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@c
@c COPYRIGHT (c) 1988-1998.
@c On-Line Applications Research Corporation (OAR).
@c All rights reserved.
@c
@c $Id$
@c
@chapter Clock Driver
@section Introduction
The purpose of the clock driver is to provide a steady time
basis to the kernel, so that the RTEMS primitives that need
a clock tick work properly. See the @code{Clock Manager} chapter
of the @b{RTEMS Application C User's Guide} for more details.
The clock driver is located in the @code{clock} directory of the BSP.
@section Clock Driver Global Variables
@subsection Major and Minor Number
The major and minor numbers of the clock driver are made available via
the following variables.
@itemize @bullet
@item rtems_device_major_number rtems_clock_major;
@item rtems_device_minor_number rtems_clock_minor;
@end itemize
The clock device driver is responsible for declaring and
initializing these variables. These variables are used
by other RTEMS components -- notably the Shared Memory Driver.
@b{NOTE:} In a future RTEMS version, these variables may be replaced
with the clock device driver registering @b{/dev/clock}.
@subsection Ticks Counter
Most of the clock device drivers provide a global variable
that is simply a count of the number of clock driver interrupt service
routines that have occured. This information is valuable when debugging
a system. This variable is declared as follows:
@example
volatile rtems_unsigned32 Clock_driver_ticks;
@end example
@section Initialization
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 @i{m} microseconds, where @i{m} is equal to
@code{BSP_Configuration.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 @code{rtems_interrupt_catch} to install
your clock interrupt service routine:
@example
rtems_interrupt_catch (Clock_ISR, CLOCK_VECTOR, &old_handler);
@end example
Since there is currently not a driver entry point invoked at system
shutdown, many clock device drivers use the @code{atexit} routine
to schedule their @code{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 @code{ticks_per_timeslice} field of the Configuration
Table is 0.
@section System shutdown
Many drivers provide the routine @code{Clock_exit} that is scheduled
to be run during system shutdown via the @code{atexit} routine.
The @code{Clock_exit} routine will disable the clock tick source
if it was enabled. This can be used to prevent clock ticks after the
system is shutdown.
@section Clock Interrupt Subroutine
It only has to inform the kernel that a ticker has elapsed, so call :
@example
@group
rtems_isr Clock_isr( rtems_vector_number vector )
@{
invoke the rtems_clock_tick() directive to announce the tick
if necessary for this hardware
reload the programmable timer
@}
@end group
@end example
@section IO Control
The clock driver must supply a handler for the IO control device driver
entry point. This functionality is used by other components -- notably
the Shared Memory Driver to install a wrapper for the clock interrupt
service routine. The following shows the functionality required:
@example
@group
rtems_device_driver Clock_control(
rtems_device_major_number major,
rtems_device_minor_number minor,
void *pargp
)
@{
error check the argument pointer parameter
if the command is "ISR"
invoke the clock interrupt service routine
else if the command is "NEW"
install the requested handler
@}
@end group
@end example
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