USB2251 datasheet(32/43 Pages) MICROCHIP

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 2009 - 2015 Microchip Technology Inc.

DS00002005A-page 32

USB2250/50i/51/51i

8.0

AC SPECIFICATIONS

8.1

Oscillator/Crystal

Parallel Resonant, Fundamental Mode, 24 MHz

350 ppm.

Note 8-1

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 8-2, "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 8-2

Each of these capacitance values is typically approximately 18 pF.

FIGURE 8-1:

TYPICAL CRYSTAL CIRCUIT

TABLE 8-1:

CRYSTAL CIRCUIT LEGEND

Symbol

Description

In Accordance With

Crystal shunt capacitance

Crystal manufacturer’s specification (See Note 8-1)

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 8-2, "Capacitance

Formulas" (See Note 8-2)

FIGURE 8-2:

CAPACITANCE FORMULAS

XTAL1

(CS1 = CB1 + CXTAL1 )

XTAL2

(CS2 = CB2 + CXTAL2 )

1 M

Crystal

C1 = 2 x (CL – C0) – CS1

C2 = 2 x (CL – C0) – CS2

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