📄 hal-clocks-and-timers.html
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><H1
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><A
NAME="HAL-CLOCKS-AND-TIMERS"
>Clocks and Timers</A
></H1
><P
>These interfaces contain definitions related to clock and timer
handling. They include interfaces to initialize and read a clock for
generating regular interrupts, definitions for setting the frequency of
the clock, and support for short timed delays.</P
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN2833"
>Clock Control</A
></H2
><TABLE
BORDER="5"
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WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>HAL_CLOCK_INITIALIZE( period )
HAL_CLOCK_RESET( vector, period )
HAL_CLOCK_READ( pvalue )</PRE
></TD
></TR
></TABLE
><P
>These macros provide control over a clock or timer device that may be
used by the kernel to provide time-out, delay and scheduling
services. The clock is assumed to be implemented by some form of
counter that is incremented or decremented by some external source and
which raises an interrupt when it reaches a predetermined value.</P
><P
><CODE
CLASS="FUNCTION"
>HAL_CLOCK_INITIALIZE()</CODE
> initializes the timer
device to interrupt at the given period. The period is essentially the
value used to initialize the timer counter and must be calculated from
the timer frequency and the desired interrupt rate. The timer device
should generate an interrupt every <CODE
CLASS="VARNAME"
>period</CODE
> cycles.</P
><P
><CODE
CLASS="FUNCTION"
>HAL_CLOCK_RESET()</CODE
> re-initializes the timer to
provoke the next interrupt. This macro is only really necessary when
the timer device needs to be reset in some way after each interrupt.</P
><P
><CODE
CLASS="FUNCTION"
>HAL_CLOCK_READ()</CODE
> reads the current value of the
timer counter and puts the value in the location pointed to by
<CODE
CLASS="PARAMETER"
>pvalue</CODE
>. The value stored will always be the
number of timer cycles since the last interrupt, and hence ranges
between zero and the initial period value. If this is a count-down
cyclic timer, some arithmetic may be necessary to generate this value.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN2845"
>Microsecond Delay</A
></H2
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><PRE
CLASS="PROGRAMLISTING"
>HAL_DELAY_US(us)</PRE
></TD
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></TABLE
><P
>This macro provides a busy loop delay for the given number of
microseconds. It is intended mainly for controlling hardware that
needs short delays between operations. Code which needs longer delays,
of the order of milliseconds, should instead use higher-level
functions such as <CODE
CLASS="FUNCTION"
>cyg_thread_delay</CODE
>. The macro
implementation should be thread-safe. It can also be used in ISRs or
DSRs, although such usage is undesirable because of the impact on
interrupt and dispatch latency.</P
><P
>The macro should never delay for less than the specified amount of
time. It may delay for somewhat longer, although since the macro uses
a busy loop this is a waste of cpu cycles. Of course the code invoking
<CODE
CLASS="FUNCTION"
>HAL_DELAY_US</CODE
> may get interrupted or timesliced,
in which case the delay may be much longer than intended. If this is
unacceptable then the calling code must take preventative action
such as disabling interrupts or locking the scheduler.</P
><P
>There are three main ways of implementating the macro:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>a counting loop, typically written in inline assembler, using an outer
loop for the microseconds and an inner loop that consumes
approximately 1us. This implementation is automatically thread-safe
and does not impose any dependencies on the rest of the system, for
example it does not depend on the system clock having been started.
However it assumes that the cpu clock speed is known at compile-time
or can be easily determined at run-time.
</P
></LI
><LI
><P
>monitor one of the hardware clocks, usually the system clock. Usually
this clock ticks at a rate independent of the cpu so calibration is
easier. However the implementation relies on the system clock having
been started, and assumes that no other code is manipulating the clock
hardware. There can also be complications when the system clock wraps
around.
</P
></LI
><LI
><P
>a combination of the previous two. The system clock is used during
system initialization to determine the cpu clock speed, and the result
is then used to calibrate a counting loop. This has the disadvantage
of significantly increasing the system startup time, which may be
unacceptable to some applications. There are also complications if the
system startup code normally runs with the cache disabled because the
instruction cache will greatly affect any calibration loop.
</P
></LI
></OL
></DIV
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CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN2860"
>Clock Frequency Definition</A
></H2
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><TD
><PRE
CLASS="PROGRAMLISTING"
>CYGNUM_HAL_RTC_NUMERATOR
CYGNUM_HAL_RTC_DENOMINATOR
CYGNUM_HAL_RTC_PERIOD</PRE
></TD
></TR
></TABLE
><P
>These macros are defined in the CDL for each platform and supply the
necessary parameters to specify the frequency at which the clock
interrupts. These parameters are usually found in the CDL definitions
for the target platform, or in some cases the CPU variant.</P
><P
><TT
CLASS="LITERAL"
>CYGNUM_HAL_RTC_NUMERATOR</TT
> and
<TT
CLASS="LITERAL"
>CYGNUM_HAL_RTC_DENOMINATOR</TT
> specify the resolution
of the clock interrupt. This resolution involves two separate values,
the numerator and the denominator. The result of dividing the
numerator by the denominator should correspond to the number of
nanoseconds between clock interrupts. For example a numerator of
1000000000 and a denominator of 100 means that there are 10000000
nanoseconds (or 10 milliseconds) between clock interrupts. Expressing
the resolution as a fraction minimizes clock drift even for
frequencies that cannot be expressed as a simple integer. For example
a frequency of 60Hz corresponds to a clock resolution of
16666666.66... nanoseconds. This can be expressed accurately as
1000000000 over 60.</P
><P
><TT
CLASS="LITERAL"
>CYGNUM_HAL_RTC_PERIOD</TT
> specifies the exact value
used to initialize the clock hardware, it is the value passed as a
parameter to <TT
CLASS="LITERAL"
>HAL_CLOCK_INITIALIZE()</TT
> and
<TT
CLASS="LITERAL"
>HAL_CLOCK_RESET()</TT
>. The exact meaning of the value
and the range of legal values therefore depends on the target
hardware, and the hardware documentation should be consulted for
further details.</P
><P
>The default values for these macros in all HALs are calculated to give
a clock interrupt frequency of 100Hz, or 10ms between interrupts. To
change the clock frequency, the period needs to be changed, and the
resolution needs to be adjusted accordingly. As an example consider
the i386 PC target. The default values for these macros are:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>CYGNUM_HAL_RTC_NUMERATOR 1000000000
CYGNUM_HAL_RTC_DENOMINATOR 100
CYGNUM_HAL_RTC_PERIOD 11932</PRE
></TD
></TR
></TABLE
><P
>To change to, say, a 200Hz clock the period needs to be halved to
5966, and to compensate the denominator needs to be doubled to 200. To
change to a 1KHz interrupt rate change the period to 1193 and the
denominator to 1000.</P
><P
>Some HALs make this process a little easier by deriving the period
arithmetically from the denominator. This calculation may also involve
the CPU clock frequency and possibly other factors. For example in the
ARM AT91 variant HAL the period is defined by the following
expression:</P
><TABLE
BORDER="5"
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>((CYGNUM_HAL_ARM_AT91_CLOCK_SPEED/32) / CYGNUM_HAL_RTC_DENOMINATOR)</PRE
></TD
></TR
></TABLE
><P
>In this case it is not necessary to change the period at all, just
change the denominator to select the desired clock frequency. However,
note that for certain choices of frequency, rounding errors in this
calculation may result in a small clock drift over time. This is
usually negligible, but if perfect accuracy is required, it may be
necessary to adjust the frequency or period by hand.</P
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