39.13 XOSC32K Electrical Specifications

Table 39-15. 32.768 kHz Crystal Oscillator (XOSC32K) AC 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.	Typ.	Max.	Units	Conditions
XOSC32_1	FOSC_XOSC32	XOSC32 Oscillator Crystal Frequency	—	32.768	—	kHz	XIN32, XOUT32 Secondary Osc, ±100 ppm maximum
XOSC32_3	CXIN32	XOSC32 XIN32 parasitic pin capacitance	—	2.9	—	pF	—
XOSC32_5	CXOUT32	XOSC32 XOUT32 parasitic pin capacitance	—	3.2	—	pF	—
XOSC32_11	CLOAD_X32 ⁽²⁾	32.768 kHz Crystal Load Capacitance	—	—	12.5	pF	XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1
XOSC32_13	ESR_X32	32.768 kHz Crystal ESR	—	—	70	KΩ	XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1 CLOAD = 12.5 pF
XOSC32_15	TOSC32	TOSC32 = 1/FOSC_XOSC32	—	30.5176	—	µs	See parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17	XOSC32_ST^(1,3)	XOSC32 Crystal Stabilization Time	—	16000	—	TOSC	Crystal stabilization time only not Oscillator Ready
XOSC32_19	FOSC_XCLK32	Ext Clock Oscillator Input Frequency (the XIN32 pin)	—	32.768	—	kHz	XOSC32K.XTALEN = 0, ±100 ppm maximum
XOSC32_21	XCLK32_DC	Ext Clock Oscillator Duty Cycle	40	50	60	%	XOSC32KCTRLn.XTALEN = 0
XOSC32_23	XCLK32_FST	XIN32 Clock Fail Safe Time-out Period	—	4*1/(ULPRC32K_1/2^OSC32KCTRL.CFDCTRL.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. The Crystal Load Capacitor calculation is as follows: 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 C_XIN32 and C_XOUT32 within 4 pF of each other, assume CXTAL_EFF = ((CXIN32+CXOUT32)/2) Note: Averaging C_XIN32 and C_XOUT32 will effect final calculated C_LOAD value by less than 0.25 pF. Equation 1: MFG C_LOAD Spec = {( [C_XIN32 + C1] * [C_XOUT32 + C2] ) / [C_XIN32 + C1 + C2 + C_XOUT32] } + estimated oscillator PCB stray capacitance Assuming C1 = C2 and C_XIN32 ~= C_XOUT32, 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)) For example: XTAL Mfg C_LOAD Data Sheet Spec = 12 pF PCB XTAL Trace Capacitance = 2.5 pF C_XIN32 pin = 6.5 pF, C_XOUT32 pin = 4.5 pF. Therefore, C_{XTAL_EFF} = ((C_XIN32+C_XOUT32) / 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 User Selectable in XOSC32K.STARTUP. Figure 39-3. XTAL