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Synchronize Task To Absolute Time

Posted by dbeckwith01 on January 31, 2017

In our application, we want data collection and transmission to be synchronized to absolute time. For example, we want a specific transmission to occur every hour at the top or bottom of the hour. If we use the standard FreeRTOS APIs such as software timers or task delays, then our system is based on relative time from the last power up or reset.

My question is what is the best way to go about synchronizing specific events to absolute time. My thinking was to use a separate hardware timer that maintains an epoch. This epoch timer would get set to absolute time via GPS or some other method. Once this timer expired it would single an event to the task that maintained a count. Once that count reached count % 3600 == 0 then it would be time to transmit.

Thoughts? Is there an easier way to do this?

Thanks, Darren

Synchronize Task To Absolute Time

Posted by heinbali01 on January 31, 2017

There will be many solutions for that.

I would also choose for a time source other than the kernel timer tick, just to keep them independent. I often use NTP as a time source and use one of the hardware timers to continue counting. Sometimes a small correction must me made due to XTAL's that aren't perfect.

Care must be taken when a few seconds are skipped, after a time correction. I would calculate the expiration time and wait for e.g. count > 0x58904C8D, and do not use % and ==.

Starting in January 2038, the signed 32-bits notation (time_t) will become negative. So it might be worth using unsigned or move to a 64-bit notation. Regards.

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