57.14 XOSC32 Electrical Specifications

Table 57-16. XOSC32K Electrical Specifications (1)
AC CHARACTERISTICSStandard Operating Conditions: VDDIO = 2.5V to 3.6V (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—6.3—pF—
XOSC32_5CXOUT32XOSC32 XOUT32 parasitic pin capacitance—6.3—pF—
XOSC32_11CLOAD_X32 (3) 32.768 kHz Crystal Load Capacitance——12.5pF SUPC_CR.XTALSEL = 1
XOSC32_13ESR_X32 32.768 kHz Crystal ESR——100kΩ SUPC_CR.XTALSEL = 1, Cload = 9 pF
XOSC32_14——50kΩSUPC_CR.XTALSEL = 1, Cload = 12.5 pF
XOSC32_15TOSC32TOSC32 = 1/FOSC_XOSC32—30.5176—µsSee parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17XOSC32_ST (2)XOSC32 Crystal Start-up Time ——508001/LP32K_1Fixed counter value of Slow RC
XOSC32_19FOSC_XCLK32Ext Clock Oscillator Input Freq (XIN32 pin)——44kHzSUPC_CR.XTALSEL = 0
XOSC32_21XCLK32_DCExt Clock Oscillator Duty Cycle405060%SUPC_CR.XTALSEL = 0
XOSC32_23XCLK32_FSTXIN32 Clock Fail Safe Time-out Period—(4)—µsUsed with 32K oscillator frequency monitor

(CKGR_MOR.XT32KFME) and error status of PMC_SR.XT32KERR

Note:
  1. VDDIO = VDDIN = 3.3V.
  2. This is for guidance only. A major component of crystal start-up time is based on the 2nd 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.
  3. 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 XTAL PCB layout
    • For CXIN and CXOUT within 4 pF of each other, Assume CXTAL_EFF = ((CXIN+CXOUT) / 2)
      Note: Averaging CXIN and CXOUT will effect final calculated CLOAD value by less than the tolerance of the capacitor selection.

    Equation 1:

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

    • Assuming C1 = C2 and CXin ~= CXout, 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))

    For example,

    • XTAL Mfg CLOAD Data Sheet Specification = 18 pF
    • XTAL PCB trace Capacitance = 2.5 pF
    • CXIN pin = 6.5 pF, CXOUT pin = 4.5 pF, therefore CXTAL_EFF = ((CXIN+CXOUT) / 2)

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

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

      C1 = C2 = (36 - 5.5 - (2 * 2.5))

      C1 = C2 = 25.5 pF (always rounded down)

      C1 = C2 = 25 pF (i.e., for hypothetical example crystal external load capacitors)

      User C1 = C2 = 25 pF CLOAD (max.) spec

  4. 4 * Period of RC Main Oscillator.