The GPS-18 does NOT output 1PPS until it acquires lock. Then the 1PPS
stays on even if lock is lost, running from the internal crystal. I
have not checked, but once it reacquires lock I presume it jumps to the
correct second. All outputs including 1PPS are CMOS levels, so it
drives both low and high.
David
On 7/21/13 9:02 AM, Jim Lux wrote:
I don't have a GPS-18 in front of me, and I'm modifying some software
remotely, and I ran across an issue that someone on this list probably
knows off the top of their head.
Does the GPS-18 put out 1pps pulses even if it hasn't got a fix yet?
That is, when you apply power, does it just start putting out 1pps
pulses (at some random phase), and then when it gets a fix, it snaps
to the correct timing.
The manual says that it puts out 1pps pulses until power is removed,
so I assume that when the GPS signal goes away, it keeps on as best it
can. I would assume that the clock isn't great, but it's reasonably
close to 1pps.
What's the behavior when it reacquires? Say the free-running 1pps is
something like 1.1 Hz, so in 10 seconds, we've got an extra 1pps. Does
it then skip pulses, or what?
And a sort of background hardware question...
The manual is also not very forthcoming on the electrical
characteristics.. is it a pull up with a switch pulling it down to
ground? or does it source significant current? That is, if you wanted
to "wire or" the output from the GPS-18 with another 1pps source,
would it be "diodes" or "resistors" you'd use.
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