[Thumbgps-devel] Raspberry PI on Piday; better yet, generate our own system tick with a PPS synced PLL

[Patrick Maupin]

On Wed, Mar 14, 2012 at 12:59 PM, tz <thomas at mich.com> wrote:

> The ethernet port is USB based and has a USB connector,

Hm, yet more latencies to characterize.  If one of the USB ports is
OTG, maybe we could just use a Linux gadget device interface and have
this connect to the router through USB?

> Better yet, we can move the system tick itself off to a GPIO pin
> causing an interrupt, and use a PLL frequency multiplier that syncs
> exactly to 100x (or 1000x) the PPS.  The PPS could be a second
> interrupt if there was no other easy way to indicate the UTC boundary
> from the other pulses.  The GPIO interrupt can sample the internal
> clock and do what it needs for the sub-tick timing.  I don't know
> which other peripherals, but maybe they could read the sub-tick or
> even the "time of day in microseconds" from our system over I2C or
> SPI.
>
> This could be a fairly simple circuit (worries about noise, drift...),
> or there's a chip...

Yes, there are lots of chips available for this sort of thing.
Probably don't need anything too fancy.