Rate monotonic help

[Joel Sherrill]

On 07/25/2011 02:51 PM, Matthews, Lee wrote:
> Hi Joel,
>
> Thanks for your reply. I set the clock tick using :
>
> #define CONFIGURE_MICROSECONDS_PER_TICK 8195
>
> Basically, I'm working on a digital filter and I need a function to execute every 8.195ms (or 122Hz), hence the above value.
>
> I was indeed aiming for a tick to equal 1/122 of a second. Please excuse my noob question, but why do you think that that can't be right? By setting it to this value am I messing up the correct functioning of RTEMS?
>
It won't change the functioning but the clock tick is
the basis for all timeouts and delays.  So all of your
timeouts will be in terms of 1/122 of a second.

If that works as a reasonable common divisor for every
timing loop or delay in the system, then all is OK.  You just
often see rates like 100Hz, 1000Hz, etc. and those are
just match up nicely with this clock tick rate. :)

But for rate monotonic periods, it should be OK since
the task (assuming all preemptive and this is highest
priority) will run first after each clock tick.

*** This is all a balancing act of the timing ***
*** granularity requirements of the set of tasks ***
*** in your application.  There is no universally ***
*** right or wrong answer. ***

FWIW Depending on RTEMS version, you will be able to
get time of day down to the nanosecond though.

--joel
> Best wishes,
> Lee
> ________________________________________
> From: Joel Sherrill [joel.sherrill at oarcorp.com]
> Sent: 25 July 2011 18:50
> To: Matthews, Lee
> Cc: 'rtems-users at rtems.org'
> Subject: Re: Rate monotonic help
>
> What is your clock tick configured as?  This is the
> CONFIGURE_MICROSECONDS_PER_TICK configuration
> parameter.
>
> FWIW your code assumes that 1 tick == 1/122 of
> a second from what I can tell and that can't be
> right.
>
> --joel
>
> On 07/25/2011 12:44 PM, Matthews, Lee wrote:
>> Hi
>>
>> I’m developing some software that uses RTEMS on an Aeroflex Gaisler
>> LEON3 processor that is running on a Pender GR-CPCI-AX2000 development
>> board.
>>
>> I’d like to setup a task that is called 122 times per second. I’m
>> currently trying to use the rtems rate_monotonic functions but I’m not
>> having much luck.
>>
>> I have used the example code from the rtems_user_guide as a guide.
>>
>> My function looks like the following :
>>
>> rtems_task Periodic_task()
>>
>> {
>>
>>        rtems_name name;
>>
>>        rtems_id period;
>>
>>        rtems_status_code status;
>>
>>        unsigned int flag=0;
>>
>>        unsigned long count=0;
>>
>>        unsigned int i=0;
>>
>>        printf( "Starting periodic function : 61Hz clock output.\n");
>>
>>        printf( "Turning GPIO OFF.\n");
>>
>>        gpiolib_set(port1,1,0); // Set GPIO0 to OFF
>>
>>        for(count=0;count<1000000;count++) {}
>>
>>        printf( "Exiting wait loop.\n");
>>
>>        name = rtems_build_name( 'P', 'E', 'R', 'D' );
>>
>>        status = rtems_rate_monotonic_create( name,&period );
>>
>>        if ( status != RTEMS_SUCCESSFUL ) {
>>
>>              printf( "rtems_monotonic_create failed with status of
>> %d.\n", status );
>>
>>              exit( 1 );
>>
>>        }
>>
>>        while ( 1 ) // RTEMS ticks per second set to 122.
>>
>>        {
>>
>>              if ( rtems_rate_monotonic_period( period, 1 ) ==
>> RTEMS_TIMEOUT )  break;
>>
>>              /* Perform some periodic actions */
>>
>>              if(flag==0)
>>
>>              {
>>
>>                    gpiolib_set(port1,1,1); // Set GPIO0 to ON
>>
>>                    flag=1;
>>
>>              }
>>
>>              else
>>
>>              {
>>
>>                    gpiolib_set(port1,1,0); // Set GPIO0 to OFF
>>
>>                    flag=0;
>>
>>              }
>>
>>        }
>>
>>        /* missed period so delete period and SELF */
>>
>>        status = rtems_rate_monotonic_delete( period );
>>
>>        if ( status != RTEMS_SUCCESSFUL ) {
>>
>>              printf( "rate_monotonic_delete failed with status of
>> %d.\n", status );
>>
>>              exit( 1 );
>>
>>        }
>>
>>        status = rtems_task_delete( RTEMS_SELF );
>>
>>        /* should not return */
>>
>>        printf( "rtems_task_delete returned with status of %d.\n", status );
>>
>>        exit(1);
>>
>> }
>>
>> I’d like for the function to turn one of the GPIO lines on and off
>> repeatedly, but all that seems to happen is that the code within the
>> while ( 1 ) loop executes once, then stops.
>>
>> The code that I use to create and start this task is the following :
>>
>> Task_name1 = rtems_build_name( 'P', 'E', 'R', 'D' ); // Set task name
>> to 'PERD'
>>
>>                  status = rtems_task_create(Task_name1, 1,
>> RTEMS_MINIMUM_STACK_SIZE * 2,
>> RTEMS_DEFAULT_MODES,RTEMS_DEFAULT_ATTRIBUTES,&Task_id1);
>>
>>                  if(status!=RTEMS_SUCCESSFUL)
>>
>>                  {
>>
>>                                  printf("rtems_task_create unsuccesful\n");
>>
>>                                  exit( 0 ); // Exit
>>
>>                  }
>>
>>                  else
>>
>>                                  printf("Created Periodic task\n");
>>
>>                  status = rtems_task_start( Task_id1, Periodic_task, 1
>> ); // Start Periodic task
>>
>>                  if(status!=RTEMS_SUCCESSFUL)
>>
>>                  {
>>
>>                                  printf("rtems_task_start unsuccesful\n");
>>
>>                                  exit( 0 ); // Exit
>>
>>                  }
>>
>> Could anyone shed some light on what I’m doing wrong please?
>>
>> Thanks in advance,
>>
>> Lee
>>
>
> --
> Joel Sherrill, Ph.D.             Director of Research&   Development
> joel.sherrill at OARcorp.com        On-Line Applications Research
> Ask me about RTEMS: a free RTOS  Huntsville AL 35805
>      Support Available             (256) 722-9985
>
>


-- 
Joel Sherrill, Ph.D.             Director of Research&  Development
joel.sherrill at OARcorp.com        On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
    Support Available             (256) 722-9985