31.14 XOSC32K Electrical Specifications

Table 31-16. 32.768 kHz Crystal Oscillator (XOSC32K) AC Electrical Specifications
AC CHARACTERISTICSStandard Operating Conditions: VDDIO = AVDD = VDD 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, ±100 ppm maximum
XOSC32_3CXIN32XOSC32 XIN32 parasitic pin capacitance—3.2—pF—
XOSC32_5CXOUT32XOSC32 XOUT32 parasitic pin capacitance—3.5—pF—
XOSC32_11CLOAD_X32(2) 32.768kz Crystal Load Capacitance——12.5pF

XOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1

XOSC32_12—————
XOSC32_13ESR_X32 32.768 kHz Crystal ESR——100KΩ

XOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1

CLOAD = 12.5 pF

XOSC32_15TOSC32TOSC32 = 1/FOSC_XOSC32—30.5176—µsSee parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17XOSC32_ST(1)XOSC32 Crystal Stabilization Time—28000(3)TOSCCrystal stabilization time only not Oscillator Ready
XOSC32_19FOSC_XCLK32Ext Clock Oscillator Input Freq (the XIN32 pin)—32.768—kHzXOSC32K.XTALEN = 0, ±100 ppm maximum
XOSC32_21XCLK32_DCExt Clock Oscillator Duty Cycle405060%XOSC32K.XTALEN = 0
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. 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 CXIN32 and CXOUT32 within 4 pF of each other, assume CXTAL_EFF = ((CXIN32+CXOUT32) / 2)
      Note: Averaging CXIN32 and CXOUT32 will effect final calculated CLOAD value by less than 0.25 pF.

      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))

      For example:

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

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

      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 = 13.5 pF (always rounded down)

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

      User C1 = C2 = 13 pF ≤ CLOAD_X32(max) spec

  3. User Selectable in XOSC32K.STARTUP.
Figure 31-2. XTAL