some pics like pic16f887 has the feature to use external resistor and capacitor to wake up the pic after long period of sleeping.
Thanks, I was trying to remember the name of that feature.
Only a limited range of PIC16F devices have that feature
631, 636, 639, 677, 684, 685, 687, 688, 689, 690, 882, 883, 884, 886, 887
none of the "enhanced" midrange PICs do (PIC16F1xxx and PIC16F1xxxx). I don't know if they have something equivalent under a different name.
A number of PIC18F devices have it.
Again, this will give you a very long pause in sleep, but it's not accurate at all.