43.12 External 32 kHz Crystal Oscillator (XOSC32K) Electrical Specifications

Table 43-15. 32.768 kHz Crystal Oscillator (XOSC32K) 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. Typical Max. Units Conditions
XOSC32_1 FOSC_XOSC32 XOSC32 Oscillator Crystal Frequency — 32.768 — kHz XIN32, XOUT32 Secondary Osc
XOSC32_3 CXIN32 XOSC32 XIN32 parasitic pin capacitance — 3 — pF —
XOSC32_5 CXOUT32 XOSC32 XOUT32 parasitic pin capacitance — 3.1 — pF —
XOSC32_11 CLOAD_X32 (2) 32.768 kz Crystal Load Capacitance — — 12.5 pF XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1
XOSC32_13 ESR_X32 32.768 kz Crystal ESR — — 70 kΩ XOSC32K.XTALEN = 1 XOSC32K.ENABLE = 1 CLOAD = 12.5 pF
XOSC32_15 TOSC32 TOSC32 = 1/FOSC_XOSC32 — 30.5176 — us See parameter XOSC32_1 for FOSC_XOSC32 value
XOSC32_17 XOSC32_ST XOSC32 Crystal Stabilization Time — 16000 (3) TOSC32 32.768 kHz Crystal (1)
XOSC32_19 FOSC_XCLK32 Ext Clock Oscillator Input Freq (XIN32 pin)(4) — 32.768 — kHz XOSC32K.XTALEN = 0
XOSC32_21 XCLK32_DC Ext Clock Oscillator Duty Cycle 40 50 60 % XOSC32K.XTALEN = 0
XOSC32_23 XCLK32_FST XIN32 Clock Fail Safe Time-out Period — 4*1/(ULPRC32K_1/2^CFDPRESC) — ms —
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. 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 ~= 2.5 pF for a tight PCB xtal layout
    • For CXIN and CXOUT within 4pF of each other, Assume CXTAL_EFF = ((CXIN+CXOUT) / 2)
      Note: Averaging CXIN and CXOUT will effect final calculated CLOAD value by less than 0.25 pF.

    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))

    EXAMPLE ONLY:

    • XTAL Mfg CLOAD Data Sheet Spec = 12 pF
    • PCB XTAL 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 = (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

    Figure 43-5. XTAL
  3. User Selectable in OSC32KCTRL.STARTUP.
  4. Minimum value in order to respect the FDPLL96M minimum input frequency parameter (FDPLL_1). Only applicable when XOSC32K is used to feed the FDPLL96M.