Starting a periodic timer from an ISR?

[panpwr]

I am trying to use _timer_start_periodic_every() from within ISR (when a button is pressed), but receives results of TIMER_NULL_ID.

When I am debugging into the function, when reaching _timer_start_periodic_every_internal(), this function always reaches following condition and returns MQX_INVALID_LWSEM error:

    /* Gain exclusive access to the timer queues */
    if (kernel_data->ACTIVE_PTR != timer_component_ptr->TIMER_TD_PTR)
    {
        if (_lwsem_wait(&timer_component_ptr->TIMER_ENTRIES_LWSEM) != MQX_OK)
        {
            _task_set_error(MQX_INVALID_LWSEM);
            return(TIMER_NULL_ID);
        } /* Endif */
    } /* Endif */

According to the MQX user's guide,  _timer_start_ functions are not included in "Functions That the ISR Cannot Call" - so it should succeed.

However, from the implementation above, I can see that it calls _lwsem_wait() which is not allowed in an ISR.

So, my questions:

1. Why can't I allocate a timer from the ISR?

2. If it is because of the lwsem_wait() usage - then why it is still allowed to use?

Thanks,

Lior.

[DavidS]

Hi Panpwr,

An ISR routine must not call a function that can block.

In the ISR you can call functions that do not block.  Setting an event would be OK.  Then in a user task that is blocked waiting for that event, have the code you want ready to run.

Please review section "Handling Interrupts and Exceptions" in "MQX_User_Guid.pdf" in the MQX4.1/doc/mqx path.

Regards,

David

[panpwr]

According to your answer, I understand that this function obviously should be listed in the user guide, under the section "Functions That the ISR Cannot Call" as well, correct?

The question is: Why is it not listed there (only _timer_cancel appear)? Maybe there is an alternative way to use this function in spite of the blocking call?

[DavidS]

Hi Panpwr,

Short answer:  It should be in that document.

I will see about getting it added.

Thank you.

Regards,

David