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><H1
><A
NAME="POWER-INTRO">Introduction</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN15587"
></A
><H2
>Name</H2
>Introduction -- eCos support for Power Management</DIV
><DIV
CLASS="REFSECT1"
><A
NAME="POWER-INTRO-INTRO"
></A
><H2
>Introduction</H2
><P
>The eCos Power Management package provides a framework for
incorporating power management facilities in an embedded application.
However its functionality is deliberately limited.</P
><P
></P
><OL
TYPE="1"
><LI
><P
>The package does not contain any support for controlling the current
power mode of any given processor, device or board. Instead it is the
responsibility of the appropriate HAL or device driver package to
implement such support, by implementing <I
CLASS="FIRSTTERM"
>power
controllers</I
>. The power management package groups these
power controllers together and provides an interface for manipulating
them.</P
></LI
><LI
><P
>The package does not contain any power management policy support.
Specifically, including this package in an application does not by
itself ever cause the system to go into low-power mode. Instead it is
the responsibility of a separate policy module, provided by
higher-level application code or by some other package, to decide when
it would be appropriate to switch from one power mode to another. The
power management package then provides the mechanisms for making it
happen.</P
></LI
></OL
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="POWER-INTRO-INCLUDE"
></A
><H2
>Including Power Management</H2
><P
>The power management package is never included automatically in an
eCos configuration: it is not part of any target specification or of
any template. Instead it must be added explicitly to a configuration
if the intended application requires power management functionality.
When using the command-line <B
CLASS="COMMAND"
>ecosconfig</B
> tool this
can be achieved using a command such as:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="SCREEN"
>$ ecosconfig add power</PRE
></TD
></TR
></TABLE
><P
>The generic eCos user documentation should be consulted for more
information on how to use the various tools. The functionality
provided by the power management package is defined in the header file
<TT
CLASS="FILENAME"
>cyg/power/power.h</TT
>. This header
file can be used by both C and C++ code.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="POWER-INTRO-MODES"
></A
><H2
>Power Modes</H2
><P
>There are four defined modes of operation:</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>active</DT
><DD
><P
>The system is fully operational, and power consumption is expected to
be high.</P
></DD
><DT
>idle</DT
><DD
><P
>There has been little or no activity for a short period of time. It is
up to the policy module to determine what constitutes a short period
of time, but typically it will be some tenths of a second or some
small number of seconds. A possible action when entering idle mode is
to reduce the system's clock speed, thus reducing the power drawn by
the cpu.</P
><P
>Note that typically this power mode is not entered automatically
whenever the idle thread starts running. Instead it is entered when
the policy module discovers that for a certain period of time the
system has been spending most of its time in the idle thread.
Theoretically it is possible to implement a policy module that would
cause a switch to idle mode as soon as the idle thread starts running,
but that could result in a great many power mode changes for no
immediate benefit.</P
></DD
><DT
>sleep</DT
><DD
><P
>The system has been idle for a significant period of time, perhaps
some tens of seconds. It is desirable to shut down any hardware that
is drawing a significant amount of power, for example a screen
backlight.</P
></DD
><DT
>off</DT
><DD
><P
>The system is powered down. Power consumption should be minimized.
Some special action may be needed before the system comes back up, for
example the user may need to press a specific button.</P
></DD
></DL
></DIV
><P
>The exact transitions that will happen are decided by the policy
module. One policy module might include transitions from active to
idle, from idle to sleep, from sleep to off, and from any of idle,
sleep or off directly back to active. Another policy module might
only use the active and off states, bypassing the intermediate ones.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="POWER-INTRO-CONTROLLERS"
></A
><H2
>Power Controllers</H2
><P
>The power management package operates primarily on power controllers.
The main functionality provided by a power controller is to switch the
power mode for some part of the system, for example the lcd display or
the cpu. A power controller consists primarily of a function which
will be invoked to switch the power mode for the part of the overall
system being controlled, plus some auxiliary data. A typical system
will include a number of different power controllers:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>Usually there will be one power controller
<TT
CLASS="VARNAME"
>power_controller_cpu</TT
> associated with the processor
or with the target platform, and provided by the corresponding HAL
package. It is this controller which is responsible for switching off
the system when entering the <SPAN
CLASS="TYPE"
>off</SPAN
> mode, which makes it
somewhat special: attempting to switch off the cpu before other
devices like the lcd display does not make sense because the cpu would
no longer be executing any instructions for the latter operation.
Therefore this power controller has to be invoked last when switching
to a lower-power mode, and similarly when switching back to a
higher-power mode it will be invoked first.</P
><P
>It should be noted that providing power management support is not a
hard requirement when porting eCos to a new processor or platform, and
many eCos ports predate the availability of power management support.
Therefore for any given platform it is distinctly possible that
<TT
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