supply set is composed of two or more supply nets. Therefore, the

    power state for a supply set is specified in terms of the supply nets that compose the set. It is the combined states of the constituent supply net which determine if there is a current powering an element and the voltage level of the supply.

    Supply sets can have special semantics in UPF, which are applied when the set is implicitly connected (see section 4.5.1) to provide power to design elements. The implicit connection semantics include simulation behavior semantics (simstate) that are specified

    for the power state. The simstate is a specification at a digital level of abstraction of the ability of the supply set's power level to support a level of operational capability. The following simstates are defined:

    a) NORMALThe power level of the supply set is sufficient to support full and

    complete operational (switching) capabilities with characterized timing.

    b) CORRUPTThe power level of the supply set is either off (one or more supply

    nets in the set are switched off terminating the flow of current) or at such a low

    level that it cannot support switching and the retention of the state of transistors

    cannot be guaranteed to be maintained even in the absence of changes or activity

    in the elements powered by the supply.

    c) CORRUPT_ON_ACTIVITYThe power level of the supply set is insufficient

    to support switching operation with characterized timing. However, the power

    level is sufficient that transistors retain their state as long as there is no switching

    activity within the elements connected to the supply.

    d) CORRUPT_SEQ_ON_CHANGEThe power level of the supply set is

    insufficient to support switching operation in registers should the register inputs

    change. The registers are insensitive to activity (non toggling glitches) on the data

    and control inputs. However, the power level is sufficient to support the switching

    operation of combinatorial logic with characterized timing.

    e) CORRUPT_SEQ_ON_ACTIVITYThe power level of the supply set is

    insufficient to support switching operation in registers if the data inputs of the

    register are active. However, the power level is sufficient to support the switching

    operation of combinatorial logic with characterized timing.

    The simstate specification provides digital-simulation tools sufficient information for approximating the power-related behavior of logic implicitly connected to the supply set with sufficient accuracy.

    NOTEWhen greater accuracy is desired or required, a mixed signal or full analog simulation must be used. Since analog simulations already incorporate power, this format provides no additional semantics for analog verification.

    Simulation results reflect the implemented hardware results only to the extent the UPF simstate specification for a given power state of a supply set is correctly specified. For example, if verification is performed with simulation of a supply set in a power state specified as having a CORRUPT_ON_ACTIVITY simstate, but the implementation is

    more accurately classified as CORRUPT_SEQ_ON_CHANGE, the simulation results

    will differ.

    NOTE--In the case above, the inaccuracy in simstate specification is conservative relative to the implemented hardware behavior. However, inaccurate specifications can be optimistic resulting in errors in the implemented hardware that simulation failed to expose.

Due to the laws of physics, a supply set shall be CORRUPT when any of the supply sets

    which create the electric circuit powering elements is switched off (the circuit is open and not closed). Therefore, it shall be an error if a simstate other than CORRUPT is

    associated with a supply set state in which the net_state of any supply net in the set is

    OFF.

5.1.2.1 Specification of min, nom, max voltage levels

    Since the power state of a supply net or port is the current supply net value, specification of the min, nom, and max voltage levels cannot be made at the supply net- or port-level. The appropriate place to specify the min, nom, and max voltage levels is in a supply set's power state specification (see xxx).

    5.1.3 Power states of power domains

    The power state of a domain is determined by the state of supply sets associated with the domain. For example, the definition of a domain's MY_DOMAIN_IS_ON power state would logically require that the primary supply set be in a NORMAL simstate (all supply

    nets of the primary supply set are on and the current delivered by the power circuit sufficient to support normal operation. Similarly, a SLEEP mode for the domain may require the primary to be in a CORRUPT state while appropriate retention and isolations

    supplies are NORMAL.

    Unlike supply nets, supply ports, and supply sets, power domains are composed of logic elements as well as power elements. Thus, the state of logic elements may be a relevant aspect to the specification of a domain's power state, e.g.,

    a) A power domain is considered to be "On" when:

    1) The logic signal controlling the power switch(es) for the domain's primary

    supply set is active (the power switch is closed or on).

    2) The logic signal(s) enabling isolation or triggering retention save or

    restore are inactive.

    b) A power domain is considered to be "Off" when:

    1) The logic signal controlling the power switch(es) for the domain's primary

    supply set is inactive (the power switch is open or off).

    2) If the isolation or retention supplies are switched, the control signals for

    those supplies are active (the power switch is on).

    3) The clock line(s) are gated

    4) The isolation enable(s) are active.

