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Re: SiRF module and bluetooth

From: Dave Riesz
Subject: Re: SiRF module and bluetooth
Date: Fri, 26 Jun 2020 18:01:40 -0700

I think I'm explaining it badly.

The GPS module (EM410) works just fine.  I can connect it directly to the UART pins on a Raspberry Pi, start the gpsd service, and everything works.

I wanted to try to isolate it from the RPi to see if I could reduce the noise reaching the GPS receiver, so I got the bluetooth serial module (HC-06).  I expected it to work something like a "wireless serial cable," but it's not so straightforward.

The GPS module connects to the bluetooth serial module.  I connect wirelessly to the bluetooth module with the builtin bluetooth adapter on the RPi board.  So on the RPi, the configured link shows up as a the serial device "/dev/rfcomm0".  Using stty, I can set the speed of /dev/rfcomm0, but the remote bluetooth module will still be running at its preset baud rate (in this case 9600bps).  Since I can't set the wireless link to 4800, I can't make the initial connection to the GPS module.

Now, I can take everything apart, physically connect to the bluetooth module, and tell it to communicate at 4800bps.  When I put everything back together, I can then stty the serial device to 4800 and speak to the GPS module.

But then I'll be stuck at 4800bps forever.

Again, the cheapo GPS modules work great with gpsd already.  I'm getting a little too ambitious with them...

Dave Riesz

On Fri, Jun 26, 2020 at 3:46 PM Gary E. Miller <> wrote:
Yo Dave!

On Fri, 26 Jun 2020 14:07:46 -0700
Dave Riesz <> wrote:

> 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.

But you said your GPS was already at 9600?

> I can only configure it by connecting
> physically to the HC-06 hardware and sending a specific AT command,

AT commands?  To a SiRF III?

> requiring me to physically disassemble and reassemble everything each
> time I start the system.

That would be a PITA.

> 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.

Got any doc?  Otherwise we are mostly guessing.

> If I were to consider a better GPS module, what should I look for?

Depends on what you want.  Do you need the Bluetooth?  Does the
Bluetooth need to be build it?  Does it need to work indoors?  Is
fast TTFF important?  For the last few generations any u-blox beat
the competing SiRF.

I have an "BT A+ GPS" that is bluetooth and MTK-3301.  Works fine for
my testing, but really old now.  Only works on the dashboard os a car.

> 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.

If you can find doc, and send me one, I'll see if I can make them work.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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