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>Clocks</TITLE
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><H1
><A
NAME="KERNEL-CLOCKS">Clocks</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN922"
></A
><H2
>Name</H2
>cyg_clock_create, cyg_clock_delete, cyg_clock_to_counter, cyg_clock_set_resolution, cyg_clock_get_resolution, cyg_real_time_clock, cyg_current_time -- Provide system clocks</DIV
><DIV
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><A
NAME="AEN931"><H2
>Synopsis</H2
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><A
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></P
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/kernel/kapi.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_clock_create</CODE
>(cyg_resolution_t resolution, cyg_handle_t* handle, cyg_clock* clock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_clock_delete</CODE
>(cyg_handle_t clock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_clock_to_counter</CODE
>(cyg_handle_t clock, cyg_handle_t* counter);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_clock_set_resolution</CODE
>(cyg_handle_t clock, cyg_resolution_t resolution);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_resolution_t cyg_clock_get_resolution</CODE
>(cyg_handle_t clock);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_handle_t cyg_real_time_clock</CODE
>(void);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_tick_count_t cyg_current_time</CODE
>(void);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-CLOCKS-DESCRIPTION"
></A
><H2
>Description</H2
><P
>In the eCos kernel clock objects are a special form of <A
HREF="kernel-counters.html"
>counter</A
> objects. They are attached to
a specific type of hardware, clocks that generate ticks at very
specific time intervals, whereas counters can be used with any event
source.
</P
><P
>In a default configuration the kernel provides a single clock
instance, the real-time clock. This gets used for timeslicing and for
operations that involve a timeout, for example
<TT
CLASS="FUNCTION"
>cyg_semaphore_timed_wait</TT
>. If this functionality
is not required it can be removed from the system using the
configuration option <TT
CLASS="VARNAME"
>CYGVAR_KERNEL_COUNTERS_CLOCK</TT
>.
Otherwise the real-time clock can be accessed by a call to
<TT
CLASS="FUNCTION"
>cyg_real_time_clock</TT
>, allowing applications to
attach alarms, and the current counter value can be obtained using
<TT
CLASS="FUNCTION"
>cyg_current_time</TT
>.
</P
><P
>Applications can create and destroy additional clocks if desired,
using <TT
CLASS="FUNCTION"
>cyg_clock_create</TT
> and
<TT
CLASS="FUNCTION"
>cyg_clock_delete</TT
>. The first argument to
<TT
CLASS="FUNCTION"
>cyg_clock_create</TT
> specifies the
<A
HREF="kernel-clocks.html#KERNEL-CLOCKS-RESOLUTION"
>resolution</A
> this clock
will run at. The second argument is used to return a handle for this
clock object, and the third argument provides the kernel with the
memory needed to hold this object. This clock will not actually tick
by itself. Instead it is the responsibility of application code to
initialize a suitable hardware timer to generate interrupts at the
appropriate frequency, install an interrupt handler for this, and
call <TT
CLASS="FUNCTION"
>cyg_counter_tick</TT
> from inside the DSR.
Associated with each clock is a kernel counter, a handle for which can
be obtained using <TT
CLASS="FUNCTION"
>cyg_clock_to_counter</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-CLOCKS-RESOLUTION"
></A
><H2
>Clock Resolutions and Ticks</H2
><P
>At the kernel level all clock-related operations including delays,
timeouts and alarms work in units of clock ticks, rather than in units
of seconds or milliseconds. If the calling code, whether the
application or some other package, needs to operate using units such
as milliseconds then it has to convert from these units to clock
ticks.
</P
><P
>The main reason for this is that it accurately reflects the
hardware: calling something like <TT
CLASS="FUNCTION"
>nanosleep</TT
> with a
delay of ten nanoseconds will not work as intended on any real
hardware because timer interrupts simply will not happen that
frequently; instead calling <TT
CLASS="FUNCTION"
>cyg_thread_delay</TT
> with
the equivalent delay of 0 ticks gives a much clearer indication that
the application is attempting something inappropriate for the target
hardware. Similarly, passing a delay of five ticks to
<TT
CLASS="FUNCTION"
>cyg_thread_delay</TT
> makes it fairly obvious that
the current thread will be suspended for somewhere between four and
five clock periods, as opposed to passing 50000000 to
<TT
CLASS="FUNCTION"
>nanosleep</TT
> which suggests a granularity that is
not actually provided.
</P
><P
>A secondary reason is that conversion between clock ticks and units
such as milliseconds can be somewhat expensive, and whenever possible
should be done at compile-time or by the application developer rather
than at run-time. This saves code size and cpu cycles.
</P
><P
>The information needed to perform these conversions is the clock
resolution. This is a structure with two fields, a dividend and a
divisor, and specifies the number of nanoseconds between clock ticks.
For example a clock that runs at 100Hz will have 10 milliseconds
between clock ticks, or 10000000 nanoseconds. The ratio between the
resolution's dividend and divisor will therefore be 10000000 to 1, and
typical values for these might be 1000000000 and 100. If the clock
runs at a different frequency, say 60Hz, the numbers could be
1000000000 and 60 respectively. Given a delay in nanoseconds, this can
be converted to clock ticks by multiplying with the the divisor and
then dividing by the dividend. For example a delay of 50 milliseconds
corresponds to 50000000 nanoseconds, and with a clock frequency of
100Hz this can be converted to
((50000000 * 100) / 1000000000) = 5
clock ticks. Given the large numbers involved this arithmetic normally
has to be done using 64-bit precision and the
<SPAN
CLASS="TYPE"
>long long</SPAN
> data type, but allows code to run on
hardware with unusual clock frequencies.
</P
><P
>The default frequency for the real-time clock on any platform is
usually about 100Hz, but platform-specific documentation should be
consulted for this information. Usually it is possible to override
this default by configuration options, but again this depends on the
capabilities of the underlying hardware. The resolution for any clock
can be obtained using <TT
CLASS="FUNCTION"
>cyg_clock_get_resolution</TT
>.
For clocks created by application code, there is also a function
<TT
CLASS="FUNCTION"
>cyg_clock_set_resolution</TT
>. This does not affect
the underlying hardware timer in any way, it merely updates the
information that will be returned in subsequent calls to
<TT
CLASS="FUNCTION"
>cyg_clock_get_resolution</TT
>: changing the actual
underlying clock frequency will require appropriate manipulation of
the timer hardware.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-CLOCKS-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
><TT
CLASS="FUNCTION"
>cyg_clock_create</TT
> is usually only called during
system initialization (if at all), but may also be called from thread
context. The same applies to <TT
CLASS="FUNCTION"
>cyg_clock_delete</TT
>.
The remaining functions may be called during initialization, from
thread context, or from DSR context, although it should be noted that
there is no locking between
<TT
CLASS="FUNCTION"
>cyg_clock_get_resolution</TT
> and
<TT
CLASS="FUNCTION"
>cyg_clock_set_resolution</TT
> so theoretically it is
possible that the former returns an inconsistent data structure.
</P
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