55.15 XOSC Maximum Clock Frequencies Electrical Specifications

Table 55-19. External XTAL and Clock Electrical Specifications
AC CHARACTERISTICS Standard Operating Conditions: VDD = AVDD = 1.71V to 3.63V (unless otherwise stated)

Operating temperature:

-40°C ≤ TA ≤ +85°C for Industrial Temperature
Param. No. Symbol Characteristics Min. Typ. Max. Units Conditions
XOSC_1 FOSC_XOSC XOSC Crystal Frequency 8 — 48 MHz XOSCCTRLn.XTALEN=1

XIN, XOUT Primary Osc
XOSC_1A TOSC TOSC = 1/FOSC_XOSC 20.833 — 125 ns —
XOSC_2 XOSC_ST (1) XOSC Crystal Start-up Time — 39700 Note (2) TOSC —
XOSC_3 CXIN XOSC XIN parasitic pin capacitance — 6.3 — pF —
XOSC_5 CXOUT XOSC XOUT parasitic pin capacitance — 5.9 — pF —
XOSC_11 CLOAD (3) XOSC Crystal FOSC = 8MHz — — 20 pF XOSCCTRLn.ENALC = n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x3

XOSCCTRLn.IPTAT = 0x2
XOSC_13 XOSC Crystal FOSC = 16MHz — — 20 pF XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x4

XOSCCTRLn.IPTAT = 0x3
XOSC_15 XOSC Crystal FOSC = 24MHz — — 20 pF XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x5

XOSCCTRLn.IPTAT = 0x3
XOSC_17 XOSC Crystal FOSC = 48MHz — — 13 pF XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x6

XOSCCTRLn.IPTAT = 0x3
XOSC_21 ESR XOSC Crystal FOSC = 8MHz — — 181 Ω XOSCCTRLn.ENALC = n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x3

XOSCCTRLn.IPTAT = 0x2

CLOAD = 20pF
XOSC_23 XOSC Crystal FOSC = 16MHz — — 180 Ω XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x4

XOSCCTRLn.IPTAT = 0x3

CLOAD = 20pF
XOSC_25 XOSC Crystal FOSC = 24MHz — — 70 Ω XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x5

XOSCCTRLn.IPTAT = 0x3

CLOAD = 20pF
XOSC_27 XOSC Crystal FOSC = 48MHz — — 70 Ω XOSCCTRLn.ENALC = _n/a

XOSCCTRLn.LOWBUFGAIN = 0x0

XOSCCTRLn.IMULT = 0x6

XOSCCTRLn.IPTAT = 0x3

CLOAD = 13pF
XOSC_33 DLEVEL MCU Crystal Osc Power Drive Level — — 1000 µW XOSCCTRLn.XTALEN=1,

ENALC = ON
XOSC_35 FOSC_XCLK Ext Clock Oscillator Input Freq (XIN pin) 0 — 48 MHz XOSCCTRLn.XTALEN=0
XOSC_37 XCLK_DC Ext Clock Oscillator (XIN) Duty Cycle 40 50 60 % XOSCCTRLn.XTALEN=0
XOSC_39 XCLK_FST Primary XIN Clock Fail Safe Time-out Period — 4*1/(DFLL_1/2^XOSCCTRLn.CFDPRESC) — µs DFLL48M in open loop,

DFLLCTRLB.MODE=0,

DFLL_1: Refer to DFLL48M Electrical Specifications
— 4*1/(DFLL_13/2^XOSCCTRLn.CFDPRESC) — DFLL48M in closed loop,

DFLLCTRLB.MODE=1,

DFLL_13: Refer to DFLL48M 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 XOSCCTRLn.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:

      (i.e. Simplified 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 hypothectical example crystal external load capacitors)

      User C1=C2=13pF CLOAD(max) specification