On 30 Jun, 2013, at 08:50 , David J Taylor <[email protected]> wrote: > From: Attila Kinali > [] > Oh.. and if you want to go the linux way and use a Raspberry Pi.. just dont! > Use a Beaglebone black instead. It uses less power and is easier to deal with. > Not to mention that you dont have all those USB related problems. > [] > Attila Kinali > =========================================== > > I've built three Raspberry Pi stratum-1 NTP servers: > > http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html > > one of which has both a Wi-Fi dongle and a DVB TV receiver stick attached, > and on none of these have I seen any USB problems. I'm using a 5.25 V 2A > power supply from ModMyPi.com. You can view the timekeeping accuracy here: > > http://www.satsignal.eu/mrtg/performance_ntp.php
You aren't necessarily showing the part where the Raspberry Pi is a bit weak, though. How well do clients which receive their time via the USB ethernet interface do? The Beaglebone Black has about three advantages going for it in this application: - The ARM CPU is about twice as fast as the Raspberry Pi's for about the same power consumption (I'm not sure this is a particular advantage for NTP, however, so I won't count it). - The Ethernet MAC core is built into the SOIC, and tightly coupled to it, so packet traffic doesn't have to sit waiting for the USB scheduler to get around to doing something with it. - The Ethernet MAC core also provides fairly good, complete IEEE1588 support. This is not of direct use to NTP but does provide a way to calibrate the software timestamps which NTP produces and consumes to better match when the packets arrive from and are transmitted by the actual hardware. I.e. you can measure the typical difference between hardware and software inbound timestamps (measuring interrupt latency), and hardware and software outbound timestamps (measuring the processing time spent in the outbound network stack) for PTP UDP packets, and then use these results to improve the symmetry of software timestamps for NTP UDP packets. There is no way I know of to measure this without the IEEE 1588 support (and the outbound number in particular is often big enough to deserve correction). - The TI SOIC also has a hardware timestamp capture peripheral (look for eCap in the documentation) which can capture PPS edge times with single-digit-nanosecond accuracy. That's a couple of orders of magnitude better than interrupt sampling and eliminates the jitter of the latter measurements. For a $5-$10 difference in price for the board I think these are worth it. The RPI makes a fine, low-power replacement for Intel hardware for this, but the Beaglebone Black has the raw material to do significantly better at this than either of them. The only problem with the Beaglebone is that it is not as popular as the Raspberry Pi, so making use of the former is going to require one to do more work on one's own to take advantage of it. Dennis Ferguson _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
