On 04/23/2012 01:56 PM, Andrey Nechypurenko wrote:
> Hi Folks,
>
> Following my tests with PWM generation using GPIO in user space [1],
> I've made the RTDM module [2] to further reduce the jitter. As a
> result, jitter was improved, but still under heavy system load the
> servo motor I am trying to control starts shaking. Now, I fill stuck
> and hope to get some help here.
>
> At one hand, I can not imaging that 800MHz ARM (BeagleBoard xM) could
> not manage to generate 20mS PWMs from RTDM driver precise enough to
> avoid sporadic servo movements. So probably I am doing something
> wrong. On the other hand, I do not see where the possible mistake can
> happen and hope that someone experienced in with Xenomai could help.
>
> There is an article about observed behavior [3] with more details, but
> the core problem, I guess, boils down to the following code fragment:
>
> void pwm_task_proc(void *arg) {
> const int which = (int)arg;
>
> // Toggling GPIO pin
> for(;;) {
> //set_data_out has offset 0x94 . Set gpio pin to 1 (up)
> iowrite32(0x40000000, gpio + 0x6094);
>
> // wait requested pulse width time (duty)
> if(0 != rtdm_task_sleep(up_period[which]))
> rtdm_printk("PWM: rtdm_task_sleep() returns error\n");
>
> //clear_data_out has offset 0x90 . Set gpio pin to 0 (down)
> iowrite32(0x40000000, gpio + 0x6090);
>
> // wait until the next pulse should start (20mS interval)
> if(0 != rtdm_task_wait_period())
> rtdm_printk("PWM: rtdm_task_wait_period() returns error\n");
> }
>
> This is the function running as a periodic task started with the following
> call:
>
> retval = rtdm_task_init(&pwm_task[i], // there is currently only one
> element in this array
> "pwm-task",
> pwm_task_proc,
> 0,
> RTDM_TASK_HIGHEST_PRIORITY,
> 20000000); // 20ms period
Do not use a thread, use a timer.
--
Gilles.
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