MrAl
This project used TMR1 running continuously in capture mode. Only two events were involved, but they were up to 1 second apart and always several milliseconds between them. There were no interrupts, but of course, sloppy coding can screw up anything. Capture can also be done using an interrupt, but again, there is little need to adjust for those interrupt latencies, because one is subtracting, say T1 from T2, and they cancel out.
My question was how to measure the 32 KHz crystal without affecting its frequency for the purpose of finding the correct load capacitors for an undocumented device. Mike's suggestion (post#10) was the most direct way, but I do not have the equipment needed to do that. So, I dug out my Wavetek frequency standard (160 MHz to 0.1 Hz, 9 digit) and compared its reading to my Racal-Dana counter with similar precision. They agreed well enough for me, i.e., to ±1 ppm or less. They could be both wrong, of course. Then I used that to check my 1 Hz source (6 places), which was very close to the calculated loop time. Finally, I used that source to clock the TMR1 capture running at 32 KHz and got a very steady 7FFF result after adjusting the load capacitors on that crystal. I am awaiting the stable frequency source mentioned above from Banggood.
John