[ECOS] Change system tick during runtime

Daniel Lidsten Daniel.Lidsten@combitechsystems.com
Wed Oct 1 13:16:00 GMT 2003

> > Is there any way of changing the system tick during runtime? I have 
> > made a quick test by letting an application provided 
> variable be used 
> > when setting the decrementer in the HAL_CLOCK_*-functions. 
> This will 
> > make the system generate tick more or less often depending on the 
> > value set. However, the drawback is that the system expects that we 
> > have a heartbeat of 100Hz and that will differ when changing the 
> > decrementer. Is there any way to get around this? Is there 
> any way to 
> > change the CYGNUM_HAL_RTC_DENOMINATOR during runtime so that the 
> > change will affect every file that use it? I want to be 
> able to have 
> > 10000 ticks per second but then i also need to let the system know 
> > that we have 10000 ticks per seconds so that not all timeouts and 
> > stuff gets all confused.
> > 
> Are you sure that you want the system clock to run at this 
> high of a frequency?  That means that every 100us you are 
> going to execute the clock handler, which oft times ends up 
> running the scheduler, etc. Unless you're on a blazingly fast 
> platform, this seems like too much overhead.
> If you just want higher granularity timers or interrupts, how about 
> using a second timer for this purpose?
> As for changing the clock frequency on the fly, I don't think eCos 
> is set up for this, nor (IMO) is it a good idea.
> Maybe if you explain a little more the *why*, we can give you 
> some ideas on the *how*.

In my system i want to run a thread at regular intervals since it is a
control thread. It can be every 1ms but also every 50ms. Since i have to
run the thread faster then i can setup an alarm in the default
configuration where 10ms is the tick rate then i thought that perhaps i
could let the system generate ticks more often. This is why i altered
the tick generator.

In your probosed solution then i should setup a HW timer to generate an
interrupt every time i want to run the thread and then let the interrupt
DSR post a semaphore or some other trigger mechanism. Did i understand
it right? Is this the best way to accomplish the thread trigger?

Regards, Daniel Lidsten

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