Forum: GNU Radio reference clock and sync_to_pps

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72573690d4ab1e21a8c8a53b4654966b?d=identicon&s=25 Juha Vierinen (Guest)
on 2009-04-28 16:07
(Received via mailing list)
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?

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?

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
745d8202ef5a58c1058d0e5395a78f9c?d=identicon&s=25 Eric Blossom (Guest)
on 2009-05-05 20:15
(Received via mailing list)
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
72573690d4ab1e21a8c8a53b4654966b?d=identicon&s=25 Juha Vierinen (Guest)
on 2009-05-06 09:37
(Received via mailing list)
>> 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.

Is there any way to know if the PLL is locked? A PLL lock is critical
for most of our applications.

juha
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