46.15 XOSC Electrical Characteristics

Table 46-20. External XTAL and Clock AC Electrical Specifications
AC CHARACTERISTICSStandard Operating Conditions: VDDREG = VDDIO = AVDD 1.71V to 3.63V (unless otherwise stated)

Operating temperature:

-40°C ≤ TA ≤ +85°C for Industrial

Param. No.SymbolCharacteristicsMin.Typ.Max.UnitsConditions (1)
XOSC_1FOSC_XOSCXOSC Crystal Frequency4—48MHzXOSCCTRLn.XTALEN = 1

XIN, XOUT Primary Osc

XOSC_1ATOSCTOSC = 1/FOSC_XOSC20.83—250nsSee parameter XOSC1 for FOSC_XOSC value
XOSC_2XOSC_ST (1)XOSC Crystal Start-up Time ——1300000 (3)TOSC Crystal stabilization time only, not Oscillator Ready

XOSCCTRLA.AGC = 1

XOSCCTRLA.BW = 1

FOSC = 48 MHz

(Note 1)
XOSC_3CXINXOSC XIN parasitic pin capacitance—2.2—pF—
XOSC_5CXOUTXOSC XOUT parasitic pin capacitance—2.2—pF—
XOSC_11CLOAD (2)Crystal load capacitance

FOSC_XOSC = 4 MHz

——20pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 500Ω

XOSC_13Crystal load capacitance

FOSC_XOSC = 8 MHz

——pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 500Ω

XOSC_15Crystal load capacitance

FOSC_XOSC = 12 MHz

——pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 230Ω

XOSC_17Crystal load capacitance

FOSC_XOSC = 16 MHz

——18pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 160Ω

XOSC_19Crystal load capacitance

FOSC_XOSC = 24 MHz

——pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 70Ω

XOSC_21Crystal load capacitance

FOSC_XOSC = 32 MHz

——12pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 80Ω

XOSC_23Crystal load capacitance

FOSC_XOSC = 48 MHz

——8pFXOSCCTRLA.AGC = 1

Crystal ESR ≤ 70Ω

XOSC_33DLEVELMCU Crystal Osc Power Drive Level——100µWXOSCCTRLA.AGC = 1
XOSC_35FOSC_XCLKExt Clock Oscillator Input Freq (XIN pin)4—48MHzXOSCCTRLA.XTALEN = 0
XOSC_37XCLK_DCExt Clock Oscillator (XIN) Duty Cycle405060%XOSCCTRLA.XTALEN = 0
XOSC_39XCLK_FSTPrimary XIN Clock Fail Safe Time-out Period—4*1/(DFLL_1/2^XOSCCTRLA.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. 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 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.
      Note:

      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))

      For example:

      • 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 hypothetical example crystal external load capacitors)

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

      Figure 46-6. XTAL
  3. Maximum start up time; user must configure XOSCCTRLA.STARTUP counter value greater than start up time.