horvat.johann@gmx.net writes:

> Hi there,
>
> I try to generate a square signal on a fox board (axis) to its gpio A
> with a duty cycle of 1ms. The generated signal is watched by an
> oscillator.
>
> My code is:
>
> #include
> #include
> #include
> #include
> #include
> #include
> #define _POSIX_C_SOURCE 199309
> #include
> #include
> #include
>
> int main(int argc, char **argv) {
> struct timespec ts={
> .tv_sec=0,
> .tv_nsec=1000000
> };
> int return_value=0;
>
> // set PA0 as output
> gpiosetdir(PORTA, DIROUT, PA0);
>
> while(1) {
> gpiosetbits(PORTA, PA0);
> return_value=nanosleep(&ts, NULL);
>
> gpioclearbits(PORTA, PA0);
> return_value=nanosleep(&ts, NULL);
> }
> return(0);
> }
>
> But whatever I will use as .tv_nsec (if smaller than 20000000), the
> duty cycle is 20 ms! I also tried usleep - the same.
> My kernel is:
> 2.6.15 #1 PREEMPT Thu Oct 30 10:41:51 CET 2008 cris unknown
> with Preempt on!

I think you posted this before, and got some ideas. But I think there's
a much greater problem. When some other process decides to run, your
process will get scheduled out, possibly for many tens of milliseconds,
and your square wave will stop oscillating until your process is
scheduled again.

Fundamentally, you don't want a multitasking operating system for
something like this. Dedicated hardware would probably be the ideal
choice. However, if you must use Linux, you should look into real-time
scheduling. Start with the man page for sched_getscheduler. It looks
like there are also some kernel config options that you will need to
turn on.

horvat.johann@gmx.net wrote:

> Hi there,
>
> I try to generate a square signal on a fox board (axis) to its gpio A
> with a duty cycle of 1ms. The generated signal is watched by an
> oscillator.
>
> My code is:
>
> #include
> #include
> #include
> #include
> #include
> #include
> #define _POSIX_C_SOURCE 199309
> #include
> #include
> #include
>
> int main(int argc, char **argv) {
> struct timespec ts={
> .tv_sec=0,
> .tv_nsec=1000000
> };
> int return_value=0;
>
> // set PA0 as output
> gpiosetdir(PORTA, DIROUT, PA0);
>
> while(1) {
> gpiosetbits(PORTA, PA0);
> return_value=nanosleep(&ts, NULL);
>
> gpioclearbits(PORTA, PA0);
> return_value=nanosleep(&ts, NULL);
> }
> return(0);
> }
>
> But whatever I will use as .tv_nsec (if smaller than 20000000), the
> duty cycle is 20 ms! I also tried usleep - the same.

Cannot work. This old kernel misses the high resolution timer (=HRT)
support.

> My kernel is:
> 2.6.15 #1 PREEMPT Thu Oct 30 10:41:51 CET 2008 cris unknown
> with Preempt on!

Try with a more recent kernel (2.6.26.6), add the RT-Preempt patch on top of
it (http://rt.et.redhat.com/download/pat....26.6-rt11.bz2), lock your
application's memory (man mlock/mlockall), give your application realtime
priority (man pthread_attr_setschedpolicy) and (may be) it will work as
expected.
It depends on the architecture your application is running on. On my PowerPC
the HRT/RT-Preempt kernel is able to hit this 1ms period with about ~80us
accuracy, independent from any other system activity.

jbe