43.11 External Crystal Oscillator and Clock (XOSC) Electrical Specifications

Table 43-14. External Crystal Oscillator and Clock 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
XOSC_1 FOSC_XOSC XOSC Crystal Frequency 0.4 — 32 MHz XOSCCTRL.XTALEN = 1

XIN, XOUT Primary Osc
XOSC_1A TOSC TOSC = 1/FOSC_XOSC 31.25 — 2500 ns See parameter XOSC_1 for FOSC_XOSC value
XOSC_2 XOSC_ST XOSC Crystal Stabilisation Time(3)

XOSC Crystal FOSC = 2 - 32 MHz

— 12300 — TOSC CL = 20pF,

XOSC.GAIN = 0,1,2,3,4
XOSC_3 CXIN XOSC XIN parasitic pin capacitance — 6.2 — pF —
XOSC_5 CXOUT XOSC XOUT parasitic pin capacitance — 3 — pF —
XOSC_11 CLOAD (2) XOSC Crystal FOSC = 0.455 MHz — — 100 pF —
XOSC_13 XOSC Crystal FOSC = 2 MHz — — 20 pF —
XOSC_15 XOSC Crystal FOSC = 4 MHz — — 20 pF —
XOSC_17 XOSC Crystal FOSC = 8 MHz — — 20 pF —
XOSC_19 XOSC Crystal FOSC = 16 MHz — — 20 pF —
XOSC_20 XOSC Crystal FOSC = 32 MHz — — 12 pF —
XOSC_21 ESR XOSC Crystal FOSC = 0.455 MHz — — 443 Ω CL = 100 pF, XOSC.GAIN = 0
XOSC_23 XOSC Crystal FOSC = 2 MHz — — 383 Ω CL = 20 pF, XOSC.GAIN = 0
XOSC_25 XOSC Crystal FOSC = 4 MHz — — 218 Ω CL = 20 pF, XOSC.GAIN = 1
XOSC_27 XOSC Crystal FOSC = 8 MHz — — 114 Ω CL = 20 pF, XOSC.GAIN = 2
XOSC_29 XOSC Crystal FOSC = 16 MHz — — 58 Ω CL = 20 pF, XOSC.GAIN = 3
XOSC_31 XOSC Crystal FOSC = 32 MHz — — 62 Ω CL = 12 pF, XOSC.GAIN = 4
XOSC_35 FOSC_XCLK Ext Clock Oscillator Input Freq (XIN pin) — — 48000 KHz XOSCCTRL.XTALEN = 0
XOSC_37 XCLK_DC Ext Clock Oscillator (XIN) Duty Cycle 40 50 60 % XOSCCTRL.XTALEN = 0
XOSC_39 XCLK_FST Primary XIN Clock Fail Safe Time-out Period — 4*1/(OSC48M_1/2^CFDPRESC) — µs
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 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 0.25 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 version of 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 = 13.5 pF (Always rounded down)

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

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

    Figure 43-4. XTAL
  3. Maximum start up time user selectable in XOSCCTRL.STARTUP.
  4. Minimum value to respect the FDPLL96M minimum input frequency parameter (FDPLL_1). Only applicable when XOSC is used to feed the FDPLL96M.