USB2512 datasheet(56/63 Pages) MICROCHIP

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USB 2.0 Hi-Speed Hub Controller

Datasheet

Revision 2.2 (02-17-12)

SMSC USB251xB/xBi

DATASHEET

Chapter 7 AC Specifications

7.1

Oscillator/Crystal

Crystal: Parallel resonant, fundamental mode, 24 MHz

±350 ppm

Figure 7.1 Typical Crystal Circuit

Figure 7.2 Formula to Find the Value of C1 and C2

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

Each of these capacitance values is typically around 18 pF.

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

SMSC IC and OEM board design

CXTAL

XTAL pin input capacitance

SMSC IC

Load capacitors installed on OEM

board

Calculated values based on Figure 7.2 (Note 7.2)

1 M

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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