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Date: Mon, 23 Apr 2012 14:44:37 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-help] Jitter while generating PWMs with GPIO from RTDM
 driver
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To: Andrey Nechypurenko <andreynech@domain.hid>
Cc: Xenomai help <xenomai@xenomai.org>

On 04/23/2012 02:27 PM, Gilles Chanteperdrix wrote:
> 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.
> 

So, rtdm_timer_init

If this still has too much overhead, you can use the OMAP3 API do
dedicate a GPTIMER to this task, and simply register an irq for the
GPTIMER which does the job (as opposed to a timer created with
rtdm_timer_init).

-- 
					    Gilles.