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From: | George Nychis |
Subject: | Re: [Discuss-gnuradio] inband pings and clock |
Date: | Sun, 05 Aug 2007 03:02:05 -0400 |
User-agent: | Thunderbird 1.5.0.12 (X11/20070604) |
Eric Blossom wrote:
On Fri, Aug 03, 2007 at 11:12:30AM -0400, George Nychis wrote:What is 'pingval' and what is its units? I didn't think most pings had a value, only empty responses for which you could compute whatever delay value you please by calculating the time between.http://gnuradio.org/trac/browser/gnuradio/branches/developers/gnychis/inband/usrp/host/lib/inband/usrp_server.mbh#L105The pingval is arbitrary.
Check!
Second question. For an application to properly timestamp outgoing packets, the application needs some general idea of the current clock value on the USRP. At first I was thinking "oh well the app can just send a ping and read the timestamp off of the response while calculating the delay." Well, the ping doesn't carry the clock back up to the application. RX packets (response-recv-raw-samples) carry the timestamp back up to the application, but there is no notion of calculating delay here.The usrp_server should be passing the timestamp back to the application. It's in the header of the packet containing the ping reply. We should add the timestamp to the response-from-control-channel
Right, this is where the current hole is. I will add a timestamp field for the response-from-control-channel.
Is the clock stored in a readable register somewhere on the USRP that a register read could be used? The delay could be calculated here between the request and response. I checked our wiki but see no register:http://gnuradio.org/trac/wiki/UsrpFPGAI think having the clock counter mapped to a register is a fine idea. Note that the list of regs at http://gnuradio.org/trac/wiki/UsrpFPGA is the existing list of regs, and hasn't been refactored to reflect the requirements for inband signaling. Also, I think it's a bad idea to use the wiki for data where the ultimate source is the source code. A link to the source would make more sense, otherwise the wiki is pretty much guaranteed to be out-of-date. (Basically we're violating the "write it once" principle)
Got ya, I'll stick with the source code. We'll work out mapping the clock to a register.
- George
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