Re: [9fans] "gpio device" for Plan 9

[erik quanstrom <quanstro@quanstro.net>]

> Of things interrelated, I wish to sample a 10kHz square wave into a GPIO,
> which I am certain the RPi will do, see my earlier post with link to RPi
> forums. This will be a constant signal (an output of a GPSDO, with
> potentially a rubidium oscillator backup). So while the 10kHz is constant,
> the users input is not, and I wish to correlate in time the users input to
> the rising edge of the 10kHz signal, as it is then to be sent over a
> network with the time data of the event.
[...]
>
> I could be wildly off here Erik in my way of thinking, but my motivation is
> to extend the syncfs module to correlate precisely in time. The example
> system used by the Indiana University had no synchronisation across the
> network - presumably the only synchronised filesystem was the local
> filesystem on the cart robot? I will also be writing some kind of memory
> bounding for the ramfs. So, the events themselves are asynchronous and
> non-deterministic, however the clock source against which the events are
> placed in time is deterministic, and allows for easy reference and auditing.
>
> Perhaps there is a better way of doing this, I am not sure. Remember, I am
> an outsider to Plan 9 ways of thinking, although I am mostly unspoiled by
> Linux ways of thinking. To be precise, my ways of thinking are MSDOS, if
> that is at all possible in this day and age. I accept any and all better
> suggestions.

it seems that the system timer is already 1mhz.  but i suppose the issue is
that there is no external reference.  do you think you could get .1 ppm
time resolution with ntp between raspberry pis?  if not, i don't think i know
enough about your setup to say much of use other than to note frequency
is not directly related to precision.  if local timing can is stable to .1ppm for
a longish period of time, then a low-frequency strobe of high precision and
known timing could be enough.  the bbc does this with the tone at the top
of the hr.

- erik