Re: [Thumbgps-devel] Project clarification

[tz <thomas@mich.com>]

A side note - Sparkfun has the Venus638 on a breakout board and it has
a PPS and even a "sync to UTC" so in 1Hz mode the first sentence start
bit is at a small fixed offset from UTC, so it has a few ways which
might make things easier downstream,.  I've been playing with them and
it is a reason I've suggested their use.

There also is some consideration that these things should be in some
kind of enclosure.  They may end up inside a router on a console port
though.  That is part of the spin-up.

Some milli-micro clarifications:

On Tue, Mar 13, 2012
 at 7:06 PM, Eric S. Raymond <esr@thyrsus.com> wrote:
> One the one hand, ordinary GPS can yield time-service accuracy down
> to a second with jitter on the close order of a hundred milliseconds.
> This is not as good as NTP, which is generally believed accurate to 10us
> (but may not be - that's part of what we want to check).

Maybe LAN NTP can go 10uS - microseconds, but pool.ntp.org is more
like 10mS - milliseconds

> To bogon-check NTP, we need time sources with about 1us accuracy that
> are cheap enough to be deployable in flocks.

1uS - One Microsecond?

> ...The product concept Patrick Maupin
> (our principal hardware designer) is focusing on is a device the size
> and shape of a thumb drive that delivers GPS info *and* PPS over USB.
> (USB would add some jitter due to polling latency but, we think, not
> enough to bust our 1ms goal.)

1ms - One Millisecond goal?

Measuring an edge inside a kernel interrupt (linuxPPS) is probably
under 10uS, maybe well under as long as interrupts aren't disabled.
It would depend on the polling interval, but USB2.0 should have lower
latency.