[time-nuts] Re: Network interface cards that support timestamping

[S McGrath]

All your points are good ones,   My point was this is the common lowest
cost approach,  better approaches exist and they range from tens of dollars
to thousands depending on the accuracy needed.

Driving the CD line with a ‘real’ serial port is good enough for ‘standard’
NTP timing applications,  its not good enough for a ‘time nuts’
implementation

Scott

On Mon, Jan 30, 2023 at 8:15 PM Trent Piepho <tpiepho at gmail.com> wrote:

> Even if you have a real serial port, still common in embedded SoCs that
> run Linux if not on PCs, and even if it still has a CD line to generate an
> interrupt, there are still issues.
>
> Modern CPUs are complex, and while the serial port might even be on the
> same chip as the CPU, they are connected through an internal network like
> an AXI bus that adds latency and jitter.  A PCI-E network card generating
> an interrupt still needs to send a MSI packet over the PCI-E network bus to
> the PCI-E controller, which then needs to send it on the CPU through some
> other kind of internal network.
>
> But that's not even the bad part.  Once the CPU gets an interrupt, there
> is still a lot of stuff that needs to happen in software before a timestamp
> is generated.  The CPU must context switch, new cachline loads from RAM for
> data and code, the base kernel interrupt dispatching, etc.
>
> With tests I did a while back with GPS PPS on an embedded Linux board,
> this was the largest source of error.
>
> Despite hardware timestampers being common on microcontrollers, SoCs that
> can run Linux never seem to have them.
>
> I wonder if one could make a useful USB timestamper with a common USB
> microcontroller board.  These have hardware counters that can timestamp a
> PPS.  It would be easy to send these timestamps over USB.  Not sending
> events about a PPS, but sending the actual timestamp of the PPS.  USB has,
> comparatively, terrible timing jitter, so timestamping the arrival of a USB
> packet is of course no good.
>
> With a bit of thought, we might think this is a step back.  We have GPS
> with an accurate clock that sends NMEA timestamps to us over an inaccurate
> UART.  Now we have a microcontroller with a semi-accurate clock sending
> timestamps to us over an even more inaccurate USB interface.  Same problem,
> but worse.
>
> But there is a difference!  Typically the clock in an USB connected
> microcontroller can use USB clock recovery.  It's disciplined to the 1kHz
> USB SOF signal from the host computer.  This lets them meet the USB timing
> spec with a cheap internal resonator.
>
> So the clock in the microcontroller that gives us timestamps is in fact
> tracking our clock.  Does that make a difference?  We can infer that the
> error in the timestamps we get reflects a matching error in the host clock
> we are trying to discipline with the PPS.
>
> On Mon, Jan 30, 2023 at 4:01 PM S McGrath via time-nuts <
> time-nuts at lists.febo.com> wrote:
>
>> Driving Pin1 only works with a ‘real’ serial port on a dedicated IRQ.  a
>> USB interface will add unpredictable latency
>>
>> On Mon, Jan 30, 2023 at 6:16 PM Lux, Jim via time-nuts <
>> time-nuts at lists.febo.com> wrote:
>>
>> > On 1/30/23 12:28 PM, S McGrath via time-nuts wrote:
>> > > Most common way of getting 1PPS into a computer without a dedicated
>> 1PPS
>> > > interface is to drive a RS-232 port’s Carrier Detect pin (pin 1 on
>> DB-9)
>> > > with the 1PPS signal
>> >
>> >
>> >
>> > back in the day when that tied directly to an interrupt with consistent
>> > latency, that works pretty well.
>> >
>> >
>> > These days, though, there's often USB hosts in the way, or some other
>> > intermediate interfaces that increases the uncertainty in timing of
>> > "when software does something" in response to "external event"
>> >
>> >
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