    In fact, prior to having the golden source (the HDL and UPF source used as input to implementation tools), the design specification might not have a completely defined supply network, but it may have a power management block and associated power control signals which will turn power switches on/off, enable/disable isolation, etc. when fully specified. At this stage of design specification, the power domain's power states might only be defined in terms of the state of logic elements. The state of the domain's supply sets will be added later and is necessary for a complete supply network specification. Through support of incremental refinement of the power state specification, early power-aware simulations can be performed with only the logic net expressions defining the states with those logic expressions transitioning to assertions to be verified in later stages when the complete power state definition can be specified.

5.1.3.1 Power states of systems and subsystems

    What constitutes a system is contextual. In one context, a system may be considered as complete by itself, e.g., one chip of a multi-chip or multi-board low power system. Although it might seem reasonable to define a "system" as that which is automatically implemented, UPF is not limited to that context and the verification of an entire system composed of multiple chips each with its own power design specification, as well as an overall power design specification for the board on which the chips are placed, needs to be supported. The power states of a system or subsystem are attributed on a design element. The use of the term system includes the term subsystem.

    As all elements of a design must be in a power domain, specification of power states for a system or subsystem are attributed to the power domain(s) that are defined for the (sub)system. As a system power state may depend on the state of more than one power domain, the power state specification for a power domain may include references to the states of domains defined on scopes in the logic hierarchy that are descendents of the ―higher level‖ power domain. (Here, ―higher level‖ refers to the location of the power domain’s scope being closer to the design’s top-level root design element relative to the

    scope of another power domain.) Therefore, UPF allows the power state for a power domain to reference the power state of any power domain whose scope is in the descendent tree of the scope of the domain.

    Assuming that the domain CORE_PD is defined on the root scope of a processor design, the power states of CORE_PD can reference lower level power domains such as CACHE_PD, ALU_PD and FP_PD in the specification of its power states. Thus, an example power state of FULL_OP for CORE_PD would reasonably require that its primary supply set is in a NORMAL simstate (all supply nets of the primary supply set are on and the voltage of the supply is sufficient for normal operations) and that the CACHE_PD, ALU_PD and FP_PD are all in an equivalent FULL_OP mode. In contrast, a NON_FP_OP mode for the CORE_PD may be defined identically to FULL_OP except that the FP_PD may be in a SLEEP mode. By allowing a higher level domain to reference lower level domain’s power states in the specification of its own power states, subsystem and system power states can be defined in UPF.

    The power state information defined for the top level power domain provides the guidance implementation tools need to optimize according to power state requirements. Consequently, the power states of the system shall define all the legal states of the system. Since it is not possible to check for completeness of the system power state specification, an implementation can only be optimized based on the power states that are defined. As stated previously, implementation tools shall only consider the supply net expressions in power state definitions. The supply net expressions are specified directly in the case of power states of supply sets or indirectly by reference to the power state of supply sets for a domain’s power states or the power states of other domains (which reference the power states of supply sets).

    NOTEAlthough the prototypical, top-down specification of power states suggests a power domain’s power states are defined in terms of the power states of supply sets and

    lower level power domains, the power state of a domain can be specified entirely in terms of the state of supply sets and/or supply nets and supply ports; i.e., the hierarchical specification can be collapsed into a (relatively) flat power state specification. Top-down, hierarchical power state specification is convenient when the power design starts prior to the existence of the complete supply network and is refined into an implementation. The flat specification of power states of domains in terms of direct references to supply nets may be quicker and more concise when the power state specification is not captured until after the supply network is specified.

    5.2 Power state name spaces

    Power states are attributed to specific objects in the design. The power states can then be referenced by specifying the object_name.upf_state where object_name can be a

    hierarchical name denoting a power domain, supply set or supply net. Power states are attributes of the object and can only be referenced via the object. Specifically, power

    states of a domain are attributes of the domain and not attributes of the scope of the domain. Thus, a design element may be shared as the scope for multiple domains, each domain containing states with the same name (e.g., sleep) without incurring a name space collision.

The following objects may have power states attributed to them:

     Power domains

     Supply sets

     Supply nets (power state is not an attribute but the state value -- of the net)

    A power state shall be defined before it can be referenced. Semantically, the transition of an object from one power state to another is an event. The state of a supply net is

    referenced as a SystemVerilog expression in the same manner that the state of a logic net is referenced. As the only other objects that have power states supply sets and power

    domains are not objects that can otherwise be referenced in expressions, the power state of a supply set or power domain can be referenced in an expression simply through the supply set or power domain name. Examples:

    supply_set_foo = SLEEP

    -- Returns TRUE if supply_set_foo is in a state consistent with state SLEEP

    ALU_PD != FULL_OP

    -- Returns TRUE if the ALU_PD is in a state inconsistent with FULL_OP 5.3 Simulation semantics of a power state

    The simulation semantics (