Hello fellow freaks
I am using the ATXMEGA256A3U RTC, with a 32kHz crystal, part number ST3215SB32768B0HPWB3.
The main specs of the crystal:
Nominal Frequency: 32768Hz
Load Capacitance: 6pF
Shunt capacitance: 0.6-1.2 pF
The main specs of the XMEGA RTC:
ESR Max (crystal load 6.5pF): 60kohm
Recommended safety factor - capacitance load matched to
crystal specification: 3 (what's this?)
The image below is the PCB, short traces and a ground plane below.
The RTC is running a little bit too fast. The time is gaining a few seconds a day.
I don't have load capacitors. In case I needed to add them, do I need an exact value or is there a range of capacitance that will make the oscillation exactly 32768Hz (+/- 20ppm).