46.12 XOSC32K Electrical Specifications

Table 46-14. 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. Typ. Max. Units Conditions
XOSC32K_1 FOSC_XOSC32K XOSC32K Oscillator Crystal Frequency — 32.768 — kHz XIN32, XOUT32

XOSC32K.XTALEN = 1
XOSC32K_3 CXIN32 XOSC32K XIN32 parasitic pin capacitance — 2.9 — pF —
XOSC32K_5 CXOUT32 XOSC32K XOUT32 parasitic pin capacitance — 3.2 — pF —
XOSC32K_11 CLOAD_X32 (2) 32.768kz Crystal Load Capacitance — — 12.5 pF XOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1
XOSC32K_13 ESR_X32 32.768kz Crystal ESR — — 70 kΩ XOSC32K.XTALEN = 1

XOSC32K.ENABLE = 1

CLOAD = 12.5 pF
XOSC32K_15 TOSC32 TOSC32 =

1/FOSC_XOSC32K

 — 30.5176 — µs See parameter XOSC32_1 for FOSC_XOSC32K value
XOSC32K_17 XOSC32K_ST(1) XOSC32K Crystal Stabilization Time — 16000 (3) TOSC32 Crystal stabilization time only not Oscillator Ready
XOSC32K_19 FOSC_XCLK32 Ext Clock Oscillator Input Freq (XIN32 pin) — 32.768 — kHz XOSC32K.XTALEN=0
XOSC32K_21 XCLK32_DC Ext Clock Oscillator Duty Cycle 40 50 60 % XOSC32K.XTALEN=0
XOSC32K_23 XCLK32_FST XIN32 Clock Fail Safe Time-out Period — 4*1/(ULPRC32K_1/2^OSC32KCTRL.CFDCTRL.CFDPRESC) — ms —
Note:
  1. 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.
  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 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 0.25pF.

    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 46-6. XTAL
  3. User Selectable in OSC32KCTRL.STARTUP.