Re: [Discuss-gnuradio] reference clock and sync_to_pps

[Eric Blossom]

On Tue, Apr 28, 2009 at 02:06:01PM +0000, Juha Vierinen wrote:
> Why does the config_mimo(MC_WE_SYNC_TO_SMA) always return true, even
> when there is no reference signal connected to the SMA connector?

It just configures the clocks.  It doesn't check to see the PLL locks.

> From what I understand, the sync_to_pps() feature works as follows:
> when the next rising edge of the PPS is encountered, the timestamps
> are set to zero. But the streaming is started before this, so that you
> have to manually deduce from the timestamps when the PPS occurs. Is
> this correct?

Yes.

> If so, I think there is one nasty special case, that occurs with a
> 1/2^32 probability, where you would have to store 1 s of data to be
> sure that the PPS leading edge has in fact (by accident) occurred
> exactly at the same time as the 32-bit sample counter has flipped from
> 2^32 -1 to 0. This could be avoided, if the sample counter is set to
> >0 and <2^32-10^8 - 1 initially. But anyway, I'm not that worried
> about this. The possibility of such an event is a thousand times less
> likely than winning in the lottery.
> 
> juha

In the future we're moving to a 64-bit time value.

Eric