Re: [Discuss-gnuradio] half duplex set_auto_tx timing problems

[Miklos Maroti]

Hi Marcus,

On Mon, Jun 3, 2013 at 8:15 PM, Marcus D. Leech <address@hidden> wrote:
>> You say that it switch from TX to RX if the next timestamp is in the
>> future and switch from RX to TX just when the timestamp is triggered.
>>
>>>> You see "L" because you are sending your bursts far enough advance.  In
>>>> my
>>>> applications, the bursts are sent to the USRP about ~5ms before the
>>>> actual
>>>> transmission.
>>
>> Ok, I will try to send them 5ms before transmit time, that should
>> work. But then synchronizing the FPGA clock with the PC clock will be
>> more trouble.
>>
> You can use set_time_now()  to set the FPGA clock to whatever you want.
>
> In fact, without such synchronization of ideas of time, it's pretty hard to
> build a TDMA-type system.  The USRP and the host delivering time-stamped
>   samples have to agree on what time it is.  If you have 1PPS pulses going
> in, you can use set_time_next_pps() to align times on the next PPS pulse.

Thanks for the idea of using set_time_now(). Setting the time to zero
(at the start of the flowgraph) makes a lot of sense (by the way,
USRP1 resets the time automatically when you start the flowgraph, but
USRP2 does not). However, you cannot use the set_time_now
periodically, because you will loose synchronization between host and
fpga. I do not think 1PPS would help either if you want to achive
cycle correct TX/RX switch. I want to let the FPGA to use its internal
timer and count everything in FPGA cycles on the host and do an
implicit time sync between the host and FPGA in order to send the next
packet 5ms before it times comes on the FPGA.

Best,
Miklos