[Tinyos-help] ftsp

[Yi-Tao Wang]

Thanks for your help Janos.

If I want the clock error between the root and another mote, would
that be the offset? Also, what's is are the timestamps in
milliseconds?

David

On Mon, Nov 3, 2008 at 8:22 AM, Janos Sallai <sallai at isis.vanderbilt.edu> wrote:
> David,
>
> The local timestamp is the time of reception of a RadioCounToLeds message
> according to the motes local clock, whereas the global timestamp is the time
> of reception of a RadioCounToLeds message according to the global time in
> the network. When a mote is synchronized, they are typically different, so
> what you're seeing is correct.
>
> Regarding the skew being zero: FTSP maintains two local state variables,
> skew and offset, which are used to translate local time to global time in
> the following way:
>
> global_time = (1 + skew) * local_time + offset;
>
> The test app print out skew*1000000, which, per the above definition, should
> be pretty close to zero.The 32kHz crystal on the mote has a +-50ppm skew
> within the whole operating temperature range. If all the motes are operating
> at the same temperature, the skew should be much less than that. I'm seeing
> mostly zeros, and -1 here and there. As long as the global times for the
> same counter value agree when the network is synchronized, seeing zeros
> should not be a problem.
>
> Please note that, according to the readme file in tests/TestFtsp, a mote
> flag is synced if the synced flag is zero.The skew value is only meaningful
> when the mote is synced (i.e. the synced flag is zero).
>
> Janos
>
> On Mon, Nov 3, 2008 at 9:55 AM, Yi-Tao Wang <yitao at cs.ucla.edu> wrote:
>>
>> Hi,
>>
>> I'm having some trouble interpreting the results of the testftsp
>> application on tinyos 2.x. The skew value is always 0 even when the
>> synched flag is false and global timestamp doesn't equal local
>> timestamp. Does anyone know why?
>>
>> Thanks,
>>
>> David
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>
>