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How PowerChoice Technology Works
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See the Seagate technology paper "Seagate' PowerChoice' Technology Provides
Unprecedented Hard Drive Power Savings and Flexibility"
http://www.seagate.com/files/docs/pdf/en-GB/whitepaper/tp608-powerchoice-tech-provides-gb.pdf

Each individual power condition builds on the capabilities of the previous
higher power condition in order to save incrementally more power. The specific
energy-saving steps implemented for each power condition are as follows:

Idle_A
        - Disables most of the servo system, reduces processor and channel
        power consumption
        - Discs rotating at full speed (7,200 RPM)
Idle_B
        - Disables most of the servo system, reduces processor and channel
        power consumption
        - Heads are unloaded to drive ramp.
        - Discs rotating at full speed (7,200 RPM)
Idle_C/Standby_Y (SAS Only)
        - Disables most of the servo system, reduces processor and channel
        power consumption
        - Heads are unloaded to drive ramp.
        - Drive speed reduced to a lower RPM (reduced RPM)
Standby_Z
        - Heads are unloaded to drive ramp.
        - Drive motor is spun down.
        - Drive still responds to non-media access host commands.

Flexibility is a key feature of PowerChoice technology, enabling commands from
the host side to customize power condition settings and direct a drive into or
out of power conditions as required. Two different options are available for
the user to modify PowerChoice technology settings, depending on the interface
used:

SAS
        Host-definable timers via mode pages
        Immediate host-commanded power transitions via Start/Stop Unit (SSU)
        command
SATA
        Host-definable timers via Set Features commands
        Immediate host-commanded power transitions via Set Features commands

