USB2512B Datasheet, PDF (44/57 Pages) Microchip Technology – USB 2.0 Hi-Speed Hub Controller

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
(C_{S1 =} C_B1 + C_XTAL1 )
C₁
C₀
Crystal _CL
C₂
XTALOUT
(C_{S2 =} C_B2 + C_XTAL2 )
TABLE 7-1: CRYSTAL CIRCUIT LEGEND
Symbol
Description
In Accordance with
C₀
C_L
C_B
C_S
C_XTAL
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 (C_L – C₀) – C_S1
C₂ = 2 x (C_L – C₀) – C_S2
Note 7-1
Note 7-2
C₀ is usually included (subtracted by the crystal manufacturer) in the specification for C_L 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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