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How many ticks till a timer expires?

Posted by mizer on December 5, 2011

Let's say I've created and started a timer for period X ticks. Now, some unknown number of ticks later I want to change it's period to Y and restart it. But ONLY do this if the new period Y will cause the timer to expire later than the presently ticking down balance of the original period X. I may have missed it but I didn't see an obvious way to do this.

I was hoping to avoid creating and running a whole slew of timers. As in my case this will take up to 24 timers. Even though they all can reference a single callback function.


RE: How many ticks till a timer expires?

Posted by Richard on December 6, 2011
There is no way of doing that currently, but you could add something like the following to timers.c [read caveats below]:

portTickType xTimerGetExpireTime( xTimerHandle xTimer )
xTIMER *pxTimer = ( xTIMER * ) xTimer;
portTickType xTimeNow, xExpireTime, xReturn;
const portTickType xMaxTime = -1;

configASSERT( pxTimer );
xTimeNow = xTaskGetTickCount();
xExpireTime = listGET_LIST_ITEM_VALUE( &( pxTimer->xTimerListItem ) );

if( xExpireTime >= xTimeNow )
xReturn = xExpireTime - xTimeNow;
xReturn = ( xMaxTime - xTimeNow ) + xExpireTime;


1) This code is completely untested, not even compiled, it is here for demonstration of how it could be done only.

2) You have to take into account that time may pass between calling this function and the function returning - especially if it is called from a low priority task.

Hope that helps.


RE: How many ticks till a timer expires?

Posted by mizer on December 6, 2011
Yes Richard. That surely helps. I pictured something like that, including the suspends, but wasn't sure how to get at the expiration time. I'll try to work with this. But for safety sake may end up with the brute force approach with an array of timers waiting for the last one to expire.

Thanks for your help.

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