Recommend 32.768KHz crystal for use with 16LF88 Timer1?

Hi,

Quick question: Can someone recommend a thru-hole crystal for use with the 16LF88 (one of the newer NanoWatt PICs with the low power optimized Timer1 oscillator). (Recommend a digikey part #?)

What I've figured out so far: Tolerance plus/minus 20 PPM ESR: 30 - 50 K max Max Drive level probably should be 1uW (which means that I probably need a series resistor to prevent overdriving the crystal) Load capacitance of crystal: 12 pF

Are the above specs correct for selecting a crystal for Timer1 use?
I don't understand the shunt capacitance parameter for crystals - from what I see on datasheets there's 1.00, 1.35, and 1.60 pF? Which one do I choose?
Will be prototyping circuit on solderless breadboard - any problems with doing this? (I know that due to the low voltage/power levels as well as differences in capacitance etc things may be different).

Thanks Kevin.

It doesn't matter much. All common 32.768kHz crystals are pretty much the same. They all have the same cut so they've all got the same temperature curve. Series resistance is somewhat variable, and the better units have lower series resistance, but when you put a multi-hundred K or M resistor in series (which you have to do to limit the crystal drive level) then the ESR becomes irrelevant in comparison.

If you'll be mass-producing this gadget, then choosing the drive-level-limiting series resistance can become somewhat difficult given the variables of crystal parameters vs PIC oscillator gain parameters as a function of voltage temperature etc. But on the bench it'll always work :-)

You will want to put a variable capacitor in there if you want to trim the frequency. You can also do this trimming in the firmware and just use fixed capacitors on either side of the crystal. Others will tell you that the capacitors are determined by crystal parameters but really the oscillator parameters are more relevant because you don't really need to match the load capacitance to get the thing to oscillate. And if it's not oscillating it doesn't matter that you've matched the crystal's load capacitance!

Not a problem that will stop oscillation at 32.768kHz, but the trimmer setting will be a bit different due to the breadboard capacitance (many pF).

Tim.