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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Clocks</TITLE
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>eCos Reference Manual</TH
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><H1
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><A
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NAME="KERNEL-CLOCKS">Clocks</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN922"
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></A
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><H2
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>Name</H2
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>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&nbsp;--&nbsp;Provide system clocks</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN931"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN932"><P
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></P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/kernel/kapi.h&gt;
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        </PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_clock_create</CODE
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>(cyg_resolution_t resolution, cyg_handle_t* handle, cyg_clock* clock);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_clock_delete</CODE
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>(cyg_handle_t clock);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_clock_to_counter</CODE
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>(cyg_handle_t clock, cyg_handle_t* counter);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_clock_set_resolution</CODE
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>(cyg_handle_t clock, cyg_resolution_t resolution);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_resolution_t cyg_clock_get_resolution</CODE
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>(cyg_handle_t clock);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_handle_t cyg_real_time_clock</CODE
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>(void);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_tick_count_t cyg_current_time</CODE
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>(void);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-CLOCKS-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>In the eCos kernel clock objects are a special form of <A
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HREF="kernel-counters.html"
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>counter</A
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> objects. They are attached to
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a specific type of hardware, clocks that generate ticks at very
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specific time intervals, whereas counters can be used with any event
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source.
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      </P
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><P
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>In a default configuration the kernel provides a single clock
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instance, the real-time clock. This gets used for timeslicing and for
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operations that involve a timeout, for example
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<TT
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CLASS="FUNCTION"
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>cyg_semaphore_timed_wait</TT
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>. If this functionality
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is not required it can be removed from the system using the
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configuration option <TT
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CLASS="VARNAME"
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>CYGVAR_KERNEL_COUNTERS_CLOCK</TT
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>.
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Otherwise the real-time clock can be accessed by a call to
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<TT
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CLASS="FUNCTION"
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>cyg_real_time_clock</TT
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>, allowing applications to
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attach alarms, and the current counter value can be obtained using
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<TT
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CLASS="FUNCTION"
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>cyg_current_time</TT
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>.
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      </P
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><P
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>Applications can create and destroy additional clocks if desired,
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using <TT
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CLASS="FUNCTION"
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>cyg_clock_create</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_clock_delete</TT
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>. The first argument to
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<TT
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CLASS="FUNCTION"
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>cyg_clock_create</TT
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> specifies the
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<A
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HREF="kernel-clocks.html#KERNEL-CLOCKS-RESOLUTION"
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>resolution</A
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> this clock
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will run at. The second argument is used to return a handle for this
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clock object, and the third argument provides the kernel with the
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memory needed to hold this object. This clock will not actually tick
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by itself. Instead it is the responsibility of application code to
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initialize a suitable hardware timer to generate interrupts at the
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appropriate frequency, install an interrupt handler for this, and
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call <TT
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CLASS="FUNCTION"
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>cyg_counter_tick</TT
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> from inside the DSR.
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Associated with each clock is a kernel counter, a handle for which can
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be obtained using <TT
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CLASS="FUNCTION"
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>cyg_clock_to_counter</TT
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>.
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      </P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-CLOCKS-RESOLUTION"
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></A
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><H2
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>Clock Resolutions and Ticks</H2
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><P
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>At the kernel level all clock-related operations including delays,
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timeouts and alarms work in units of clock ticks, rather than in units
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of seconds or milliseconds. If the calling code, whether the
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application or some other package, needs to operate using units such
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as milliseconds then it has to convert from these units to clock
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ticks.
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      </P
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><P
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>The main reason for this is that it accurately reflects the
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hardware: calling something like <TT
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CLASS="FUNCTION"
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>nanosleep</TT
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> with a
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delay of ten nanoseconds will not work as intended on any real
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hardware because timer interrupts simply will not happen that
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frequently; instead calling <TT
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CLASS="FUNCTION"
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>cyg_thread_delay</TT
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> with
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the equivalent delay of 0 ticks gives a much clearer indication that
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the application is attempting something inappropriate for the target
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hardware. Similarly, passing a delay of five ticks to
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<TT
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CLASS="FUNCTION"
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>cyg_thread_delay</TT
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> makes it fairly obvious that
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the current thread will be suspended for somewhere between four and
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five clock periods, as opposed to passing 50000000 to
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<TT
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CLASS="FUNCTION"
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>nanosleep</TT
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> which suggests a granularity that is
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not actually provided.
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      </P
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><P
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>A secondary reason is that conversion between clock ticks and units
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such as milliseconds can be somewhat expensive, and whenever possible
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should be done at compile-time or by the application developer rather
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than at run-time. This saves code size and cpu cycles.
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      </P
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><P
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>The information needed to perform these conversions is the clock
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resolution. This is a structure with two fields, a dividend and a
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divisor, and specifies the number of nanoseconds between clock ticks.
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For example a clock that runs at 100Hz will have 10 milliseconds
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between clock ticks, or 10000000 nanoseconds. The ratio between the
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resolution's dividend and divisor will therefore be 10000000 to 1, and
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typical values for these might be 1000000000 and 100. If the clock
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runs at a different frequency, say 60Hz, the numbers could be
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1000000000 and 60 respectively. Given a delay in nanoseconds, this can
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be converted to clock ticks by multiplying with the the divisor and
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then dividing by the dividend. For example a delay of 50 milliseconds
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corresponds to 50000000 nanoseconds, and with a clock frequency of
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100Hz this can be converted to
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((50000000&nbsp;*&nbsp;100)&nbsp;/&nbsp;1000000000)&nbsp;=&nbsp;5
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clock ticks. Given the large numbers involved this arithmetic normally
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has to be done using 64-bit precision and the
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<SPAN
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CLASS="TYPE"
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>long&nbsp;long</SPAN
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> data type, but allows code to run on
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hardware with unusual clock frequencies.
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      </P
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><P
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>The default frequency for the real-time clock on any platform is
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usually about 100Hz, but platform-specific documentation should be
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consulted for this information. Usually it is possible to override
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this default by configuration options, but again this depends on the
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capabilities of the underlying hardware. The resolution for any clock
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can be obtained using <TT
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CLASS="FUNCTION"
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>cyg_clock_get_resolution</TT
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>.
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For clocks created by application code, there is also a function
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<TT
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CLASS="FUNCTION"
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>cyg_clock_set_resolution</TT
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>. This does not affect
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the underlying hardware timer in any way, it merely updates the
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information that will be returned in subsequent calls to
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<TT
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CLASS="FUNCTION"
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>cyg_clock_get_resolution</TT
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>: changing the actual
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underlying clock frequency will require appropriate manipulation of
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the timer hardware.
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      </P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-CLOCKS-CONTEXT"
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></A
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><H2
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>Valid contexts</H2
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_clock_create</TT
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> is usually only called during
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system initialization (if at all), but may also be called from thread
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context. The same applies to <TT
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CLASS="FUNCTION"
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>cyg_clock_delete</TT
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>.
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The remaining functions may be called during initialization, from
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thread context, or from DSR context, although it should be noted that
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there is no locking between
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<TT
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CLASS="FUNCTION"
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>cyg_clock_get_resolution</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_clock_set_resolution</TT
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> so theoretically it is
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possible that the former returns an inconsistent data structure.
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      </P
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