Electronics > Beginners
oscilloscope probing a GPS connected to an FTDI programmer
sbsivertsen@gmail.com:
Hi,
I have an GPS that I'm note sure if it works and wants to see there any signals present. The GPS is connected to an FTDI programmer.
Is it safe to probe the signal pins with an oscilloscope or do I risk a ground loop since the FTDI is connected to my computer's USB and the computer is connected to mains ground, same as the oscilloscope?
Regards
Stig Sivertsen
hamster_nz:
I wouldn't hesitate to do this. (Not saying it is safe, just that I would put a scope on this without a second thought).
If you are concerned, use a laptop and unplug the mains, so the laptop and module are floating.
cdev:
Go for it, thats an interesting thing to do if the software you use is the right software. I may be able to steer you to some interesting software for that. Just ground the grounds together so you arent using two grounds by mistake. And its always smart to not run power from other sources. A bit of advice, a great many perfectly fine GPSs will take some experimentation with serial port settings and software resets, to start after they have been off for a while. You should start it up with the manufacturers own configuration program if possible. Once everything is set properly, the GPS connected to an antenna, it still may appear dead while it sits there trying to get a fix. You may see absolutely nothing or you may see a banner with the manufacturers name and the software revision and then an endless stream of NMEA sentences all populated with zeros. Just be patient. The default for NMEA is 4800 baud N81, I would start there.
make sure an antenna is connected and you have some kind of sky view for it. It will need a minimum of 13 uninterrupted minutes under the most ideal conditions from a cold start and typically much more before it successfully has downloaded the ephemeris data.
I cant count how many times, Ive been certain a GPS is dead when it surprises me. Suddenly you'll notice a signal, then a bit later it becomes possible for it to get another one and then another and then suddenly many more may suddenly pop in. Poof, its now working.
hamster_nz:
Also, although NMEA sentences over RS232/serial is the default protocol for most GPS modules it is not uncommon for them to be configured to use vendor specific protocols and over other interfaces (e.g. I2C or SPI). Just because RS232 is quiet it doesn't mean that the module is not functional.
Sometimes modules can be configured to fewer messages - for example for some uses position might not be important, only time. These settings can be held in the module's flash and persist over restart making the module seem "dead" until a good fix is acquired.
As a rule of thumb current standard "time for first fix" for a cold start on a multi-channel module ideal conditions is around 30 seconds from signal acquisition of the first four space vehicles - this usually works out to be well under a minute with good visibility of the sky. The "wait 15 minutes" time is for the full Almanac to be downloaded, which allows for a faster startup based on assumptions about receiver's location and the current time.
So turn it on and get a cup of coffee. if it doesn't start working after getting a cup of coffee, then go to lunch. If it still isn't working, then plug the antenna in ;)
cdev:
If its really a cold start, i.e. the hardware does not have any data in it as to where it is, and no ephemeris data, which is typical if its been off and completely away from power for a while, it will almost always need at least that 13 or so minutes, unless it has access to a network connection and can download it via the net. (thats the norm for cell phone GPSs but not others).
Unless you pre-load it with an ephemeris file you have lying around.
There is another model which is used too, where the cell phone handset is a dumb GPS receiver and all the processing is done elsewhere. Then it can wake up any time, download some data and send it to the mother ship, practically using no battery and not even lighting up or anything. Then it can get its fix more accurately too using DGPS. GPSs can be set to do this at regular intervals.
This kind of post-processing model can be used to keep track of where a cell phone is even when its officially 'turned off'.
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