RTEMS C User's Guide
Most RTEMS applications will include a clock tick
device driver which invokes the rtems_clock_tick
directive at regular intervals. The clock tick is necessary if
the application is to utilize timeslicing, the clock manager, the
timer manager, the rate monotonic manager, or the timeout option on blocking
directives.
The clock tick is usually provided as an interrupt from a counter/timer
or a real-time clock device. When a counter/timer is used to provide the
clock tick, the device is typically programmed to operate in continuous
mode. This mode selection causes the device to automatically reload the
initial count and continue the countdown without programmer intervention.
This reduces the overhead required to manipulate the counter/timer in
the clock tick ISR and increases the accuracy of tick occurrences.
The initial count can be based on the microseconds_per_tick field
in the RTEMS Configuration Table. An alternate approach is to set
the initial count for a fixed time period (such as one millisecond)
and have the ISR invoke rtems_clock_tick on the
configured microseconds_per_tick boundaries. Obviously, this
can induce some error if the configured microseconds_per_tick
is not evenly divisible by the chosen clock interrupt quantum.
It is important to note that the interval between clock ticks directly impacts the granularity of RTEMS timing operations. In addition, the frequency of clock ticks is an important factor in the overall level of system overhead. A high clock tick frequency results in less processor time being available for task execution due to the increased number of clock tick ISRs.
RTEMS C User's Guide
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