On 7/15/20 5:59 AM, Hal Murray wrote:
[email protected] said:
It seems like it would not be that hard to get the USB frame sequence phase
locked to the system clock. One would need a way to measure the phase offset
of the USB S-o-F vs the system clock, and then it's a standard process to
phase lock, with the necessary control to do this described above.
The catch is that getting the frame lined up doesn't provide you with any info
about the offset. (I'm assuming we have something like a RS-232 to USB setup
with the PPS signal connected to one of the modem control signals - Carrier
Detect is the most popular.)
If you have a knob to slew the phase of the USB signal, and things are stable,
you could move the USB signal so that it is exactly lined up. Then noise in
the system would make half the samples come up "no" and the other half come up
"yes". That probably depends on something like the noise being large relative
to the stability of the system. (You could add artificial noise.) If things
aren't that stable, you can steer the USB timing back and forth to bracket the
PPS signal.
It all seems much simpler to put a counter in the USB device to measure the
time between the PPS and the poll. That actually sounds like a fun hack.
Does anybody have a favorite dev board with a CPU and USB that would be
appropriate for doing that?
I use teensy boards (pjrc.com) for this kind of thing (Arduino sort of,
but much, much more - much faster processor with more resources, and
physically smaller).
I've used Teensy 3.1s and 3.2s - but a Teensy LC for $12 might do it.
https://www.pjrc.com/store/teensylc.html
lots of timers on the chip. You can in fact, run it off an external
clock, although if you change from the native frequency, the USB
interface won't work (but it is conceivably reprogrammable to make it work)
An Arduino might also do it.
Both have serial ports and USB interfaces.
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