Re: Real Time Clock

[Jonathan Larmour <jifl at eCosCentric dot com>]

Privat wrote:
> Dear Ecos Developers and Maintainers,
>
> i have a question regarding the real time clock.
> I have a FreeSacle which runs at 116 Mhz, which means  i have 1 000 000
> 000  ns / 116 000 000  or 8ns per cycle.
>
> I set the dividend to 1000, and the divisor to 116 which following the
> documentation (http://ecos.sourceware.org/docs-2.0/ref/kernel-clocks.html)
>  - should always yield the ratio of ns between clock ticks.
> Unfortunatly that does not result in correct values.

The kernel real time clock doesn't run at 116 MHz though! The clock speed 
for the processor is different from the clock you want the kernel to use 
to measure time - the latter drives a clock interrupt that is used to 
handle e.g. kernel alarms or timeslicing. You can't have a tick (and 
therefore a clock interrupt) every CPU cycle :-).

You would need to set it to the number of nanoseconds per clock interrupt 
you would get when the clock setting given by CYGNUM_HAL_RTC_PERIOD is 
programmed into the PowerPC decrementer.

I don't know your HAL, but if it is derived from the MPC5554demo HAL, 
which seems most likely, CYGNUM_HAL_RTC_PERIOD defaults to the expression 
(CYGHWR_HAL_POWERPC_MPC55XX_FSYS/CYGNUM_HAL_RTC_DENOMINATOR) (with name 
changed for you)

so that it becomes easy to make the denominator correspond to the kernel 
clock frequency you want. In other words, if you want the standard 100Hz 
kernel clock tick, you probably do indeed want to leave it as it is.

Jifl
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