Re: SiRF module and bluetooth

[Dave Riesz]

Gary,

Thanks for the response.  So this command you mentioned:

> stty -F /dev/rfcomm0 4800

will set the tty speed on the Linux side but the bluetooth link will still be running at 9600.  I can only configure it by connecting physically to the HC-06 hardware and sending a specific AT command, requiring me to physically disassemble and reassemble everything each time I start the system.

If it can't be done (which it's looking like), it can't be done.  I hardly spent any money on this stuff and it's not a big loss.

If I were to consider a better GPS module, what should I look for?  The SiRF modules were just so cheap that I couldn't resist them (a sack of 10 for about $20).  But I think I've gone about as far as I can go with them.

Thanks,

Dave Riesz

On Fri, Jun 26, 2020 at 1:38 PM Gary E. Miller <gem@rellim.com> wrote:

Yo Dave!

On Thu, 25 Jun 2020 22:26:25 -0700
Dave Riesz <dave@riesz.net> wrote:

> Hi, all.  I'm working with a sack of GlobalStar EM-410 GPS modules I
> found on eBay some time ago.  They're based on the SiRF Star III.

Ugh, I hope they were really cheap, those never worked well.  Not
very sensitive.

> I recently got the bright idea to hook them to some HC-06 bluetooth
> serial modules.  The results are not quite so great.  After lots of
> futzing, testing, and verifying that the individual parts all work
> correctly, but I can't get usable data over the link.

Bluetooth is a PITA, but it can be made to work.  Have you followed the
bluetooth directions?  https://gpsd.io/bt.html

> I think I know why:  The SiRF chipset starts up in 4800 baud mode and
> the HC-06 module is factory-configured for 9600 baud.

Yup, you gotta fix that.

> Gpsd wants to
> switch the receiver to 9600 but can't do so over the HC-06 link.

Thus the "-b" option to tell gpsd not to do that.  Set the rfcomm0
speed to 4800, then start gpsd at 4800 (-b -s 4800).

> I
> could reconfigure the bluetooth module to speak 4800, but that will
> cause the GPS to drop a bunch of data.

Yup.

> Has anyone successfully put together something like this?

Yup.  Loud cursing is invovled.  You can set the speed on the SiRF III
with gpsctl, but it is a PITA.  Something vaguly like this, as root:

    stty -F /dev/rfcomm0 4800
    gpsctl -a 9600 /dev/rcfomm0
    sleep 2
    gpsd -nbs 9600 /dev/rfcomm0

> Is it
> possible to flash a configuration to the GPS module to force it into
> a different startup comm speed?

Cheap GPS do not have persistent storage.

> Any other advice?

Get a modern GNSS receiver?  The difference in sensitivity and TTFF is
huge.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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