Heatbeat timer.

This class generates regular heartbeat events on a fixed fraction of a second. It is more accurate than a CPeriodic timer, because it provides a function to restore timer accuracy if it gets out of synchronisation with the system clock.

The protected RunL() function is called when the timer completes. The RunL() function in turn calls either the MBeating::Beat() or the MBeating::Synchronize() functions; MBeating is specified as a parameter to the Start() function used to start the heartbeat timer.

The relevant MBeating function may not be called immediately after the signal from the timer request has been generated, for the following reasons:

1. the RunL() of another active object may be running at the time of the signal

2. other active objects may have a higher priority than the CHeartbeat

If no heartbeat is missed, then the Beat() function is called.

If one or more heartbeats are missed then the Synchronize() function is called. It is important to bear in mind that the machine might be switched off after a few beats of the heart, and then Synchronize() will be called several days later. It is therefore essential that synchronisation is achieved as quickly as possible, rather than trying to catch up a tick at a time. In the context of an analogue clock, for instance, the clock should just redraw itself with the current time - rather than moving the hands round in steps until the time is correct.

CHeartbeat is an active object, derived from CActive (via CTimer). You should be familiar with CActive in order to understand CHeartbeat behaviour, but not necessarily with CTimer.

MBeating