Re: [ECOS] RE: Re: How to set clock related options?

[Nick Garnett <nickg at ecoscentric dot com>]

Savin Zlobec <savin@elatec.si> writes:

> Savin Zlobec wrote:
> 
> > caocangmou wrote:
> >
> >> But, I got a configuration from template(Akizuki H8/3068) :
> >>           Real-time clock numerator : 1000000000
> >>           Real-time clock denominator : 100
> >>           Real-time clock base prescale : 8192           Real-time
> >> clock period :10
> >>           OSC/Clock Frequency :20000000
> >> It seems not the case .What does period really mean?
> >>
> > CYGNUM_KERNEL_COUNTERS_RTC_PERIOD which defaults to
> > CYGNUM_HAL_RTC_PERIOD is
> > used to initialize you system clock hardware. Ecos tick is usually
> > implemented with a  hw timer which gives
> > periodic interrupts. The  CYGNUM_HAL_RTC_PERIOD is calculated so
> > that you get an interrupt every
> > 10ms (the default) and it depends on your timer hardware. Changing
> > the tick requires setting the
> > CYGNUM_HAL_RTC_NUMERATOR, CYGNUM_HAL_RTC_DENOMINATOR and
> > CYGNUM_HAL_RTC_PERIOD (if it is not automatically calculated from
> > the previous two - look how it is
> > calculated in cdl - it can only depend on your frequency - in that
> > case it is probably set to 10ms).
> 
> I just realized that what I said about the hw timer it is not entirely
> correct. The timer should only be able to generate
> an interrupt after a desired amount of time. The kernel takes care of
> the periodic aspect by resetting the timer with
> HAL_CLOCK_RESET in ISR.

Well, if the timer can be configured to generate periodic interrupts
then that is what should be done -- you can then leave the
HAL_CLOCK_RESET() macro empty. It is much better to let the hardware
keep time than try to do it in software. Unless you are very careful,
reloading the timer in HAL_CLOCK_RESET() can lead to clock drift.

-- 
Nick Garnett                    eCos Kernel Architect
http://www.ecoscentric.com      The eCos and RedBoot experts


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