50.7.12 Slow Clock Crystal Oscillator

Table 50-29. 32.768 kHz Crystal Oscillator Characteristics
Symbol	Parameter	Conditions	Min	Max	Unit
V_VDDBU	Supply voltage (VDDBU)^{(1, 3)}	–	V_IT-	3.6	V
I_VDDBU	Current consumption (VDDBU)⁽²⁾	–	–	750	nA
t_{SWITCH_TD_SLCK}	Slow clock source switching time after crystal oscillator enable to crystal oscillator ready⁽⁴⁾		⁽⁴⁾		s
f_OSC	Operating frequency	–	32.768		kHz
Duty	Duty cycle	–	40	60	%
C_PARA32K	Internal parasitic capacitance⁽²⁾	Between XIN32 and XOUT32	–	1	pF

Note:

VDDBU is the output of the power switch connected to VBAT or VDD3V3.
Simulation data.
Functional down to V_IT-of the VBAT Supply Monitor.
This time is defined as 98400 Slow RC Periods. Oscillator Enable and Status bits in Supply Controller registers.

The oscillator supports a bypass mode. See Input AC Characteristics.

Figure 50-20. 32.768 kHz Crystal Oscillator

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

where C_PCB is the 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 - 0.5) = 22 pF.

The table below summarizes recommendations for crystal selection.

Table 50-30. Recommended 32.768kHz Crystal Characteristics
Symbol	Parameter	Conditions	Min	Max	Unit
C_CRYSTAL	Crystal load capacitance	As specified by the crystal manufacturer	⁽¹⁾	⁽¹⁾	pF
ESR	Equivalent Series Resistor (R_S)	–	–	⁽¹⁾	kΩ
C_SHUNT	Shunt capacitance	–	–	⁽¹⁾	pF
P_ON	Drive level	–	15	–	μW

Note:

See Table 50-31.

Table 50-31. ESR, C_SHUNT and C_LOAD Selection
ESR (kΩ)	C_SHUNT(pF)	C_CRYSTAL(pF)
ESR (kΩ)	C_SHUNT(pF)	Min	Max
50	1	5	14
	2	5.5	13
	3	6.5	12
	4	8	10.5
70	1	5	11.5
70	2	7	10
80	1	5.5	10
80	2	8	8
90	1	6	9.5
100	1	6.5	8.5