Re: [Discuss-gnuradio] external 1pps pulse

[Matt Ettus]

Hans Glitsch wrote:
> > > I currently have four complex input signals coming in through USB
> > > and I have a 1pps timing pulse coming from a gps source that I need
> > > to synchronize the 64MHz clock to and also pass on to the PC so that
> > > I know exactly which sample it occurred on.  Can someone give me
> > > some guidance as to how I might accomplish this?
> > >
> > > Thank you,
> > > Hans
> >
> > Hi Hans,
> >
> > In the good news / bad news department, the good news is that this
> > shouldn't be hard to do with the "inband signaling" work that's
> > currently underway.  The bad news is that this work probably won't
> > be ready for a couple of months.
> >
> > In the interim, if you're willing to steal an LSB, you could modify
> > the verilog such that it reported the presence or absence of the 1pps
> > in the LSB of say, the I component of the first channel.
> >
> > Eric
>
> Hi Eric,
>
> Thanks for the reply.  I could use an LSB on one signal, although I 
> hate to lose a whole bit when I only have 12 to start with.  

It would be 1 bit out of 16, not out of 12.  We send 16 bit samples over 
the bus.

> I was hoping there might be a way to interleave another signal in with 
> the four complex ones I'm receiving in the PC.  My data rate is fairly 
> low, so another signal would not hurt much.   I haven't looked at the 
> Verilog code yet.  Would a fifth interleaved signal even be implement 
> in the context of the verilog code?  Also, what method would you use 
> to receive the 1pps signal so the verilog code could stick it into an 
> LSB, or another signal?

This is also possible.  You could put in into another channel.  See the 
code in tx_buffer.v

> The other issue is synchronizing the 64MHz clock to the 1pps.  Is 
> there any way to do this synch with the usrp's onboard clock, or will 
> I have to make an external clock?

You'll need to use an external clock.

Matt