Gabs Ricalde is tracking this path: On Sat, Jun 15, 2013 at 08:13:51AM +0800, Gabs Ricalde wrote: > As an alternative to the Net4501, the AM335x in the Beaglebone has > timers that accept an external clock up to 25 MHz (TCLKIN) and can > timestamp events on an input pin (TIMER4-TIMER7). Both sets of pins are > available on the headers after an appropriate pinmux configuration. > > I'm finishing the clocksource driver for Linux, I'm testing it using the > 10 MHz and PPS outputs of a LEA-6T. ntpd loopstats show +/- 0.1 us > offsets, frequency is a flat 0.000 PPM as expected.
Edésio On Sun, Oct 27, 2013 at 10:58:02PM -0700, Tom Van Baak wrote: > > On most CPU architectures, the low level hardware has a register that > > counts on the CPU clock. > > Merely counting the CPU clock is only half the requirement. Reading the clock > precisely when an external event occurs is the other half. > > In my mind, there are three levels of NTP precision: > > 1) The traditional scheme, in use for decades, where a PC uses interrupts > (e.g., DCD) and OS s/w acts as a crude time interval counter. This is quite > limiting, but Dave Mill's worked with what he had, and it's far better than > nothing. > > 2) The external h/w capture/counter scheme, which is what any external time > interval or time-stamping counter can provide. By using external h/w instead > of internal s/w you get a precise, low-jitter time comparison at every pulse. > The OP was talking about the picPET, a sub-microsecond $1 time-stamping > counter. But you can use a $1000 HP sub-nanosecond time interval counter if > you prefer. > > 3) An internal h/w capture/counter scheme, which is what the latest crop of > SBC offer. Here, not only is the counter tied to the CPU clock, but > timestamping is pure h/w, so all problems with s/w and latency and jitter are > removed from the equation. This is the ideal solution, not unlike how any > GPSDO is designed. > > In my mind, the modern SBC architectures with single CPU clock source (could > be OCXO or atomic), and real GPIO pins (instead of serial or USB ports) and a > native h/w capture/compare register should allow NTP to work at the 10 to 100 > ns level. If any of you could verify this, it would be time-nut worthy. > > /tvb > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
