On 01/23/2020 12:32 PM, Lukas Haase via USRP-users wrote:
Hi,
TO MY UNDERSTANDING, the USRP has an internal clock that is different from host
clock when running gnuradio (which makes sense because there are buffers etc in
between).
Example: I transmit a CW at f=1001, receive it at f=1000 and then use gnuradio
to downconvert the remaining 1 MHz I run into trouble (tried it...).
For this reason, there exist timed commands and the tune_request object with
which I can execute commands (LO tuning) at a precice time. For example, with
these commands I can phase align tuning between TX/RX at different center
frequencies:
tune_req_tx = uhd.tune_request(fcenter-1e6, 1e6)
tune_req_rx = uhd.tune_request(2*fcenter)
tune_req_rx.args=uhd.device_addr(','.join(["mode_n=integer",
"int_n_step=1000e3",]))
tune_req_tx.args=uhd.device_addr(','.join(["mode_n=integer",
"int_n_step=1000e3",]))
now = self.uhd_usrp_sink_0.get_time_now()
self.uhd_usrp_sink_0.set_command_time(now + uhd.time_spec(0.1))
self.uhd_usrp_source_0.set_command_time(now + uhd.time_spec(0.1))
self.uhd_usrp_sink_0.set_center_freq( tune_req_tx, 0)
self.uhd_usrp_source_0.set_center_freq(tune_req_rx, 0)
self.uhd_usrp_source_0.clear_command_time()
self.uhd_usrp_sink_0.clear_command_time()
The commands execute execatly at get_time_now() plus 100ms. As far as I
understand, these 100ms are to ensure that the host computer has enough time
until the USRP processes the clear_command_time function. But it does not
relate the exact point in time with anything that exists in gnuradio.
MY QUESTION: What I am unsure is how to align samples in gnuradio with the time
on the USRP. For example, suppose I have an ideal clock signal in gnuradio and
I want to perform a timed command EXACTLY at a particular sampling point (e.g.
rising edge). How would I go about this?
The actions I want to execute exactly time aligned with gnuradio include:
tuning requests, reading out sensors (PLL sensor when it settled), switching IO
pins through the GPIO interface.
For example, I would like to switch a GPIO port exactly once per period of a
signal in gnuradio and exactly at the same time (clearly there will be delays
but that's OK as long as the delay is fixed).
As another example, I would like to re-tune exactly once in every period of a
gnuradio signal. Then I would like to read out when the PLL has settled and
generate a binary indicator signal out of it. Plotting the original signal and
the indicator signal should tell me exactly (at least sample accuracy) how long
the PLL took to settle *relative* to the signal in gnuradio.
Thank you very much,
Luke
Whatever "dance" you're using to set the USRP time, (presumably
something like set_time_unknown_pps), you need to have it derive the
USRP time register from the host time. The normal code that is
emitted in GRC for "unknown_pps" just resets the USRP time to zero.
But you can arrange for it to be the host time (+1 second or
something) instead.
You haven't indicated whether you're using GRC, or "naked" Gnu Radio
programming.
General synchronization "things" are discussed in the knowledge base, here:
https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices
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