[GitHub] [incubator-nuttx] patacongo edited a comment on pull request #1726: nRF52: extend systimer support

[GitBox <gi...@apache.org>]

patacongo edited a comment on pull request #1726:
URL: https://github.com/apache/incubator-nuttx/pull/1726#issuecomment-687904015


   > Let me look at STM32.
   
   It appears that STM32 sets a zero interval timeout.  So the interrupt and call to nxsched_alarm_expiration() will occurr immediately.
   
       553   /* Set up to receive the callback when the counter overflow occurs */
       554
       555   STM32_TIM_SETISR(g_tickless.tch, stm32_tickless_handler, NULL, 0);
       556
       558   /* Initialize interval to zero */
       559
       560   STM32_TIM_SETCOMPARE(g_tickless.tch, g_tickless.channel, 0);
       561
       562   /* Setup compare channel for the interval timing */
       563
       564   stm32_tickless_setchannel(g_tickless.channel);
       565
       566   /* Set timer period */
       567
       569   STM32_TIM_SETPERIOD(g_tickless.tch, UINT32_MAX);
       579
       580   /* Initialize the counter */
       581
       582   STM32_TIM_SETMODE(g_tickless.tch, STM32_TIM_MODE_UP);
       583
       584   /* Start the timer */
       585
       586   STM32_TIM_ACKINT(g_tickless.tch, GTIM_SR_UIF);
       587   STM32_TIM_ENABLEINT(g_tickless.tch, GTIM_DIER_UIE);
   
   Am I interpreting that correctly?  The match value is set to zero, the timer is started, then interrupts are enabled.
   
   Calling nxsched_timer_expiration() has the effect of calling nxched_timer_start() to start the interval timer.  So I think that these have the same:  Setting up an initial 0-length interval or calling nxsched_tiner_expiration() directly.
   
   NOTICE that tickless mode does not have the drift that you were concerned about when using the oneshot timer for a periodic event:  The tickless mode uses both a free-running and a one-shot timer (which might be one or two times).  So it gets the time that the interval timer is processed directly from the free-running timer.


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