[time-nuts] yet another version of PIC 1pps code, for 16F1823

[John Beale]

I got some of the cheap Pletronics OHM40480526 26MHz OCXO parts for 
experiments (couldn't resist $2 each :-). For this purpose I modified the 
Richard H McCorkle/Tom Van Baak code for generating a 1 pps output from a 
PIC, to work with 26 MHz input, and also target the PIC 16F1823 since 
that's the part I had handy. But, being new to the PIC16F family it took 
some time to get all the configuration bits correct. So in case anyone else 
is in that position, here's the code I ended up with:

http://bealecorner.org/best/measure/time/source/PIC16F1823-26MHz-1pps_asm.txt

My M12T+ GPS board is coming on a slow boat from China, so I don't yet have 
any real standard to compare this with.

But I set up two OCXOs driving two PICs, and compared the two 1pps outputs 
on a 2-ch scope, after tweaking my 10k 10-turn trim pot feeding the ADJ pin 
on one part to be pretty close in frequency to the other. I see a slow 
steady increase in phase lag, accumulating 167 ns in 7 minutes, so I have a 
frequency difference between my two oscillators of 1.67E-7/420 = 4E-10 or 
0.4 ppm. For better measurements I guess I want a PIC-TIC or equivalent.

-John Beale