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| From: | LD Zhang |
| Subject: | Re: [Discuss-gnuradio] How do I capture of the time of USRP N210 samples with host computer system time? |
| Date: | Thu, 3 Jan 2013 23:55:09 -0800 |
Hello, Thanks for the note below. I am reading an older email exchange between Josh Blum and a guest. Josh mentioned that there is a downstream block (in GRC I guess) that can use the timestamp tag to decide which samples are to be stored. Also there appears to be metadata utility to use. Maybe these magic functions such as stream tags (or) and get_time_last_pps to get the job done. So far I have used none of these utilities. My goal here is trying to avoid using the PPS if possible. Since the data gathering is done infrequently, I tend to think that the PPS can be avoided. I am trying not to use the PPS at least for this phase of the development. Yes for the next phase with enhanced capabilities, the external reference need will resurrect again. But it is better not to use the PPS right now, unless I am wrong here and need re-education of the USRP. The mimo cable is not an option since the 2 units are not co-located. So is it possible to do the downstream block and metadata stream tags in GRC (currently using UHD source block and file sink block)? Thanks very much, LD From: John Malsbury [mailto:address@hidden You can get the synchronize two USRPs with an external reference or a USRP MIMO cable, and specifying the time to start streaming for both. On Thu, Jan 3, 2013 at 11:18 PM, LD Zhang <address@hidden> wrote: Hi Folks, We are testing 2 USRP N210 units with data gathering (using GRC source and file sink). The command from the host to the N210 is sent at the approximately simultaneous time (referenced to system NTP time). However, the samples gathered from the 2 units appear to differ by as much as 0.4-0.5 seconds! The intention is to gather data at the 2 units at approximately the same time as the pre-programmed system time commands. The gathering is done with the top_block.py code generated from GRC. What happens inside the USRP appears to be beyond my control here. So the natural thing to do here is to time-stamp the samples with host computer system time. I hope that we don’t have to use an external reference for the USRP. Isn’t there an internal USRP clock that is continuously counting. If we can get the USRP clock counts to tie together the samples counts and the host computer system time, we would be in good shape. Is this a good idea? Your thoughts and help are appreciated. LD Zhang
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