3.3.9.2 External 32 kHz Crystal Oscillator (XOSC32K) Electrical Specifications

Table 3-14. 32.768 kHz Crystal Oscillator (XOSC32K) AC Electrical Specifications
Standard Operating Conditions: V_DDIO = AVDD = 1.8V to 5.5V (unless otherwise stated) Operating temperature: -40°C ≤ T_A ≤ +125°C for extended temperature
Param. No.	Symbol	Characteristics	Min.	Typ.	Max.	Units	Conditions
XOSC32_1	f_{OSC_XOSC32}	XOSC32 crystal oscillator frequency	—	32.768	—	kHz	—
XOSC32_3	C_XIN32	XOSC32 XIN32 parasitic pin capacitance	—	3.5	—	pF	—
XOSC32_5	C_XOUT32	XOSC32 XOUT32 parasitic pin capacitance	—	3.6	—	pF	—
XOSC32_11	C_{LOAD_X32}⁽²⁾	32.768 kHz crystal load capacitance	—	—	9	pF	XOSC32KCTRL.LPMODE = `1` XOSC32KCTRL.XTALEN = `1` XOSC32KCTRL.ENABLE = `1`
XOSC32_12	C_{LOAD_X32}⁽²⁾	32.768 kHz crystal load capacitance	—	—	12.5	pF	XOSC32KCTRL.LPMODE = `0` XOSC32KCTRL.XTALEN = `1` XOSC32KCTRL.ENABLE = `1`
XOSC32_13	ESR_X32	32.768 kHz crystal ESR	—	—	50	kΩ	XOSC32KCTRL.LPMODE = `1` XOSC32KCTRL.XTALEN = `1` XOSC32KCTRL.ENABLE = `1`
XOSC32_14	ESR_X32	32.768 kHz crystal ESR	—	—	100	kΩ	XOSC32KCTRL.LPMODE = `0` XOSC32KCTRL.XTALEN = `1` XOSC32KCTRL.ENABLE = `1`
XOSC32_15	T_OSC32	T_OSC32 = 1/f_{OSC_XOSC32}	—	30.5176	—	µs	See parameter XOSC32_1 for F_{OSC_XOSC32} value
XOSC32_17	XOSC32_ST⁽¹⁾	XOSC32 Crystal Star-up Time	—	51500	⁽³⁾	TOSC	XOSC32KCTRL.LPMODE = `1` Crystal stabilization time only not oscillator ready
XOSC32_18	XOSC32_ST⁽¹⁾	XOSC32 Crystal Star-up Time	—	5800	⁽³⁾	TOSC	XOSC32KCTRL.LPMODE = `0` Crystal stabilization time only not oscillator ready
XOSC32_19	f_{OSC_XCLK32}	Ext clock oscillator input freq (XIN32 pin)	—	32.768	—	kHz	XOSC32KCTRL.XTALEN = `0`
XOSC32_21	XCLK32_DC	Ext clock oscillator duty cycle	—	50	—	%	XOSC32KCTRL.XTALEN = `0`
XOSC32_23	XCLK32_FST	XIN32 clock fail safe time-out period	—	4 x T_OSC32⁽⁴⁾	—	µs	—
Note: This is for guidance only. A major component of crystal start-up time is determined by the parasitics of the third-party crystal manufacturer, which 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 standard trace width = 0.175 mm, height = 36 μm, thickness = 113 μm) Using these measurements, a typical crystal PCB capacitance is approximately 2.5 pF with a PCB layout keeping the crystal traces as short as possible For C_XIN and C_XOUT within 4 pF of each other, assume C_{XTAL_EFF} = ((C_XIN + C_XOUT)/2) Note: Averaging C_XIN and C_XOUT will affect the final calculated C_LOAD value by less than the tolerance of the capacitor selection. EQUATION 1: MFG C_LOAD spec = {( [C_XIN + C1] x [C_XOUT + C2] )/[C_XIN + C1 + C2 + C_XOUT] } + estimated oscillator PCB stray capacitance Assuming C1 = C2 and C_XIN is approximately equal to C_XOUT, the formula can be further simplified and restated to solve for C1 and C2 as follows: EQUATION 2 (Simplified Equation 1): C1 = C2 = ((2 x MFG C_LOAD spec) - C_{XTAL_EFF} - (2 x PCB capacitance)) EXAMPLE ONLY: Crystal MFG C_LOAD spec = 12 pF PCB crystal trace capacitance = 2.5 pF C_XIN pin = 6.5 pF C_XOUT pin = 4.5 pF C_{XTAL_EFF} = ((C_XIN + C_XOUT)/2) C_{XTAL_EFF} = ((6.5 + 4.5)/2) = 5.5 pF C1 = C2 = ((2 x MFG C_LOAD spec) - C_{XTAL_EFF} - (2 x PCB capacitance)) C1 = C2 = (24 - 5.5 - (2 x 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 3-1. XTAL User selectable in XOSC32KCTRL.CSUT[1:0]. Period of four safe clocks, monitored by the Clock Failure Detection system, watching for XOSC32K clock activity.