USB251 datasheet(44/57 Pages) MICROCHIP

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USB251xB/xBi

DS00001692C-page 44

 2010 - 2015 Microchip Technology Inc.

7.0

AC SPECIFICATIONS

7.1

Oscillator/Crystal

Crystal: Parallel resonant, fundamental mode, 24 MHz

350 ppm.

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.

Note:

The USB251xB/xBi contains an internal 1 M

 resistor between the XTALIN and XTALOUT pins.

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

C 1

C 2

C L

Cry s ta l

XT AL O U T

(C S2 = C B2 + C XTAL2 )

XT AL IN

(C S1 = C B1 + C XTAL1 )

C 0

C1 = 2 x (CL – C0) – CS1

C2 = 2 x (CL – C0) – CS2

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