Re: pulse-per-second API status

[Alan Barrett <apb%cequrux.com@localhost>]

On Fri, 01 Nov 2013, Paul_Koning%Dell.com@localhost wrote:
> I don't know this API.  But my first reaction when I saw the 
> designation "PPS" is to think of GPS timekeeping boxes and other 
> precision frequency sources that have a PPS output.  On those 
> devices, the PPS output is divided down from the main oscillator 
> frequency, i.e., you can expect accuracies of 10^-9 for modest 
> price crystal oscillators, 10^-10 to 10^-12 for higher end stuff 
> -- and jitter in the nanosecond range or better.
>
> It seems rather confusing to have another interface that goes by 
> the same name but has specs 6 or more orders of magnitude worse. 
> How about a different name that avoids this confusion?

It's exactly the same interface.  Something in the external 
timekeeping box is hooked up to one of the modem control lines on 
a serial port; the modem control line is hooked up to an interrupt 
(or something like an interrupt); the interrupt is hooked up to 
something in the kernel that records the time that the interrupt 
occurred.

The difference is only one of interrupt latency.  With plain old 
serial ports, the modem control line can be hooked up to a CPU 
interrupt pin using low-latency electronics.  With USB, if I have 
understood correctly, the "interrupt" is faked by some sort of 
polling interface with much higher latency and jitter.

--apb (Alan Barrett)