49.14 XOSC32K Electrical Specifications

Table 49-18. External 32.768 kHz XTAL and Clock Electrical Specifications
AC CHARACTERISTICSStandard Operating Conditions: VDD = AVDD = 1.62V to 3.63V (unless otherwise stated)

Operating temperature: -40°C ≤ TA ≤ +85°C for Industrial

Param. No.SymbolCharacteristicsMin.Typ.Max.UnitsConditions
XOSC32_1FOSC_XOSC32XOSC32 Oscillator Crystal Frequency—32.768—kHzXIN32, XOUT32 Secondary Osc
XOSC32_3CXIN32XOSC32 XIN32 parasitic pin capacitance—4.2—pF—
XOSC32_5CXOUT32XOSC32 XOUT32 parasitic pin capacitance—4.2—pF—
XOSC32_11CLOAD_X32 (3) 32.768kz Crystal Load Capacitance——9pFXOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1

XOSC32_13ESR_X32 32.768kz Crystal ESR——70kΩXOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1

XOSC32_15TOSC32TOSC32 = 1/FOSC_XOSC32—30.518—µsSee parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17XOSC32_ST (1)XOSC32 Crystal Start-up Time —10000Note (2)TOSC32—
XOSC32_19FOSC_XCLK32Ext Clock Oscillator Input Freq (XIN32 pin)—32.768—kHz±100ppm max

XOSC32K.XTALEN=0

XOSC32K.ENABLE = 1

XOSC32_21XCLK32_DCExt Clock Oscillator Duty Cycle40—60%XOSC32K.XTALEN=0

XOSC32K.ENABLE = 1

XOSC32_23XCLK32_FSTXIN32 Clock Fail Safe Time-out Period—4 / (LP32K_1 / 2^(CFDCTRL.CFDPRESC) )—msLP32K_1: Refer to OSCULP32K Electrical Specifications
Note:
  1. 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.
  2. Maximum start up time user selectable in XOSC32K.STARTUP.
  3. 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 ~= 2.5pF for a tight PCB Xtal layout
    • For CXIN32 and CXOUT32 within 4pF of each other, Assume CXTAL_EFF = ((CXIN32+CXOUT32) / 2)
      Note: Averaging CXIN32 and CXOUT32 will effect final calculated CLOAD value by less than 0.25pF.

EQUATION 1:

MFG CLOAD Spec = {( [CXIN32 + C1] * [CXOUT32 + C2] ) / [CXIN32 + C1 + C2 + CXOUT32] } + estimated oscillator PCB stray capacitance

  • Assuming C1 = C2 and CXIN32 ~= CXOUT32, the formula can be further simplified and restated to solve for C1 and C2 by:

EQUATION 2: (Simplified Equation 1)

C1 = C2 = ((2 * MFG CLOAD spec) - CXTAL_EFF - (2 * PCB capacitance))

EXAMPLE ONLY:

  • XTAL Mfg CLOAD Data Sheet Spec = 12pF
  • PCB XTAL trace Capacitance = 2.5pF
  • CXIN32 pin = 6.5pF, CXOUT32 pin = 4.5pF. Therefore CXTAL_EFF = ((CXIN32+CXOUT32) / 2)

CXTAL_EFF = ((6.5 + 4.5)/2) = 5.5pF

C1 = C2 = ((2 * MFG CLOAD spec) - CXTAL_EFF - (2 * PCB capacitance))

C1 = C2 = (24 - 5.5 - (2 * 2.5))

C1 = C2 = (24 - 5.5 - 5)

C1 = C2 = 13.5pF (Always rounded down)

C1 = C2 = 13pF (i.e. for hypothetical example crystal external load capacitors)

User C1=C2=13pF ≤ CLOAD_X32(max) spec