USB5537B datasheet(67/79 Pages) MICROCHIP

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 2012 - 2015 Microchip Technology Inc.

DS00001682C-page 67

USB5537B

7.0

AC SPECIFICATIONS

7.1

Oscillator/Crystal

Crystal: Parallel resonant, fundamental mode, 25 MHz

30 ppm

External Clock: 50% duty cycle

 10%, 25 MHz  30 ppm, jitter < 100 ps rms

Note 7-1

C0 is usually included (subtracted by the crystal manufacturer) in the specification for CL and should

be set to 0 for use in the calculation of the capacitance formulas in Figure 7-2. However, the PCB

itself may present a parasitic capacitance between XTALIN and XTALOUT. For an accurate

calculation of C1 and C2, take the parasitic capacitance between traces XTALIN and XTALOUT into

account.

Note 7-2

Consult crystal manufacturer documentation for recommended capacitance values.

FIGURE 7-1:

TYPICAL CRYSTAL CIRCUIT

TABLE 7-1:

CRYSTAL CIRCUIT LEGEND

Symbol

Description

In Accordance With

Crystal shunt capacitance

Crystal manufacturer’s specification (Note 7-1)

Crystal load capacitance

Total board or trace capacitance

OEM board design

Stray capacitance

Microchip IC and OEM board design

CXTAL

XTAL pin input capacitance

Microchip IC

Load capacitors installed on OEM

board

Calculated values based on Figure 7-2 (Note 7-2)

FIGURE 7-2:

FORMULA TO FIND THE VALUE OF C1 AND C2

Crystal

XTAL2

(CS2 = CB2 + CXTAL2 )

XTAL1

(CS1 = CB1 + CXTAL1 )

C1 = 2 x (CL – C0) – CS1

C2 = 2 x (CL – C0) – CS2

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