1. you force it somewhere in the program to return to low

2. you PIC is reset for some reason (Watchdog maybe, decoupling cap missing, etc).
All the code and fuses settings are here.

I soldered the crystal directly on the µC's pins, a 100nF cap is there...

although it is much slower , period goes from 3uS to 13uS
Strange. The pulse should be 800ns (nano seconds).


Okay, I'll give it a try with another 16F690...