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>Alarms</TITLE
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><H1
><A
NAME="KERNEL-ALARMS">Alarms</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN1016"
></A
><H2
>Name</H2
>cyg_alarm_create, cyg_alarm_delete, cyg_alarm_initialize, cyg_alarm_enable, cyg_alarm_disable -- Run an alarm function when a number of events have occurred</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN1023"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
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></P
><TABLE
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/kernel/kapi.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_alarm_create</CODE
>(cyg_handle_t counter, cyg_alarm_t* alarmfn, cyg_addrword_t data, cyg_handle_t* handle, cyg_alarm* alarm);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_alarm_delete</CODE
>(cyg_handle_t alarm);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_alarm_initialize</CODE
>(cyg_handle_t alarm, cyg_tick_count_t trigger, cyg_tick_count_t interval);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_alarm_enable</CODE
>(cyg_handle_t alarm);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_alarm_disable</CODE
>(cyg_handle_t alarm);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-ALARMS-DESCRIPTION"
></A
><H2
>Description</H2
><P
>Kernel alarms are used together with counters and allow for action to
be taken when a certain number of events have occurred. If the counter
is associated with a clock then the alarm action happens when the
appropriate number of clock ticks have occurred, in other words after
a certain period of time.
</P
><P
>Setting up an alarm involves a two-step process. First the alarm must
be created with a call to <TT
CLASS="FUNCTION"
>cyg_alarm_create</TT
>. This
takes five arguments. The first identifies the counter to which the
alarm should be attached. If the alarm should be attached to the
system's real-time clock then <TT
CLASS="FUNCTION"
>cyg_real_time_clock</TT
>
and <TT
CLASS="FUNCTION"
>cyg_clock_to_counter</TT
> can be used to get hold
of the appropriate handle. The next two arguments specify the action
to be taken when the alarm is triggered, in the form of a function
pointer and some data. This function should take the form:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void
alarm_handler(cyg_handle_t alarm, cyg_addrword_t data)
{
…
}
</PRE
></TD
></TR
></TABLE
><P
>The data argument passed to the alarm function corresponds to the
third argument passed to <TT
CLASS="FUNCTION"
>cyg_alarm_create</TT
>.
The fourth argument to <TT
CLASS="FUNCTION"
>cyg_alarm_create</TT
> is used
to return a handle to the newly-created alarm object, and the final
argument provides the memory needed for the alarm object and thus
avoids any need for dynamic memory allocation within the kernel.
</P
><P
>Once an alarm has been created a further call to
<TT
CLASS="FUNCTION"
>cyg_alarm_initialize</TT
> is needed to activate it.
The first argument specifies the alarm. The second argument indicates
the number of events, for example clock ticks, that need to occur
before the alarm triggers. If the third argument is 0 then the alarm
will only trigger once. A non-zero value specifies that the alarm
should trigger repeatedly, with an interval of the specified number of
events.
</P
><P
>Alarms can be temporarily disabled and reenabled using
<TT
CLASS="FUNCTION"
>cyg_alarm_disable</TT
> and
<TT
CLASS="FUNCTION"
>cyg_alarm_enable</TT
>. Alternatively another call to
<TT
CLASS="FUNCTION"
>cyg_alarm_initialize</TT
> can be used to modify the
behaviour of an existing alarm. If an alarm is no longer required then
the associated resources can be released using
<TT
CLASS="FUNCTION"
>cyg_alarm_delete</TT
>.
</P
><P
>The alarm function is invoked when a counter tick occurs, in other
words when there is a call to <TT
CLASS="FUNCTION"
>cyg_counter_tick</TT
>,
and will happen in the same context. If the alarm is associated with
the system's real-time clock then this will be DSR context, following
a clock interrupt. If the alarm is associated with some other
application-specific counter then the details will depend on how that
counter is updated.
</P
><P
>If two or more alarms are registered for precisely the same counter tick,
the order of execution of the alarm functions is unspecified.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-ALARMS-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
><TT
CLASS="FUNCTION"
>cyg_alarm_create</TT
>
<TT
CLASS="FUNCTION"
>cyg_alarm_initialize</TT
> is typically called during
system initialization but may also be called in thread context. The
same applies to <TT
CLASS="FUNCTION"
>cyg_alarm_delete</TT
>.
<TT
CLASS="FUNCTION"
>cyg_alarm_initialize</TT
>,
<TT
CLASS="FUNCTION"
>cyg_alarm_disable</TT
> and
<TT
CLASS="FUNCTION"
>cyg_alarm_enable</TT
> may be called during
initialization or from thread or DSR context, but
<TT
CLASS="FUNCTION"
>cyg_alarm_enable</TT
> and
<TT
CLASS="FUNCTION"
>cyg_alarm_initialize</TT
> may be expensive operations
and should only be called when necessary.
</P
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