BSP and Device Driver Development Guide
The initialization routine is responsible for programming the hardware that will periodically generate an interrupt. A programmable interval timer is commonly used as the source of the clock tick.
The device should be programmed such that an interrupt is generated
every m microseconds, where m is equal to
rtems_configuration_get_microseconds_per_tick(). Sometimes
the periodic interval timer can use a prescaler so you have to look
carefully at your user's manual to determine the correct value.
You must use the RTEMS primitive rtems_interrupt_catch to install
your clock interrupt service routine:
rtems_interrupt_catch (Clock_ISR, CLOCK_VECTOR, &old_handler);
Since there is currently not a driver entry point invoked at system
shutdown, many clock device drivers use the atexit routine
to schedule their Clock_exit routine to execute when the
system is shutdown.
By convention, many of the clock drivers do not install the clock
tick if the ticks_per_timeslice field of the Configuration
Table is 0.
BSP and Device Driver Development Guide
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