I'm using a 32kHz crystal oscillator with timer1 on a PIC16F1825 to provide more accurate timing for events (I want to do something every 20 minutes). The crystal circuit looks easy enough to design but I'm not sure whether a resistor is necessary to reduce the drive level. I'm running at 3.0V.
I'm using an ABS25-32.768KHZ-4-T crystal from Abracon as the crystal. It has a CL of 12.5pF. Thus I figure C1 and C2 at 18pF. The problem is Rs. The crystal maximum drive level is specified at 1uW and the ESR is 50k but I cannot figure out if the PIC (in LP mode) will overdrive the crystal if Rs is zero. Can I assume that LP will be the "right level". I've seen a number of circuits where Rs is 100k and a number where it isn't present. Thus my confusion.