58.7.7 32.768 kHz Crystal Oscillator

Table 58-44. 32.768 kHz Crystal Oscillator Characteristics
Symbol	Parameter	Conditions		Min	Typ	Max	Unit
V_DDBU	Supply voltage range (VDDBU)	–		V_IT-⁽¹⁾	–	3.6	V
I_VDDBU	Current consumption (VDDBU)⁽²⁾	–		–	–	3.1	μA
t_START	Startup time⁽²⁾	ESR < 50 kΩ	C_M = 0.6 fF	–	–	3.7	s
		ESR < 50 kΩ	C_M = 3 fF	–	–	0.8	s
		ESR < 90 kΩ	C_M = 0.6 fF	–	–	5.0	s
		ESR < 90 kΩ	C_M = 3 fF	–	–	1.0	s
f_OSC	Operating frequency	–		32.7	–	32.8	kHz
Duty	Duty cycle	–		40	50	60	%
C_PARA32K	Internal parasitic capacitance⁽²⁾	Between XIN32 and XOUT32		–	1.7	–	pF

Note:

In this table, V_IT- corresponds to the negative-going input threshold voltage of VDDBU POR (see the table VDDBU POR Characteristics). Operation of the device backup section is granted down to the VDDBU POR V_IT- threshold.
Simulation data

The 32.768 crystal oscillator supports a Bypass mode. See Input AC Characteristics.

Figure 58-35. 32.768 kHz Crystal Oscillator

C_LEXT32K = 2x(C_CRYSTAL – C_PARA32K – C_PCB / 2)

where C_PCB is the single-ended (ground-referenced) parasitic capacitance of the printed circuit board (PCB) on XIN32 and XOUT32 tracks. As an example, if the crystal is specified for a 12.5 pF load, with C_PCB=1 pF (on XIN32 and on XOUT32), C_LEXT32K = 2 x (12.5 - 1.7 - 0.5) = 20.6 pF.

The table below summarizes recommendations for 32.768 kHz crystal selection.

Table 58-45. Recommended 32.768 kHz Crystal Characteristics
Symbol	Parameter	Conditions	Min	Typ	Max	Unit
C_CRYSTAL	Crystal load capacitance	As specified by the crystal manufacturer	6	–	12.5	pF
R_S	Equivalent series resistor	–	–	50	90	kΩ
C_M	Motional capacitance	–	0.6	–	3	fF
C_SHUNT	Shunt capacitance	–	0.6	–	2	pF
P_ON	Drive level	–	0.2	–	–	μW