Parallax GPS sensor PMB-688 1PPS
David B
Posts: 591
I'm trying to set up my propeller development board as a frequency counter. To get better accuracy, I wanted to tune its clock to a GPS signal.
I've got an old Garmin GPS sensor that has a 1 pulse-per-second output, which is ideal for tuning the prop clock, but the Garmin isn't very sensitive and has to be mounted outside to get a fix.
I've also got a Parallax PMB-688 GPS sensor. It's much more sensitive than the Garmin, and can get a fix sitting on the workbench, but it doesn't bring the internal module's 1 pulse-per-second connection to a user-accessible pin.
Somewhere I heard that with some GPS sensors, the beginning edge of the start bit of the serial pulse is as accurate as the 1 PPS signal - does anyone know if that is true for the PMB-688?
(edit - I mean the leading edge of the first start pulse of the first character of each seconds messages)
I've got an old Garmin GPS sensor that has a 1 pulse-per-second output, which is ideal for tuning the prop clock, but the Garmin isn't very sensitive and has to be mounted outside to get a fix.
I've also got a Parallax PMB-688 GPS sensor. It's much more sensitive than the Garmin, and can get a fix sitting on the workbench, but it doesn't bring the internal module's 1 pulse-per-second connection to a user-accessible pin.
Somewhere I heard that with some GPS sensors, the beginning edge of the start bit of the serial pulse is as accurate as the 1 PPS signal - does anyone know if that is true for the PMB-688?
(edit - I mean the leading edge of the first start pulse of the first character of each seconds messages)
Comments
But the internal SIRFStar III chipset was supposed to have a 1 PPS output, so I popped the lid off.
The chip inside looked a lot like the chip from a web specification for a part EB-3631CX, as in these references -
www.digictech.com/UploadFiles/201072293031780.pdf
www.bnxgps.com/PIC/PIC/20109301528370.pdf
which show pin 20 as a 1 PPS output.
I clamped a needle onto my scope probe and checked the pin for the signal. I couldn't see anything at all on the scope, but by feeding the pin signal to a digital counter, saw the regular pulse per second coming from the pin.
(Unlike my Garmin, with a 50% duty cycle, evidently the SIRFStar 1PPS signal is only true for like 700 nanoseconds, and my old scope just wasn't showing it visually.)
The Parallax (Polestar) module's white wire was unattached to anything, so I soldered a jumper from pin 20 of the SIRFStar chip to the connection to the white wire.
It worked! The propeller detects the SIRFStar 1 PPS signal with only about 5 clock cycle differences from second to second.
The SIRFStar III chipset has an excellent reputation, and seems to work as well as advertised right on my desktop inside the house, so with this 1PPS signal as a reference, I can use my propeller breadboard as a better than a part-per-million frequency counter.
I've had some experience with the "688", too, but info on it is scarce, unlike most everything else. Curious.
Looks like you're using the external antenna. I bought one but haven't used it, works like a champ
indoors unassisted.
Have you reconfigured the 688? I managed to change it from 4800 to 9600 baud, but in the process accidently programmed it to send data in SIRF binary mode instead of NMEA ASCII mode. Now I can see that it's delivering regular bursts of serial characters but they're not human readeable.
So far my attempts to convert it back to NMEA mode have failed. Maybe in binary mode, it requires the reconfiguration command to also be in binary mode? I'll have to work on that.
I mainly wanted the unit for the 1PPS signal, which is working great, but it would be nice to get the rest of the data from it as well.
Now you get my meaning.
I only wanted the RMC sentence, but the 688 was woofing several others.
I remembered the SiRF config app that I got with my Rikaline and it worked
with the 688.
Someone posted of a situation similar to yours (somehow got it into binary),
but on the old Forum. I posted the app there. Searching, I can't find it here.
PE -- Hey, I found it afterall. Here's a link to that thread and therein a ZIP
with the goods:
http://forums.parallax.com/showthread.php?125402-Can-t-Read-GPS&highlight=SiRF
Bean
"60Hz" wanders. 2 secs / wk?
I was able to find a SIRF Binary manual, and it looked straightforward to compose and send a binary message to convert the 688 to NMEA, but no matter how carefully I followed the example, I just couldn't get it to work. I'd sure like to why not because that binary mode lets you make all sorts of requests from the sensor.
Bean, that's a good idea, but the problem would be that at any given time, I'd have no idea how accurate the timing was. Plus, it's fun to hack around with these GPS sensors. Once I get a frequency counter working, I would be curious how the 60 Hz varies minute to minute, hour to hour, day to day.