2.6 About the PPU

The PPU takes your software-programmed power domain policy and then controls the low-level hardware control signaling. The PPU enables re-usability by separating device and technology specifics, and the provision of a common software interface.

The PPU has the following interfaces:

Software interface
For high-level policy control and configuration.
Clock control interface
For high-level clock control.
Device control interface
For low-level device control and for ensuring device quiescence. The interfaces are:
  • The device interface, that consists of one or more Low Power Interfaces (LPIs).
  • The device control interface, that includes clock enables, resets, and isolation control.
Power Control State Machine (PCSM) interface
For controlling low-level technology-specific power switch and retention controls.

The following figure shows the PPU interfaces.

Figure 2-7 PPU interfaces
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The PPU provides technology-independent hardware and software interfaces for controlling domain power modes in co-ordination with device quiescence. The device interface uses either a single P-Channel or one or more Q-Channels. A PPU that uses one or more Q-Channels as the device interface is called a Q-Channel PPU. A PPU that uses a P-Channel as the device interface is called a P-Channel PPU. See the AMBA® Low Power Interface Specification, Arm® Q-Channel and P-Channel Interfaces for more information about the LPI.

The Power Control State Machine (PCSM) is a technology-dependent state machine for the sequencing of power switch chains and retention controls, that can include RAM and register retention. The PCSM executes power mode changes under PPU direction. The interface between the PPU and the PCSM is a P-Channel.

The following figure shows a high-level illustration of how the PPU and PCSM controls connect to each other, and to a power-gated domain. The dotted lines indicate the implementation-dependent components and signal connections.

Figure 2-8 Example PPU connections to a power-gated domain
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The PPU is a configurable component that can support different power domain scenarios. Many PPU features are optional or configurable. Software can read the PPU Identification Registers to discover which features a PPU supports.

PPU operation

The PPU uses power modes, such as on (ON), off (OFF), and full retention (FULL_RET), to represent the various power conditions of a domain. It has extensive support to reflect the various combinations of logic and memory power states into which a domain can be set.

Software can use these modes as either a:

Static policy
A request to enter a mode directly.
Dynamic policy
Sets the minimum mode, so the PPU can autonomously change mode above this minimum, depending on the hardware inputs.

A P-Channel PPU also supports operating modes, which are configurations of the power modes. The meaning of each operating mode is specific to one or more components within the domain. The P-Channel PPU has an interrupt output that can indicate events such as the completion of power mode transitions and the completion of operating mode transitions.

See the Arm® Power Policy Unit Architecture Specification, version 1.1 for more information about the PPU.

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