Re: [gpsd-dev] RaspbPi HOWTO

[Fred Wright]

On Tue, 3 May 2016, Hal Murray wrote:
> address@hidden said:
> > The GPS Hat should just work.  The GPS Hat will also provide an RTC for both
> > the ODroid and the RPi.
>
> Is there really a RTC on a GPS HAT?  Which one?
>
> The battery is to keep the RTC equivalent inside the GPS chip going.  It's
> the difference between cold start and warm start.  That clock is not normally
> available to the outside world.

Indeed.  It would be useful if that clock were made available, though
there would have to be some additional information to show that it's not
"real GPS time".  Also, it almost certainly keeps GPS time rather than
UTC, which isn't appropriate for the usual NMEA sentences.  Not that that
stops some receivers from reporting GPS time as if it were UTC when they
haven't yet acquired the offset. :-)

BTW, the processor chip in the BBB does include an RTC, but the BBB
doesn't provide a battery for it, probably due to lack of space.  It's not
completely useless, since it allows *warm* boots to come up with the
correct time if the RTC has been set.

On Tue, 3 May 2016, Clark B. Wierda wrote:

> I have an Adafruit Ultimate GPS Breakout coming that I intend to try on the
> BBB.  I have a few GPS devices of the handheld and mouse USB variety that I
> intend to try as well.

In case you didn't see it already, there's:

http://www.toptechboy.com/tutorial/beaglebone-black-gps-lesson-1-hooking-up-the-adafruit-ultimate-gps/

and

https://www.youtube.com/watch?v=QjgkfY8-gxM

I'm not aware of any *manufactured* GPS capes for the BBB other than this
one:

http://www.yantrr.com/products/m2m-cape-for-beaglebone/vayup

Which isn't unreasonably priced (even when the required external antenna
is included) and works well, but shipping from India adds another $34 to
the cost.

That's probably the only plausible choice for a BBB HOWTO, since building
your own board seems beyond the HOWTO scope.

On Tue, 3 May 2016, Frank Nicholas wrote:

> With regards to WiFi, if I ping a Pi, sometimes 1-3 pings fail, before
> the WiFi seems to "wake up".  I’ve not been able to track this down to
> a chipset or USB specific (it happens on the Pi 3, with built-in
> Broadcom WiFi).  All my scripts that automate things with my Pi’s that
> are on WiFi, start with a “ping -c 4 -q $host”.  ALWAYS by the 4th
> ping, the Pi is responding.  With the Ethernet, always by the 2nd ping,
> the Pi is responding.

Aside from the wakeup issue, the latency on WiFi is typically horrible and
inconsistent, making it a terrible choice for connecting a time server.

The single-packet drop in the wired case is probably due to ARP.  When
there's no ARP cache entry for the destination, typical practice is to
send an ARP request *instead of* the IP packet that it was trying to send
(hoping to get a response before the retransmission).

On Tue, 3 May 2016, Hal Murray wrote:

> > Conveniently, the build doesn't require either USB or WiFi.
>
> On a Pi, the Ethernet is on USB.

That by itself makes it a questionable choice for a time server. :-)

Fred Wright