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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] time tags for transition time using USRPs |
Date: | Wed, 27 Apr 2016 16:39:09 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 |
Hi Henning, to only answer the first part of your questions: On 04/27/2016 03:22 PM, Henning
Bredenberg wrote:
Hi Martin,Nearly! In fact, the USRP itself realizes that in hardware -- which is much, much better, because that hardware "knows" and runs at a rate directly linked to the sampling rate and LO frequencies; if your PC waited the 240ms of "PC clock time", that might be anything between 238 and 300ms "USRP time", if we add non-deterministic delay of the USB transport for commands. So: there's a feature in the FPGA image that allows you to specify "hey, FPGA, I want you to execute that "start streaming" command *exactly* at the clock tick nr 12147, and it will do that. Assume i want to add a propagation delay of 240ms: Setting 'tx_time' at the begin of the burst to 240ms will realize that?No, you should first set the device time to something sensible (or just roll with it being set to zero on device initialization) and use relative times to that. Minor caveat: if you're using a device with adjustable master clock rate (mainly: B2xx and E31x), and you change the MCR after setting the time, the host won't know when that change of "tick rate" happened, and the conversion between timestamps and tick numbers will be pretty random. Best regards, Marcus
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