Hal Murray via devel writes: > I'd like to move the current kernel mode PLL to user space. I think modern > CPUs are fast enough for that to make sense. I haven't done any > experimentation.
That doesn't buy you what you seem to think it does. This in-kernel PLL (at least on Linux) benefits from two things your userland code will not be getting: it can read the raw and the corrected time stamps of the PPS pulse in a single kernel call for getting a much better grip on what the XO really is doing at the moment and the control loop gets called at very regular intervals (the tick interrupt). The latter is why the code is no longer activated by default, because the kernel is now tickless and the former is unfortunately tied to configuration option for the in-kernel PLL which can't be set when the tickless kernel is chosen, so you won't be able to use it whether you are ion kernel mode or not. I have some experimental code that reads both types of timestamps from userland, but two roundtrips to the kernel for each of them increases the jitter on their first-order difference (which is what you'll need for the loop control) so much that you'll need lots more averaging than you'd like. Running usermode code from a highres timer is quite easily doable, but I haven't tried what happens if I trigger at 100 or even 250Hz. Triggering at 1Hz has some pretty large jitter that would either necessitate a control strategy that doesn't assume equally spaced samples or you'd need to reduce the jitter by successive interval shortening (that works on the average, but can fail badly and you'd need to filter out such samples). I have some code that reads a PPS off an I²C bus using that method and it still has about 1ms of jitter on it (down from around 4…10ms without it). The PPS timestamp for the same PPS event has around 20…50 times lower jitter than that and most of it is caused by the USB interface I'm using to get it registered (the single "real" serial is used by the DCF77 receiver and no GPIO on that computer, so it has to go in via a virtual serial port). Regards, Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ SD adaptation for Waldorf rackAttack V1.04R1: http://Synth.Stromeko.net/Downloads.html#WaldorfSDada _______________________________________________ devel mailing list devel@ntpsec.org http://lists.ntpsec.org/mailman/listinfo/devel
