Clock and Hardware Timer usage

Cedric Aubert cedric_aubert at
Thu Jul 3 11:01:34 UTC 2003


> -----Original Message-----
> From: Victor V. Vengerov [mailto:vvv at]
> Sent: Thursday, July 03, 2003 12:27 PM
> To: cedric_aubert at
> Cc: RTEMS-USERS; Joel Sherrill
> Subject: Re: Clock and Hardware Timer usage
> Cederic,
> Cedric Aubert wrote:
> >
> >To me, hardware need to RTEMS for all software kind
> >use :
> >
> >- one varying HW interval timer
> >- one RTC
> >
> >That all.
> >
> I have missed part of discussion, but I'll add my 2
> Consider following:
> - not every hardware has RTC, although it may keep
>   (think about CD/cassete player which is running in
>    mode when turned off and handle timer interrupts
to keep
>    clock)

It's not a problem, still not use RTC and simple use
the varying
HW timer at 1 sec frequency interruption like we are
talking about
with joel. :-)

It's mandatory to have always some timer IT, to known
timer rollover
and for still have timeslice tasking. In fact if no
timeout is need
by the software user, you will have the period clock
as today concept.

But if some user application need timeout, you will
have always the
period clock IT but also the user timeout IT on the
same HW Timer.

> - Frequently, RTC chip has so simple interface that
it is hard to
>   use from software efficiently. There are   many
RTC chips
>   which are I2C/SPI accessible. Access to them is
slow enough
>   to perform it in everyday operations. I beleive,
even access to
>   PC clock may be slow enough: although it has
simple I/O port
>   interface, actual delays may be hidden in

Not a problem to, read the RTC only at boot time and
still keep alive
current TOD with the same at least 1 second interrupt
from the varying
timer. :-)

> So, even if your concept is correct, it may be hard
to apply it
> to the current state of technology.

I don't think so. I have today a proprietary RTOS who
run with
this same concept.

It's difficult to understand this concept of one only
varying timer.
I have talk about this with many compagny like OSE,
VxWorks,... no one
understand very well that, and of course don't have
RTOS are very very often base with a fixed time of
timer interrupt.
Some one have a second timer and implement a part of
this concept.
But this concept is more performant.

The advantage of this concept, it's to have precision
without performance
deterioration. And still have time slice and other
periodic event.

> Please, sorry if my comment do not fit into context
of this
> discussion: I have not read/understand it
> Victor
> -- 
> Victor V. Vengerov
> OKTET Ltd.
> Ulianovskaya st., 1, office 533, St.-Petersburg
198504 Russia
> phone: +7 812 4284384(work),+7 812
9389372(mobile),+7 812 4281653(home)

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