Rhapsody tm() function is not precise

Resolving The Problem

Modifying the <lang>_CG::Framework::TimerResolution should help, but the Rhapsody timer is only a "rough" clock based on system ticks.

For example, say a timeout is set to 300ms, and the timer resolution is 100ms (the default), how long after the timeout was set will it expire?

Answer: Somewhere between 200ms and 300ms, on average after 250ms.

Why? The answer relates to the framework timer implementation. The framework timer manager keeps track of the time using a counter that is updated on every timer tick. When a timeout is set the timer manager calculates the timeout expire time as the current-stored-time + timeout-time (for example 16000 + 300).

However since the current-stored-time represents the time on the last tick, the next tick will occur anywhere in (0, timer-resolution-time), and on average after timer-resolution-time/2. So, the current-stored-time + timeout-time is missing the time that passed since the last tick.

Timeout latency: Some of the OS adaptors' implementation of the tick-timer is synchronous (via sleep(interval)) this means that there is a built-in latency (the time taken to process the expired timeouts)
that can get to be significant when the timeout is very long (involves many timer ticks).

In general a timeout will expire between (timeout-time - timer-resolution-time) and timeout-time.

In addition, if your statechart is in the middle of processing an event, that event needs to complete before the timeout is handled.

Just the process of printing out the time (say from a printf) of the timeout actually causes latency in processing it. So the numbers you are looking at may be skewed.

The creation of the timer itself is done by the timer manager.
The number used (default is 100) is the one set in the C_CG.Framework.TimerResolution property.

If you want a rough timeout then the tm function is ideal.

Consider the following options for a more accurate timer:

• Use the Windows Multimedia Timer that can have a resolution up to 1 ms.
  
  
• Create your own timer class that would use a hardware timer to timeouts accurate to say a uS. This timer class would then call a triggered operation or even a "Synchronous" event.