[ntp:questions] Cheap ($29.90) GPS receiver

Rob nomail at example.com
Tue Apr 20 07:49:56 UTC 2010

unruh <unruh at wormhole.physics.ubc.ca> wrote:
> I think that depends on how you set up the chip. I think you can set it
> up so it will interrupt on each byte. This is not terribly efficient for
> a serial port driver, but if it is timing you want, then that is
> probably what you want.

It will give you little more than a warm feeling, I think.

> Well, actually you probably want the interrupt
> to occur at the beginning of the first start bit of the first
> character, which could in theory on a 4800bd give you  sub ms resolution
> ( and maybe much better) .

The serial chip does not do that.  It will send the interrupt at the
time it has assembled a full character.  That will probably be sometime
in the middle of the stopbit, when it has had the opportunity to sample
the state of the stopbit so that it can properly report any framing error.

Anyway, you can try to do any timing on the serial message you want, as
long as the receiver does not define the exact timing of the messages
it sends, it will be worth nothing.

And from my limited experience with only a few receivers from different
manufacturers, even the difference in timing after a couple of restarts
(or the drift over time) is changing so much that there really is no
reason to believe that the messages are sent out with more accuracy
than that of a multitasking OS putting the messages in a buffer queue
and then sending them out by a UART asynchronously.


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