[USRP-users] Precise transmit control of X310

Marcus Müller marcus.mueller at ettus.com
Wed Oct 28 06:01:26 EDT 2015


Hi Carel,

to be honest, that's a good question and I can't answer it, because I
wasn't around when the timespec_t was changed from having a counter for
"ticks" in the fractional seconds to double fractional nanoseconds, to
double fractional seconds later on.
Rationale here might be:
an integer for the fractional seconds would only make sense if you're
actually counting in multiples of the tick rate of the individual device
that will execute the command; it doesn't make sense to tell a device
that runs with a fixed rate of 100MHz to tune at 12.5ns, but with a B200
that might as well run at 32MHz, that might make a lot more sense than
using whole nanoseconds. And then, there are 4G-compatible tick rates
like 184.32MHz ... . So the double gives you enough precision let you
correctly represent any "ticked" time for any of the foreseeable devices.

Best regards,
Marcus

On 28.10.2015 06:32, Carel Combrink wrote:
>
> Hi Marcus,
>
> Thank you for the answers, it helps a lot. 
>
>     Regarding the time stamps: Internally (and on the wire), the time
>     stamps are transmitted as to fixed point numbers for the full
>     seconds and the fractions of the second. 
>
>  
> Out of curiosity (if I may ask), why is uhd::time_spec_t using a
> double instead of something like a unsigned long for the fraction
> part? The timespec struct (in time.h) handles it using a time_t and a
> long. 
>
> Regards,
>
>

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