Re: [Linuxptp-users] How pch2sys calcs the offset

[Richard C. <ric...@gm...>]

On Thu, Jul 25, 2013 at 03:01:22PM +0000, Ledda William EXT wrote:
> 
> *offset = (tdst1.tv_sec - tsrc.tv_sec) * NS_PER_SEC + tdst1.tv_nsec - tsrc.tv_nsec + interval / 2;
> 
> Can someone explain to me why the half interval is added at the end?

* We have two clocks, TSRC and TDST.
* We want to know the offset,

     OFFSET = TDST - TSRC.

* We take three readings, in order, TDST1, TSRC, and TDST2.
* We assume that the TSRC reading occured exactly half way between TDST1
  and TDST2.

     TDST = TDST1 + (TDST2 - TDST1) / 2

     OFFSET = TDST1 + (TDST2 - TDST1) / 2 - TSRC

     OFFSET = TDST - TSRC + (TDST2 - TDST1) / 2

HTH,
Richard