Re: [Discuss-gnuradio] Synchronized packet transmission

[Josh Blum]

On 01/02/2011 12:59 PM, Sangho Oh wrote:
> Hello Josh,
> 
> I have used this command
> 
> dev->send(&buff.front(), samps_to_send,
> md, uhd::io_type_t::COMPLEX_FLOAT32,
> uhd::device::SEND_MODE_FULL_BUFF,  seconds_in_future + 0.1);
> 
> The reference time for seconds_in_future is not clear to me. When does the
> timer starts?

that last parameter is a timeout. In the example, the samples are sent
several seconds in the future, so the timeout should wait at least that
long. See the docs for that call:

http://www.ettus.com/uhd_docs/doxygen/html/classuhd_1_1device.html#a11cea7a2131f2764e9afa4eab34bd8d5

The metadata specifies the timestamp of the packet:

http://www.ettus.com/uhd_docs/doxygen/html/structuhd_1_1tx__metadata__t.html

> I guess there are large imprecise delay between gnuradio to USRP2 FPGA
> (gnuradio -> ethernet -> FPGA).
> 

on the order of 10s of microseconds if the CPU is keeping up.

> If I synchronize two USRPs using sync_to_pps(), just wondering how to
> control the transmit timing from the reference timing stamp, which is reset
> every second?
> 

The time will be set at the next pps to the desired value, and increment
forever after (its a 64 bit timer). So nothing is reset or rolls over
every second.

http://www.ettus.com/uhd_docs/doxygen/html/classuhd_1_1usrp_1_1single__usrp.html#a7fdb91ce9c7dd16a2c4cf210ebc8105f

-josh