67.6.3 32.768 kHz Crystal Oscillator Characteristics

Table 67-22. 32.768 kHz Crystal Oscillator Characteristics
Symbol Parameter Conditions Min Typ Max Unit
fOSC Operating Frequency Normal mode with crystal – 32.768 – kHz
tSTART Startup Time Cm> 3fF – – 1200 ms
IDDON Current Consumption ESR < 50k ohm CCRYSTAL32 = 12.5 pF – 440 900 nA
CCRYSTAL32 = 6 pF 600 900
ESR < 100k ohm CCRYSTAL32 = 12.5 pF 800 1200
CCRYSTAL32 = 6 pF 700 1200
CPARA32 Internal Parasitic Capacitance Between XIN32 and XOUT32 1.4 1.6 1.8 pF
Figure 67-4. 32 kHz Oscillator Schematics
Table 67-23. Recommended 32.768 kHz Crystal Characteristics
Symbol Parameter Conditions Min Typ Max Unit
ESR Equivalent Series Resistor Crystal at 32.768 kHz – – 100 kΩ
– Duty Cycle – 40 50 60 %
Cm Motional Capacitance Crystal at 32.768 kHz 3 – 8 fF
CSHUNT Shunt Capacitance Crystal at 32.768 kHz 0.6 – 2 pF
CCRYSTAL32 Allowed Crystal Capacitance Load(1) From crystal specification 6 – 12.5 pF
PON Drive Level – – – 0.2 µW
Note: The value of the external capacitors can be determined by using the following formula:

CLEXT32 = (2 x CCRYSTAL32) - CBOARD - (CPARA32 x 2)

where:

  • CLEXT32 is the external capacitor value which must be soldered from XIN to GND and XOUT to GND
  • CCRYSTAL32 is crystal targeted load
  • CBOARD is the external board parasitic capacitance (from XIN to GND or XOUT to GND)
  • CPARA32 is the internal parasitic capacitance