rosserial: time syncronization across multiple sensor, connected to same ros master

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Hello,

I have an imu, and odometry sensor, connected to ros master tru rosserial_tivac. Occasionally I get warnings like:

Timestamps of odometry and imu are x seconds apart from robot_pose_ekf, which indicates that timestamps of the sensor messages that are received differ more than 1 second.

I have looked at the source code of node_handle.h and I see that, as long as spinOnce() is called, the rosserial system sycronizes the clocks.

/* occasionally sync time */
if (configured_ && ((c_time - last_sync_time) > (SYNC_SECONDS * 500))) {
  requestSyncTime();
  last_sync_time = c_time;
}

• How can I make sure that timeStamps across my sensors are syncronized?
• Is there a way to force the rosserial into syncronization?
• Provided both microcontrollers (running the sensors) were syncronized at some point, how can the time difference between them differ by 1 seconds in such a short amount of time?

I have done further code reading and found out this is how rosserial sycronizes time. There is a systick generator, that counts the time since the program has started in milliseconds. This is accomplished by the systick interrupt handler, and is connected to internal clock, well not even a clock, just a crystal at 80mhz, and of course its frequency will vary with temperature.

void setNow(Time & new_now){
   uint32_t ms = hardware_.time();
   sec_offset = new_now.sec - ms / 1000 - 1;
   nsec_offset = new_now.nsec - (ms % 1000) * 1000000UL + 1000000000UL;
   normalizeSecNSec(sec_offset, nsec_offset); }

So it just records offsets, and when time is requested, it will report time taking sec_offset and nsec_offset

How can I improve this? I could modify my hardware to add a RTC? or maybe I could force time sync updates?

Any ideas/recomendations/solutions greatly appreciated.

Best Regards, Can