[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs.PPSdecoding cards

[Peter Monta]

Hi Tom,

> [ USB time transfer ]
>
> It seems to me that if the read path and the write path are different it
breaks down.
>
> ... But turning that into precise time requires some kind of calibration
of the actual code path delays. In other words, it sounds to me like your
method is valid for frequency transfer but not time transfer.

Yes, but I think the same could be said of other I/O mechanisms.  Even a
nominally symmetric physical layer, such as Ethernet, becomes a little
asymmetric once the general-purpose CPU, software, drivers, interrupts are
added in to the Tx and Rx paths.  Same for PCIe, although those paths are
nearly all hardware.

I'm sure USB is somewhat worse than PCIe.  Let me dig up the old code and
try a one-way calibration against a PCIe GPIO signal.  The PCIe write
should get out to the wire in 500 ns or so, and the Teensy (or a TIC) can
measure that against the USB event that comes along later.  Should allow a
reasonable estimate of the fixed delay (better than measuring the
round-trip total USB transaction time at the CPU and dividing by 2).  The
fixed delay will be system-specific, though, requiring a calibration for
each unique hardware platform, unless the USB round-trip time is small
enough to be ignored by the application.

Certainly PCI, or better, a CPU GPIO pin, is vastly preferable to USB; but
USB is very convenient and available.  If USB can be coaxed into good
performance, so much the better.

Cheers,
Peter