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TomS
02-09-2007, 10:41 AM
I'm building a disciplined 10MHz reference oscillator and need to lock it to the 1pps output of a gps rcxr. The problem is I can't drill any holes in the walls of my apartment and otherwise have no way of running a cable from outside to inside. I'm wondering if I could use a pair of eb500 Bluetooth modules to transmit the serial data and 1pps pulse. Of course I would be using a propeller chip at both ends. Any comments, suggestions? IR or LED's won't work unless I want to run 50 ft of cables through my bedroom/living room.

Thanks
Tom

Martin Hebel
02-09-2007, 11:31 AM
How is this signal sent? Is it serial data, or a digital output from the unit? sorry, haven't worked with this before, but it would guide the discussion.

-Martin

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StampPlot - GUI and Plotting, and XBee Wireless Adapters (http://www.selmaware.com/)
Southern Illinois University Carbondale, Electronic Systems Technologies (http://www.siu.edu/~isat/est)

TomS
02-09-2007, 09:59 PM
There is serial data (NMEA 0183 sentences) and 1 pulse every second. The serial communications would have to be between the pulses and the inside unit would have to discriminate between data and pulses. The one second rep rate of the pulses with a 20 - 100mSec width leaves plenty of time for serial communications. I haven't used Bluetooth yet and need to know if it can be done.



Tom

bambino
02-09-2007, 10:06 PM
TomS,

I have used the eb500 and found that I could pick up a signal outside my window very well, as well as certain other rooms in the house, but it did have it's limits.

It would not pick up at the other end of the house, say corner to corner. I could link up in my room and walk through the house outside and around to my window and not go below a 50% signal. However, I can't say what type of building materials went into the construction of the building.

As a side note though, a lot of the circuitry on the eb500 is to adjust the radios 3.3V logic to basic Stamp 5V. I think the eb100 is striped of all that and is more compatible with the prop. Its worth looking into anyway if your going that route.

Last I looked there about $25 dollars cheaper.

Check out A7 engineering for the different versions.

PS, The com on the eb500 defaults to 9600 baud, but it can be adjusted higher, by how much I don't remember.

Martin Hebel
02-09-2007, 10:17 PM
What I'm trying to understand is whether this 1pps pulse is serial data or not. It sounds like it is not. In that case it will be difficult to send it using the bluetooth module since they expect a standard baud format, and a pulse would simply be garbage to it. As I understand it, the NMEA string is typically 4800 8-N-1, so how does the pulse get put in there?

Looking at this, http://www.leapsecond.com/museum/palisade/, it appears the 1pps signal appears on a pin other than the serial data. My recommendation would be to use a set of 433MHz units Parallax sells since they don't really accept serial data to be transmitted, but control the output based upon the RX bit state - a 'dumb' transmitter as opposed to the 'smart' ones. If you also need serial data, Bluetooth or XBee modules are an appropriate choice.

I've used the 433MHz to send pulse-width data, so should be ok there.

-Martin

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StampPlot - GUI and Plotting, and XBee Wireless Adapters (http://www.selmaware.com/)
Southern Illinois University Carbondale, Electronic Systems Technologies (http://www.siu.edu/~isat/est)

bambino
02-09-2007, 10:32 PM
Thanks Martin, I totally musted the pulse thing.

TomS, The eb is strictly a serial port transciever. To the user it looks just like a serial port on your computer, except for a command mode where you can adjust it's parameters. There is some pins on it that are reserved for future development of anolog signals but at the present it does not support PWM.

David B
02-09-2007, 11:44 PM
Even if you could get a good signal from the GPS receiver, won't the spread-spectrum, channel-hopping of Bluetooth completely negate the precision of the GPS 1PPS pulse? I've read some reports that Bluetooth has as much as 40-50 ms of latency. Or is there some Bluetooth mode that guarantees maybe one microsecond or better latency? That's what you'd need.

If you live close enough to a WWV or LORAN transmitter, you might just as well get a WWV or a LORAN-C time-transfer receiver to deliver you a timing pulse, which I would guess would give much better precision that GPS through a Bluetooth link, not as good as direct-wired GPS but still probably better than a microsecond, according to the NIST website.

Once I hooked my Garmin GPS 1PPS signal to my propeller and used it with the propeller counter to do some measurements of crystal frequency stability. It worked great. I could touch the propeller clock crystal case with my finger and watch the frequency change from the warmth of my finger!

TomS
02-09-2007, 11:56 PM
Thanks Everybody,

I didn't know about the Bluetooth latency issue. Even with a large average it would probably render it useless. The 1pps is separate from the serial stream. I had hoped to combine them. It looks like I also can't do that with Bluetooth. I don't live close enough to use wwv reliably so its gps. The 433MHz units may be the way to go.

Tom