67.7.5 32.768 kHz Crystal Oscillator

Table 67-37. 32.768 kHz Crystal Oscillator Characteristics
Symbol	Parameter	Conditions		Min	Max	Unit
V_DDBU	Supply voltage range (VDDBU)⁽¹⁾	–		V_IT-	3.60	V
I_DDBU	Current consumption (VDDBU)⁽²⁾	–		–	3.1	µA
t_START	Start-up time⁽²⁾	R_S⁽³⁾ < 50 kΩ	C_M = 0.6 fF	–	3.7	s
		R_S⁽³⁾ < 50 kΩ	C_M = 3 fF	–	0.8	s
		R_S⁽³⁾ < 90 kΩ	C_M = 0.6 fF	–	5.0	s
		R_S⁽³⁾ < 90 kΩ	C_M = 3 fF	–	1.0	s
f_OSC	Operating frequency	–		32.7	32.8	kHz
duty	Output duty cycle⁽²⁾	–		40	60	%
C_PARA32k	Internal parasitic capacitance⁽²⁾	Between XIN32 and XOUT32		2		pF
R_F	Internal resistor⁽²⁾	Between XIN32 and XOUT32		6		MΩ

Note:

In this table, VIT- refers to the negative-going input threshold voltage of the VBAT POR (see Table 67-34). The operation of the backup section of the device is granted down to the VBAT POR VIT- threshold.
Simulation data
R_S is the crystal’s equivalent series resistor.

Figure 67-32. 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 - 2 - 0.5) = 20 pF

The table below summarizes recommendations for 32.768 kHz crystal selection.

Table 67-38. Recommended 32.768 kHz Crystal Characteristics
Symbol	Parameter	Conditions	Min	Max	Unit
C_CRYSTAL	Crystal load capacitance	As specified by the crystal manufacturer	6	12.5	pF
R_S	Equivalent series resistor	–	–	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