USB2512AI View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

USB2512AI

USB 2.0 Hi-Speed Hub Controller

Microchip Technology 

USB251xB/xBi

7.0 AC SPECIFICATIONS

7.1 Oscillator/Crystal

Crystal: Parallel resonant, fundamental mode, 24 MHz 350 ppm.

Note: The USB251xB/xBi contains an internal 1 M resistor between the XTALIN and XTALOUT pins.

FIGURE 7-1:

TYPICAL CRYSTAL CIRCUIT

XTALIN

(CS1 = CB1 + CXTAL1 )

C1

C0

Crystal CL

C2

XTALOUT

(CS2 = CB2 + CXTAL2 )

TABLE 7-1: CRYSTAL CIRCUIT LEGEND

Symbol

Description

In Accordance with

C0

CL

CB

CS

CXTAL

C1

C2

Crystal shunt capacitance

Crystal load capacitance

Total board or trace capacitance

Stray capacitance

XTAL pin input capacitance

Load capacitors installed on OEM

board

Crystal manufacturer’s specification (Note 7-1)

OEM board design

Microchip IC and OEM board design

Microchip IC

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

FIGURE 7-2:

FORMULA TO FIND THE VALUE OF C1 AND C2

C1 = 2 x (CL – C0) – CS1

C2 = 2 x (CL – C0) – CS2

Note 7-1

Note 7-2

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.

Each of these capacitance values is typically around 18 pF.

DS00001692C-page 44

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