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RTX os_itv_set microsec periodic wakeup

sanjoy saha

I wrote a small application using Keil RTX for smartfusion device and probed RTX overhead using Cortex-M3 systick timer (in Digital CRO). I have changed the OS_TICK macro to 50 micro sec. (Even though as per manual its suggested to set it >= 1msec). Using RTX as per our design we wanted to run the foreground processing frame every 100 micro sec. When probing the wave pattern in CRO i see RTX overhead time of ~= 34 micro sec (This changes with OS_TICK value). Am i doing something wrong or this is how it is suppose to behave ? (We won't be able to achive time resolution in micro sec using RTX os)

In task phaseA i set os_itv_set (2). The periodic wakeup interval is 2 sys ticks (i.e 100 micro sec). i switch on and off LED_A.

When i probe the via in eval board for LED_A (D1). I see every 134.2 micro sec the On operation is performed.

File: RTC_Conf_CM.c

// </h>
// <h>SysTick Timer Configuration
// =============================
//   <o>Timer clock value [Hz] <1-1000000000>
//    Set the timer clock value for selected timer.
//    Default: 100000000  (100MHz)
#ifndef OS_CLOCK
 #define OS_CLOCK       100000000
#endif

//   <o>Timer tick value [us] <1-1000000>
//    Set the timer tick value for selected timer.
//    Default: 10000  (10ms)
#ifndef OS_TICK
 #define OS_TICK        50
#endif

File: main.c

#include <RTL.h>
#include "a2fxxxm3.h"                   /* A2FxxxM3x definitions             */

OS_TID t_phaseA;                        /* assigned task id of task: phase_a */

#define LED_A      0x01
#define LED_On(led)     GPIO->GPIO_OUT &= ~led
#define LED_Off(led)    GPIO->GPIO_OUT |=  led

 __task void phaseA (void) {
  os_evt_wait_and (0x0001, 0xffff);    /* wait for an event flag 0x0001    */
  os_itv_set (2);

  for (;;) {
    os_itv_wait ();

    LED_On (LED_A);
    LED_Off(LED_A);

  }
}

__task void init (void) {

  GPIO->GPIO_0_CFG = 5;                  /* Configure GPIO for LEDs          */
  GPIO->GPIO_1_CFG = 5;
  GPIO->GPIO_2_CFG = 5;
  GPIO->GPIO_3_CFG = 5;
  GPIO->GPIO_4_CFG = 5;
  GPIO->GPIO_5_CFG = 5;
  GPIO->GPIO_6_CFG = 5;
  GPIO->GPIO_7_CFG = 5;
  GPIO->GPIO_OUT  |= 0xFF;


  t_phaseA = os_tsk_create (phaseA, 0);  /* start task phaseA                */
  os_evt_set (0x0001, t_phaseA);         /* send signal event to task phaseA */
  os_tsk_delete_self ();
}



int main (void) {
  WATCHDOG->WDOGENABLE = 0x4C6E55FA;    /* Disable the watchdog              */
  os_sys_init (init);                   /* Initialize RTX and start init     */
}