Alarms

Description

Kernel alarms are used together with counters and allow for action to be taken when a certain number of events have occurred. If the counter is associated with a clock then the alarm action happens when the appropriate number of clock ticks have occurred, in other words after a certain period of time.

Setting up an alarm involves a two-step process. First the alarm must be created with a call to cyg_alarm_create. This takes five arguments. The first identifies the counter to which the alarm should be attached. If the alarm should be attached to the system's real-time clock then cyg_real_time_clock and cyg_clock_to_counter can be used to get hold of the appropriate handle. The next two arguments specify the action to be taken when the alarm is triggered, in the form of a function pointer and some data. This function should take the form:

void
alarm_handler(cyg_handle_t alarm, cyg_addrword_t data)
{
    …
}
      

The data argument passed to the alarm function corresponds to the third argument passed to cyg_alarm_create. The fourth argument to cyg_alarm_create is used to return a handle to the newly-created alarm object, and the final argument provides the memory needed for the alarm object and thus avoids any need for dynamic memory allocation within the kernel.

Once an alarm has been created a further call to cyg_alarm_initialize is needed to activate it. The first argument specifies the alarm. The second argument indicates the absolute value of the attached counter which will result in the alarm being triggered. If the third argument is 0 then the alarm will only trigger once. A non-zero value specifies that the alarm should trigger repeatedly, with an interval of the specified number of events.

Alarms can be temporarily disabled and reenabled using cyg_alarm_disable and cyg_alarm_enable. Alternatively another call to cyg_alarm_initialize can be used to modify the behaviour of an existing alarm. If an alarm is no longer required then the associated resources can be released using cyg_alarm_delete.

The alarm function is invoked when a counter tick occurs, in other words when there is a call to cyg_counter_tick, and will happen in the same context. If the alarm is associated with the system's real-time clock then this will be DSR context, following a clock interrupt. If the alarm is associated with some other application-specific counter then the details will depend on how that counter is updated.

If two or more alarms are registered for precisely the same counter tick, the order of execution of the alarm functions is unspecified.

Valid contexts

cyg_alarm_create cyg_alarm_initialize is typically called during system initialization but may also be called in thread context. The same applies to cyg_alarm_delete. cyg_alarm_initialize, cyg_alarm_disable and cyg_alarm_enable may be called during initialization or from thread or DSR context, but cyg_alarm_enable and cyg_alarm_initialize may be expensive operations and should only be called when necessary.