USB2640 datasheet(51/60 Pages) MICROCHIP

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DS00001947C-page 51

USB2640/USB2641

10.0 AC SPECIFICATIONS

10.1

Oscillator/Crystal

Parallel Resonant, Fundamental Mode, 24 MHz

350 ppm.

Note 10-1

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 10-2, "Capacitance

Formulas". However, the OEM PCB itself may present a parasitic capacitance between XTAL1 and

XTAL2. For an accurate calculation of C1 and C2, take the parasitic capacitance between traces

XTAL1 and XTAL2 into account.

Note 10-2

Each of these capacitance values is typically approximately 18 pF.

FIGURE 10-1:

TYPICAL CRYSTAL CIRCUIT

TABLE 10-1:

CRYSTAL CIRCUIT LEGEND

Symbol

Description

In Accordance With

Crystal shunt capacitance

Crystal manufacturer’s specification (See Note 10-1)

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 10-2, "Capacitance

Formulas" (See Note 10-2)

FIGURE 10-2:

CAPACITANCE FORMULAS

C1 = 2 x (CL – C0) – CS1

C2 = 2 x (CL – C0) – CS2

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