n3xx: Support clock_source=external with time_source=gpsdo #570
Conversation
Previously, the GPSDO PPS could only be selected if GPSDO was also providing refclk. This change allows a use case of externally provided refclk with the internal GPSDO PPS. This is useful, for example, when wanting to use a clock with lower phase noise for sampling, but wanting to use GPSDO PPS for coarse time sync across distribued radios. Note the HW and FPGA designs already support this combination, and it was just be filtered as invalid by software.
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@draeman-synoptic We deliberately do not support this on N310. The difference between external/external and external/gpsdo is that for the former case, we assume people are providing matching PPS and 10 MHz. Mismatched PPS and 10 MHz is one of those issues that come up a lot with customer support etc. and is a common mistake for setting up synchronized system. One of our engineers spoke about this at GRCon '19: https://youtu.be/MZe-hh0NRiY external/gpsdo is guaranteed to not have PPS and 10 MHz matched, so we are likely to fail setup&hold times. Now it's not like your USRP will explode if you do that, and maybe your application is resilient enough to deal with bad synchronization (what's a few samples give or take), but we're hesitant to simply enable this feature given the risk of folks de-syncing their devices. |
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@mbr0wn Thanks for the feedback and the link's to Daniel's talk, which I just watched in its entirety. I can understand how supporting unmatched PPS/10MHz causes a lot of confusion for users trying to setup synchronized sampling. My use case is more of a niche - there are a few N320s that are separated by a few miles and that do not have access to wired networking. I don't need synchronous sampling, I just need to latch a "roughly common" system time across the distributed radios (plus or minus a handful of microsecond is fine). I'm also not concerned with other common pitfalls such as the fact that they'll slowly drift apart due to the oscillator not being GPS-disciplined. I appreciate the info and understand why this causes more trouble than it's worth across a broad userbase, especially when my use case is rather uncommon. We'll continue to apply a patch like this out-of-tree as needed for our experiments. |
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Just to close this out: We are not comfortable making this feature available to the wider public, so we'll be closing this PR. |
Update list of valid sync sources on N320 so that external refclk can be used in conjunction with internal GPSDO PPS time source.
Pull Request Details
Description
We have a use case that requires using an external refclk to get very low phase noise in the receive chain, but we wanted to use the internal GPSDO PPS to provide coarse time synchronization across distributed radios. A quick review of the schematic and FPGA suggest this case should work fine, but the combination was not in the list of valid sync sources for the radio.
Related Issue
Which devices/areas does this affect?
Impacts supported set of sync sources on N3xx radios.
Testing Done
I've done rudimentary testing on an N320 to confirm the clock locks and PPS is arriving as expected, and more thorough validation across several distributed N320s is underway.
Checklist
MPM compat, noc_shell, specific RFNoC block, ...)