49.13 XOSC Electrical Specifications

Table 49-17. External XTAL and Clock Electrical Specifications
AC CHARACTERISTICSStandard Operating Conditions: VDD =AVDD = 1.62V to 3.63V (unless otherwise stated)

Operating temperature: -40°C ≤ TA ≤ +85°C for Industrial

Param. No.SymbolCharacteristicsMin.Typ.Max.UnitsConditions
XOSC_1FOSC_XOSCXOSC Crystal Frequency0.4—32MHzXOSCCTRL.XTALEN=1

XIN, XOUT Primary Osc

XOSC_1ATOSCTOSC = 1/FOSC_XOSC31.25—2500ns—
XOSC_2XOSC_ST (1)XOSC Crystal Start-up Time —15600Note (2)TOSCCLOAD = 20pF
XOSC_3CXINXOSC XIN parasitic pin capacitance—7—pF—
XOSC_5CXOUTXOSC XOUT parasitic pin capacitance—4.5—pF—
XOSC_11CLOAD (3) XOSC Crystal

FOSC = 0.4 MHz

——100pFXOSCCTRL.GAIN = 0x0
XOSC_13 XOSC Crystal

FOSC = 2 MHz

——20XOSCCTRL.GAIN = 0x0
XOSC_15 XOSC Crystal

FOSC = 4 MHz

——20XOSCCTRL.GAIN = 0x1
XOSC_17 XOSC Crystal

FOSC = 8 MHz

——20XOSCCTRL.GAIN = 0x2
XOSC_19 XOSC Crystal

FOSC = 16 MHz

——20XOSCCTRL.GAIN = 0x3
XOSC_20 XOSC Crystal

FOSC = 32 MHz

——20XOSCCTRL.GAIN = 0x4
XOSC_21ESR XOSC Crystal

FOSC = 0.4 MHz

——5600ΩXOSCCTRL.GAIN = 0x0

CLOAD= 20pF

XOSC_23 XOSC Crystal

FOSC = 2 MHz

——330XOSCCTRL.GAIN = 0x0

CLOAD= 20pF

XOSC_25 XOSC Crystal

FOSC = 4 MHz

——240XOSCCTRL.GAIN = 0x1

CLOAD= 20pF

XOSC_27 XOSC Crystal

FOSC = 8 MHz

——105XOSCCTRL.GAIN = 0x2

CLOAD= 20pF

XOSC_29 XOSC Crystal

FOSC = 16 MHz

——60XOSCCTRL.GAIN = 0x3

CLOAD= 20pF

XOSC_30 XOSC Crystal

FOSC = 32 MHz

——55XOSCCTRL.GAIN = 0x4

CLOAD= 20pF

XOSC_35FOSC_XCLKExt Clock Oscillator Input Freq (XIN pin)0—24MHzXOSCCTRL.XTALEN=0
XOSC_37XCLK_DCExt Clock Oscillator (XIN) Duty Cycle40—60%XOSCCTRL.XTALEN=0
XOSC_39XCLK_FSTPrimary XIN Clock Fail Safe Time-out Period—4 / (RC16M_1C / 2^(CFDPRESC.CFDPRESC))—µsRC16M_1C: Refer to OSC16M Electrical Specifications
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. Maximum start up time user selectable in XOSCCTRL.STARTUP.
  3. 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 ~= 2.5pF 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 0.25pF.

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 version of equation 1)

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

EXAMPLE ONLY:

  • XTAL Mfg CLOAD Data Sheet Spec = 12pF
  • PCB XTAL trace Capacitance = 2.5pF
  • CXIN pin = 6.5pF, CXOUT pin = 4.5pF. Therefore CXTAL_EFF = ((CXIN+CXOUT) / 2)

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

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

C1 = C2 = (24 - 5.5 - (2 * 2.5))

C1 = C2 = 13.5pF (Always rounded down)

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

User C1=C2=13pF ≤ CLOAD(max) spec