43.12 External 32 kHz Crystal Oscillator (XOSC32K) Electrical Specifications

Table 43-15. 32.768 kHz Crystal Oscillator (XOSC32K) Electrical Specifications
AC CHARACTERISTICS			Standard Operating Conditions: VDD and VDDIO 2.7V to 5.5V (unless otherwise stated) Operating temperature -40°C ≤ TA ≤ +85°C for Industrial
Param. No.	Symbol	Characteristics	Min.	Typical	Max.	Units	Conditions
XOSC32_1	FOSC_XOSC32	XOSC32 Oscillator Crystal Frequency	—	32.768	—	kHz	XIN32, XOUT32 Secondary Osc
XOSC32_3	CXIN32	XOSC32 XIN32 parasitic pin capacitance	—	3	—	pF	—
XOSC32_5	CXOUT32	XOSC32 XOUT32 parasitic pin capacitance	—	3.1	—	pF	—
XOSC32_11	C_LOAD_X32 ⁽²⁾	32.768 kz Crystal Load Capacitance	—	—	12.5	pF	XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1
XOSC32_13	ESR_X32	32.768 kz Crystal ESR	—	—	70	kΩ	XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1 CLOAD = 12.5 pF
XOSC32_15	TOSC32	TOSC32 = 1/FOSC_XOSC32	—	30.5176	—	us	See parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17	XOSC32_ST	XOSC32 Crystal Stabilization Time	—	16000	(3)	TOSC32	32.768 kHz Crystal ⁽¹⁾
XOSC32_19	FOSC_XCLK32	Ext Clock Oscillator Input Freq (XIN32 pin)⁽⁴⁾	—	32.768	—	kHz	XOSC32K.XTALEN = 0
XOSC32_21	XCLK32_DC	Ext Clock Oscillator Duty Cycle	40	50	60	%	XOSC32K.XTALEN = 0
XOSC32_23	XCLK32_FST	XIN32 Clock Fail Safe Time-out Period	—	4*1/(ULPRC32K_1/2^CFDPRESC)	—	ms	—
Note: This is for guidance only. A major component of crystal start-up time is based on the Second party crystal MFG parasitics that are outside the scope of this specification. If this is a major concern the customer would need to characterize this based on their design choices. CRYSTAL LOAD CAPACITOR CALCULATION GIVEN: Standard PCB trace capacitance = 1.5 pF per 12.5 mm (0.5 inches) (i.e. PCB STD TRACE W = 0.175 mm, H = 36 μm, T = 113 μm) Xtal PCB capacitance typical therefore ~= 2.5 pF for a tight PCB xtal layout For CXIN and CXOUT within 4pF of each other, Assume CXTAL_EFF = ((C_XIN+C_XOUT) / 2) Note: Averaging CXIN and CXOUT will effect final calculated C_LOAD value by less than 0.25 pF. EQUATION 1: MFG C_LOAD Spec = {( [C_XIN + C1] * [C_XOUT + C2] ) / [C_XIN + C1 + C2 + C_XOUT] } + estimated oscillator PCB stray capacitance Assuming C1 = C2 and C_XIN ~= C_XOUT, the formula can be further simplified and restated to solve for C1 and C2 by: EQUATION 2: (Simplified Equation 1) C1 = C2 = ((2 * MFG C_LOAD spec) - C_{XTAL_EFF} - (2 * PCB capacitance)) EXAMPLE ONLY: XTAL Mfg C_LOAD Data Sheet Spec = 12 pF PCB XTAL trace Capacitance = 2.5 pF C_XIN pin = 6.5 pF, C_XOUT pin = 4.5 pF. Therefore C_{XTAL_EFF} = ((C_XIN+C_XOUT) / 2) C_{XTAL_EFF} = ((6.5 + 4.5)/2) = 5.5 pF C1 = C2 = ((2 * MFG C_LOAD spec) - C_{XTAL_EFF} - (2 * PCB capacitance)) C1 = C2 = (24 - 5.5 - (2 * 2.5)) C1 = C2 = (24 - 5.5 - 5) C1 = C2 = 13.5 pF (Always rounded down) C1 = C2 = 13 pF (i.e., for hypothetical example crystal external load capacitors) User C1 = C2 = 13 pF ≤ C_LOAD_X32 (max) spec Figure 43-5. XTAL User Selectable in OSC32KCTRL.STARTUP. Minimum value in order to respect the FDPLL96M minimum input frequency parameter (FDPLL_1). Only applicable when XOSC32K is used to feed the FDPLL96M.